AC MEC 225 Theoryx

1

UNESCO-NIGERIA TECHNICAL &

VOCATIONAL EDUCATION

REVITALISATION PROJECT-PHASE II

YEAR II- SE MESTER II

THEORY

Version 1: December 2008

NATIONAL DIPLOMA IN

MECHANICAL ENGINEERING TECHNOLOGY

REFRIGERATION AND AIR

CONDITIONING

COURSE CODE: MEC225

MECHANICAL ENGINEERING TECHNOLOGY

REFRIGERATION AND AIR CONDITIONING MEC225

COURSE INDEX.

WEEK 1. 1. UNDERSTAND THE BASIC PRINCIPLES OF REFRIGERATION

1.1 Introduction……..………………..………..…1

1.2 DEFINITION OF REFRIGERATION….…2

1.3 Methods of Refrigeration……………………3

WEEK 2. 2.0 KNOW THE BASIC TOOLS AND THE EQUIPMENT USED IN

REFRIGERATION PRACTICE

2.1 COMMON HAND TOOLS IN REFRIGERATION WORKSHOP

2.2 GENERAL HAND TOOLS

2.3 SPECIALIZED SERVICE EQUIPMENTS

WEEK 3. 3.0 UNDERSTAND THE VARIOUS TYPES OF PRACTICAL

REFRIGERATION CYCLES

3.1 Know the Thermodynamic Principles of Refrigeration

3.2 REVIEW OF FIRST LAW OF THERMODYNAMIC

3.3 Refrigeration System and Cycles

3.4 System Representation

3.5 REFRIGERATION CYCLE

WEEK 4. 4.0 KNOW THE THERMODYNAMICS PRINCIPLE OF

REFRIGERATION

4.1 THE REVERSED CARNOT CYCLE

4.2 SIMPLE VAPOUR COMPRESSION REFRIGERATION SYSTEM

4.3 Mechanism of A VCR System

4.4 UNIT OF REFRIGERATION

4.5 REFRIGERATION SYSTEM CAPACITY

4.6 MASS OF REFRIGERANT CIRCULATED PER SECOND

4.7 VOLUME FLOW RATE OF VAPOUR

4.8 Coefficient of Performance (COP)

4.9 CONDITIONS FOR HIGHEST COP

4.10 THE CARNOT’S PRINCIPLE

WEEK 5. 5.0 KNOW THE FUNCTIONS AND THE PROPERTIES OF

REFRIGERANTS

5.1 REFRIGERANTS

5.2 REFRIGERANTS: QUALITIES OF DESIRED

REFRIGERANTS

5.3 CLASSIFICATIONS

5.4 COMMON REFRIGERANT TYPES

5.5 PROPERTIES OF REFRIGERANTS

5.6 PHYSICAL PROPERTIES OF REFRIGERANTS

5.7 REFRIGERANT PIPING MATERIALS AND ITS EFFECTS

5.8 REFRIGERANT STORAGE AND SAFETY

5.9 REFRIGERANT LEAK DETECTION

5.10 REFRIGERANT CHARGING



5.13 REFRIGERATIONS OILS

WEEK 6. 6.0 UNDERSTAND THE FUNCTIONS AND PROPERTIES OF

REFRIGERANTS AND KNOW THE REASONS FOR AND THE

METHODS OF LUBRICATION IN REFRIGERETION

6.1 REFRIGERANTS

6.2 REFRIGERANTS: QUALITIES OF DESIRED REFRIGERANTS

6.3 CLASSIFICATIONS




6.7 PRIMARY AND SECONDARY REFRIGERANT.




6.11 REFRIGERATION LUBRICATING OILS

6.12 METHOD OF LUBRICATION

WEEK 7. 7.0 KNOW THE PROCEDURE OF CHARGING

REFRIGERATION CIRCUIT



WEEK 8. 8.0 KNOW THE VARIOUS APPLICATIONS OF REFRIGERATION

8.1 REFRIGERATION INDUSTRY AND APPLICATIONS

8.2 CLASSIFICATION OF APPLICATION OF

REFRIGERATION SYSTEM

WEEK 9. 9.0 KNOW THE FUNCTION OF AN AIR CONDITIONING SYSTEM FOR A

BUILDING

9.1 AIR CONDITIONING SYSTEM

9.2 AIR CONDITION PLANT

9.3 OPERATION PRINCIPLES OF AIR CONDITIONING

PLANT

WEEK 10. 10.0 KNOW HOW TO CALCULATE COOLING LOADS FOR

REFRIGERATION AND AIR CONDITIONING SYSTEMS

10.1 COOLING LOADS AND CALCULATION

WEEK 11. 11.0 KNOW THE FUNCTION OF AN AIR CONDITIONING

SYSTEM FOR A BUILDING CONTD

11.1 REFRIGERATED COOLING OR AIR CONDITIONING

11.2 PACKAGED AIR CONDITIONING

11.3 SPLIT SYSTEM AIR CONDITIONING

WEEK 12. 12.0 KNOW HOW TO SERVICE REFRIGERATION SYSTEM

12.1 REFRIGERATION SYSTEM SERVICE

12.2 SAFETY PRECAUTIONS

12.3 ADJUSTMENT OF CONTROLS

12.4. THERMOSTATIC EXPANSION VALVES

WEEK 13. 13.0 KNOW THE FUNCTION OF COMPONENTS OF AN AIR

CONDITIONING SYSTEM FOR A BUILDING

13.1 HEAD PRESSURE CONTROLLERS

13.2 PRESSURE CONTROLS

13.3 ACCESSIBLE AND OPEN COMPRESSORS



14.1 ADDING OIL TO OPEN/ACCESSIBLE HERMETIC COMPRESSORS

14.2 REPLACING WELDED HERMETIC COMPRESSORS

14.3 AIR OR OVERCHARGE

14.4 GAS CHARGING METHODS



15.1 STEAM PAN HUMIDIFIER

15.2 ELECTRODE HUMIDIFIERS

15.3 WATER ATOMIZING HUMIDIFIERS

A15.4 CONDENSERS

15.5 WATER PUMPS

WEEK 1

1.0 UNDERSTAND THE BASIC PRINCIPLES OF REFRIGERATION

LEARNING OUTCOME: CONCEPTS AND PRINCIPLES AND DEFINITIONS.

1.1 Introduction: Refrigeration and air-conditioning are now an integral part of modern

life. The application of this technology has made possible provision of every

commodities and services which have made life more comfortable, easier, safer and

healthier.

Fig 1.0 Picture of refrigeration and Air-conditioning hard wares.

Fig 1.1 Picture of refrigeration and Air-conditioning hard wares.

Making cold depends on a few simple principles. Of these, the most important is that

heat always flow from a warm to a cool body and never reverse.

Fig 1.2 Heat flow

Warm

Cool

Heat

To cool a warm object therefore, we need to place it near a cold one like a block of

ice, or in a cold well, pond or lake as is obtained in countries in cold regions. Using

natural ice and cold water to remove heat provided mans first cooling systems.

Fig 1.3 Pictures of ice blocks

Typical heat flow principles are thus so illustrated. If we put some ice cubes in a jar

with a thermometer was in fig4, heat flowing from the room and surrounding air

melts the cubes, if the room temperature is above 00c. To melt 1 kg of ice, 339.94 kg

of energy in the form of heat must be absorbed.

Fig 1.4 Heat flow

Fig. 1.5 Ice melts

Conceivably, we could cool an entire room with ice cube, but the process would not be

efficient and ultimately cumbersome.

If a concentrated heat source is applied to a jar, fig 2, water formed from the ice cubes boils

when it reaches a temperature of 1000c. Once again, heat is flowing from a warm body to a

cooler one. To make the water change to steam, a gas like vapor, we need to ad a certain

quantity of heat energy to the 1kg of water. This heat quantity is the enthalpy of

vaporization of water the steam, there is no temperature change heat absorbed by ice in

melting is enthalpy of fusion.

Fig.1.6 Superheating

When all the water changes to steam, further heat addition causes super heating,

indicating that the temperature of the steam is higher that corresponding to its

pressure fig1.6.

Water boils at 1000C when subjected to atmospheric pressure on surface of a liquids

pressure. Increasing the pressure on surface of a liquid raises the reverse of

vaporization also at constant temperature. For a given pressure of the liquid,

condensation occurs at the same temperature as vaporization.

Refrigerants are fluids that, with few exceptions, boil at low temperatures at

atmospheric pressure. For example, ammonia at 1 atm (1.013 bar) boils at – 33.30,

while F-12 (Freon 12) boils at -300c- at the same pressure. Boiling and condensation

process of refrigerants are the same as for water.

The phenomena discussed above are mainly heat transfer processes. The aspect of

refrigeration can come in when the principle of heat transfer is so applied to bring

about an artificial change in a well regulated cold state production that is

reproducible irrespective of the location or environment.

Applying the concepts of heat transfer using refrigerants as a medium or system fluid

the fig. 5 represents an insulated box. With normal atmospheric pressure the liquid

boils at -150C absorbing about 1315KJ of heat energy during vaporization. Since little

heat can pass through the insulated box, heat needed for vaporization will come

from the air or other contents of the box. Thus, we have a crude system for

producing refrigeration.

Fig.1.7 Boiling Ammonia

Fig 1.8 Recovering Ammonia

This cycle can be repeated by replacing the condensed ammonia in insulated box.

This process can also be carried out the same result in different locations provided

the parameters are the same. The process so far explained is a typical refrigeration

process.

1.2 DEFINITION OF REFRIGERATION

In the light of the foregoing, refrigeration can be defined as the artificial withdrawal

of heat producing in a substance or within a space temperature lower than that

which would exist under the natural influence of surrounding. It is the science of

providing and maintaining temperature below that of surroundings. These definitions

are quite in agreement with the American Society of the Heating Refrigeration and

1.3 Methods of Refrigeration

The following means are available for achieving cold state. These methods are

capable of producing a specific degree of cold state which is reproducible if the

parameters are maintained. The methods are briefly discussed below:

(i). Dissolution of certain salt in water: When certain salts such as sodium

chloride, salt patre etc. are dissolved in water, they absorb heat. This

property has been used to produce refrigeration. By this method, the

temperature of water can be lowered much below 00C, the freezing

temperature of water. Calcium chloride lowers the water temperature up to

around -500C while sodium chloride up to – 200C. The salt used for

refrigeration has to be regained by evaporating the solution. Also there is a

limitation on the amount of salt to be put in the solution. On one hand, the

refrigeration produced is quite small and on the other hand, the regaining

process of salt is so cumbersome that this is not feasible for commercial

exploitation. However when some salts are used with water it lowers the

freezing temperature of water by taking off its latent heat from the solution.

It is in this sense that salt may be employed as secondary refrigerant to

obtain freezing below 00C. Areas of application are numerous and include

Breweries, chemical plants, diaries, food processing, ice cream, ice plant,

meat packaging, skating ring etc.

ii. By coolant, i.e. by lowering the temperature of a coolant. To achieve cooling,

eat is removed from a coolant and hence this lowers the temperatures of the

coolant.

If Q is heat removal rate from the coolant (we call it the cooling load),

then Qout = MCP (Tf – Ti) - - - - - (i)

This is called the refrigerating effect

Where M = Coolant flow rate

Cp = Specific heat of coolant

Tf = Coolant temperature at entry

Ti = Coolant exit temperature

= Temperature of refrigerated space

Then Ti > Tf i.e

Qin = heat leakage into system

= MCP (Ti – Tf) - - - (1.2)

iii. By change of phase for example, cooling is brought about by removal of heat

a phase change, i.e. heat of sublimation or vapourisation.

. Heat of vapourisation = hfg ⇒ from liquid to gas

. Heat of sublimation = hsg ⇒ from solid to gas

. Heat of welding or fusion = hcf ⇒ from solid to liquid.

Examples

(a). Use of packaged ice to achieve refrigeration:

Tf

Qin

Ti

Initial mass of ice = Mi

Initial internal energy = Ui

Final mass of ice = Me

Final internal energy = Ue

Also exist condition, dm, h2, U2 etc.

Therefore, energy balance on control volume

dQ = Heat leakage into control space

V2 = Very small specific volume and so could be Ignored.

dW=0

dQ

dm2

U2 =(Uf)2

Chilled space (space whose

Temperature you want to

Ice

Hence,

DQ + Mi Ui = dm2 h2 +Me Ue

DQ = dm2 (h2) – (MiUi – MeUe)__________ (1.3)

U refers to the internal energy of all the content in the chilled space, ⇒ ice, chilled

air, food etc.

and

dm2 = Mi – Me

∴ DQ = (Mi – Me) h2 – (Mi Ui – Me Ue)

= heat removed from everything in the chilled space.

DQ therefore is the refrigeration that must be removed, if heat is control space must

be maintained at that of the melting ice.

h2 refers to only the liquid water coming out, i.e h2 = (hf)2

If we ignore the internal energy of other things except that for ice,

then,

DQ = (hf2) [Mi – Me] – (Mi – Me) Usf

Fig 1.9 Picture of icebox with item stored for cool

(b). Change of phase by heat of vapourisation:

Popular vapour compression cycles of the heat of vapourisation of refrigerant. This is

achieved by controlling pressure at which this occurs.

Fig 1.10 Diagram of V. C. Refrigeration System representation

(c). Use of dry ice or solid carbon dioxide:

The dry ice can be produced by cryogenics. Use of dry ice involves use of

sublimation of the packaged ice but operates at a lower temperature. This is what is

used in most cases to preserve ice cream.

(vi). Expansion of a liquid.

If the expansion occurs entirely in the liquid state, fall in temperature will be quite

small as in a- a1. Expansion that leads in liquid vapour phase will lead to higher

temperature drop in (b – bi) the fluid. This expansion process of a liquid is employed

in vapour compression cycle.

Fig 1.11 Refrigeration by liquid expansion

(v). Steady-flow Expansion of a Gas:

b

b1

a

a1

T

S

The throttling process for real fluid comes into this category. Throttling may lead to

cooling depending on the condition at (1). Suppose the flow passage were

converging (velocity) V2 >V1 so then h2< hi. Ideal fluids will reach a stage where h2

in less than h1 i.e T2<T1.

So, if flow passage converges, we can produce cooling effect.

H1 + V21 = h2 + V22

i.e h1 – h2 =½ (V22 – V

21)

So if V2 > V1, h2<h1

(vi). By Expansion Turbines

The expansion of gas in a turbine while producing work will produce some cooling

effect. This is used in aircraft air conditioning.

For a turbo jet, a kg of air is bled out and passed through heat exchanger. The air

leaving the compressor has high temperature and pressure and in the turbine, its

temperature and pressure are lowered and discharged into the aircraft cabin.

Humidity is low at high attitude even if temperature is comfortable. For a plane that

is in motion, we get the cooling air from the atmosphere we don’t need fan.

(v). Electrical Process

By electrical process, cooling can be produced by either of these two process.

(a). Molecular alignment of a material under a magnetic field:

If we impose magnetic field on a specimen, cool it under the influence with

the circulation of helium, we shall be arranging the molecules as those of iron

filings, and by removing the effect of electricity, or magnetic field electrons

try to revert to their disorderly behaviour and in so doing absorb heat and so

cause cooling.

(b). Peltier effect: This is the principle of thermocouple.

Voltage differential is created if two junctions are put in cold and hot

junctions are put in cold and hot junctions.

Therefore, if we impose an e.m.f, we create cold at the cold junction and

heat in hot junction.

(c). Purposes of Refrigeration

The followings are major and encompassing purpose of refrigeration.

(i) To extract as much heat at possible from a cold researviour with the

minimum expenditure of work.

(ii). To create an environment most suitable for preserving food, fruits,

meat and related products thus reducing spoilage to minimum level.

(iii). To create a conducive environment for good and sweet or tasteful

ripening of fruits.

(iv). To enable certain agricultural products that are seasonal to be

available at all seasons.

(v). For moderating temperatures in industrial environment.

WEEK 2

2.0 KNOW THE BASIC TOOLS AND THE EQUIPMENT USED IN

REFRIGERATION PRACTICE

2.1 COMMON HAND TOOLS IN REFRIGERATION WORKSHOP

Air conditioning, heating and refrigeration technicians must be properly use hand tools and

specialized equipment relating to this field. Technicians must use the tools and equipment

intended for the job.

2.2 GENERAL HAND TOOLS

(i) Portable electric drills:- These are used extensively by refrigeration and air conditioning

technicians. They are available in cord type (115v) or cordless (battery operated)

(ii) Flaring tools: - The flaring tools has s flaring bar to hold the tubing, a slid- on yoke and a feed

screw with flaring core and handle. Several sizes of tubing can be flared with this tool.

(iii) Swaging tools: - Swaging tools are available in punch type and lever type

Fig 2.1 Working tools

(iv) Tube benders: - Three types of tube benders may be used: spring type, lever type and to a

lesser extend, gear type. These tools are used for bending soft copper and aluminum.

(v) Plastic tubing shear: - A plastic tubing shear cuts plastics tubing and non-wire-reinforced or

synthetic hose.

(vi) Tubing pinch-off tool: - A tubing pinch – off tool is used to pinch shut the short stub of tubing

often provided for service, such as the service stub on a compressor. This tool is used to

pinch shut this stub before sealing it by soldering.

(vii) Metal workers hammer: - A metal workers hammers straightens and form sheet metal for

duct work.

2.3 SPECIALIZED SERVICE EQUIPMENTS

(i) Gage manifold: - This is one of the most important of all pieces of refrigeration and

air conditioning service equipment. It normally includes the compound gage, the

manifold, valves, and hoses. The four valve design has separate valves for the

vacuum, how –pressure, high pressure and refrigerant cylinder connections

Fig 2.2 Manifold gauges

(ii) Refrigerant charger: - This device is used to accurately charge a system with

refrigerant. It can charged by pressure or by refrigerant weight. It has both digital

and analog displays and charges R-12, R-22, R-500 AND R-502.

(iii) Electronic charging scale: - It allows a technician to accurately charge refrigerant by

weigh. This can be done manually or automatically. The amount of refrigerant to be

charged into the system can be programmed.

(iv) Halide leak detector: A hide leak detector detects refrigerant leaks. It is used with a

acetylene or propane gas. When the detector is ignited, the flame heats a copper disc. Air

for the combustion is drain through the attached hose. The end of the hose is passed over or

near fitting or other area where a leak the refrigerant will be drain into the hose and contact

the copper disc. This breaks down the halogen refrigerant into other compounds and

changes the colour of the flame. The colour change from green to purple, depending on the

size of the leak.

(iv) Vacuum pump: Vacuum pumps designed specifically for servicing air conditioning

and refrigeration systems remove the air and non-condensable gases from the

system. This is called evacuating the system and is necessary because the air and

non-condensable gases take up space contain moisture and cause excessive

pressures.

(v) Refrigerant recovery recycling station: It is illegal to vent refrigerant to the air. The

refrigerant from refrigeration and air-conditioning system is pumped into a cylinder

or container at this station where it is stored until it can be changed back into the

system if it meets the requirements or transferred to another approved container

for transportation to a refrigerant reclaiming facility.

(vii) Compressor Oil Charging Pump: This is use specifically for charging refrigeration

compressors with oil without pumping the compressor down.

WEEK 3

3.0 UNDERSTAND THE VARIOUS TYPES OF PRACTICAL REFRIGERATION CYCLES

3.1 LEARNING OUTCOME: Know the Thermodynamic Principles of Refrigeration

Refrigeration operation is governed by thermodynamic principles. It involves energy

transactions in its operation and the energy accounting is performed. This energy

accounting is performed. This is done using basic laws of thermodynamics. Before

reviewing these laws, it is appropriate to highlight and explain terms associated with

refrigeration system operation. These terms are briefly defined below.

. Isobaric Process: Constant pressure process, no pressure

change. During the process, remains constant. This can also

be referred to as isopiestic process.

. Isentropic Process: Constant entropy process, no change

in entropy. Entropy is a state or order or disorder in a

system.

. Isenthalpic Process: This is a constant enthalpy process.

During the process, the enthalpy remains unchanged.

. Isobaric Process: Constant Pressure Process, NO pressure Change

During the process, Pressure remain constant The can also be referred to an

isopiestic process

. Isentropic Process: Constant isentropy, no change in entropy. Entropy is the

state of disorder in a system.

. Isenthalpic Process: Constant enthalpy, no change in enthalpy

. Adiabatic Process: No heat transfer, Q = O

A process in which no heat is transferred across the boundary

. Isochoric Process: In this process, the volume remains constant throughout.

It can also be called isometric process

. Isothermal Process: In this case the temperature remains constant during

the process

• Psychrometry deals with the state of atmosphere with respect to moisture content. It

deals with the thermal properties of air and the control and measurement of the

moisture content of air in addition to the stud of effects of atmospheric moisture on

commodities and human comforts.

• Dry-Bulb Temperature, Tab: The actual temperature of gas or mixture of gases

indicated by error –free temperature measuring device

• Wet-bulb Temperature, Twb: It is the temperature obtained by an accurate

thermometer having a wick moistened with distilled water and the air stream across

the wet-bulb flows with a velocity of 270mm

• Dew point Tdp: It is the temperature at which the liquid droplets just appear when the

moist air is cooled continuously.

• Absolute humidity: It is the amount of water vapour per unit volume of the gas

• Relative humidity φ; It is the ratio of the actual partial pressure of water vapour in the

moist air to the saturation pressure of water vapour corresponding to the dry-bulb

temperature

φ = Existing partial pressure of water vapour, Pv

The saturation pressure of pure water vapour

at the same temerpature Bs

= Pv/Ps - - - - - - (1)

• Humidity ratio, w: This is called specific humidity it is defined as the amount of water

vapour in the moist air per unit mass of the dry air in a given volume.

Using the perfect gas relation for air and water vapour for a given volume V of the

Moist air,

Mv = Pv V/(Rv T) - - - -- - - (2)

and the mass of air is

Ma = (P - Pv) V/Ra T) - - - - (3)

Where P = The total pressure of the moist air

From definition

� � ��

� ��

. ��

� 0.622 ��

- - (4)

Ra = 8317/28.96 = 287.2 KJ/Kg-K - Gas Constant

Molecular wt

Rv = 8317/18 = 462KJ/Kg-K

Ra – Gas Constant of dry air

Rv = Gas constant of water vapour

• Saturated air: A mixture of dry air and enough water vapour all at the same dry-bulb

temperature

• Degree of saturation u: it is defined as a ratio of the weight of the water vapour at an

given temperature associated with the unit mass of dry air to the weight of water

vapour (Ws) associated with unit weight of saturated air at the same temperature

� � ��

- - - - - (5)

Using equation 4 for w and Ws, it is found as:

µ = [Pv /(p-Pv) ] [(P-Ps)/Ps] - - - (6)

µ = � ��/ ��

�/�� - - - - (7)

and φ = µ / [1 – (1-µ) Ps/P] - - - (8)

� � ��

��

��

� ��

� � �!��

For moist air at 303k (30oC)

Dew point = 288K (15oC)

The total pressure = 1 bar

Obtain

(i) Relative humidity (ii) Degree of saturation

Solution

From steam table,

Ps = Saturation pressure at 303k = 0.04242

Pv = Vapour pressure at 288K = 0.01744

φ = [Pv/ (p-pv)] [(P-Ps)/Ps]

Pv = 0.0174 bar

Ps = 0.04242 bar

P = 1bar

"#$%# � � & '.'�(')��'.'�(')* &�'.'�('�'.')+)+�

'.')+)+ * � 0.39

Specific Quantities: In case of moist air, the specific enthalpy, specific volume, specific

entropy specific humidity etc are expressed per unit mass of the dry air (moisture free air). If

the air is moist having specific humidity.

Specific enthalpy: h = ha + w hv

Specific volume: v = Va + w Vv

Specific entropy: S = Sa + W Sv

Where ‘a’ and ‘v’ stand for dry air and water vapour respectively

The specific volume of most air can be obtained from

V = Ra Tdb /P – Pv)

Enthalpy, h = Cp Tdb + w hv

Specific enthalpy of saturated vapour, hv = hdp + 1.884 (Tdb – Tdp)

Hence h = 1.004Tdb +w [hdp + 1.884 (Tdb – Tdp)]

The wet-and dry-bulb temperature have been used to determine the saturation pressure

corresponding to dew point.

Dr. Carriers equation:

• Pv = Pwb = (P – Pwb) (Tdb – Twb) - - - (1)

1940 1.44 Twb

The ferret’s equation

Pv = Pwb – 0.00066p (Tdb – Twb) .�/01 +(2.�3�4(+.4 5

The Apjohn’s equation

Pv = Pwb – P �/01 /61�

�3''

For temp below 273.15K, the carrier equation becomes

Pv = Pw - �/61�61� �/01/61

�((3'.'7 /61

Pv = Pwb - �/61�61� �/01/61

�3)(�.)) /61

Data obtained or given

Tdb = 30oC

Twb = 20oC

Barometric reading = 740mm Hg

To determine

(a) Dew point and R. H

(b) Degree of saturation

(c) Specific humidity

(d) Specific Volume

(e) Specific enthalpy

Solution

Ambient pressure = ()'(8' 1.013 � 0.9863;<=

Saturation pressure at due point

Pv = Pw - ��61� �/01/61

�3)(�.)) /61

From table, Pwb = 0.02337bar

Hence Pv = 0.0237 – (0.9863 – 0.02337) (30.20)

1547 – 1.44 x 20

= 0.02337 - 0.96293 x 10

1518.2

= 0.02337 – 0.0063426

= 0.017027 bar

D.P = 15oC (dew point temp).

Relative humidity, R. H = ��

� �

Ps = 0.04242 bar

= 0.017027 = 0.4014

0.042242

The degree of saturation, µ =

� & ��

* &� ��

*

= & '.'�('+('.7482'.'�('+(* &'.7)244

'.')+)+*

= 0.3909

The specific humidity, w =

= '.8++�� '.'�('+(

'.7482'.'�('+(

= '.'�'37'4'.787+(2 � � '.'�'7+83>?

>? @A B=C <D=

Specific volume,

� E FGH� � ��

I 287.2 303 0.9863 � 0.017027 103

= 4('+�.8 '.787+(2 103 � 0.8978 K2/LM of dry air

Specific enthalpy, h = 1.004Tdb + w (2538.9 + 1.884 (Tdb – Tdp)

= 1.004 x 30 x 0.0109265 [2528.9 + 1.884 (30-15oC)]

30.12 + 27.94 = 58.06 KJ/kg

3.2 REVIEW OF FIRST LAW OF THERMODYNAMIC

It is already a generally accepted concept and hypothesis that energy cannot be created nor

destroyed, but could be transferred from one form to another. This is known as the

principles of conservation of energy. It forms the basis of many of the scientific laws

including the first law of thermodynamics.

Energy is defined as the capacity of a system for interacting with and influencing its

environment or surroundings. It is the central figure or factor in all interactions between

systems. All interactions involve the flow, one way or the other of energy between the

systems involved. The flow of energy occurs either by way of heat or in the form of work, the

variety of work being extensive.

Heat and work are energy in transits (i.e. is flow) from one system to another. They may not

be said to be contained in any system. They are only recognized as such during interaction.

Systems do contain energy, but not work nor heat. Energy contained in a system is internal

energy, the sum total of the kinetic energy (or translation rotation and vibration) and

potential energy of all, along with the energy of chemical bonding and unclear bonding

among all the particles of matter making up the system. The reference to potential energy

suggests immediately that some datum or reference level must be identified from which

such energy is reckoned. The datum is usually arbitrary. In thermodynamics, the concern is

ultimately with changes in internal energy and other system properties notwithstanding the

arbitraries of the datum. When energy content of a system is reckoned relative to a different

but equally arbitrary datum, the resulting energy property of the system is known as

enthalpy.

Therefore, energy may flow from one system to another by means of either heat or work

but it may not be said to be contained in these form. This is part of the substance of the first

law of thermodynamics which states that “Energy may be transferred to and from a system

by means of either heat or work. In any event, energy is conserved; it can neither be created

nor destroyed”. Put differently; “when any closed system is taken through a cycle, the net

work delivered to the surrounding is proportional to the net heat taken from the

surrounding

This can be expressed mathematically

As

∑ @ OP � ∑ B�

Or Q P � Q �

This means that some work can be converted to heat in a system and some heat converted

conversely into work

Thus, W =Q

Where W = Work

and Q = Heat

For a non-flow energy equation, Heat absorbed by the system = Increase in internal energy +

work done by the system

Hence Q = ∆u +W - - - - - - (1)

Where ∆u = increase in internal energy = ∆K + ∆P

This agrees with the energy conservation law and is therefore a statement of the first law of

thermodynamics

3.3 Refrigeration System and Cycles

3.3.1 First law of thermodynamics and System Principles

The first law can be represented by the relationship

W = Q

Where W = work

And Q = Heat

This means that some work can be converted into heat is system and conversely some heat

can be change into work

For a non flow energy equation, the can be represented thus

Q = ∆ u + w

In general, in a thermodynamic system, Heat absorbed = Change is internal energy + work

Q1 - Q2 = ∆V + W

Over a cycle ∆u = O, and W = Q1 – Q2

For a Carnot engine, heat Q1 is received at high temperature T1 from heat source. The

engine then converts some of this heat into work, W, and finally rejects the remainder

energy Q2 into a sink at T2 2 cold space)

The thermodynamic efficiency of the engine is given by

Carnot = RS � RT

RS � 1 � RT

RS

T1

HOE

T2

HOE

Q1

Q2

Sink

W

Source

For a Carnot refrigerator or reversible engine, R1 work W is done o the engine to take away

heat Q1 from a source at T1 (cold space). This work, W, is converted into heat Q2 which is

rejected into the sink at T2

The efficiency of this system is given by

= RS

RT RS

The efficiency is called the coefficient of performance

The purpose of a refrigerator is TD extracts as much heat as possible from a cold reservoir

with minimum expenditure of work. The performance

Of a refrigerator is measured by the coefficient of performance.

3.4 System Representation

This diagram is a flow diagram of a simple vapour compression system.

The parts include:

1. Evaporator

2. Suction line

3. Vapour compressor

4. Discharge line

5. Condenser

6. Receiver tank

7. Liquid line

8. Refrigerant flow control

Evaporator

(Inside)

Valve (Refrigerant flow

control)

Section

Line Discharge

Line

Liquid

Line

Receiver Tank

Fig 3.1 Refrigeration system representation

Assignment

Briefly describe the function and features of each of the various parts of a vapour compression

system as outlined above. Use also diagrams for your illustration

3.5 REFRIGERATION CYCLE

As the refrigerant circulates through the system, it passes through a number of changes in

state or condition. Each of this stage of change is called process. The refrigerant starts at

some initial state, passes through a series of processes in a definite sequence and returns to

the initial condition. This series of processes is called a cycle.

The charts below are the various cycle representations based on the given relationships.

Mother Chart V

P

d

c

a

b Qin

S=c

T1

T2

Liquid Vapour

e

Vapour Out

Liquid

P.V Chart

a

S

H

d

c

a

b Qin

S=c

T1

T2 e

Qout

c

d

V

T

c

a

b Qin

T1

T2

T.S Chart

b

Qout

d

Isentropic Chart H

P

d

c

a

b Qin

S=c

T1

T2 e

Qout

The cycle description for the processes using these charts include:

1. a – b ⇒ Irreversible throttling expansion ( )T ,∆∆P no heat loss

2. b-c ⇒ reversible isothermal expansion (evaporator) Q is absorbed at T1 (constant

temperature)

3. c-d ⇒ reversible adiabatic compression (not heat transfer)

4. d-e ⇒ Isobaric cooling of super heated vapour to saturation

5. e-a ⇒ Isothermal condensation of vapour to saturated liquid (Q, lost at T2)

bc h - h =inQ

ad h - h =outQ

isentropic is b-a since h h ba =but

acin h - h Q =∴

bc h - h =

effect ingRefrigerat =

Also

inout

in

Q-Q

Q ... =POC

( )bcad

bc

h - h - h - h

h - h =

cd

ac

h - h

h - h =

3.4.1 Refrigeration Effect (qc)

This is defined as the quantity of heat absorbed from the refrigerated space per unit mass of

refrigerant. It is the amount of costing produced by a system.

ac h - h =cq

41 h - h =

31 h - h =

1s T ∆=

( )T41 S-S =

This is sometimes called valuable commodity. Theoretically, this equal to the latent heat of

vapourisation. In actual practice, the refrigerant is at the condensing temperature before it

enters the expansion valve (point ‘a’ in the diagram) and must be first reduced to the

evaporator temperature. That is, only part of the refrigerant vapours in the evaporator.

H

P

4

3

1

2 21

T

4

3

1

2 21

S

T1

Latent heat fc h - h =λ

bcc h - h q E. . ==R

Loss (part of λ to cool

from T1 → T2) fb h - h =

f b c

d e a T2

P

H

Saturation Line

T1

WEEK 4

4.0 KNOW THE THERMODYNAMICS PRINCIPLE OF REFRIGERATION

4.1 THE REVERSED CARNOT CYCLE

A reversed Carnot cycle is a refrigeration cycle and consists of four reversible processes. The

fig. T-S diagram below is used to describe the processes.

(a) 4-1, Isothermal expansion at low temperature 41 T =T heat transfer in cold

chamber for an isothermal process,

W =Q

( ) 1-41-4411 W Q S - S T ==∴

5416 Area =

Qout

2 2

4

5 6

S

T

Qin

T1-T4

T2-T3

1

S3-S4 S1-S2

Cycle on T-S Diagram

(b) 1-2, isentropic compression from T1 to T2. The compression is also adiabatic

0 Q 2-1 =∴

212-1 u-u W =and

fig 4.1

(c) 2-3, Isothermal compression at high temperature (energy rejection)

32 T =T

Also ( ) ( )412322 S-ST- S =− ST

5623 Area W- Q- 3-23-2 ===

The –ve sign is because of the negative heat transfer. This means that the heat transfer is a

loss here. Heat is rejected into the atmosphere.

Compressor

Evaporator

Condenser

Expander

1 4

3 2

System Diagram

(d) 3-4, Isentropic expansion from T3 to T4. This is the same as T2 to T1. The

expansion is also adiabatic

0 Q 4-3 =∴

and 4-343 U- =−W

Fig 4.2 System representation

Compressor

Qin

Qout

4 1

2 3

We

Expander

We

For this cycle, heat received at low temperature = Refrigerating Effect.

( )411 S - S T =

For a cycle, i.e. 1→2→3→4→1

∫ ∫= Q W

Heat is taken from (4) to (1), Qin = 1T s∆

Heat is taken out from (2) to (3), Qout = 2T s∆

rejectedHeat - receivedHeat work =Net

( ) ( )∫ = 412411 S-ST - S - ST W

2s1s - T T∆∆=

( ) ( ) ( )411212s S-S T-T =−∆= TT

The –ve sign shows that work must be supplied to perform the cycle. Thus, the external

energy supplied to perform the cycle ( ) ( )4112 S-S T-T =

( ) s∆= 12 T-T

For Carnot cycle, heat engine,

∫ =absorbed

doneWork

Heat

outQ

W =

( )

2

12

T

T-T =

( )

2

12

T

T-T =

2

12

2

24

Q

Q-Q

Q

Q-Q ==

2

1

Q

Q - 1 =

2

1

T

T - 1 =

For a refrigeration cycle (reversed Carnot cycle), the valuable commodity here is Qin and heat

extracted = 1Ts∆

work

commodity Valuable P. O. .

NetC =

suppliedenergy

Effect ingRefrigerat

External=

This is a measure of the performance of the system. Hence,

( )( )

( )( )4112

411

12s

1s

T-T

S-S T

T P. O. .

SSTTC

−=

−∆∆=

12

1

T - T

T =

12

1

1

2 Q - Q

Q

1 - T

T1

==

Evidently, the COP of a Carnot refrigeration cycle is a sole function of upper and lower

temperatures.

COP can be raised by raising the top temperature and or by making ( )12 TT − small.

For effective exchange of heat to occur, then, the temperature of space, T0 from T1

and the space where you are depositing heat, its temperature Tb must be less than

T1, i.e. for heat to be extracted from inside the room, 1a T T , >aT and for heat to be

sent into the environment, b2 T T , >bT .

The Carnot cycle is practically not feasible since isothermal energy rejection requires

extremely slow motion followed by isentropic process during which the piston

should move at extremely faster rate. Such motion is mechanically not obtainable.

On the other hand if phase change is allowed for isothermal energy transfers, the

T

Tb

3 2 2 Tb

(Condenser)

Ta

(Room)

S

4 T1 1

Ta

compression will be so wet that the same will lead to severe mechanical difficulties

since the liquid present inside the compressor may get squeezed causing bursting of

the cylinder or other components due to practically incompressible temperature of

liquid.

As such, the Carnot cycle serves as the basic ideal cycle meant for comparison of

other cycles. The deviation of various values in other cycles will be for measure of

presence of irreversibility in various processes.

4.1.1 Summary

For heat engine, Carnot efficiency

∫ =2

12

T

T - T ……………………………………. (1)

For reversed carnot cycle,

1→2→3→4→1, Heat is taken in from (4) to (1), ≈∆= 1sTQin that is taken out from (2) to

(3).

2s T ∆=outQ ………………………………………….. (0)

Qin

T

S

Qout

2 T3

T1 1 4

s∆

2

( )12s work TTNet −∆= ………………………………….. (1)

1sin Q effect Re Tgfrigeratin ∆== ……………………….. (2)

work

effect ingRefrigerat ..

NetPOC =

( ) 12

1

12s

1s

T - T

T

T -

T =

∆∆=T

…………………………… (3)

4.2 SIMPLE VAPOUR COMPRESSION REFRIGERATION SYSTEM

A vapour compression refrigeration system is an improved type of air refrigeration system in

which a suitable working substance, termed as refrigerates used. It condenses and

evaporates at temperatures and pressure close to atmospheric conditions. The refrigerates,

usually used for this purpose are ammonia (NH3), carbon dioxide (CO2) and sulphur dioxide

(SO2). The refrigerant used, does not leave the system, but is circulated throughout the

system alternatively condensing and evaporating. In evaporating, the refrigerant absorbs its

latent heat to the circulating water of the water. The vapour compression refrigeration

system is therefore a latent heat pump, as it pumps its latent heat from the brine and

delivers it to the cooler.

The vapour compression refrigeration system is universally employed for all purpose

refrigeration. It’s is generally used for all industrial purposes form small domestic

refrigerators to big air conditioning plant.

4.2.1 Advantage and Disadvantages of VCR system over Air Refrigeration

System

(i) It has smaller size for given capacity of refrigeration temperature

(ii) It has less running cost

(iii) It can be employed over a large range value

(iv) The coefficient of performance is quite high

4.2.2 Disadvantages

(i) The initial cost is high

(ii) They prevent of leakage of the refrigerant is the major problem in VCR

system.

4.3 Mechanism of A VCR System

Low pressure

liquid

Insulated

Chamber

Evaporator

Fig4. 3 Diagrammatic Representation of Simple VCR System

The figure show schematic diagram of a simple vapour compression refrigeration system. It

consists of the following five essential parts

Expansion valve or

refrigerant control valve

Receiver

Low pressure vapour Pressure

Gauge

Low

pressure

side

Compressor

High pressure side

Pressure gauge High pressure

vapour

Condenser

High pressure liquid

vapour mixture

1. Compressor: The low pressure and temperature vapour refrigerant from evaporate

is drawn into the compressor through the inlet or suction value A, where it is

compressed to high pressure and temperature. This high pressure and temperature

vapour refrigerant is discharged into the condenser through the delivery or

discharge value B.

2. Condenser: the condenser or cooler consists of coils of pipe in which the high

pressure and temperature vapour refrigerant is cooled and condensed. The

refrigerant, while passing through the condenser, gives up its latent heat to the

surrounding condensing medium which is normally air or water.

3. Receiver: The condenser Liquid refrigerant from the condenser is stored in a vessel

known as receiver from where it is supplied to the evaporator through the

expansion valve or the refrigerant control value.

4. Expansion value: It is also called throttle value or refrigerant control value. The

function of the expansion valve is to allow the liquid refrigerant under high pressure

and temperature to pass at a controlled rate after reducing its pressure and

temperature. Some of the liquid refrigerant evaporates as it passes through the

expansion value the greater portion is vaporized in the evaporator at the low

pressure and temperature.

5. Evaporator: An evaporator consists of coils of pipe in which the liquid –vapour

refrigerant at low pressure and temperature is evaporated and changed into vapour

refrigerant at low pressure and temperature. In evaporating, the liquid vapor

refrigerant absorbs its latent heat of vaporization from the medium (air water or

brine) which is to be cooled.

In any compression refrigeration system, there are two different pressure conditions. One is

called the high pressure side and the other is known as low pressure side. The high pressure

side includes the discharge line (i.e. piping from delivery value B to the condenser),

condenser, receiver and expansion value. The low pressure side includes the evaporator,

piping from the expansion value to the evaporator and the suction line (i.e. piping from the

evaporator to the suction value A).

4.4 UNIT OF REFRIGERATION

The unit used in refrigeration industry is tone of refrigeration (TR). A tone of refrigeration is

defined as the amount of refrigeration effect produced by the uniform melting of one tone

(1000Kg) of ice from and at 0oC in 24 hours. It is equivalent to the rate of heat transfer

needed to produce 1 tonne (20001b) of ice at 0oC from water at 0

oC (32

oF) in one day.

Since latent heat of ice which is enthalpy of solidification of water is given in S. I unit as

334.94Kd/kg and British unit 144Btu/1b,

∴ ITR = 1000x 335 in 24hrs

= 1000 x 335

24 x 60

= 232.6 KJ/min

= 3.88KW /ton

In actual practice, one tone of refrigeration is as equivalent to 210 KJ/min or 3.5 Kw (or 3.5

KJ/S)

In British unit therefore, 1 ton of refrigeration =

2000 1b x 144

24 x 60

= 2000BTU /min

= 12,000BTU/hrs

4.5 REFRIGERATION SYSTEM CAPACITY

This is the rate at which the system removes heat from the refrigerated space. The first

expression was based on an equivalent effect of melting ice. Melting 1 ton of ice per 24 hr day

will absorb heat at the rate of 1 ton = 2000 1b.

λ = 144 BTU

= 334.944KJ/Kg

∴ 1 ton of refrigeration (of ice per 24hr day) will absorb heat at the rate of = 12,000BTU

/hr

= 210KJ/min

= 3.54KW

System capacity depends on

(a) Lot of refrigerant circulated per unit time

(b) Refrigerating effect of refrigerant

∴ Refrigeration system capacity, Qe = Mqe (KJ/S nkw)

Where qe = refrigerating effect in KJ/Kg

In = mass flow rate in Kg/S

Weight of refrigerant circulated is given by

Wt = 3.5 >V�WXYZ �

�.[

Note that the refrigerating capacity is actually an energy transfer rate and as such is an expression of

power.

4.6 MASS OF REFRIGERANT CIRCULATED PER SECOND

The mass of refrigerant which must be circulated per second per kilowatt of refrigerating

capacity for any given operating conditions is found by dividing the refrigerating effect per

kilogram at the given conditions into 1Kw.

4.7 VOLUME FLOW RATE OF VAPOUR

When 1kg of refrigerant vaporizes, the volume of saturated vapour produced depends on

the refrigerant employed and on the vaporizing tempo. For any one refrigerant, the volume

of vapour depends only on the vaporizing temp and increase as the vaporizing temp (and

pressure) decreases. When the vaporizing temp of the refrigerant is known, the volume of

vapour produced per unit mass (specific volume) can be determined directly from the

saturated vapour tables. Once the specific volume of the vapour is known, the total volume

of vapour generated in the evaporator per unit time can be found by multiplying the

refrigerant mass flow rate by the specific volume of the vapour.

V = MV

Where

V = the total volume of vapour generated is the evaporator in mb/S

M = the mass flow rate of refrigerant in kg/s

V = the specific volume of the vapour at the vaporizing temp in

m3/kg

4.8 Coefficient of Performance (COP)

Before any evaluation of the performance of a refrigeration system can be made, an

effectiveness term must be defined. The index of performance is not called efficiency,

because that term is reserved for the ratio of output to input. The ratio of output to input

would be misleading if applied to a refrigeration system because the output in process 2-3 is

usually wasted. However, the philosophy of the performance index of the refrigeration cycle

is the same as efficiency since it represent the ratio

Amount of desired commodity

Amount of expenditure

The performance term in refrigeration cycle is called the coefficient of performance it is

defined as the ratio of the refrigerating effect to the work done or heat supplied to do work

Hence

COP = Useful refrigeration

Net Work

= hc – ha

hd - hc

= refrigeration effect

Work input

= heat extracted from system /cycle

Network supplied /cycle

P

d e a

b e

= heat absorbed from refrigerated space

Heat energy equivalent energy supplied to the compressor

= Refrigerated effect

Heat of compression

The two terms which make up the COP must be in the same unit so that the COP is therefore

dimensionless.

4.9 CONDITIONS FOR HIGHEST COP

A high COP is desirable because it indicates that a given amount of refrigeration requires a

small amount of work. What then can we do in a Carnot cycle to maintain a high COP.

IF we express the COP of the Carnot refrigerator is terms of the temps of the reservoirs for

cycle analysis, the heat transferred in a reversible process is

Qrev = ∫ Tds

Net work

2 3

4 1

S

T

21oC

2 3

T

The above analysis gives a false impression that we have complete control over T1, and T2.

This highlights the limitations imposed by temperature on COP. The temperature

recruitments is governed by vaporizing temp of refrigerant and the condenser heat load. For

example, if the refrigeration system must maintain a cold room at – 18oC and can reject heat

to atmosphere at 21oC, these two terms are limitations with which the cycle must abide.

The two temperature are shown in dotted liens I the T.S diagram. During the heat rejection

process, the refrigerant temperature must be higher than 21oC. During the refrigeration

process, the refrigerant temperature has to be lower than – 18oC in order is transfer heat

from the cold space to the refrigerant. The cycle that result us 1-2 – 3-4-1

4.10 THE CARNOT’S PRINCIPLE

The reversible process is considered the perfect and most efficient process. It is assumed for

this process, that there is no loss of energy. This process is applied in principle to

thermodynamic engines. A thermodynamic engine is a device which receives heat at some

high temperature from a heat source. It converts this heat into work and rejects remainder

into a sink.

4 1

S

-18oC

DT

A Frenchman, Sadi Carnot in reviewing an engine working on thermodynamically reversible

process states that no engine can be more efficient than a reversible engine working

between the same limits of temperature. Consider a thermodynamically reversible engine R

working between temperature limits of source T1 and sink T2(Fig D1).

Fig. 4.3 Reversible engine

Source T1

R

Sink T2

W

Q - W

Q

Let this engine receive Q units of heat at a given period of time from a source whole

temperature is T1. Let this engine convert W units of this heat into work and

WEEK 5

UNDERSTAND THE VARIOUS TYPES OF PARCTICAL

REFRIGERATION CYCLES & KNOW THE FUNCTIONS AND

PROPERTIES OF REFRIGERANTS

REFRIGERANTS

A refrigerant is a medium of heat transfer through phase change

such as evaporation at low temperature and pressure, with some

exception where the sensible energy transfer occurs. There are

many substances which are used in refrigeration system for

energy transfer purposes. These refrigerants are classified under

the following headings.

A. Based on Working Principle

These refrigerants are categorized under common and secondary

refrigerants. The common refrigerants pass through compression,

cooling or condensation, expansion and evaporation or warming

up doing cyclic. In case of phase change media such NH3, R-12, R

22, CO2 sulphur dioxide etc, the heat transfer is associated with

phase change while sensible energy transfer takes place with air.

A medium which does not undergo the cyclic process in

refrigeration system but is used only as a medium of heat transfer

is often referred to as a secondary refrigerant. E.g water, brine

solutions of sodium chloride calcium chloride.

Examples of secondary refrigerators include water, brine

solutions of sodium chloride, calcium chloride etc.

B. Based on safety

Safety consideration as categorized into three:

(i). Safety refrigerants: These are non-toxic, non-inflammable

and include the following.

R - 11 - Trichloromono- fluoromelth are

R – 12 - Dichlorodifluoromethane

R - 13 - Monochloro-trifuoro-methane

R – 14 - Carbon Tetrafluriode

R - 21 - Dchlorofluoromethane

R – 22 - Chlorodifluoromethane

R - 113 - Trichdifluoromethane


- Methyl chloride

- Carbon dioxide

- Water

(ii). Toxic and somewhat flammable refrigerants.

These include, dichloroethylene, methyl formate,

ethylchoride, sulphurdioxide, ammonia.

(iii). Very flammable refrigerants: They are such compounds like

Butane, Isobutane, propane, ethane, methane, ethylene.

C. Based on Chemical Composition:

These refrigerants are grouped under the following categories:

(i). Halocarbon compounds: The are obtained after replacing

one or more hydrogen atoms in a hydrocarbon ethane or

methane with halogens (chloride, bromine or fluorine).

(ii). Cyclic organic compound are group in a class like

dichlorohexafluorocyclobutane.

Monochloroheptafluorocycleclobutane etc.

(iii). Azeotropes are those mixtures of two or more refrigerants

which behave as if they are compounds.

(iv). Miscellaneous: This group contains various compounds which

cannot be grouped in the above categories. They are of

about 700 series with their molecular weight as the last

numbers. They include water, air, carbon dioxide, sulphur

dioxide.

(v). Oxygen and nitrogen compounds are grouped in one

category and as given 600 series.

(vi). Inorganic compounds: They comes under two classes

(a). Cyogenic

(b). Non- cryogenic

Cryogenic fluids are those which are employed for achievement of

temperatures in the range of 113k to OK, Nitrogen, Oxygen,

Helium, Hydrogen are used to achieve temperature below 113K.

Non-cryorgenic refrigerants are those inorganic compounds which

are employed above the cryogenic ranges such as water,

ammonia, sulphur dioxide. These are grouped into 700 series.

(vii). Unsaturated compounds such as ethylene, propylene etc are

grouped into 1000 series.

REFRIGERANTS: QUALITIES OF DESIRED REFRIGERANTS

Any refrigerant should be capable of absorbing heat at low temperature

and below ambient, and be capable of rejecting heat preferably to

ambient. Selection of each refrigerant depends on P.T data at

saturation, an enthalpy of vapourization. The higher than enthalpy the

better. So refrigerant should have a higher enthalpy of vapourization

and the higher, the smaller the plant.

- Odour should not be obnoxious

- Refrigerants with high specific volume means big compressor. So

the better the sp. vol. the better especially at entry to compressor.

- Should not be corrosive so it does not attack containers.

- Should not be flammable to avoid out break of five and so

eliminates possibility of explosion.

- In vapour compressor system, refrigerant should operate at temp

above its freezing point.

- Should be stable to keep its properties same throughout its

working cycle.

- Should not be soluble or react with lube oil.

- Should have high thermal conductivity so in to increase mass rate

on high K gives less residence time.

- Non injurious or poisonous to food, should not be toxic.

CLASSIFICATIONS

- Halo carbon compounds are most common as refrigerants e.g

Freon.

- Gzeotropes.

- Some hydrocarbons, especially in oil companies.

- Some inorganic compounds litte CO2.

- Some unsaturated hydrocarbons organic compounds.

A. The Helogens

The Ferons contains one or more of the halogenne made up of

chlorine, fluorine and Bromixe. F11, F12, or R12, etc sold under

several trade names e.g. freous, genetrons, isotrons, avetrous

(see table).

A. Azeotrope: Is a physical mixture of two chemically pure

substances which cannot be separated by distillation. It will

possess properties different from the two substances but behaves

like another separate pure substance e.g R500 = F12 (73.8% by

mass) + F152 cas (26.2% by mass).

Chemical Designation Chemical Name Chemical Formula Bond

F11 11 Trichloromonflur CCL3F

oromethane

R11

12 Dichlorofluoro

methane CCL2F2

CL

C CL CL

CL

CL

C F CL

F

22 Monochlorodi- CCLHF2

fluoromethane or CHCLF2

40 Methychloride CH3CL

113 Trichloroflu-

oroethene CCL2F-CF2CL

114 Dichlorotetraflu

oroethene CCLF2 –CCLF2

H

C F CL

F

H

C CL H

H

CL

C F

CL

F

C CL

F

F

C CL

F

F

C CL

F

COMMON REFRIGERANT TYPES

There are different types of refrigerants ranging from water to

synthetic fluids.

Hydrocarbon refrigerants such as ethane, propane, butane and

isobutene are used especially in the petroleum industry. Many

other types of fluids have be tested as refrigerants and are now

discarded for better synthetic ones. The most important types of

such refrigerants is the halocarbon family.

The most commonly used halocarbon refrigerants are R-12, R-22

and the intermediary R-500.

(i) Ammonia: - Ammonia is the only refrigerant outside of the

halocarbon group that is being used to any great extent at the

present time. Although ammonia is toxic and also some what

flammable and explosive under certain conditions, its excellent

thermal properties make it the predominant refrigerant in the

production end of the food industry. Ammonia has advantage of

being an environmentally safe refrigerant.

(ii) Refrigerant-ii (CFC)

It has a boiling temperature at standard pressure of 74.7of

(23.7oc). operating pressures at standard ton conditions are

2.94psia (0.3bar) evaporating and 18.19psia (1.25bar) condensing

due to its low operating pressure,R-11 is employed only with

centrifugal compressors and mainly in air conditioning systems for

small office buildings, factories, department stores, etc. its is

considered as a safe refrigerant and also has been used as a

solvent and as a secondary refrigerant. However, it has one of the

highest ozone destruction potentials.

(iii) refrigerant-12(CCL2F2):- the chemical name of this fluid is

dichlorodifluoromethane

It is probably the most widely used of all of the refrigerants. It is a

safe refrigerant in that it is non toxic, non flammable and non

explosive. Further more, it is a highly stable compound that is

difficult to break down even under extreme operating conditions.

However if brought into contact with an open flame or an electrical

heating element R-12 will decomposes into highly toxic products.

It is suitable refrigerant for use in high, medium and lo-

temperature applications and with all three types of compressor.

(iv) Refrigerant-22 (CHCLF2):- the chemical name of R-22 is

monochlorodifluoromethane. It has a boiling point at atmospheric

pressure of -41.4oF(-40.8oc).it is developed originally as a low

temperature refrigerant, it has been used in the post in domestic

and industrial low temperature systems down to evaporator

temperature as low as -125oF (-87oc). its primary use today is in

packaged air conditioners, where because of space limitations, the

relatively small compressor displacement required is a decided

advantage.

(v) Refrigerant-500:- this refrigerant is an azeotropic mixture of

73.8% R-12 and 26.2% R-152a. An azeotropic mixture is that

mixture comprised of specific proportions of liquids which behaves

as a single pure compound. Such a liquid therefore boils at

constant temperature under a constant pressure.

Refrigerant-500 is used only in commercial and industrial units. Its

normal boiling temperature is-33.3oc and latent heat of

vaporization at 5of is 45.8 cal/kg.

PROPERTIES OF REFRIGERANTS

Chemical properties of refrigerant

(i) Flammability: - Refrigerants such as ethane, propane e.t.c are

highly flammable. Ammonia is also somewhat flammable and

becomes explosive when mixed with air in the rate of 16 to 25

percent of gas by volume. Good refrigerants used in domestic and

industrial purpose should not be flammable

(ii) Toxicity: - some non-toxic refrigerant (i.e. all fluorocarbon

refrigerants) when mixed with certain percentage of air become

toxic. Toxic refrigerant are not used in domestic refrigerant and

comfort air conditioning the use of toxic refrigerant is only limited

to cold storages..

(iii) Solubility of Water: - water is only slightly soluble in R-12 the

solution formed is very slightly corrosive to any of the common

metals. Solubility of water with refrigerant should be reducing to

minimal.

(iv) Miscibility :- the ability of refrigerant to mix with oil is called

miscibility. The degree of miscibility. The degree miscibility

depends upon the temperature of the oil and pressure of the

refrigerant vapour. The refrigerant should not in any form mix with

the oil within the crank case

(v) Effect on perishable materials: - The refrigerants used in cold

storage

Plant and in domestic refrigerators should be such that incase of

leakage, it should have no effect on the perishable materials.

PHYSICAL PROPERTIES OF REFRIGERANTS

(i) Stability and inertness: - An ideal refrigerant should not

decompose at any temperature normally encountered in the

refrigerating system. It should function for long period of time with

out dissociating into other compound.

(ii) Corrosive property: - The corrosive property of a refrigerant must

be taken into consideration while selecting the refrigerant. The

Freon group of refrigerant is non-corrosive with all metals

ammonia is used only with iron or steel. Sulphur dioxide is non-

corrosive to all in the absence of water good refrigerants should

not have corrosive tendency.

(iii) Viscosity: - The refrigerant in the liquid and vapour states should

have low viscosity. The heat transfer through the condenser and

evaporator is improved at low viscosities.

(iv) Thermal conductivity: - The refrigerant in the liquid and vapour

states should have high thermal conductivity. It requires the heat

transfer coefficients in evaporator and condensers.

(v) Dielectric strength: - this is a measure of the resistance that the

refrigerant offers to the flow of electric current it is important in

hermetically sealed units in which the electric motor is expose to

the refrigerant.

(vi) Leakage tendency: - the leakage tendency of a refrigerant should

be low. If there is a leakage of refrigerant, it should be easily

detectable the leakage occurs due to opening in the joints or flaws

in material used for construction.

(vii) Cost: - this is important in high capacity refrigerating system like

industrial and commercial the ammonia, being the cheapest is

widely used in large industrial plants such as cold storage and ice

plants.

PRIMARY AND SECONDARY REFRIGERANT.

(i) Primary Refrigerant: - These are that working fluid which passes

through the refrigerating cycle of evaporation, recovery

compression, condensation and expansion. for example ammonia,

Freon, so2 , methyl chloride and co2 e.t .c

(ii) Secondary refrigerants: - these are those working fluids which act

as cooling, medium but do not undergo the refrigerating cycle. Air,

brine and any other freezing solution are its example

REFRIGERANT PIPING MATERIALS AND ITS EFFECTS

In general, the type of piping material employed for refrigerant

piping depend upon the size and nature of the installation, the

refrigerant used and the cost of materials and labour. The

materials most frequently used for refrigerant piping are black

steel, wrought iron, copper, and brass. All these may not be

suitable with ammonia attacks non ferrous metals.

Copper tubing has the advantage of being lighter in weight, more

resistance to corrosion, and easier to install them either wrought

iron or black steel.

i. PIPE JOINTS

Depending on the type and size of the piping, joints for refrigerant

piping may be severed, flanged flared, welded, brazed or soldered

when refrigerant pressures are below 250sil (17bar), screwed

joints may be used on pipe sized up to 80mm. for higher

pressures, screwed joint are limited to pipe sized B5mm and

smaller. Above these sizes, flanged joints of the tongue and

groove type should be used. Welting is probably the most

commonly used method of joining iron and steel piping.

ii. LOCATION

In general, refrigerant piping should be located so that it does not

present a safety hazard, obstruct the normal operation and

maintenance of the equipment, or restrict the used of adjoining

spaces.

iii. VIBRATION AND NOISE

In most cases, the vibration and noise in refrigerant piping

originates not in the piping itself but in the connected equipment.

How ever, regardless of the source, vibration, and the objectional

noise associated with it is greatly reduced by proper piping design.

REFRIGERANT STORAGE AND SAFETY

Most refrigerants are supplied and stored in large pressure vessels

holding about 60kg of liquid and vapour when full. These must be

stored upright, with caps in place, in cool and well ventilated

stores located well away from boiler rooms, or areas in which

operations presenting fire hazards- e.g. welding are carried out.

Similarly, when cylinders are used in the workshop or on site,

brazing torches or welding sets must not be used close to them.

Given a pressure cylinder containing liquid and vapour, plus a

source of intense heat, one has all the necessary components to

cause a leth at explosion.

The requirement for storage of cylinders in well ventilated rooms

covers both the need to remove any refrigerant which might

escape, and the need to keep the cylinders as cool as possible. In

practical terms, storage are temperature should not reach levels at

which excessive refrigerant pressure is generated. An upper limit

of not more than 2070kpa is recommended.

At all times bear in mind that refrigerants have been specially

developed to remove a lot of heat quickly through any surface on

which they boil. If liquid refrigerant sprays onto your skin, you will

apply the necessary first aid for the treatment of cold burn.

If you have to open a circuit, or purge it, make sure that the liquid

cannot spray on you.

REFRIGERANT LEAK DETECTION

Leaks cannot be tolerated in any refrigeration system, and leak

detecting equipment must be well maintained and regularly used

during maintenance checks as well as installation work. The

methods which can be used with specific refrigerants are listed

below in increasing order of efficiency.

1. Sulphur candles: When lit and exposed to air containing

ammonia vapour, these give off a white cloud of ammonium

chloride or ammonium sulphide. This method cannot be used

to pin point leak position.

2. litmus Paper: Moist red limits paper will turn blue if exposed

to ammonia vapour, but cannot be used with any of the

halogen family refrigerants.

3. Bubbles tests: Soapy water, a washing up liquid, or better,

still a purpose developed leak detector will indicate the

locations of leaks by the formation of bubbles by escaping

refrigerant. However, this type of test can only be made on

piping or fittings known to be at higher pressure than that of

the atmosphere. Test solutions applied low temperature, low

pressure suction lines could cause considerable damage

because the liquid could be drawn into the pipes.

4. Halide test lamps: Detectors fuelled with propane, butane, or

methylated spirit can be used to locate fluorocarbon

refrigerants leaks. The detector includes a fuel tank which is

or can be pressurized to supply fuel at the at a steady and

controlled pressure and a jet to admit the fuel to a burner.

When lit, the burner flame is supported by oxygen in the air

which is drawn through the tube used as a sensing probe.

The probe is passed slowly over the joint or surfaces being

leak tested. If any fluorocarbon refrigerants are drawn into

the tube, the colour of the lamp flame will change to green

or blue, depending on the quantity of gas passed over the

burner element.

This type of detector can only be used with non-flamable

gases and care must be taken to avoid ignity any other

gases or material.

5. Electronic leak detectors: A wide range of electronic

detectors is available. All are extremely sensitive. Battery

operated models for use on site will pick up leaks which give

a little as 14gm per year.

The refrigerant is sensed by a plug- element, exposed to air

drawn through a probe or tube. Its presence will be

indicated by a flashing lamp, an audible bleep or buzz or a

meter reading each increasing in speed or intensity as more

refrigerant passes over the element.

WEEK 6

6.0 UNDERSTAND THE FUNCTIONS AND PROPERTIES OF REFRIGERANTS

AND KNOW THE REASONS FOR AND THE METHODS OF LUBRICATION

IN REFRIGERETION

6.1 REFRIGERANTS

A refrigerant is a medium of heat transfer through phase change such as evaporation at low

temperature and pressure, with some exception where the sensible energy transfer occurs.

There are many substances which are used in refrigeration system for energy transfer

purposes. These refrigerants are classified under the following headings.

6.1.1 A Based on Working Principle

These refrigerants are categorized under common and secondary refrigerants. The common

refrigerants pass through compression, cooling or condensation, expansion and evaporation

or warming up doing cyclic. In case of phase change media such NH3, R-12, R 22, CO2 sulphur

dioxide etc, the heat transfer is associated with phase change while sensible energy transfer

takes place with air.

A medium which does not undergo the cyclic process in refrigeration system but is used only

as a medium of heat transfer is often referred to as a secondary refrigerant. E.g water, brine

solutions of sodium chloride calcium chloride.

Examples of secondary refrigerators include water, brine solutions of sodium

chloride, calcium chloride etc.

6.1.2 B. Based on safety

Safety consideration as categorized into three:

(i). Safety refrigerants: These are non-toxic, non-inflammable and include the following.

R - 11 - Trichloromono- fluoromelth are


R - 13 - Monochloro-trifuoro-methane

R – 14 - Carbon Tetrafluriode

R - 21 - Dchlorofluoromethane

R – 22 - Chlorodifluoromethane

R - 113 - Trichdifluoromethane


- Methyl chloride

- Carbon dioxide

- Water

(ii). Toxic and somewhat flammable refrigerants.

These include, dichloroethylene, methyl formate, ethylchoride, sulphurdioxide,

ammonia.

(iii). Very flammable refrigerants: They are such compounds like Butane, Isobutane,

propane, ethane, methane, and ethylene.

6.1.3 C. Based on Chemical Composition:

These refrigerants are grouped under the following categories:

(i). Halocarbon compounds: The are obtained after replacing one or more hydrogen

atoms in a hydrocarbon ethane or methane with halogens (chloride, bromine or

fluorine).

(ii). Cyclic organic compound are group in a class like dichlorohexafluorocyclobutane.

Monochloroheptafluorocycleclobutane etc.

(iii). Azeotropes are those mixtures of two or more refrigerants which behave as if they

are compounds.

(iv). Miscellaneous: This group contains various compounds which cannot be grouped in

the above categories. They are of about 700 series with their molecular weight as

the last numbers. They include water, air, carbon dioxide, sulphur dioxide.

(v). Oxygen and nitrogen compounds are grouped in one category and as given 600

series.

(vi). Inorganic compounds: They come under two classes

(a). Cryogenic

(b). Non- cryogenic

Cryogenic fluids are those which are employed for achievement of temperatures in the

range of 113k to OK, Nitrogen, Oxygen, Helium, and Hydrogen are used to achieve

temperature below 113K.

Non-cryogenic refrigerants are those inorganic compounds which are employed above the

cryogenic ranges such as water, ammonia, sulphur dioxide. These are grouped into 700

series.

(vii). Unsaturated compounds such as ethylene, propylene etc are grouped into 1000 series.

6.2 REFRIGERANTS: QUALITIES OF DESIRED REFRIGERANTS

Any refrigerant should be capable of absorbing heat at low temperature and below ambient,

and be capable of rejecting heat preferably to ambient. Selection of each refrigerant

depends on P.T data at saturation, an enthalpy of vaporizations. The higher than enthalpy

the better. So refrigerant should have a higher enthalpy of vaporization and the higher, the

smaller the plant.

- Odour should not be obnoxious

- Refrigerants with high specific volume means big compressor. So the better the sp.

vol. the better especially at entry to compressor.

- Should not be corrosive so it does not attack containers.

- Should not be flammable to avoid out break of five and so eliminates possibility of

explosion.

- In vapour compressor system, refrigerant should operate at temp above its freezing

point.

- Should be stable to keep its properties same throughout its working cycle.

- Should not be soluble or react with lube oil.

- Should have high thermal conductivity so in to increase mass rate on high K gives

less residence time.

- Non injurious or poisonous to food, should not be toxic.

6.3 CLASSIFICATIONS

- Halo carbon compounds are most common as refrigerants e.g. Freon.

- Azeotropes.

- Some hydrocarbons, especially in oil companies.

- Some inorganic compounds litte CO2.

- Some unsaturated hydrocarbons organic compounds.

6.3.1 A. The Helogens

The Freons contains one or more of the halogens made up of chlorine, fluorine and Bromixe.

F11, F12, or R12, etc sold under several trade names e.g. freous, genetrons, isotrons, avetrous

(see table).

Azeotrope: Is a physical mixture of two chemically pure substances which cannot be separated

by distillation. It will possess properties different from the two substances but behaves like

another separate pure substance e.g R500 = F12 (73.8% by mass) + F152 cas (26.2% by mass).

Chemical Designation Chemical Name Chemical Formula Bond

F11 11 Trichloromonflur CCL3F

oromethane

R11

12 Dichlorofluoro

methane CCL2F2

22 Monochlorodi- CCLHF2

fluoromethane or CHCLF2

CL

C CL CL

CL

CL

C F CL

F

H

C F CL

F

40 Methychloride CH3CL

114 Trichloroflu-

oroethene CCL2F-CF2CL

114 Dichlorotetraflu

oroethene CCLF2 –CCLF2

H

C CL H

H

CL

C F

CL

F

C CL

F

F

C CL

F

F

C CL

F


There are different types of refrigerants ranging from water to synthetic fluids.

Hydrocarbon refrigerants such as ethane, propane, butane and isobutene are used

especially in the petroleum industry. Many other types of fluids have be tested as

refrigerants and are now discarded for better synthetic ones. The most important types of

such refrigerants is the halocarbon family.

The most commonly used halocarbon refrigerants are R-12, R-22 and the intermediary R-

500.

(i) Ammonia: - Ammonia is the only refrigerant outside of the halocarbon group that is

being used to any great extent at the present time. Although ammonia is toxic and also

some what flammable and explosive under certain conditions, its excellent thermal

properties make it the predominant refrigerant in the production end of the food industry.

Ammonia has advantage of being an environmentally safe refrigerant.

(ii) Refrigerant-ii (CFC)

It has a boiling temperature at standard pressure of 74.7of (23.7

oc). operating pressures at

standard ton conditions are 2.94psia (0.3bar) evaporating and 18.19psia (1.25bar)

condensing due to its low operating pressure,R-11 is employed only with centrifugal

compressors and mainly in air conditioning systems for small office buildings, factories,

department stores, etc. its is considered as a safe refrigerant and also has been used as a

solvent and as a secondary refrigerant. However, it has one of the highest ozone destruction

potentials.

(iii) refrigerant-12(CCL2F2):- the chemical name of this fluid is dichlorodifluoromethane

It is probably the most widely used of all of the refrigerants. It is a safe refrigerant in that it is

non toxic, non flammable and non explosive. Further more, it is a highly stable compound

that is difficult to break down even under extreme operating conditions. However if brought

into contact with an open flame or an electrical heating element R-12 will decomposes into

highly toxic products. It is suitable refrigerant for use in high, medium and lo-temperature

applications and with all three types of compressor.

(iv) Refrigerant-22 (CHCLF2):- the chemical name of R-22 is monochlorodifluoromethane.

It has a boiling point at atmospheric pressure of -41.4oF(-40.8

oc).it is developed originally as

a low temperature refrigerant, it has been used in the post in domestic and industrial low

temperature systems down to evaporator temperature as low as -125oF (-87

oc). its primary

use today is in packaged air conditioners, where because of space limitations, the relatively

small compressor displacement required is a decided advantage.

(v) Refrigerant-500:- this refrigerant is an azeotropic mixture of 73.8% R-12 and 26.2%

R-152a. An azeotropic mixture is that mixture comprised of specific proportions of liquids

which behaves as a single pure compound. Such a liquid therefore boils at constant

temperature under a constant pressure.

Refrigerant-500 is used only in commercial and industrial units. Its normal boiling

temperature is-33.3oc and latent heat of vaporization at 5

of is 45.8 cal/kg.


6.5.1 Chemical properties of refrigerant

(i) Flammability: - Refrigerants such as ethane, propane e.t.c are highly flammable.

Ammonia is also somewhat flammable and becomes explosive when mixed with air

in the rate of 16 to 25 percent of gas by volume. Good refrigerants used in domestic

and industrial purpose should not be flammable

(ii) Toxicity: - some non-toxic refrigerant (i.e. all fluorocarbon refrigerants) when mixed

with certain percentage of air become toxic. Toxic refrigerant are not used in domestic

refrigerant and comfort air conditioning the use of toxic refrigerant is only limited to cold

storages..

(iii) Solubility of Water: - water is only slightly soluble in R-12 the solution formed is very

slightly corrosive to any of the common metals. Solubility of water with refrigerant should

be reducing to minimal.

(iv) Miscibility: - the ability of refrigerant to mix with oil is called miscibility. The degree

of miscibility. The degree miscibility depends upon the temperature of the oil and pressure

of the refrigerant vapour. The refrigerant should not in any form mix with the oil within the

crank case

(v) Effect on perishable materials: - The refrigerants used in cold storage

Plant and in domestic refrigerators should be such that incase of leakage, it should have no

effect on the perishable materials.


(i) Stability and inertness: - An ideal refrigerant should not decompose at any

temperature normally encountered in the refrigerating system. It should function for long

period of time with out dissociating into other compound.

(ii) Corrosive property: - The corrosive property of a refrigerant must be taken into

consideration while selecting the refrigerant. The Freon group of refrigerant is non-corrosive

with all metals ammonia is used only with iron or steel. Sulphur dioxide is non-corrosive to

all in the absence of water good refrigerants should not have corrosive tendency.

(iii) Viscosity: - The refrigerant in the liquid and vapour states should have low viscosity.

The heat transfer through the condenser and evaporator is improved at low viscosities.

(iv) Thermal conductivity: - The refrigerant in the liquid and vapour states should have

high thermal conductivity. It requires the heat transfer coefficients in evaporator and

condensers.

(v) Dielectric strength: - this is a measure of the resistance that the refrigerant offers to

the flow of electric current it is important in hermetically sealed units in which the electric

motor is expose to the refrigerant.

(vi) Leakage tendency: - the leakage tendency of a refrigerant should be low. If there is a

leakage of refrigerant, it should be easily detectable the leakage occurs due to opening in

the joints or flaws in material used for construction.

(vii) Cost: - this is important in high capacity refrigerating system like industrial and

commercial the ammonia, being the cheapest is widely used in large industrial plants such as

cold storage and ice plants.

6.7 PRIMARY AND SECONDARY REFRIGERANT.

(i) Primary Refrigerant: - These are that working fluid which passes through the

refrigerating cycle of evaporation, recovery compression, condensation and expansion. for

example ammonia, Freon, so2 , methyl chloride and co2 e.t .c

(ii) Secondary refrigerants: - these are those working fluids which act as cooling, medium but do

not undergo the refrigerating cycle. Air, brine and any other freezing solution are its example


In general, the type of piping material employed for refrigerant piping depend upon the size

and nature of the installation, the refrigerant used and the cost of materials and labour. The

materials most frequently used for refrigerant piping are black steel, wrought iron, copper,

and brass. All these may not be suitable with ammonia attacks non ferrous metals.

Copper tubing has the advantage of being lighter in weight, more resistance to corrosion,

and easier to install them either wrought iron or black steel.

6.8.1 PIPE JOINTS

Depending on the type and size of the piping, joints for refrigerant piping may be severed,

flanged flared, welded, brazed or soldered when refrigerant pressures are below 250sil

(17bar), screwed joints may be used on pipe sized up to 80mm. for higher pressures,

screwed joint are limited to pipe sized B5mm and smaller. Above these sizes, flanged joints

of the tongue and groove type should be used. Welting is probably the most commonly used

method of joining iron and steel piping.

6.8.2 LOCATION

In general, refrigerant piping should be located so that it does not present a safety hazard,

obstruct the normal operation and maintenance of the equipment, or restrict the used of

adjoining spaces.

6.8.3 VIBRATION AND NOISE

In most cases, the vibration and noise in refrigerant piping originates not in the piping itself

but in the connected equipment. How ever, regardless of the source, vibration, and the

objectional noise associated with it is greatly reduced by proper piping design.


Most refrigerants are supplied and stored in large pressure vessels holding about 60kg of

liquid and vapour when full. These must be stored upright, with caps in place, in cool and

well ventilated stores located well away from boiler rooms, or areas in which operations

presenting fire hazards- e.g. welding are carried out. Similarly, when cylinders are used in

the workshop or on site, brazing torches or welding sets must not be used close to them.

Given a pressure cylinder containing liquid and vapour, plus a source of intense heat, one

has all the necessary components to cause a leth at explosion.

The requirement for storage of cylinders in well ventilated rooms covers both the need to

remove any refrigerant which might escape, and the need to keep the cylinders as cool as

possible. In practical terms, storage are temperature should not reach levels at which

excessive refrigerant pressure is generated. An upper limit of not more than 2070kpa is

recommended.

At all times bear in mind that refrigerants have been specially developed to remove a lot of

heat quickly through any surface on which they boil. If liquid refrigerant sprays onto your

skin, you will apply the necessary first aid for the treatment of cold burn.

If you have to open a circuit, or purge it, make sure that the liquid cannot spray on you.


Leaks cannot be tolerated in any refrigeration system, and leak detecting equipment must

be well maintained and regularly used during maintenance checks as well as installation

work. The methods which can be used with specific refrigerants are listed below in

increasing order of efficiency.

6. Sulphur candles: When lit and exposed to air containing ammonia vapour, these

give off a white cloud of ammonium chloride or ammonium sulphide. This method

cannot be used to pin point leak position.

7. Litmus Paper: Moist red limits paper will turn blue if exposed to ammonia vapour,

but cannot be used with any of the halogen family refrigerants.

8. Bubbles tests: Soapy water, a washing up liquid, or better, still a purpose developed

leak detector will indicate the locations of leaks by the formation of bubbles by

escaping refrigerant. However, this type of test can only be made on piping or

fittings known to be at higher pressure than that of the atmosphere. Test solutions

applied low temperature; low pressure suction lines could cause considerable

damage because the liquid could be drawn into the pipes.

9. Halide test lamps: Detectors fuelled with propane, butane, or mentholated spirit

can be used to locate fluorocarbon refrigerants leaks. The detector includes a fuel

tank which is or can be pressurized to supply fuel at the at a steady and controlled

pressure and a jet to admit the fuel to a burner. When lit, the burner flame is

supported by oxygen in the air which is drawn through the tube used as a sensing

probe. The probe is passed slowly over the joint or surfaces being leak tested. If any

fluorocarbon refrigerants are drawn into the tube, the colour of the lamp flame will

change to green or blue, depending on the quantity of gas passed over the burner

element.

This type of detector can only be used with non-flammable gases and care must be

taken to avoid igniting any other gases or material.

10. Electronic leak detectors: A wide range of electronic detectors is available. All are

extremely sensitive. Battery operated models for use on site will pick up leaks which

give a little as 14gm per year.

The refrigerant is sensed by a plug- element, exposed to air drawn through a probe

or tube. Its presence will be indicated by a flashing lamp, an audible bleep or buzz or

a meter reading each increasing in speed or intensity as more refrigerant passes

over the element.

6.11 REFRIGERATION LUBRICATING OILS

The lubricants used in refrigeration systems must do more than protecting moving

compressor parts against wear. A film of oil is used to seal the suction and discharge values

of a compressor or the shaft seal of an open type compressor. Oil also acts as a coolant,

transferring mechanically generated heat from the crankcase to the shell of a compressor. It

dampens noise in hermetic systems, it just not attack the electric insulation. It must remain

fluid at low temperatures and mix well with refrigerants such as R22. It must not contain

waxes or other suspended matter which might clog a capillary tube or the orifice of an

expansion valve. Finally, it must remain effective for the life of an hermetically sealed

compressor which can be over 20years. The choice of oil and nomination of acceptable

alternatives can only be made by the compressor manufacturers. It is not advisable to use

other than the recommended lubricants.

Contaminations of oil by moisture or moist air cannot be tolerated. Oil containers must

therefore be stored in dry well ventilated rooms and not opened until the moment the oil is

to be used.

It must be remembered that oil is constantly circulated around systems charged with the

refrigerants. The fluorocarbon refrigerants all dissolved in oil and oil is carried away from the

compressor crankcase in refrigerants as they are pumped around the systems. Pipelines and

heat exchangers must therefore be designed to help oil flow back to the crankcase.

6.11.1 COMPRESSOR LUBLICATING OILS

The fact that the compressor lubricating oil usually comes in to contact with, and often

mixed with the system refrigerant makes it necessary that the oil used to lubricate

refrigeration compressors be special prepared for that purpose some of the more important

properties of the oil that must be considered when selecting the compressor lubricating oil

are :-

(1) Chemical stability: - The important of chemical stability is emphasized by the fact that it is

necessary for the compressor lubricating oil to perform its lubricating function continuously

and effective without undergoing change for long period of time.

(2) Dielectric strength: - The dielectric strength of oil is a measure of the resistance that oil

offers to the flow of electric current. If is expressed in terms of the voltage required to cause

an electric current to arc across a gap between two poles immersed in the oil. Since any

moisture, dissolved metal or other impurities contained in the oil will lower its dielectric

strength, a high dielectric strength is an indication that the oil is relatively free of

contaminants.

(3) Viscosity:- viscosity is defined as the resistance that a fluid offers to flow, with regard to the

lubricating oil, viscosity may also be defined as a measure of the “body” of the oil or of the

ability of the oil to perform its lubricating function by forming a protective film or coaling

between the parts separated and preventing wear. In order to provide adequate lubrication

for the compressor, the viscosity of the oil must be maintained within reasonable limits.

(3) Pour, Cloud And Floc Points: - the pour point of oil is the lowest temperature at which the oil

will flow or “pour”, when tested under certain specified conditions of two oil having the

same viscosity; one may have a higher pour point than the other because of a greater wax

tent.

Since all lubricating oils contain a certain amount of paraffin, wax will precipitate from any

oil if the temperature of the oil becomes cloudy at this point, the temperature at which the

wax begins to precipitate from the oil is called the cloud point of the oil.

The flash point of the oil is the temperature at which wax will start to precipitate from a

mixture of 90% R-12 and 10% oil by volume.

6.12 METHOD OF LUBRICATION

Methods of lubricating the compressor vary some what depending upon the types and size

of the compressor and upon the individual manufacturer. However, for the most part,

lubrication methods can be grouped into two general types:- splash and forced feed.

(i) Splash method: - in this, the compressor crank case acts as an oil sump and is filled

with oil to a level approximately even with the bottom of the main crank bearings

with each revolution of the crank shaft, the connecting rod and crank shaft dip into

the oil causing the oil to be splashed up on the cylinder walls bearings and other

rubbing surfaces.

Another modified type of splash lubrication, sometimes called flooded lubrication,

employs slinger rings, dises, screws or similar devices to raise the oil to a level above

the crankshaft or main bearing, from where it is allowed to flood over the bearing s

and or feed through oil channels to the various rubbing

(2) Forces Feed: - in the forces feed method of lubrication, the oil is forced under

pressure through oil tubes and/or rifle drilled passage in the crankshaft and

connecting rods to the various rubbing surfaces. After performing its lubricating

function, the oil drains by gravity back into a sump located in the crankcase of the

compressor, usually at the end of the crankshaft. Since most oil pumps are

automatically reversible, the direction of crank rotation is not usually critical with

regard to compressor lubrication.

Assignment

1. What do you understand by the following teams associated with lubricants?

(i). Pouring point

(ii). Flash or fire point

(iii). Close point

(iv). Dielectric strength

2. Sketch oil pump mechanism and describe the methods of lubrication.

7.0 KNOW THE PROCEDURE OF CHARGING REFRIGERATION CIRCUIT


Refrigerant can be charged in liquid or gaseous form, using one of several types of

equipments. As a general rule, liquid charging methods are used on new equipment

requiring a known weight of refrigerant. The method is fast and extremely accurate when

charges do no exceed the limits 4.5kg of transparent, calibrated refrigerant vessels.

Fig 7.1 Liquid refrigerant charging

User

Typewritten Text

Week 7

Gas charging is normally used to top-up systems. The procedure is slow, but enables gas

volumes to be accurately controlled in accordance with the readings of suction and

discharge gauges, an ammeter showing the power drawn by the compressor motor and the

absence of bubbles in liquid-line sight glasses.

Fig 7.2 Vapour refrigerant charging

7.1.1 CYLINDER POSITIONS

The logical positions of refrigerant cylinders are:

(a). For liquid charging –cylinder inverted above the system, charging value down.

(b). For gas charging – charging vertical, value uppermost, beneath the system.

Fig 7.3 Refrigerant charging system

7.1.2 CHARGING EQUIPMENT OPTIONS

1. Conventional charging hoses: These are used to connect main items of equipment.

2. High vacuum hoses: These are flexible seamless metal construction.

3. Service manifolds: These are arranged for one suction and one discharge gauge; and

either three or four flared connectors for lines from the vacuum pump, refrigerant

cylinder, and the appliance to be tested. High vacuum designs have special,

diaphragm – type soft – seat shut off values, a vacuum port and oversize internal

passages.

fig 7.4 Compound gauge

4. Conventional gauges: These are normally of 60mm diameter with threaded pipe

connectors. Normal calibration ranges are 0-3400kpa for pressure gauges and

760mm Hg- 800kpa for compound types.

5. High vacuum gauges: These are electronic types. The range covered should be

20mmto zero enabling unit pressures to be watched throughout the dehydration

process which starts at approx. 210C.

6. Vacuum Pumps: Conventional and high vacuum types are available in portable sets

suitable for site work. The high vacuum models use special high quality paraffin –

based oil. Its vapour pressure is not more than 0.005mm at 37.70C; and a vacuum

pump cannot pull a total absolute pressure less than the vapour pressure of its

sealing oil.

7. Charging stations: Many models, portable and immovable are available. They

normally incorporate a vacuum pumps; transparent, calibrated refrigerant vessels

for each important fluorinated refrigerant; gauges, valves; piped circuits, and

connections to complete.


Most refrigerants are supplied and stored in large pressure vessels holding about 60kg of

liquid and vapour when full. These must be stored upright, with caps in place, in cool and

well ventilated stores located well away from boiler rooms, or areas in which operations

presenting fire hazards- e.g. welding are carried out. Similarly, when cylinders are used in

the workshop or on site, brazing torches or welding sets must not be used close to them.

Given a pressure cylinder containing liquid and vapour, plus a source of intense heat, one

has all the necessary components to cause a leth at explosion.

The requirement for storage of cylinders in well ventilated rooms covers both the need to

remove any refrigerant which might escape, and the need to keep the cylinders as cool as

possible. In practical terms, storage are temperature should not reach levels at which

excessive refrigerant pressure is generated. An upper limit of not more than 2070kpa is

recommended.

At all times bear in mind that refrigerants have been specially developed to remove a lot of

heat quickly through any surface on which they boil. If liquid refrigerant sprays onto your

skin, you will apply the necessary first aid for the treatment of cold burn.

If you have to open a circuit, or purge it, make sure that the liquid cannot spray on you.

WEEK 8

8.0 KNOW THE VARIOUS APPLICATIONS OF REFRIGERATION

8.1 REFRIGERATION INDUSTRY AND APPLICATIONS

In the early days of mechanical refrigeration, the equipment available was bulky, expensive

and not too efficient. Also it was of such a nature as to require that a mechanic or operating

engineer will be on duty at all times. This limited the use of mechanical refrigeration to a few

large applications.

It is earlier stated that refrigeration is principally carried out for a number of reasons which

can be narrowed down to a desire to bring about cooling by reducing the temperature of a

space or product. It may be to change the state of a product, thereby transforming it from

liquid to solid. A typical example is water and similar liquid products being changed into ice.

Again, it may be directed at maintaining the state of a product as is obtainable in food

presentation. Manufacturing of certain product may require specific temperature condition

for a given property of product to be obtained, hence the need for refrigeration. Also certain

dimensional accuracies can only be obtained during manufacture under controlled

temperature condition. For effective performance of some control instruments and panels,

limits are set on the temperature of the control room to avoid overheat resulting from

instruments and power control equipment.

In the light of the above, refrigeration industries are varied and target the following six

principals’ areas:

In the early days of mechanical refrigeration, the equipment available was bulky, expensive

and not too efficient. Also it was of such a nature as to require that a mechanic or operating

engineer will be on duty at all times. This limited the use of mechanical refrigeration to a few

large applications.

It is earlier stated that refrigeration is principally carried out for a number of reasons which

can be narrowed down to a desire to bring about cooling by reducing the temperature of a

space or product. It may be to change the state of a product, thereby transforming it from

liquid to solid. A typical example is water and similar liquid products being changed into ice.

Again, it may be directed at maintaining the state of a product as is obtainable in food

presentation. Manufacturing of certain product may require specific temperature condition

for a given property of product to be obtained, hence the need for refrigeration. Also certain

dimensional accuracies can only be obtained during manufacture under controlled

temperature condition. For effective performance of some control instruments and panels,

limits are set on the temperature of the control room to avoid overheat resulting from

instruments and power control equipment.

In the light of the above, refrigeration industries are varied and target the following six

principal areas:

8.2 CLASSIFICATION OF APPLICATION OF REFRIGERATION SYSTEM

For convenience of study, refrigeration applications may be grouped into six general

categories (1) domestic refrigeration (2) commercial refrigeration, (3) industrial refrigeration

(4) marine and transportation refrigeration (5) comfort air conditioning and (6) industrial air

conditioning. It is aperients in the following discussion that the exact limits of these areas

are not precisely defined and that there is considerable overlapping between the several

areas.

8.2.1 DOMESTIC REFRIGERATION

Domestic refrigeration is rather limited in scope being concerned primary with household

refrigerators and home freezers. However, because the numbers of units in service is quite

large, domestic refrigeration represent a significant portion of this refrigeration industry.

Fig 8.1 a Domestic

Refrigerators

Fig 8.1 b Domestic Refrigerators

8.2.2 COMMERCIAL REFRIGERATION

Commercial refrigeration is concerned with the design, installation and maintenance of

refrigerated fixtures and the type used by retail stores, restaurants, hotels and institution for

the storing, displaying processing and dispensing of perishable commodities of all type.

Fig 8.2 Display cabinets

8.2.3 INDUSTRIAL REFRIGERATION

Industrial refrigeration is often confused with commercial refrigeration because the divisions

between these two are not clearly defined. As a general role, industrial applications are

larger than commercial applications and have the disguising feature of requiring on

attendant on duty, usually a licensed operating engineer. Typical industrial applications are

ice plants, large food creameries and industrial plants such as oil refineries chemical plants

and rubber plants.

Fig 8.3 Industrial Refrigerators

8.2.4 MARINE AND TRANSPORTATION REFRIGERATION

Application falling into this category could be listed partly under commercial refrigeration

and partly under industrial refrigeration. However, both these areas of specialization have

grown to sufficient size to warrant special instruction.

Fig 8.4 Refrigerated truck

8.2.5 AIR CONDITIONING

As the name implies, air conditioning is concerned with the condition of the air in some

designated areas on space. This usually involves control not only of the space. This usually

involves controls not only of the space temperature but also of spaces humidity and air

motion, along with the filtering and clearing of the air.

Air-conditioning application are of two types, either comfort or industrial according to their

purpose. Any air-conditioning that has as its primary function the conditioning of air for

human comfort air-conditioning are in homes, schools offices, house of workshop, hotels,

retail stores public buildings factorize auto mobiles, buses, trains, planer and ships.

Any air-conditioning that does not have as its primary purpose the conditioning of air for

human comfort is called industrial air-conditioning. This does not necessarily mean that

industrial air-conditioning systems cannot serve also as comfort air-conditioning

coincidentally with their primary function often this secondary function is served although

not always.

The application of industrial air-conditioning are almost without limit both in number and in

variety generally peaking, the function of industrial air-conditioning system are to

(1) Control the moisture content of hydroscopic materials.

(2) Govern the rate of chemical and biochemical reactions

(3) Limit the variations in the size of precision manufactured articles because of thermal

expansion and contraction and

(4) Provide clean, filtered air that is often essential to trouble free operation and to the

production of quality products.

Fig 8.5 Air conditioning system

WEEK 9

9.0 KNOW THE FUNCTION OF AN AIR CONDITIONING SYSTEM FOR A BUILDING

9.1 AIR CONDITIONING SYSTEM

Air conditioning is generally treatment of air in order to control simultaneously its

temperature, humidity, cleanliness and distribution to meet pre-determined requirements

of the conditioned space.

Fig 9.1 Air condoning system

Major reasons for the use of air condition include:

1. Comfort air conditioning:- To promote human comfort.

2. Process and product A/C:- For the maintenance of proper conditions for the manufacture,

processing and preservation of material and equipment.\

3. Control of industrial Environment A/C:- To maintain efficiency, health and safety of workers

at safe tolerance limits.

Application of air conditioning can be found in to major grouping namely.

1. Industrial air condition:- This include such area as.

a. Laboratories

b. Printing industries

c. Pharmaceutical industries

d. Photographic industries

e. Control rooms (Electrical/Electrics)

2. Comfort air conditioning: This include areas like:

a. Eating and amusement locations

b. Store/super markets

c. Residences

d. Large buildings e.g offices, hotels, hospitals etc.

e. Transportation e.g car, buses, plane.

9.2 AIR CONDITION PLANT

The function of an air conditioning plant is to provide air which is:

i. Without dust

ii. at the correct temperature

iii. at the correct humidity

Air condition when correctly used implies more than just temperature control. There are

four conditions which affect human comfort and these are:

a. The temperature of the surrounding air

b. The humidity (moisture content) of the air

c. The purity of the air

d. The movement of the air

True air conditioning implies that all four of there atmospheric conditions for human

comfort are met. To meet these conditions, there are basically seven functions which a

complete air conditioning plant should be capable of satisfying. These seven functions

include

1. Cooling 5. filtering

2. Heating 6. circulation

3. Humidifying 7. ventilation

4. de-humidifying

Fig 9.2 Evaporative cooler

9.3 OPERATION PRINCIPLES OF AIR CONDITIONING PLANT

Air is drawn in from the atmosphere and is passed through filters. After the air has passed

through filters, it is then passed over a cooling coil which may make the temperature of the

air to fall below the “dew point”. If the temperature of the air falls below the dew point,

some of the water moisture is condensed and extracted from the air; the cooling coils could

thus have the effect of both cooling and de-humidifying the air. They could then be heated

to the correct temperature, in the heater and then prayed with water to the required

humidity before being delivered through the ducting is the conditioned space.

In order to condition air as per required, it is important to have knowledge of the condition

of air that is being drawn in. if the condition of the in-coming air is known, then it can be

decided whether or not it will be necessary to cool it, humidify it, dehumidify it, heat it or do

all of the above processes other things.

It is a common practice to exhaust about 1/3 of this air and draw in a quantity direct from

the atmosphere to replace this. This is particularly desirable for air conditioning for human

occupancy.

ASSIGNMENT

Sketch a window air conditioner, label it properly and briefly explain the function of the

major component parts.

WEEK 10

10.0 KNOW HOW TO CALCULATE COOLING LOADS FOR REFRIGERATION

AND AIR CONDITIONING SYSTEMS

10.1 COOLING LOADS AND CALCULATION

Cooling load is the rate at which heat must be removed from the refrigerated space or

material in order to produce and maintain the desired temperature condition.

Some of the sources of heat in the refrigerated space include:

1. Heat leak by conduction from outside into the refrigerated space through the insulated

walls. This is called wall gain load.

2. Heat by radiation through glass windows and other transparent items into the

refrigerated space.

3. Heat by warm outside air entering through open doors, or through cracks around

windows and doors. This is called air change load. This air change load can be due to air

infiltration into the refrigerated space or ventilation airs to make up for require air in a

conditioned space for the occupants.

4. Heat given off by warm products entering into the refrigerated space. This is called

product load.

5. Heat given off by people occupying the refrigerated space.

6. Heat given off by heat producing equipment located inside the refrigerated space.

10.1.1 WALL GAIN LOAD

The wall gain load is given.

by Q = A x U x DT x 24hr x 60min x 60sec

where A = sum of wall area of the refrigerated space

DT = temperature change (corrected for solar radiation from table)

U = overall heat transfer coefficient.

Where U = 1

h1 = Connection coefficient (surface conductance) inside walls

ho = Convection coefficient (surface conductance) outside walls

t1 = Wall thickness

k1 = Thermal conductivity

Product load (including containers and packing materials)

For storage temperature above freezing point of product (24 - hr)

Qp = Wp x C x (T2 – T1) x 24 hrs

Desired cooling time (hr)

Where

Wp = weight of product

C = specific heat capacity

T1 = entering temperature

T2 = storage temperature

For storage temperature below freezing point of product (24 - hr)

Qpf = Wp x Cf x (T1 – Tf) + W λf + Wp x Cf x (Tf – T2)

Tf = freezing point

λ = latent heat of fusion

From the above, the quantity of heat given off by product in cooling from entering

temperature to its freezing temperature is

Qp = Wp x Cf x (T1 - Tf)

Also the quantity of heat off by the product of in solidifying or freezing is given by

Qpp = Wλf

The quantity of heat given off by the product in cooling from its freezing temperature to

final storage temperature is given by

Qps = {Wp x Cf x (Tf – T2) }

10.1.2 CHILLING RATE FACTOR

This means that factor introduction into chill load calculation to compensate for the uneven

distribution of chilling load. The effect of the chilling rate factor is to increase the product

load calculation by an amount sufficient to make the average hourly cooling rate approx

equal to the hourly load at the peak condition. This results in the selection of larger

equipment having sufficient capacity to carry the load during the initial stages of chilling.

Qp = Wp x C x (T2 – T1)

Chilling time(s) x chilling rate factor

Qp = WCDT x 24 hr

Chilling rate x MAT

W = wt of product

C = specific heat capacity

DT = change in temperature

MAT = Max allowable time.

Change load:

QA = M (ho - hi)

Where QA = air change load (kw)

M = mass of air entering space (kg/s)

ho = enthalpy of outside air (KJ/kg)

Also the heat load due to infiltration can be calculated for a known volume of air.

If V1 = the infiltration of air

Va = specific volume of ambient air

ho = enthalpy of ambient air

hi = enthalpy of inside air

Then QA = (Vi / Va) (ho – hi)

WEEK 11

11.0 KNOW THE FUNCTION OF AN AIR CONDITIONING SYSTEM FOR A

BUILDING CONTD

11.1 REFRIGERATED COOLING OR AIR CONDITIONING

Refrigerated air conditioning is similar to commercial refrigeration because of the same

components that are used to cool the air: (1) the evaporator, (2) The compressor, (3) the

condenser and (4) metering devices. These components are assembling in several ways to

accomplish the same goal, refrigerated air to cool space.

11.2 PACKAGED AIR CONDITIONING

The four components are assembled into two basic types of equipments for air conditioning

purposes: packaged equipment and split system equipment. With package equipment all of

the components are building into one cabinet. It also called self –contained equipment.

Package equipment may be located beside the structure or on top of it. In some instances the

heating equipment is built into the same cabinets.

Fig 11.1 Package unit installation

11.3 SPLIT SYSTEM AIR CONDITIONING

In spit system air conditioning the condenser is located outside, remote from the evaporator,

and uses interconnected refrigerant lines. The evaporator may be located in the attic, a crawl

space, or a closet for up flow or down flow applications. The fan to blow the air across the

evaporator may be included in the heating equipment, or a separate fan may be used for air

conditioning system.

Fig 11.2 Split air conditioning system

11.3.1 THE EVAPORATOR

The evaporator is the component that absorbs heat into the refrigeration system; it is a

refrigeration coil made of aluminum fins on either type attached to the coil to give it more

surface area for better heat exchange.

The evaporator coil has several designs for airflow through the coil and draining the

condensate water the coil, depending on the installation. The different designs are known as

the A coil the slant coil and the H coil.

Fig 11.3 Evaporator with a coil case

11.3.2 THE A COIL

The A coil is used for upflow; downflow, and horizontal flow applications. It consists of two

coils with their circuits side by side and spread apart at the bottom in the shape of the letter

A, in the figure below. When used for upflow or downflow, the condensate pan is at the

bottom of A pattern. When used for horizontal flow, a pan is placed at the bottom of the coil

and the coil is turned on its side. The air flows through A coil is through the coil of the coil. It

can not be from side to side with the two coils in series. A coil is not the best coil application

for horizontal application. When horizontal air flow is needed slant or H coils may be more

desirable.

Fig 11.4 An A coil

11.3.3 THE SLANT COIL

The slant coil is one piece coil mounted in the duct on an angle (usually 60o) or slant to give

the coil more surface area. The slant of the coil causes the condensate water to drain to the

coil causes the condensate water to slant. The coil can be used for upflow, down flow, or

horizontal flow when designed for these applications.

Fig 11.5 A slant coil

11.3.4 THE H COIL

The H coil is normally applied to horizontal applications although it can be adopted to

vertical applications by using special drain pan configuration. The drain is normally at the

bottom of the H pattern.

Fig 11.6 An H coil

11.4 COMFORT AIR CONDITIONING SYSTEM

In comfort air conditioning, the air is brought into the required dried bulb temperature and

relative humidity for the human health, comfort and efficiency. If sufficient data of the

required condition is not given, then it as assumed to be 21oC dry bulb temperature and

50o/o relative humidity. The sensible heat factor is generally kept as follow;

For residence or private office = 0.9

For restaurant or busy office = 0.8

Auditorium or cinema hall = 0.7

Ball room dance hall etc =0.6

The comfort air conditioning may be adopted for homes, offices, shops, restaurant, theatre,

hospital, schools etc .

11.5 INDUSTRIAL AIR CONDTIONING SYSTEM

It is an important system of air conditioning these days in which the inside dry bulb

temperature and relative humidity of the air is kept constant for proper working of the

machines and for the proper research and manufacturing processes. Some of the

sophisticated electronic and other machines need a particular dried bulb temperature and

relative humidity. Sometimes, these machines also required a particular method of

psychometric processes. These types of air conditioning system is used in textile mills, paper

mills, machine parts manufacturing plants, tools room, photo processing plant etc.

11.6 UNITARY AIR CONDITIONING SYSTEM

In this system, factory assembled air conditioners are installed in or adjacent to the space to

be conditioned. The unitary air conditioning systems are of the following two types,

1 window units: These are self-contained units of small capacity of 1 TR 3TR and are

mounted in a window or through the wall. They are employed to condition the air of

one room only. If the room is bigger in size, then two or more unit are installed.

2 Vertical packed unit: These are self contained unit of bigger capacity of 5-20TR and

are installed adjacent to the space to be conditioned. This is very useful for

conditioning the air of a restaurant, bank or small office.

11.7 CENTRAL AIR CONDITIONING SYSTEM

This is the most important types of air conditioning system, which is adopted, when the

cooling capacity required is 25TR or more. It is also adopted when the air flow is more than

300m3/min or different zone in a building are to be conditioned.

WEEK 12

12.0 KNOW HOW TO SERVICE REFRIGERATION SYSTEM

12.1 REFRIGERATION SYSTEM SERVICE

For simplicity, service operations have been divided into two groups. This deals with work on

refrigeration systems. A number of relevant procedures have been described previously, and will not

be repeated. For example, from the previous discussion details procedures to be followed when

cutting and jointing pipelines; brazing or silver soldering; evacuating and leak testing; and charging

with either gaseous or liquid refrigerants. Noise and vibration precautions, and drive kit adjustment

methods are also described previously; and many service pointers are included in earlier lessons

describing specific system components.

12.2 SAFETY PRECAUTIONS

Precautions to be taken when handling refrigerants or other chemicals, and flushing out or brazing

pipelines which have contained refrigerants, have also been detailed earlier. We must however

repeat the following points,

a) Wear suitable protective clothing (including safety glasses or goggles) whenever refrigerants are

handled, or refrigeration systems opened to atmosphere

b) Always purge refrigerants to the outside atmosphere when a system is to be emptied

c) Do not braze or weld systems containing refrigerants or refrigeration oil

d) Do not blow through systems which might contain oil, with oxygen or other potentially explosive gases

e) Use only suitable materials, and good practices, when working on any type of system.

12.3 ADJUSTMENT OF CONTROLS

A number of adjustments do not require circuits to be opened - especially if systems have adequate

provision for instrumentation. We will review them before passing on to more complex work, involving

opening refrigeration systems.

12.4. THERMOSTATIC EXPANSION VALVES

Before adjusting the superheat screw of a TEV, because an evaporator is receiving too much or too

little refrigerant, make sure that the sensing bulb is firmly connected to the suction line and is correctly

positioned), and that neither the bulb nor the capillary has been damaged and part or all of the bulb

charge lost. Check also that all capillary tubes from the refrigerant distributor (if fitted) are undamaged

and passing refrigerant.

Fig 12.1 TEV superheat test positions

If the above checks prove satisfactory, check the valve superheat as follows.

a) Using (preferably) an electronic thermometer, or alternatively a sensitive dial thermometer with a

gas-filled bulb, clamp the thermocouple or bulb securely to the suction line at point A.

b) Using a self-sealing access valve at point B, fit a gauge to read the evaporator outlet pressure and,

using a pressure/temperature chart, convert this to the equivalent temperature. (N.B. suction

temperature can be calculated from the pressure at the compressor suction port, minus say 1 °C or a

more accurate estimate of pressure drop in the suction line; but this is not as accurate as the method

first described.) Subtract temperature B from temperature A to determine the superheat of the

system.

For example:

Temperature at A ...................5 °C (41 °F)

Pressure at B = 62 kPa

(9 pslg) which if using

R12 is equivalent to ................0 °C (32 °F)

Therefore superheat............... 5 °C ( 9 °F)

Too low a superheat means that too much refrigerant is entering the cooling coil, the suction pressure is

high, and the Suction line may frost back to the compressor. Too high a superheat means that the

evaporator is starved. The suction pressure Is low, and only part of the cooling coil is fully used. To

adjust superheat, remove the cap from the adjustment screw, and turn it not more than one quarter of

a revolution at a time. Note the effect on the temperature at point A (remembering that it can take up

to half an hour for a valve to settle down •gain). Unfortunately, TEV manufacturers have not

standardized settings - one cannot, for example, say 'turn anti-clockwise to reduce superheat'. To be

safe, look up the technical data on the valve concerned, or be prepared to spend time getting the

correct setting by trial and error. If It is necessary to strip a TEV for any reason, the system should be

pumped down and valves off so that the valve is at a slight positive refrigerant pressure - say 14 kPa (2

psi). This system pressure will resist the entry of air or other contaminants.

12.5 CAPILLARY TUBES

Instructions of questionable value appear in some service manuals, in the form of advice on clearing

blockages from capillary tube refrigerant controls. The procedures -normally involving the use of a

hand pump, and hydraulic oil to force the contaminant out of the capillary - are usually effective. But

the cost of the operation, in terms of materials and labour, often exceeds that of a shiny new capillary

control of guaranteed quality. If a capillary tube is blocked, remove and replace both it and the liquid-

line strainer. If it is necessary to change evaporator or condenser, replace both capillary and liquid-

line strainer.

12.6 SOLENOID VALVES

The most frequent cause of trouble with solenoid valves - be they liquid-line, hot gas bypass, or

reverse-cycle/defrost controls are:

a) dirt, sludge or other system contaminants; or

b) valve bodies distorted by excessive heat when brazed or soldered into place.

Either may result in any of the common symptoms 'valve will not open'; 'valve will not close'; Valve is

not seating properly' or 'the coil has burned out'. Cross-sections through various types of solenoid

valve appear in Fig. below (reversing) and (liquid-line).

In the event of trouble with a solenoid valve, pump down to a slight positive pressure and

check it for wax, carbonized oil or other contaminants. If any are present, strip and

thoroughly clean the valve and renew the refrigerant strainer ahead of the valve, as well

as the strainer in the entry port of the valve (if applicable). Distorted valve bodies can only be

replaced. If doing this, note the procedures detailed in the previous lessons point the flame

away from the valve body. If possible favour designs with longer, not shorter sweat

connection 'tails'. Valves must be installed in the same plane (horizontal or vertical) as the

original, and be of the same make and type - some will only work if plungers are vertical (i.e.

the body is horizontal) but others have strong enough coils to work in any plane.

Fig 12.2 Liquid line solenoid valves

12.7 HOT GAS BYPASS VALVES

These capacity regulating valves are installed in a bypass between the discharge line from the

compressor and (ideally) an entry port between the expansion valve and distributor. They

modulate towards their fully open position as suction pressure falls. Figure below shows a

typical installation layout. The set point pressure is adjusted by the control spring, and the

valve remains closed so long as evaporator pressure is above the preset level. If it falls below

the set point, the valve starts to open and hot gas is injected into the entrance to the

evaporator. This increases suction pressure, and the load on the compressor, and prevents

frosting back or liquid slugging. Note from Fig. below that a solenoid valve is installed in the

discharge line ahead of the bypass valve (it is wired in parallel with the liquid-line solenoid

valve) to prevent the refrigerant pump-down system from being bypassed and rendered

inoperative. To adjust such a valve, fit a suction gauge and coarse-set the desired suction line

pressure by adjusting the liquid-line valve. Finer settings can then be made by retensioning

the adjustment spring. Bypass valve problems are not common if systems are free from

contaminants. Two types of defects can be found.

a) Valve will not open - due to the seat or piston being affected by sludge or other

contaminants. Pump down to a slight positive pressure and strip and clean the valve

b) Valve will not close - due to one of:

i) presence of contaminants - see (a) above

ii) bypass tube blockage or restriction -clean or replace bypass, and strip and clean valve

iii) diaphragm failure - replace valve power element.

Fig 12.3 Hot gas bypasses line

WEEK 13

13.0 KNOW THE FUNCTION OF COMPONENTS OF AN AIR CONDITIONING

SYSTEM FOR A BUILDING

13.1 HEAD PRESSURE CONTROLLERS

Two main types of controller are available:

a) Pressure operated - actuating dampers at one or more evaporator fan outlets or electronic

motor speed controllers;

b) Temperature operated - actuating electronic motor speed controllers or cycling one or

more fan motors on/off in accordance with ambient conditions. The second type of controller

may be less effective than the first, since there is not necessarily a fixed relationship between

system pressures and ambient temperatures. Controllers should be set to be fully effective

(i.e. motor at lowest speed, or dampers fully closed) at a liquid-line pressure slightly above

the minimum level nominated by the manufacturer of the compressor and then operated to

increase condensing efficiency as refrigerant pressures/temperatures increase. A typical 'as

fitted' circuit is as shown in Fig. below. Head pressure controllers can only be re-set using

specific instructions issued by their

manufacturers.

Fig 13.1 Head pressure controller layout

13.2 PRESSURE CONTROLS

A typical pressure control section is illustrated in Fig. below. To adjust control settings, fit suction and

discharge pressure gauges and make only small changes to settings before checking effects on system

procedures. Control operation, and the accuracy of existing settings, can be tested by 'rigging' the

equipment. For a high pressure control, progressively blank off an air-cooled condenser with a sheet

of hardboard, and note the pressure at which the control operates (or fails to operate!). For a low

pressure control, system pressures can be steadily reduced by throttling down at the liquid-line valve.

In either case, be ready to remove the 'obstruction' quickly if pressure changes are greater, and occur

more quickly, than expected.

Fig. 13.2 Pressure control

13.3 ACCESSIBLE AND OPEN COMPRESSORS

13.3.1 COMPRESSOR SHAFT SEALS

The weakest point of an open compressor is its shaft seal, which is likely to leak oil and refrigerant as

the result of mechanical damage by misaligned drive kits, as well as 'fair wear and tear1. A typical shaft

seal is illustrated in Fig. below. The seal is contained between a step (or 'shoulder') in the shaft, and a

bolt-on seal (or 'gland') cover. The shaft passes through the seal and cover, to carry the half shaft or

vee belt pulley.

The wearing surfaces of a seal are held in place by a spring, or bellows. This is slipped over the shaft,

and secured at the shaft 'shoulder' by a sleeve. At its front end, the spring presses a carbon sealing

ring against a matching metallic surface. The two wearing surfaces are lapped to optical standards of

flatness and covered by a film of recirculated refrigeration oil, which provides the ultimate seal against

differing pressures at each side of the assembly. A gasket is normally used inside the seal end cover,

which is itself machined to close tolerances around the shaft aperture. A shaft seal leak is indicated

first by loss of refrigerant - which can be detected by a lamp or electronic refrigerant detector -and

then by refrigeration oil seeping through the seal. To replace a seal, the compressor should be

pumped down to a slight positive pressure (14 kPa (2 psi)), switched off, and its service valves front-

seated. N.B. This procedure is followed every time, it is necessary to open an open or accessible

hermetic sealed compressor or system component.

The seal cover is then carefully unbolted, leaving two bolts at opposite extremes of the cover until last,

so that any excess pressure can be released in a controlled fashion. When the cover has been removed,

the seal components can be removed by hand, or using a seal withdrawal tool. The wearing surfaces of

seals which are leaking but not badly damaged can be repaired by carefully relapping.

Fig 13.3 compressor shaft seal section

13.3.2 COMPRESSOR VALVE PLATES

The valve plate and suction/discharge reed assemblies can be easily removed after an open or

accessible hermetic compressor has been pumped down. Valve plates can be relapped if slightly

worn, but there is little future in attempting to repair damaged valve reeds. Note that new

gaskets should always be fitted when a compressor Component is removed for scrutiny or repair.

Fig 13.4 Valve plate assembly

13.3.3 STRIPPING COMPRESSORS

After systems are pumped down, and seal end covers removed, compressors can be separated from

their service valves and the Oil poured into suitable containers. Examine the oil to see if it contains any

swarf or contaminants, but do not use it again in the compressor. After removing the rear (or motor)

end cover and gasket, the compressor pistons, connecting rods and crankshaft can be removed in that

order. Con rods and pistons must be marked, to ensure that they are replaced in their original

positions. Any damaged items must be replaced and all components should be stored in clean

refrigeration oil until it is time for them to be cleaned (preferably using R11 or a non-toxic, non-

explosive cleansing/grease solvent; caustic soda and trichloroethylene are not recommended) and

replaced. Do not forget Shaft or other bearings, which must be examined for fit and wear and must be

replaced if their condition is less than excellent. Oil strainers and channels must also be carefully

inspected, and any contaminants removed.

When re-assembling compressors, take care not to use the original gaskets (or other sealing materials)

or oil. All components (including the crankcase and other castings) must be cleaned and examined to

ensure they are free of defects, then coated with fresh, dry refrigeration oil. Care must be taken not to

over-tighten bolts, and the use of torque-indicating wrenches is desirable. We list a logical order in

which components can be refitted.

a) Crankshaft bearings (often grooved or marked to indicate which side should contact the thrust

surfaces of the crankshaft).

b) Crankshaft (inserted from the front (seal) end) then the seal end cover.

c) Oil pump end bearing head. Adjust the end play of the crankshaft by shims between the bearing head

and housing. Double check the end play with a feeler gauge after the bearing head is positioned and

tightened.

d) Remove the seal end cover, and insert the shaft seal. Replace the end cover.

e) Install the oil pump and suction strainer.

f) Assemble and test (i) valve plates and reeds, and (ii) the cylinder unloader mechanism, if used.

g) Fit piston compression oil rings, if used.

h) Assemble pistons, con rods, and (if used) cylinder liners,

i) Properly position the crankshaft, insert pistons and con rods through cylinders, and complete the

assembly to the crankshaft.

j) Refit the valve plate and cylinder head,

k) Replace the compressor on its base, and connect the service valves.

l) Insert a new charge of dry refrigeration oil.

m) Evacuate and leak test the compressor,

n) Return the unit to operation.

Fig 13.6 Section of accessible hermetic compressor

Fig 13.5 Piston and con rod assembly

WEEK 14



14.1 ADDING OIL TO OPEN/ACCESSIBLE HERMETIC COMPRESSORS

Never pour refrigeration oil into a compressor crankcase - it is likely to be contaminated by air and

moisture in the process. With the compressor pumped down as instructed, remove the oil filler plug,

and insert new, dehydrated refrigeration oil using a fully-primed hand pump. Make sure that air

entering the can to replace oil used is passed through a drier and that both the compressor and the oil

can are sealed immediately the correct quantity of oil has been added.

Fig. 14.1 Hand pump

14.2 REPLACING WELDED HERMETIC COMPRESSORS

The replacement of a welded hermetic compressor after a motor burn-out is potentially the most

critical operation we are likely to perform. Unless the job is planned and carried out to the highest

standards, there is every possibility that the replacement compressor will also burn out. Both

equipment and labour costs are too high for repeat burn-outs to be acceptable. The first essential is, to

be 100 per cent certain that it is necessary to fit a replacement before proceeding beyond the test stage.

The electrical checks to be made to establish whether motor windings or insulation have been

irrevocably damaged are detailed in the next chapter. Mechanical defects which justify replacing a

compressor are as follows.

a) Components being jammed, or 'frozen' - normally follows lubrication problems, but can occur as

the result of tight tolerances between piston and cylinder or stator and rotor. In this case, it may be

possible to get a compressor "running by increasing the starting torque of its motor (using 'starting

gear' or fitting it for C.S.R. operation) arid/or running the motor backwards for a minute or two.

b) The unit 'jumping' from its internal mounting springs as the result of mishandling (normally the

consequence of an appliance not standing upright during transit). The symptom is an abnormally high

noise level (hammering), frequently followed by loss of pumping efficiency as the result of internal

pipelines fracturing.

c) Damage to suction or discharge valves, or other components, resulting in unacceptably low

pumping efficiency. This condition can be positively checked if the compressor does not incorporate

gauge ports, by installing line-tap or Schrader valves close to suction and discharge tubing connections

to the compressor casing. The most serious damage is that which results from a compressor motor

burning out. This causes oil to carbonize, form sludge, and contain acids as well as pieces of charred

winding insulation and electrical materials. All traces of these contaminants must be removed from the

system when the compressor is replaced, since their presence inevitably leads to a repeat burnout.

Burned out compressors must be handled with caution. If the seal of the service (or 'slave') tube is

broken, the characteristic smell of burned windings is unmistakable and the compressor oil will be

dark, evil-smelling and contaminated with carbonised particles. Contaminated refrigerant should be

purged to fresh air, using a disposable length of tubing, before the compressor is removed; and

protective clothing (goggles and gloves) must be worn to prevent refrigerant, oil or acids from injuring

skin or the eyes. Compressor replacement procedures are given below.

a) With power off, remove all electric conductors and accessories. Using a pipe cutter, cut suction and

discharge lines close to (within 50 mm of) the compressor housing, unless service valves are fitted.

Release the external mountings and remove the burned-out compressor.

b) Remove the original capillary tube and refrigerant strainer.

c) Using a purpose-made pump, thoroughly flush out the remainder of the system with R11.

Continue until no traces of oil or contaminants emerge from the suction line and/or suction-

line accumulator.

d) Fit a new capillary tube, an over-sized liquid-line strainer/drier, the new compressor and a

suction-line filter-drier (or 'burn-out'/'clean up' kit) immediately upstream of the

compressor. If this does not have service valves, install Schrader valves before and after the

suction-line drier. All electrical accessory items (capacitors, relay, overload etc.) must be

replaced with new equipment.

e) Carefully silver solder all new joints whilst washing through the system with dry nitrogen

applied at low pressure.

f) Evacuate the system, using either the Triple Evacuation Method at 710 mm (28 in) Hg or,

preferably, a high vacuum pump recording 250 microns or better on an electronic vacuum

gauge. Under no circumstances use the compressor as a vacuum pump.

g) Leak test, by making certain that gauge pressures do not increase (other than in step with

room temperature changes) for three hours if using the Triple Evacuation Method, or one hour

if using an electronic high vacuum kit.

h) Using a fully-equipped charging station, add the correct refrigerant (and if applicable oil)

charge.

i) Remove the charging equipment, remake all electrical equipment and start up the new

compressor.

j) Check the pressure drop across the suction-line filter-drier, using a differential pressure

gauge if possible and check the amperage drawn by the compressor.

k) After one hour, recheck the pressure drop. Ideally, this should not exceed the values.

I) After three hours, again check the pressure drop. If this exceeds three times the value

quoted, fit new liquid and suction-line driers. This procedure must be repeated until the

pressure drop stabilizes at or below the levels indicated above, for permanent operation.

m) Where the replacement compressor is a second or third replacement, i.e. the system is

one in which there have been several burn-outs, an oil sample should be taken and acid-tested

when the suction line drier pressure drop is at an acceptable level. Only if the oil is confirmed

to be acid-free can the operation be regarded as satisfactorily completed.

The above procedures are fully current, many being based on Tecumseh recommendations.

The sequence of operations and end objectives is of course the same in systems using welded

or accessible hermetic compressors. The only changes are those resulting from variations in

evacuation and leak testing procedures when working on large systems.

14.3 GAS CHARGING METHODS

Conventional procedures are: to charge gas from a vertical cylinder, standing charging valve

upwards; or to charge liquid from an inverted cylinder with the charging valve beneath the

liquid refrigerant level. This is no longer an invariable rule, since du Pont now packs some Freon

refrigerants in cylinders with a new type of charging valve. This enables liquid or gaseous

refrigerant to be selected at the valve, with the cylinder remaining upright.

14.4 AIR OR OVERCHARGE

Abnormally high condensing pressures can result both from an overcharge of refrigerant and

from air in the condenser. The question 'which' is not difficult to solve. If a head pressure gauge is

fitted and the system switched off, then:

a) If there is an overcharge of refrigerant, the condensing pressure will fall slowly but steadily until it is

equivalent to room temperature.

b) If there is air in the condenser, pressure will remain well above that of the equivalent room

temperature.

Fig 14.3 Charging cylinder positions

14.5 PRESSURES IN WELDED HERMETIC SYSTEMS

The ready availability of line-piercing valves which can be easily and permanently installed in lines up

to I" has removed the major difficulty in diagnosing faults in small all-welded systems. They are not

expensive, and their use can only help to avoid errors and repeat service calls. When piping up larger,

field-assembled systems the use of Schrader valves is desirable, even when compressor and liquid line

valves are provided. This type of valve costs even less than line-tap models and has many uses. In split

systems, refrigerant can be charged at either condensing or air handling units. Suitably located valves

make it quick and easy to adjust TEV superheat or the settings of head pressure controllers or bypass

valves, to take oil samples, or to check pressure at or through any part of the system.

14.6 ROTOLOCK WELDED COMPRESSOR VALVES

An increasing proportion of welded hermetic compressors incorporate suction and discharge valves,

which again help to eliminate guesswork. Some other 'cans' incorporate provision for the installation

of service valves by equipment manufacturers - or distributors - who consider that service costs and

standards are more important than minimal first costs. The following notes will be of help to those

fitting replacement compressors. Some compressors have 'spuds' brazed into their casings. If screw-

on caps and seals are removed from 'spuds', Rotolock service valves can be screwed onto the fittings.

Alternatively, suction or discharge line terminals can be silver-soldered to the screw-on capping rings,

using appropriate precautions against over-heating compressor casings.

A second type of fitting uses a steel adaptor, which fits over and is silver-soldered to stub tube

connections emerging from the compressor casing. (Don't forget to cut the end seals from stubs

before fixing the adaptors!) The other end of the adaptor is threaded to receive a screw-on Rotolock

valve (which has a capped service or charging port). Electrically or mechanically operated recording

Instruments covering a variety of applications are available - some forming in Integral part of control

systems. Figure below, Illustrates a two-pen, 24-hour DB and WB temperature recorder which is typical

of a wide range of instruments available.

Fig 14.4 Rotorlock valve adaptor Fig 14.5 DB/WB temperature recorder

REVISION QUESTIONS

1 The plant for a commercial cold room includes a 0-5 kW semi-hermetic compressor charged with R12. It is

suspected that air has entered the system, and service pressure gauges have been installed.

a) List two major observations made using the gauges which would confirm the presence of air.

b) Enumerate two other ways in which the presence of air could be confirmed.

c) Describe in detail the method used to eliminate the air.

d) What signs would confirm that air has been present, the compressor valve cover and valve plate having been

removed for inspection?

2 a) Make a labeled line sketch showing the layout of a commercial refrigeration evaporator and an externally equalized

thermostatic expansion valve.

b) With the help of your diagram, explain briefly how the TEV superheat setting can be checked and altered.

WEEK 15



Several different types of humidifier are available, all designed to admit steam or spray small water

droplets into the supply airstream where the moisture will be vapourised and immediately increase

RH. Again, a wide range of capacities is available, equipment being rated in terms of the moisture it will

distribute, or the steam it will generate, in a given period of time - usually one hour. The types most

often used are listed below.

15.1 STEAM PAN HUMIDIFIER

Steam pan humidifiers Water is gravity-fed into a closed container which contains one or more electric

elements. If the control circuit is made, elements are energized and boil some of the water. The steam

passes out of the boiling pan and is fed directly, or through a supply line connecting the pan and steam

distributing nozzles, into the supply airstream which has left the evaporator. Water which has been

boiled is replaced from a break tank fitted with a ball-type make-up valve. When space RH reaches the

set point, the humidifier de-energizes the heating elements. This arrangement, illustrated in Fig.

below, is simple and economical. Drawbacks are that there is a time lag between the need for steam

being sensed, and its being generated, and that in hard water areas heating elements will - like those in

electric kettles - become coated with scale, which increases the time taken to produce steam. This can

of course be removed using a standard descaling chemical, but if treatment is neglected the humidifier

will cease to operate effectively. Water bleed-off or water drainage and refilling cycles initiated by

time clocks are often provided, to reduce scaling.

Fig 15.1 Steam pan humidifiers

15.2 ELECTRODE HUMIDIFIERS

Electrode humidifiers The control sequences and water supply arrangements of this type are as

described above, but steam is generated by the action of electrodes, so that there are no heating

elements which can be scaled over by hard water. Scale does of course form

accumulates in the container and takes much longer to affect the efficiency of the system.

Sophisticated control systems can be used to adjust the current drawn according to the pH of the

water and, where necessary, to boil or

its chemical characteristics is suitable for the

Electrode humidifiers are more expensive than steam pan types, but offer

characteristics lead to heavy scale formation. The av

by the absence of scale on heating

LECTRODE HUMIDIFIERS

The control sequences and water supply arrangements of this type are as


elements which can be scaled over by hard water. Scale does of course form as water bo

container and takes much longer to affect the efficiency of the system.


water and, where necessary, to boil or drain off and replace part or all of the water in the boiler until

its chemical characteristics is suitable for the equipment.

Electrode humidifiers are more expensive than steam pan types, but offer advantages where water

characteristics lead to heavy scale formation. The average time taken to produce steam is minimized

by the absence of scale on heating elements.

The control sequences and water supply arrangements of this type are as


as water boils, but

container and takes much longer to affect the efficiency of the system.


e part or all of the water in the boiler until

advantages where water

erage time taken to produce steam is minimized

Fig 15.2 Electrode humidifiers

15.3 WATER ATOMIZING HUMIDIFIERS

Water atomizing humidifiers The spinning disc type shown below uses a horizontal, quickly-rotating

surface to atomize (make into tiny droplets) water sprayed onto it, or admitted in drops from an

overhead supply grid. Rotary drum construction features a horizontal drum or brush construction, in

which only its top portion rises above water level. As the drum rotates it sprays a shower of fine

droplets into the airstream which passes through the drum into the conditioned space.

Water atomizers are economical in terms of first cost, and not affected by scale formation. It is

however necessary to install effective water eliminators to prevent any droplets which are not

completely vaporized from being carried over into ducts or conditioned space. Efficiency will vary with

changes in the temperatures of the water or room air.

Fig 15.3 Water atomizers

15.4 CONDENSERS

15.4.1 AIR-COOLED

Structural details are as described in Chapter 6, and the use of motor speed controllers or fan outlet

dampers is more common in tropical installations than would at first sight seem likely. It must be

remembered that condensers operating in extremely hot

condensing pressures by day in extreme heat. When

may become effectively oversized and be unable to maintain liquid refrigerant at pressures high enough to

ensure satisfactory system operation. Whilst it is not necessary to build solar screens for properly designed

condensers, it is desirable to take advantage of any shade provided by buildings when outdoor

temperatures are at their peak.

Fig 15.4 Air-cooled packaged chiller



remembered that condensers operating in extremely hot climates are 'oversized' to secure acceptable

condensing pressures by day in extreme heat. When temperatures fall overnight, or in winter,

oversized and be unable to maintain liquid refrigerant at pressures high enough to



temperatures are at their peak.

packaged chiller



climates are 'oversized' to secure acceptable

temperatures fall overnight, or in winter, condensers

oversized and be unable to maintain liquid refrigerant at pressures high enough to



15.4.2 WATER-COOLED

Apart from systems with welded hermetic compressors (which generally use tube-in-tube, or tube-in-shell

condensers because they are cheapest) the use of shell-and-tube models is standard in units with 7-5 kW

(10 hp) or larger motors. The use of evaporative condensers is rare, since they are not compatible with

factory assembled, packaged the equipment.

Fig 15.5 water-cooled packaged chiller

15.5 WATER PUMPS

For air-conditioning needs - whether circulating chilled or condenser water -centrifugal

pumps such as the one shown in Fig. below are invariably used. These operate on similar

principles to the centrifugal compressors described earlier in this chapter. Water enters at

the centre of a fast-turning impeller; it is thrown to the outside of the impellers, the

converging blades of which add to its speed; and it collects in a volute leading to a discharge

port at right angles to the direction of entry. In many cases, the casing can be rotated to

enable the discharge connection to be made to pipework at several optional positions

around 'the clock'.

Two pump speeds are most likely to be used on 50 Hz supplies: 1 500 or 3 000 r.p.m.

synchronous. The faster the speed, the greater the head against which the pump can deliver

water, but the more noise it generates. There is also a risk that pumps running at the same

speed as centrifugal compressors may set up sympathetic vibrations which affect both, so

the lower running speed is usually favoured.

The bodies of pumps designed to handle water are likely to be of cast-iron construction,

trimmed in bronze with a bronze or stainless-steel shaft, and bronze impeller(s) - like

compressors, pumps can have several stages to enable them to operate against higher

pressures - and bearings. Water leaks are normally avoided by the use of rotary shaft seals.

The old fashioned stuffing box is rarely seen on new equipment.

There are several means of connecting the motor and the pump. If the pump is mounted on

a baseplate a direct-drive coupling is usually employed, but some prefer to use pulleys and

vee belts. The most foolproof arrangement is that used in accessible hermetic compressors

with both pump and motor on a common shaft and secured within a single-piece or close-

coupled body assembly.

Whilst installation and instrumentation needs are covered in the previous discussion, it

should be noted that pumps should always have a valve, an anti-vibration coupling and

provision for the use of a pressure gauge on both suction and discharge sides. The use of a

water strainer immediately before the pump is essential. In many cases, a second pump is

piped into the circuit ready for use if the first fails. This entails some complicated pipework,

and, in order to avoid the need for this, an increasing number of manufacturers now offer

factory-assembled dual-pump assemblies, with both the running and standby pumps

completely plumbed in at the factory. One such assembly is shown in Fig. below.

There is little basic difference between pumps designed to handle water, brine or

refrigerants. All components in contact with the liquid being handled must be chemically

compatible with it at operating temperatures, but operating principles are little changed.

Fig 15.6a Centrifugal water pump

Fig 15.6b Dual pump assembling

entrifugal water pump

ual pump assembling

15.6 AIR HANDLING UNITS

The main duties of any air handling unit, whether a factory package or field

assembled, must be capable of some or all of the following.

a) Mixing return and ventilation air s

b) Filtering all air to required standards of efficiency.

c) Cooling, or cooling and

Dehumidification, with or without reheating the total airstream or heating, or

heating and humidification.

d) Avoiding carrying over into ductwork or conditione

deposited on cooling coils during the dehumidification processes or by water

or fed into the air stream by humidifiers.

Fig 15.7 Central plant air handling unit

e) Discharging all air at a pressure

ductwork, supply grilles and diffusers, and all airhandling

requirements can be satisfied in one of several ways. We shall note most in passing, but study in

detail only those in widespread use in our specific (geographical) areas of interest.

The main duties of any air handling unit, whether a factory package or field

assembled, must be capable of some or all of the following.

a) Mixing return and ventilation air supplies.

b) Filtering all air to required standards of efficiency.

c) Cooling, or cooling and


heating and humidification.

d) Avoiding carrying over into ductwork or conditioned space, any moisture

deposited on cooling coils during the dehumidification processes or by water

or fed into the air stream by humidifiers.

Central plant air handling unit

e) Discharging all air at a pressure equaling the sum of the resistances to airflow offered by

grilles and diffusers, and all airhandling unit components.


in widespread use in our specific (geographical) areas of interest.

The main duties of any air handling unit, whether a factory package or field-


d space, any moisture

deposited on cooling coils during the dehumidification processes or by water spray

airflow offered by

unit components. Each of these


in widespread use in our specific (geographical) areas of interest.

15.6.1 AIR MIXING

Fresh, ventilation air is normally drawn into plant-rooms through weatherproof louvred intakes fitted with

volume dampers and mesh screens, which prevent the entry of leaves, birds and insects. It is filtered before

being mixed with return air in either a plenum, or a mixing box fitted with opposed-blade dampers to

regulate the volume of each type. In regions with wide seasonal variations in temperature, air intake

volumes are often varied by thermostatic controls reacting to changes in external temperatures, to

reduce demands for refrigeration or heating at times when the ambient air temperatures approach

design air-off-coil levels.

15.6.2 FILTRATION

There are three types of filter for normal use:

a) Throwaway types, of foamed plastic or glass fibre construction.

b) Cleanable types, normally using metal 'wool* or fibre as the filtering medium. This can be

washed clean, and may be coated with oil or other viscous material to increase its efficiency,

c) High efficiency filters, used dry and made from pleated mats of wool felt, cellulose

fibre, or synthetic materials. The choice of material in this and other types of filter depends upon the

efficiency required, and the velocity of the air to be filtered.

Efficiency ratings are usually expressed in terms of the percentage of air contaminants of a

specified size, removed when the air passes through the filter at a known velocity. For

example, 95 per cent at 5 micron is a good standard filter performance; with a normal high

efficiency filter rating being 80 per cent at 0-5 micron; both at a velocity of 2-54 m/s (500

ft/min). Where high standards of air cleanliness must be maintained, differential pressure

gauges can be used to sense increased resistance to airflow resulting from the accumulation of

dirt on filters. This can be read directly, or the pressure change used to operate a signal to change

the filter when resistance reaches a preset level. Automatic roll filters which react to the same

information by automatically winding on a new section of filter material, are very effective, but their use is

limited by first costs. Electrostatic or electronic air filters are discussed later.

15.6.3 COOLING COILS

All air-cooling or heating coils are of copper-tube, aluminum-fin construction, with spacing

options varying from 6 to 14 fins per 25 mm. Fins are often corrugated to increase the

turbulence of air passing through the coil, and thus increase its heat transfer capacity.

Moisture carry-over is prevented by limiting air velocity to approximately 3-0 m/s (600

ft/min). Direct-expansion coils These must be in refrigeration-quality copper tubing with

formed return bends (preferably silver-soldered) and guaranteed for pressures of 2 410 to 2 760

kPa (350-400 psi). Evaporators of this type (and also air-cooled condensers) are normally

tested for leaks when first manufactured. This is done by charging them with dry nitrogen at

the required pressure, completely immersing them in back-lit, colored water and detergent

tanks and looking for escaping gas. This does not prevent the possibility of leaks developing

later, following mechanical or other damage. Measurements for copper tubing are at present

given in British units and | in to I in outside diameter (OD) are the most used tube sizes,

depending upon the capacity of the coil. Refrigerant flow is evenly balanced over the tubes by

a distributor located immediately after the TEV. Coils are often divided into two or more

separate circuits, each with its own TEV and distributor. Coil divisions can be made in the

vertical plane (face control) or in depth (depth control). Depth control might, for example, use

two rows of coil for each of two circuits. This approach requires fine balancing, as the first two

or three rows of a coil will always remove more heat than the next two or three, if both sections

are at identical refrigerant temperatures. TEVs must therefore be set to match the design cooling

capacity of each section. This layout has the advantage that the entire face of the coil is always at a

suitable refrigerant temperature. It provides better humidity control than a face-controlled coil,

when capacity controls interrupt the supply of refrigerant to one complete section. Chilled-water

coils The main difference between direct-expansion and chilled-water coils is that water designs

use only one circuit, Capacity control is provided by modulating the volume of chilled water

passing through the coil, or by using face and bypass dampers to vary the quantity of air passing

over it. The use of one continuous tube would result in excessive water pressure drops and, to

avoid this, headered construction is used to supply a number of circuits in parallel. Air vent plugs

are usually provided at the top of coils, and drainage plugs at low level. Water inlets are normally at

the top of coils. Condensate disposal Cooling coils - and control valves - must be provided with full-

size condensate trays, to collect moisture removed from conditioned air. These trays are

normally of pressed-steel construction, insulated with sprayed-on polystyrene or polyurethane

to prevent condensation forming on external surfaces. They also include provision for the

connection of drainage lines.

Fig 15.8 Refrigerant distributor layout

15.6.4 Heating coils

Electric heating coil construction, and safety provisions, has already been described. We

must also be familiar with hot-water and steam heating coils; which are sometimes needed

in even the hottest of climates, to treat air used in industrial processes. General

construction methods are similar to those used for chilled water, but coil depth is only one or

two rows and headered construction not therefore necessary. These coils are

designated for use with a specific heat medium.

Low pressure hot water (LPHW) The maximum temperature of water leaving a low

pressure boiler is 121 °C (250°F). Typical water conditions through coils are: entry at 82 °C

(180 °F) and exit at 71 °C (160 °F). Air quantities over LPHW coils normally produce

velocities between 2-0 and 4-0 m/s (400 and 800 ft/min).

Medium pressure hot water (MPHW) This type, which is seldom used, has a

maximum temperature of water leaving the boiler only slightly higher than

that of LPHW types. In practice, water temperatures at coils are generally of the order of

127 °C (260 °F) at entry and 99 °C (210°F) at discharge.

Steam

There is a wide variety of operating ranges of steam boilers and gauge pressures at coil

entries can be between 34 kPa (5 psig) and 1 380 kPa (200 psig). These pressures are

equivalent to temperatures of 108 °C (227 °F) and 198 °C (388 °F). Face velocities of air

usually fall in the 2-54 to 6-60 m/s (500 to 1 300 ft/min) range. Coil tubes are normally

larger (25 mm (1") OD) than those used for water, and installations must include steam

traps and condensate return lines. Capacity control is by hot-water or steam flow-control

valves. Water valves are likely to be of three-way, bypass design, and steam valves

two- connection throttling types in supply lines.

15.6.5 Fans

It is unusual to find airhandling units of factory-assembled or 'modular' types, which do not use

centrifugal fans. These are normally double-width, double-inlet types with forward-inclined

blades; and consist of scroll(s), impeller(s) and cone(s), and bearing support pedestals. Sleeve-type

bearings are normally used when motor sizes do not exceed 2-25kW (3 hp), plummer blocks being

fitted for larger capacities. Fan speeds can normally be varied by altering the pitch of a motor

pulley designed for use with either one or two vee belts (although the use of direct-drive, variable-

speed electric motors is likely to increase as electronic controllers become more competitive). The

main components of a centrifugal fan, and the varying names which they may be called, are

illustrated below. Other types of centrifugal fans available have backward-inclined, radial or

airfoil blades. The main advantages of centrifugal fans as a class are compact size, reasonable

efficiency and low noise levels in applications against head pressures up to approximately 250 mm

(10 in) water gauge (wg).

Fig 15.9 Centrifugal fan components

AC MEC 225 Theoryx

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