BY Mr. ADE S.L.
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BY
Mr. ADE S.L.
5.1 Introduction
The mechanism used for lowering or producing low temp. in a body or a space, whose temp. is already below the temp. of its surrounding, is called the refrigeration system.
Here the heat is being generally pumped from low level to the higher one & is rejected at high temp.
Objectives
• Basic operation of refrigeration and AC systems
• Principle components of refrigeration and AC systems
• Thermodynamic principles of refrigeration cycle
• Safety considerations
Generic Refrigeration Cycle
Refrigeration
The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions.
It also includes the process of reducing heat & maintaining the temp. of a body below the general temp. of its surroundings.
Contd….
In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temp. of its surroundings.
Coefficient of performance(COP)
C.O.P is a measure of efficiency of a refrigeration cycle/
system.
It is the ratio of refrigerating effect to the energy spend.
Refrigerating effect is the amount of heat removal/
absorbed from the substance to be cooled.
The energy spend may e in the form of work in VCR or
heat in VAR.
REFRIGERATORS AND HEAT PUMPS
The objective of a refrigerator is to remove heat
(QL) from the cold medium; the objective of a heat
pump is to supply heat (QH) to a warm medium.
The transfer of heat from a low-temperature
region to a high-temperature one requires
special devices called refrigerators.
Another device that transfers heat from a
low-temperature medium to a high-
temperature one is the heat pump.
Refrigerators and heat pumps are essentially
the same devices; they differ in their
objectives only.
for fixed values of
QL and QH
Refrigerator & Refrigerant
A refrigerator is a reversed heat engine or a heat pump which takes out heat from a cold body & delivers it to a hot body.
The refrigerant is a heat carrying medium which during their cycle in a refrigeration system absorbs heat from a low temp. system & delivers it to a higher temp. system.
Unit of refrigeration
The capacity of refrigeration unit is generally expressed
in “Tons” of refrigeration.
The rate of removal of heat in cooling operation was
expressed in terms of kilograms or tons of ice required
per unit time usually or a day.
One ton of refrigeration
The quantity of heat required to remove from one ton ice
within 24 hours when initial condition of water is zero
degree centigrade, because the same cooling effect will
be given by melting the same ice.
Uses of Systems
• Cooling of food stores and cargo
• Cooling of electronic spaces and equipment
• CIC (computers and consoles)
• Radio (communications gear)
• Radars
• Sonar
• Air conditioning for crew comfort
5.2 Typical Refrigeration
Cycle
Refrigeration Cycle
In refrigeration system the heat is being generally pumped from low level to higher one & rejected at that temp.
This rejection of heat from low level to higher level of temp. can only be performed with the help of external work according to second law of thermodynamics.
Contd….
The total amount of heat being rejected to the outside body consist of two parts:-
- the heat extracted from the body to be cooled .
- the heat equivalent to the mechanical work required for extracting it.
Vapour compression cycle
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor Expansion
Device
1 2
3
4
Vapour compression cycle
Vapour compression cycle
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor Expansion
Device
1 2
3
4
The superheated vapour enters the
compressor where its pressure is
raised
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor Expansion
Device
1 2
3
4
Vapour compression cycle
Vapour compression cycle
20
Low pressure liquid refrigerant in
evaporator absorbs heat and
changes to a gas
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor Expansion
Device
1 2
3
4
The high pressure superheated gas
is cooled in several stages in the
condenser
Condenser
Evaporator
High
Pressure
Side
Low
Pressure
Side
Compressor Expansion
Device
1 2
3
4
Vapour compression cycle
Vapour compression cycle
Components
• Refrigerant
• Evaporator/Chiller
• Compressor
• Condenser
• Receiver
• Thermostatic expansion valve (TXV)
Evaporator/Chiller
• Located in space to be refrigerated
• Cooling coil acts as an indirect heat exchanger
• Absorbs heat from surroundings and vaporizes
• Latent Heat of Vaporization
• Sensible Heat of surroundings
Compressor
• Superheated Vapour: • Enters as low press, low temp vapour
• Exits as high press, high temp vapour
• Temp: creates differential (DT) promotes heat transfer
• Press: Tsat allows for condensation at warmer temps
• Increase in energy provides the driving force to circulate refrigerant through the system
Condenser
• Refrigerant rejects latent heat to cooling medium
• Latent heat of condensation (LHC)
• Indirect heat exchanger: seawater absorbs the heat and discharges it overboard
Receiver
• Temporary storage space & surge volume for the sub-cooled refrigerant
• Serves as a vapor seal to prevent vapor from entering the expansion valve
Expansion Device
• Thermostatic Expansion Valve (TXV)
• Liquid Freon enters the expansion valve at high pressure and leaves as a low pressure wet vapor (vapor forms as refrigerant enters saturation region)
• Controls:
• Pressure reduction
• Amount of refrigerant entering evaporator controls capacity
Vapour absorption refrigeration system
Vapour absorption refrigeration system
In the absorption refrigeration system, refrigeration effect is
produced mainly by the use of energy as heat. In such a system, the refrigerant is usually dissolved in a liquid. A concentrated solution of ammonia is boiled in a vapour generator producing ammonia vapour at high pressure. The high pressure ammonia vapour is fed to a condenser where it is condensed to liquid ammonia by rejecting energy as heat to the surroundings. Then, the liquid ammonia is throttled through a valve to a low pressure. During throttling, ammonia is partially vapourized and its temperature decreases.
Vapour absorption refrigeration system
This low temperature ammonia is fed to an evaporator
where it is vaporized removing energy from the
evaporator. Then this low-pressure ammonia Vapour is
absorbed in the weak solution of ammonia. The
resulting strong ammonia solution is pumped back to
the Vapour generator and the cycle is completed. The
COP of the absorption system can be evaluated by
considering it as a combination of a heat pump and
a heat engine
Vapour absorption refrigeration system
Domestic refrigerator
Domestic refrigerator
ICE plant
ICE plant
The simple VCR cycle is used in primary circuit using ammonia as a refrigerant and brine is a secondary circuit.
This is indirect method of cooling used for ice production.
The ice can contain water which reject heat to the brine which is circulated in secondary circuit.
In evaporator the heat of brine is transfer to the refrigerant in primary circuit and brine is cooled.
ICE plant
The vapour refrigerant form in evaporator is suck by compressor then it compressed to a high pressure and this is condensed in a condenser with the help of cooling water.
The high pressure liquid ammonia is collected in the receiver and then it is passed through expansion valve.
The throttle liquid ammonia at low pressure and temperature enter in an evaporator, so brine in cooled and ammonia absorb heat and form vapour.
5.3 Psychrometry and
Air Processes
1. Atmospheric air
Atmospheric air is not completely dry but a mixture of dry air and water vapor.
In atmospheric air, the content water vapor varies from 0 to 3% by mass.
The processes of air-conditioning and food refrigeration often involve removing water from the air (dehumidifying), and adding water to the air (humidifying).
2. The thermal parameters of moist air
(1) Dry bulb temperature t Dry bulb temperature is the temperature of the air, as
measured by an ordinary thermometer. The temperature of water vapor is the same as that of
the dry air in moist air. Such a thermometer is called a dry-bulb thermometer
in psychrometry, because its bulb is dry.
(2) Wet bulb temperature tWB: Wet bulb temperature is thermodynamic adiabatic
temperature in an adiabatic saturation process, and measured by a wet bulb thermometer.
(3) Dew point temperature tDP: When the unsaturated moist air is cooled at constant
vapor pressure or at constant humidity ratio, to a temperature, the moist air becomes saturated and the condensation of moisture starts, this temperature is called dew point temperature of the moist air.
(4) Relative humidity Ф: Relative humidity is defined as the ratio of the mole
fraction of the water vapor in a given moist air to the mole fraction of water vapor in a saturated moist air at the same temperature and the same atmospheric pressure.
Relative humidity is usually expressed in percentage (%).
From the ideal gas relations, relative humidity can be expressed as
satw
w
satw
w
P
P
x
x
,,
(3) Dew point temperature tDP: When the unsaturated moist air is cooled at constant
vapor pressure or at constant humidity ratio, to a temperature, the moist air becomes saturated and the condensation of moisture starts, this temperature is called dew point temperature of the moist air.
(4) Relative humidity Ф: Relative humidity is defined as the ratio of the mole
fraction of the water vapor in a given moist air to the mole fraction of water vapor in a saturated moist air at the same temperature and the same atmospheric pressure.
Relative humidity is usually expressed in percentage (%).
From the ideal gas relations, relative humidity can be expressed as
satw
w
satw
w
P
P
x
x
,,
(5) Degree of Saturation μ: Degree of saturation is defined as the ratio of the humidity
ratio of moist air w to the humidity ratio of saturated moist air wsat at the same temperature and atmosheric pressure.
(6) Humidity ratio (Moisture Content) w:
The humidity ratio is the mass kg of water vapor
interspersed in each kg of dry air.
It should be noted that the mass of water refers only to the moisture in actual vapor state, and not to any moisture in the liquid state, such as dew, frost, fog or rain.
The humididy ratio, like other several properties to be studied- enthalpy and specific volume-is based on 1kg of dry air.
(7) Specific Volume/Moist Volume v: Specific volume of moist air v , m3/kgdry is defined
as the total volume of the moist air (dry air and water vapor mixture) per kg of dry air.
(8) Specific Enthalpy: Specific enthalpy of moist air h (kJ/kgdry) is
defined as the total enthalpy of the dry air and water vapor mixture per kg of dry air.
Enthalpy values are always based on some datum plane.
Usually the zero value of the dry air is chosen as air at 0℃, and the zero value of the water vapor is the saturated liquid water at 0℃.
Psychrometric chart
Psychrometric chart
A psychrometric chart graphically represents the thermodynamic properties of moist air.
It is very useful in presenting the air conditioning
processes.
The psychrometric chart is bounded by two
perpendicular axes and a curved line:
1) The horizontal ordinate axis represents the dry
bulb temperature line t , in℃ ;
2) The vertical ordinate axis represents the
humidity ratio line w , in kgw/kgdry.air
3) The curved line shows the saturated air, it is
corresponding to the relative humidity Ф=100% .
The psychrometric chart incorporates seven parameters and properties.
They are dry bulb temperature t , relative humidity Ф , wet bulb temperature tWB, dew point temperature tDP , specific volume v, humidity ratio w and enthalpy h.
①Dry-bulb temperature t is shown along the bottom axis of the psychrometric chart.
The vertical lines extending upward from this axis are constant-temperature lines. ②Relative humidity lines Ф are shown on the
chart as curved lines that move upward to the left in 10% increments.
The line representing saturated air ( Ф= 100% ) is the uppermost curved line on the chart.
And the line of Ф = 0% is a horizontal ordinate axis itself.
2. Main air handing processes and their variations in properties
(1) Sensible cooling along a cooling coil, or sensible heating along a heating coil
The sensible cooling happens when the air is cooled without altering the specific humidity.
During this process, the relative humidity of the air will increase.
The sensible cooling can only take place under the condition when the temperature of the cooling coil is not below the dew point temperature of the air being processed.
WB
Dp
humidity
φ r
elative φ
=100%
3
2b
2a
1b1a w h
um
idit
y r
ati
o
t wet-bulb temp
t
dry-bulb tempdew-point temp
WB
Dp
humidity
φ r
elat
ive φ
=100%
3
2b
2a
1b1a w
hum
idit
y ra
tio
t wet-bulb temp
t
dry-bulb tempdew-point temp
The sensible heating is
similar to sensible cooling,
but with the dry bulb
temperature increasing.
It should be noted that there
should be no water within
the heating system because
the evaporation of the water
will increase the specific
humidity of the air.
Cont.….
(2) Adiabatic humidification and dehumidification using a humidifier or chemical dehumidifier
WB
Dp
humidity
φ r
elativ
e φ=100%
3
2b
2a
1b1a w h
um
idit
y r
atio
t wet-bulb temp
t
dry-bulb tempdew-point temp
The adiabatic humidification occurs
when water vapor, of which
temperature is near the wet bulb
temperature of the moist air, is added
to the air .
A humidifier performs this function by
supplying the water vapor.
During the adiabatic humidification
process along the constant wet bulb
temperature line, the specific humidity
of air will increase.
Reduction in dry bulb temperature will
happen as the evaporated water will
absorb heat .
5.4 Air Conditioning
• Purpose: maintain the atmosphere of an enclosed space at a required temp, humidity and purity
• Refrigeration system is at heart of AC system
• Types Used: • Self-contained
• Refrigerant circulating
• Chill water circulating
Air conditioning
Air conditioning is the science which deals with the
supply and maintaining desirable internal atmospheric
condition irrespective of external condition.
Air conditioning may be defined as simultaneous
control of temperature, humidity, motion of air and
purity of air within the enclosed space.
Classification of air conditioning
a) According to purpose.
i) comfort air conditioning system
ii) Industrial air conditioning
b) According season of year.
i) Winter air conditioning
ii) Summer air conditioning
iii) Year round (All weather air conditioning)
Classification of air conditioning
c) According to equipment arrangement
i) central air conditioning
ii) unitary air conditioning
d) According to working substance used.
i) All air system
ii) chilled water system
iii) Air water system
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