Heat Exchangers

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BP CASPIAN SEA HEAT EXCHANGERS

HEAT EXCHANGERS (MD-029)

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REV 1: Heat Exchangers (MD-029) Page 2 of 93

REVISI0N HISTORY Revision Number Date Comments

Rev: 0 19-08-2004

Rev. 1 03-11-2004

Comments Received from ODA and Trainers

Signed off by Douglas Milne 06-11-04

ISSUE 1


TOPICS COVERED INCLUDE

Serpentine

Fin Fan Coolers

Co- Current and Counter Current Flow

Shell and tube heat exchangers ( Fixed head, floating head, hairpin

tube)

Hairpin as a condenser

Plate exchangers

Finned tube

Maintenance of heat exchangers

Heat Exchange media

Specific Heat capacity

Thermal Stability

Lagging properties

Hazards associated with heat exchangers

Parameters for measurement during operation

Heater Treaters and fired heaters

Refrigeration and Chilling Units

Reboilers

Video Cassette: Heat Exchangers (Tech 027)


CONTENTS


1. INTRODUCTION

2. MAJOR PARTS OF A HEAT EXCHANGER

2.1. MAJOR PARTS OF A TUBE & SHELL HEAT EXCHANGERS

2.1.1. SHELL

2.1.2. TUBES

2.1.3. TUBE SHEETS

2.1.4. CHANNEL

2.1.5. BAFFLES

2.2. FLOATING HEAD HEAT EXCHANGER

3. DESCRIBE THE FUNCTION & OPERATION OF HEAT EXCHANGERS

3.1. FUNCTION OF SHELL & TUBE HEAT EXCHANGERS

3.1.1. REBOILERS

3.1.2. CONDENSERS

3.1.3. HEATERS

3.1.4. COOLERS

3.2. PLATE TYPE HEAT EXCHANGERS

3.3. PARALLEL PLATE HEAT EXCHANGER

3.3.1. HEAT TRANSFER FACTORS IN PLATE HEAT EXCHANGERS

3.3.2. ADVANTAGES OF PLATE HEAT EXCHANGERS

3.3.3. DISADVANTAGES OF PLATE HEAT EXCHANGERS

3.4. USES OF PLATE HEAT EXCHANGERS


3.5. OPERATION OF HEAT EXCHANGERS

4. GLOSSARY

5. AIR COOLERS

5.1. INTRODUCTION

6. FIN-FAN COOLER

6.1. MAJOR PARTS OF A FIN-FAN COOLER

6.1.1. HOUSING

6.1.2. FINNED TUBES

6.1.3. FAN ASSEMBLY

6.2. OPERATION OF A FIN-FAN COOLER

7. GLOSSARY

8. FIRED HEATERS

8.1. INTRODUCTION

9. FIRED HEATERS & THEIR MAJOR PARTS , FUNCTION & OPERATION

9.1. MAJOR PARTS OF A HORIZONTAL COIL FIRED HEATER

9.1.1. HOUSING

9.1.2. FIREBOX

9.1.3. TUBES

9.1.4. CONVECTION TUBES

9.1.5. SHOCK TUBES

9.1.6. RADIENT TUBES

9.1.7. BURNERS

9.1.8. FUEL GAS BURNER

9.1.9. FUEL OIL BURNER

9.1.10. STACK

9.2. MAJOR PARTS OF A VERTICAL COIL FIRED HEATER

9.2.1. HOUSING

9.2.2. BURNERS


9.2.3. TUBES

9.2.4. RADIATING CONE

9.2.5. BAFFLE SLEEVE

9.2.6. STACK

9.3. FIRED HEATER OPERATION

9.4. PRODUCT FLOW

9.5. COMBUSTION CONTROL

9.6. DRAFT

9.7. COKING

10. OPERATION OF A STEAM BOILER

10.1. DEAERATING

10.2. CHEMICAL DOSING

10.3. WATER TUBE BOILER OPERATION

11. GLOSSARY

12. REFRIGERATION

12.1. INTRODUCTION

12.2. USES OF REFRIGERATION

12.2.1. BASIC PRINCIPLES USED IN REFRIGERATION

12.2.2. ADIABATIC CHANGE

12.2.3. ISOTHERMAL CHANGE

12.2.4. THE JOULE-THOMSON EFFECT

12.3. SUMMARY

13. VAPOUR COMPRESSION

13.1. INTRODUCTION

13.2. THE VAPOUR COMPRESSION REFRIGERATION CYCLE

13.2.1. COMPRESSION

13.2.2. CONDENSATION

13.2.3. EXPANSION


13.2.4. EVAPORATION

13.2.5. RE-CIRCULATION

13.3. SUMMARY

14. REFRIGERANTS

14.1. INTRODUCTION

14.2. PRIMARY REFRIGERANTS

14.2.1. LOW BOILING POINT

14.2.2. LOW FREEZING POINT

14.2.3. HIGH LATENT HEAT

14.2.4. HIGH CRITICAL TEMPERATURE

14.2.5. LOW CRITICAL PRESSURE

14.2.6. NON – CORROSIVE

14.2.7. NON - TOXIC, NON – FLAMMABLE

14.2.8. NON - REACTIVE WITH AND EASILY SEPARATED FROM OIL

14.3. COMMON PRIMARY REFRIGERANTS

14.3.1. AMMONIA

14.3.2. CHLORO-METHANE (METHYL CHLORIDE)

14.3.3. SULPHUR DIOXIDE

14.3.4. PROPANE

14.3.5. HALOCARBON REFRIGERANTS (OFTEN CALLED CFCS)

14.4. SECONDARY REFRIGERANTS

14.4.1. VERY LOW FREEZING POINT

14.4.2. LOW VISCOSITY

14.4.3. CHEAPNESS

14.5. COMMON SECONDARY REFRIGERANTS

14.5.1. BY THE ADDITION OF A SOLUTE TO WATER

14.5.2. BY THE ADDITION OF A LIQUID TO WATER

14.6. SUMMARY


15. SAFE & EFFICIENT OPERATION

15.1. INTRODUCTION

15.2. REFRIGERATOR PERFORMANCE

15.2.1. REFRIGERATION CAPACITY

15.2.2. REFRIGERATION EFFECT

15.2.3. COEFFICIENT OF PERFORMANCE (C.O.P.)

15.3. HAZARDS ASSOCIATED WITH REFRIGERATION SYSTEMS

15.3.1. THE NATURE OF THE REFRIGERANT MATERIALS

15.3.2. LOW TEMPERATURES

15.3.3. MECHANICAL NOISE

15.4. SUMMARY



1. INTRODUCTION

Many hydrocarbon processes require changers in fluid temperatures. This

means we must transfer heat to or from the fluids. In the last two modules,

you have learned about equipment used to directly heat and cool fluids.

Boilers and fired heaters burn fuel to provide the energy needed to heat

fluids. Fin-fan coolers and cooling towers use air to remove heat from fluids

in a process.

We can also use one fluid to heat or cool another fluid, if there is a large

difference in their temperatures. Steam can be used to heat process fluids.

Air or seawater can be used to cool hot process fluids.

A heat exchanger is the device used to transfer heat between fluids. Heat

exchangers are common in hydrocarbon process plants. In part 1 of this

module, we will discuss the parts of the shell and tube heat exchanger. In

part 2, we will discuss the fraction and operation of heat exchangers used in

process operations.


2. MAJOR PARTS OF A HEAT EXCHANGER

Heat exchangers use a shell and tube arrangement to transfer heat between

fluids. One fluid is passed through the shell, or housing, or the exchanger. A

second fluid is passed through a tube bundle inside the shell. The two fluids

do not mix because they are physically separated. Heat is transferred from

one fluid to the other by conduction through the tube walls.

Figure 1 – Floating Head Heat Exchanger

This figure shows the major parts of a heat exchanger. Refer to this diagram

for the discussion that follows.

2.1. MAJOR PARTS OF A TUBE & SHELL HEAT EXCHANGERS

There are many designs of shell and tube heat exchangers. We will discuss

the most common type called the floating head heat exchanger. Figure 1

shows the parts of a typical floating head heat exchanger.


2.1.1. SHELL

The shell is the metal housing of the exchanger. In the shell, fluid flows

around the tubes.

Heat is conducted from shell to tube, or from tube to shell, depending upon

the temperature of the fluids flowing through them. The flow of heat is always

from the hotter fluid to the colder fluid.

The inlet and outlet pipes for the shell body are called shellside nozzles. Inlet

and outer pipes for the tubs are called tubside nozzles.

2.1.2. TUBES

The series of small tubes in a heat exchanger is called a tube bundle. The

tubes are positioned inside the shell of the exchanger. There are normally

several, closely packed tubes of a fairly small diameter.

Many small tubes provide a larger surface area then a single large tube. For

example fifteen 1” tubes occupy the same space as one 6 “ tube. However,

fifteen 1” tubes provide 2 ½ times the surface area of the single 6” tube. More

heat can be transferred through the larger surface area of the many small

tubes in a tube bundle.

2.1.3. TUBE SHEETS

Tubes are held together and supported at each end by a plate called a tube

sheet. The tubes and the tube sheets from a solid unit that fastened inside

the shell. Figure 2 shows a tube bundle with a tube sheet on each end.


Figure 2 – Tube Sheets

Fluid enters and exits the tube bundle through the tube sheets.

2.1.4. CHANNEL

The channel is an inlet and /or outlet chamber for the tubes. The channel is

covered at one end, and connected to a tube sheet at the other end. The

tubeside inlet nozzles direct fluid into the channel. From the channel, fluid is

forced into the tube bundle, through the tube sheet. As fluid exits the tube

sheet, it flows into the outlet channel and through the tubeside outlet nozzle.

2.1.5. BAFFLES

Baffles are metal plates used to change the direction or fluid flow.

Figure 3 – Baffles


Baffles direct the flow of shellside fluid around the tubes in the tube bundle,

increasing contact between the fluid and the tube surfaces.

Segmental Baffles

Baffles are used to slow down and direct the flow of shellside fluid around the

tubes. The baffles ensure longer contact time between the shellside fluid and

the tubes, allowing more heat exchange.

Figure 4 – Impingement Baffle

The impingement baffle is used at the shellside inlet nozzle to protect the tube

bundle from the force of the fluid entering the shell.

Impingement Baffle

A special baffle, called an impingement baffle, is normally used at the

shellside inlet of the exchanger. An impingement baffle is shown in figure 4.

At high inlet flow, the pressurised fluid entering the shell will erode the tubes.

To reduce erosion, an impingement baffle is used between the inlet nozzle

and the tubes. The impingement baffle absorbs most of the force of the

incoming fluid without restricting flow.


Channel Baffle

Another special baffle is sometimes used in the channel of the exchanger.

This baffle divides the channel into two separate chambers, separating the

tubeside inlet section from the outlet section. Channel baffles are used in

double-pass heat exchangers.

Figure 5 – Heat Exchanger Flow-Through

In a double-pass arrangement, a channel baffle separates the tubeside inlet

fluid from the outlet fluid.

Figure 5 (a) shows a single-pass flow arrangement. Fluid enters one end of

the tube bundle and leaves at the other end. This is the simplest pattern.

Figure 5 (b) shows a double-pass flow arrangement. By placing a baffle in

the exchanger channel, tubeside fluid enters only half of the tubes. The fluid

must go to the end of the tube bundle, then return through the other half of

the tubes. This arrangement exposes the tubeside fluid to the shellside fluid

two times.


2.2. FLOATING HEAD HEAT EXCHANGER

Figure 6 shows a cutaway drawing of the floating head heat exchanger.

In the floating head heat exchanger, the tube sheet on one end of the tube

bundle is not attached to the shell. It is free to move, or float, back and forth

inside the shell. This allows the tube bundle to expand and contact when

subjected to hot fluids. With one end of the tube bundle free to float, the

tubes will not bend from thermal expansion.

Figure 6 – Floating head Heat Exchanger

The floating heat exchanger is a double pass exchanger. One end of the

tube bundle is free to move, to allow for thermal expansion.


3. DESCRIBE THE FUNCTION & OPERATION OF HEAT EXCHANGERS

A heat exchanger is a device used to transfer heat from a hot fluid to a cold

fluid.

Three factors determine the amount of heat transfer in an exchanger:

The initial temperature difference between the two fluids –

The greater the difference in temperature in temperatures, the

more heat exchanged.

The length of contact time between the two fluids – The

greater the time of contact, the greater the change in outlet

temperatures.

The exposed surface area in the exchanger – The greater the

surface are of contact between the fluids, the more heat

exchanged.

3.1. FUNCTION OF SHELL & TUBE HEAT EXCHANGERS

Shell and tube heat exchangers have many uses in hydrocarbon processing.

The name of a heat exchanger is usually a guide to its use.


3.1.1. REBOILERS

A shell and tube boiler is a heat exchanger used to reheat liquids. Figure 7

shows a typical reboiler.

Figure 7 – Reboiler

Superheated steam flows through the tube side of the exchanger, heating the

liquid flowing through the shell.

The purpose of the boiler is to reheat liquid bottoms product from a distillation

column. Super-heated steam is admitted to the tube side of the exchanger.

The liquid bottoms product from the column flows through the shell side.

Liquid that is vaporized in the reboiler goes out at the top of the shell. The

remaining liquid flows over vertical plate, called a weir, and out of the bottom

of the reboiler shell.

The reboiler in figure 7 is called a kettle reboiler because of the large area

inside the shell above the tube bundle. The large area provides space for

vapours to collect.


3.1.2. CONDENSERS

A shell and tube condenser is a heat exchanger in which hot vapours are

cooled and condensed.

Figure 8 – Condenser

Condensers cool gases to change them into liquids. Cool ware flowing

through the tube side of the exchanger cools hot gas in the shell, causing the

gas to condense.

Shell and tube condensers are sometimes used with oil or NGL fractionation

columns, to cool overhead gases. The overhead gas enters the shell of the

condenser tube bundle. The cold water flowing through the tubes absorbs

the heat in the gas. This allows most of the gas to condense and leave the

shell liquid.


3.1.3. HEATERS

A shell and tube heater is a heat exchanger used to increase the temperature

of a cold feedstock.

Figure 9 – Heater

Heaters are used to heat cold feedstock. Hot fluid from a process flows

through the shell side of the exchanger, warming the cooler feedstock flowing

through the tubes.

The heater in figure 9 accomplishes two functions. Cold feedstock is heated

and hot product is cooled. This is an example of using one hydrocarbon

product to change the temperature of another hydrocarbon product. Hot

bottoms product from a fractionation column can be used to heat feedstock

going into the column. In exchange, the cold feedstock cools the bottoms

product.


3.1.4. COOLERS

A shell and tube cooler is a heat exchanger used to cool a product.

Figure 10 – Cooler

Coolers are used only to cool a product. Cold water flows through the tube

side of the exchanger, cooling hot liquids flowing through the shell.

The cooler in figure 10 is similar to the heater in figure 9. In the heater, both

products benefit from a heat exchange. In the cooler, the only purpose is to

cool the product. Water is commonly used as the cooling agent in coolers.

Once the cooling agent has served its purpose, it is often recirculated through

a cooling tower for reuse in the cooler.

3.2. PLATE TYPE HEAT EXCHANGERS

A number of special types of heat exchanger have been developed to give

improved heat transfer under conditions where shell and tube types are

unsatisfactory. One of these is the plate heat exchanger.


3.3. PARALLEL PLATE HEAT EXCHANGER

The plate heat exchanger consists of a pack of separate parallel plates. The

middle sections of these plates are corrugated and each plate has two holes

or flow orifices. Figure 11 illustrates the types of plate used.

Seal Spacer Seal

Corrugations

Figure 11 - Plate Heat Exchanger

Alternate plates have the flow orifices as shown in the figure.

The plates are standard and an exchanger may be made up of two hundred

or more of these standard units. Each plate is fitted with two seals (or

gaskets) and a spacer. The fittings are also standard.

One of the seals is shaped so that it fits around the centre corrugated section

and one of the orifices. The other is circular and surrounds the remaining

orifice.

Figure 12 illustrates how these seals (or gaskets) are positioned on the

plates.

Seals are made of rubber or a similar material and usually fit into grooves on

the surface of each plate.


The plates are arranged alternately as illustrated in Figure 12 and positioned

between end covers. The whole assembly is held together by clamping bolts

so that the seals between each plate are compressed and leak proof.

Figure 12

The plates are arranged so that there are two separate paths through the

exchanger between alternate plates. The size of these paths (i.e. the space

between each of the plates) is established by the thickness of the seals. It is

usually less than 6 mm.

Figure 13 shows how the two fluids flow through these two separate paths

within the exchanger.

Figure 13 - Plate Arrangement


Note that only four plates are shown - an actual plate exchanger may have

200 or more. Also the end covers, through which the fluids are introduced into

the unit, are not shown.

The cold fluid: bypasses plate D and flows down the space between plates

D and C. It then bypasses plate B and flows up the space between plates B

and A. The whole process continues with other plates to the left of plate A,

which are not shown.

The hot fluid: bypasses plate A and flows down the space between plates B

and C. It then bypasses plate C and flows up through the space between

plates D and E (not shown). The whole process continues with other plates to

the right of plate D.

Note how the position of the plates, holes and seals produces the two

separate paths through which the two fluids flow.

Heat transfer from one fluid to the other is by conduction through the

corrugated walls of each plate. These plates can be made very much thinner

than the tubes of a shell and tube heat exchanger. Accordingly, heat transfer

rates are much higher.

3.3.1. HEAT TRANSFER FACTORS IN PLATE HEAT EXCHANGERS

The factors that contribute to the high heat transfer rates in plate heat

exchangers are as follows:

the thin walls separating the two fluids allow for rapid heat

transfer by conduction

the corrugations on the plates give increased surface area

over which heat transfer can take place

narrow pathways, corrugations and constant changes of

direction produce a high degree of turbulence (even at low

flow rates) which improves heat transfer by convection within

each fluid.


The high heat transfer rates enable this type of heat exchanger to be used

effectively in situations where the temperature difference between the two

fluids is small.

3.3.2. ADVANTAGES OF PLATE HEAT EXCHANGERS

These are:

high heat exchange rates are possible

the plates are independent units which can be removed,

added to or rearranged for different duties

any plate can be replaced rapidly without removing the others

both sides of each plate are fully accessible for inspection and

cleaning.

Plate heat exchangers tend to have lower capital costs than shell and tube

types because the heat transfer surface can be made from sheet metal, which

is less expensive than tube manufacture. This is of particular benefit if

expensive materials of construction such as stainless steel are necessary.

3.3.3. DISADVANTAGES OF PLATE HEAT EXCHANGERS

These are:

the seals are a weak point - failure will result in a leak of one

or other of the fluids

the working temperatures and pressures are limited by quality

and durability of the seals (safe operating conditions are

normally below 250°C and 20 bar).


3.4. USES OF PLATE HEAT EXCHANGERS

Since they can be readily taken apart for cleaning and inspection, plate heat

exchangers are used in the food and beverage industries. Their use in the

chemical industry depends on the relative cost for the particular application

compared with a conventional shell and tube heat exchanger.

Note that the plate heat exchanger described is only one of the many designs

available. However, all are based on the principles discussed in this lesson.

3.5. OPERATION OF HEAT EXCHANGERS

We will use a typical plant process to show how heat exchangers maintain

desired temperature levels of process fluids. Figure 14 is a simplified

diagram of a process that removes light natural gases from raw light naphtha.

Figure 14 – Processing Light Naphtha


Seven different heat exchangers are used to regulate the temperatures of

fluids at different points in the process.

The heat exchangers in the diagram are identified by the numbers 1 to 7 and

by their names. Four different types of heat exchangers are used in this

process. Notice that each type of heat exchanger has a different symbol.

Figure 15 shows the four symbols.

Figure 15 – Heat Exchanger Symbols

The circle is each symbol represents the shell of the exchanger. The jagged

line through the symbol representing the tube bundle.

We will start at the reboiler for the deethanizer and see how each heat

exchanger affects the process.

Circles and identify the reboilers. Both reboilers perform the same

function. They reheat the bottom products from the columns. The purpose of

reheating is to vaporize any natural gases trapped in the liquid naphtha.

In the columns, ethane, propane, butane, pentane, and some of the light

naphtha vapours rise to the top of the columns. The hot gases leave the

columns and go into condensers and . The gases are cooled in the

condensers and changed back into liquids. The liquid, along with any gas

that will not condense, is collected in reflux drums. The liquid in the reflux

drums is fed back into the columns to cool the top of the columns. The

uncondensed gas flows out of the flux drum to the fuel gas system.

The bottoms product leaves the columns and goes into heaters and .

The heat from the bottoms product is used to warm the feedstock into the


columns. The bottoms product from the deethanizer becomes the feedstock

for the debutaniser column.

The bottoms product from the debutaniser column, which gave up some of its

heat in a heater , goes into a cooler . Colder water is used in the cooler

to further reduce the temperature of the light naphtha, before it goes for

further processing.


4. GLOSSARY

Bottoms Product: The liquid product, which leaves the bottom of

a column or vessel, after separation of lighter

gases.

Condensation: The changing of a vapour to a liquid by cooling.

Conduction: The transfer of heat energy between objects by

direct contact.

Erode: To wear away.

Overhead Production: The vapour separated from a liquid product,

which leaves the top of a column or vessel.

Thermal Expansion: The expansion or increase in size of solids and

liquids when they are heated.

Baffle: A baffle that changers the direction of the flow

of a liquid, as in a heat exchanger.

Impingement: The collision of a liquid or gas with a baffle at

high velocity.

Shell: The internal compartment of a heat exchanger

formed by the exchanger housing.

Tube Bundle: A set of parallel tubes fixed to one or two tube

sheets.


5. AIR COOLERS

5.1. INTRODUCTION

Temperature Control Is Very Important In Gas And Oil Processing. As you

learned in the last module, fired heaters and boilers provide a source of heat

for raising temperatures of process fluids. In this module you will learn about

two equipment items which are used to lower the temperature of process

fluids.

Fin-fan coolers are covered in part 1 of the module. Fin-fans are used in

almost all process operations to cool process fluids, lubricating oil, and

refrigerants.

Part 2 of the module covers cooling towers. Cooling towers are commonly

used to cool hot water for reuse in heat exchangers.

You will learn how both of these devices use ambient air to cool hot fluids,

using the principles of evaporation and heat exchange. When you are

assigned to a plant you will be responsible for operation of this equipment and

for keeping it in good working order.


6. FIN-FAN COOLER

Fin fan coolers are used in gas and oil processing plants to cool hot liquids.

They are also used to cool and condense pressurized gases.

Figure 16 – Fin-Fan Coolers

Fin-Fan coolers are used to cool hot process fluids. Some processes require

several fin-fan coolers to lower the temperature of fluids to the desired level.

The fin-fan cooler consists of two main assemblies, the finned tubes and the

fan assembly.

The cooling of a hot gas or liquid takes place inside the finned tubes. Figure

17 shows a section of a finned tube.

Figure 17 – Finned Tube


Fins expand the surface area of the tube, improving heat transfer.

Fins are thin metal pieces attached to a tube. They expose more surface to

the air for cooling.

Liquid heats the walls of the finned tubes. Air blows across the tube fins and

carries some of the heat away. This cools the liquid in the tubes. You can

see in figure 18 that finned tubes have more surface area exposed to the air.

The fins allow more heat transfer.

Figure 18 – Air Cooling

Finned tubes provide a large surface area, allowing greater heat transfer.

6.1. MAJOR PARTS OF A FIN-FAN COOLER

In a fin-fan cooler the fan assembly blows air over the finned tube assembly,

cooling the fluid in the tubes. Figure 19 shows how these assemblies are

connected together in a typical fin-fan cooler. The following section describes

the major parts of the cooler. Refer to figure 19 to help identify each part as

you read its description.


Figure 19 – Fin-Fan Cooler

6.1.1. HOUSING

The housing is made of heavy sheet metal. It forms an enclosure over the

finned tubes. The enclosure ensures that all the air rises through the fan. Air

is drawn in from below the cooler and is blown out of the top of the housing.

Most fin-fan coolers are located high above the ground, so that hot air close

to the ground is not drawn into the cooler.


6.1.2. FINNED TUBES

Hot liquid gas enters the fin-fan cooler through a large diameter pipe called an

inlet header. Many finned tubes connect to the header. Each finned tube

receives some of the hot liquid or gas. The finned tubes carry the hot fluid

across the cooler. The cooled fluid is collected in an outlet header on the

opposite side of the cooler. Air blowing across the finned tubes cools the

fluid.

6.1.3. FAN ASSEMBLY

The fan assembly is a type of rotating equipment a motor and gear box turns

the fan shaft to rotate the blades. Figure 20 shows the main parts of the fan

assembly and their location in the cooler.

Figure 20 – Fin-fan Cooler

This is an example of an induced draft fan, since the assembly draws air up

through the cooler. Refer to this figure as you read the description of each

major component.


Fan

The fan circulates air through the cooler. The moving air picks up the heat

from the finned tubes. Fan are designed to produce either forces or induced

draft. A forced draft fan pushes air through a cooler. An induced draft

fan pulls air through a cooler.

Most fin-fan coolers use induced draft. The fin-fan cooler in figure 21 uses an

induced draft fan

The fan blades may have either a fixed or variable pitch angle. Pitch angle

refers to the angle that the blade is turned above or below the horizontal

plane. At an increased pitch angle, the blades will draw more air through the

cooler. At a lower pitch angle, the blades will draw a smaller volume of air.

The pitch angle of the blades can be changed manually or automatically. The

pitch angle is changed to maintain a desired temperature of fluid at the outlet

header.

Figure 21 – Fin-Fan Cooler Fan Blade Pitch

Pitch is the angle above or below the horizontal plane. Increased pitch will

draw more air through the fan, increasing the cooling effect.


Fan Ring

The fan ring is a metal ring that fits around the top of the cooler housing. The

fan blades rotate inside the ring. The fan ring prevents air from leaking past

the fan. The ends of the fan blades are positioned very close to the fan ring.

Fan Mount

The fan mount is attached to the frame of the fin-fan unit, as shown in figure

21 the shaft of the fan goes through the fan mount. The fan mount serves

two purposes:

To support the weight of the fan

To house bearing that allow the shaft to rotate freely and

prevent it from moving from side to side

Seal Disc

The seal disc is a round piece of metal shaped like a plate. It is attached to

the fan, above the centre, or hub, of the fan. It prevents air leaking past the

hub of the fan blades.

Vibration Switch

A vibration switch is used to stop the fan motor automatically if the fan begins

to vibrate through gearing failure etc. The tips of the fan blades are very

close to the fan ring. If the fan begins to vibrate, the tips of the blades will hit

the fan ring. This will damage both the fan ring and the fan blades. The

vibration switch is usually mounted under the mount, as shown in figure 21.

Fan Motor

An AC electric motor is used to drive the fan. A start/stop switch is located

near the motor. Another start/stop switch may be located in the plant control

room.


Gearbox

The gearbox is used to reduce the speed of the fan so that it rotates more

slowly than the motor. The gears also change the direction of drive from

horizontal to vertical. The gearbox is filled with oil, to provide lubrication for

the rotating gear assemblies.

6.2. OPERATION OF A FIN-FAN COOLER

Fin-fan coolers maintain the desired process operating temperature during

the daily ambient temperature changes.

Variable pitch fan blades adjust to process conditions by changing the angle

of the blades. The pitch angle increases or decreases the amount of air

passing over the finned tubes.

Fixed pitch blades must be started and stopped to change the airflow over the

tubes. The more air, which passes over the tubes, the greater the cooling

effect on the process fluid.

Operators normally start and stop fin-fans locally (not by remote control). By

starting the equipment in the field, the operator can check the coolers visually

(by eye) and audibly (by ear) to make sure they are operating correctly.

Because of the fin-fan design, sand and dust can collect between the fins on

the tubes. The sand acts as an insulator, reducing the heat transfer from the

process to the surrounding air. To remove this sand build up, fin-fan coolers

must be washed down regularly. They are washed when they isolated.


7. GLOSSARY

Ambient: Of the surrounding atmosphere. Ambient

temperature is the temperature of the

surroundings.

Evaporation: The change of a liquid into vapor at

temperatures below the liquids boiling point.

Humidity: Water vapor in the air. In the summer, the

humidity in Ras Tanura is very high.

Drift: Tiny droplets of water that are blown away by

the air.

Header: A large diameter pipe to which a number of a

smaller pipes are joined.

Heat Exchanger: A device used to transfer heat from one fluid to

another, normally through a shell and tube

arrangement.

Hub: The centre of the fan, where the blades are

connected to the shaft.

Louver: A fin, or bar, on the side of a cooling tower

which opens and closed to adjust air flow.

Makeup Water: Fresh water added to replace water lost by

evaporation and drift.

Pitch: The angle of a fan blade above or below the

horizontal plane.

Sump: A reservoir serving as a drain or receptacle for

liquids. Also called a water basin.


8. FIRED HEATERS

8.1. INTRODUCTION

Heat is required for crude oil refining and processing. Most process operation

plants need a source of heat to process hydrocarbons or run utilities. In this

module, you will learn about fired heaters and boilers, the two main heat

sources used in oil gas processing. As a plant operator, you will be

responsible for operating this equipment.

In part 1 of this module, we will discuss the fired heaters used in process

operations. Fired heaters are found in most refinery units. They are

commonly called fired heaters, fire pipe stills, or simply heaters. Fired

heaters are used to heat liquid hydrocarbon feedstock as the first step in

fractional distillation.

In part 2 of the module, we will discuss steam boilers. A steam boiler is a

fired heater. It produces steam by raising the temperature of water above its

boiling point. Steam is used to drive steam turbine engines and as a source

of heat in some hydrocarbon process.


9. FIRED HEATERS & THEIR MAJOR PARTS , FUNCTION & OPERATION

Process operations uses two types of fired heaters to heat hydrocarbons.

The two types are vertical coil fired heaters and horizontal coil fired heaters.

The main difference between the fired heaters is the positions of the heating

coils. The major parts of each type of fired heater are basically the same.

Figure 22 – Fired heaters

The fired heater consists of a combustion chamber surrounded by a coil of

tubes. In a vertical coil fired heater the tubes run vertically up and down the

fired heater. Tubes are positioned horizontally in a horizontal coil.


9.1. MAJOR PARTS OF A HORIZONTAL COIL FIRED HEATER

9.1.1. HOUSING

The fired heater housing is a large rectangular steel box. The housing

contains the heating coils and the burners. The housing is sometimes called

the setting.

9.1.2. FIREBOX

The firebox is the area inside the housing where the fuel is burned. The

inside walls and floor of the firebox are lined with a special heat-resistance

material called the refectory lining. This lining is usually made of special heat-

resistance bricks. The purpose of the refractory lining is to reflect heat from

the burners back to the radiant tubes.

Figure 23 – Horizontal Coil Fired Heater Parts


The drawing shows the need of the fired heater cut away. The individual

tubes are connected to form one continuous coil through the fired heater.

9.1.3. TUBES

The tubes in a fired heater are connected together to form a continuous coil.

The tube coil carries the fluid to be heated through the fired heater. There are

three types of tubes in the coil. Each type of tube is heated in a different way.

9.1.4. CONVECTION TUBES

Convection tubes are located in the upper section of the fired heater. These

tubes absorb heat from the flue gas flowing upward from the burners. The

flue gas passes between the convection tubes on its way to the stack.

9.1.5. SHOCK TUBES

Shock tubes are located between the radiant section and the convection

section. They are called shock tubes because they receive the most heat.

They absorb both radiant heat and convection heat.

9.1.6. RADIENT TUBES

Radiant tubes are located along the walls of the firebox in the lower section of

the fired heater. Fluid flowing through the radiant tubes absorbs heat radiated

directly from the burners and heat reflected from the refractory lining.

9.1.7. BURNERS


The burners mix the fuel with oxygen in the air. This ensures smooth and

continuous burning. The burners may burn fuel oil. The fuel and air mixture

is ignited just in front of the burner.

9.1.8. FUEL GAS BURNER

A fuel gas burner is designed to mix the gas evenly with the air. Figure 24 is

a cross section drawing of a fuel gas burner. The figure shows how the air is

mixed with fuel gas to produce the best combustion. As fuel gas flows to the

burner, it sucks air in through the primary port. Additional air enters through

the secondary port, mixing with the fuel gas near the flame.

Figure 24 – Fuel Gas Burner

Air enters the burner through the primary port and secondary port and mixes

with the fuel gas.


9.1.9. FUEL OIL BURNER

One type of fuel oil burner uses steam to heat and atomise the fuel oil (break

it into a fine mist) to make it burn more completely. Figure 25 shows this

operation. Air is mixed with the fuel mist around the tip of the burner.

Figure 25 – Fuel Oil burner

Steam is used to heat and atomise the fuel oil, improving combustion.

9.1.10. STACK

The stack is a vertical duct that rises from the top of the fired heater. It allows

exhaust from the firebox to escape safely, high into the atmosphere.

The stack damper is located in the stack. The damper is very similar to the

butterfly valve you studied in an earlier module. The damper is used to

control the draft (air flow) in the fired heater.


Figure 26 – Stack Damper

The damper in the exhaust stack adjusts the flow of air, or draft, through the

fired heater.

9.2. MAJOR PARTS OF A VERTICAL COIL FIRED HEATER

9.2.1. HOUSING

The housing is a vertical cylinder. It is lines with a refractory material that

reflects heat back onto the radiant tubes. The vertical coil fired heater

occupies less ground space than the horizontal coil fired heater with the same

capacity.


Figure 27 – Vertical Coil Fired heater

A vertical coil fired heater occupies less ground space than a horizontal coil

fired heater with the same capacity.

9.2.2. BURNERS

The fired heater may be fired by fuel gas or fuel oil, using the same types of

burners found in the horizontal coil fired heater.

9.2.3. TUBES

Radiant tubes are arranged around the walls of the fired heater. They receive

radiant heat directly from the burners. Additional heat is reflected from the

burners. Additional heat is reflected from the refractory lining. The tubes

extend up into the convection section, where flue gases are concentrated on

their way to the stack.


9.2.4. RADIATING CONE

A radiating cone is located near the top of the fired heater. The radiating

cone prevents the heat of the burners from passing straight up into the fired

heater stack. The radiating cone reflects the heat outward toward the walls of

the fired heater, where the tubes are located. The cone functions to

concentrate the heat in the fired heater housing.

9.2.5. BAFFLE SLEEVE

A baffle sleeve is connected to the radiating cone. It creates a narrow

passage through the convection section. The baffle sleeve forces all of the

flue gas to pass close to the tubes in the convection section. In this way, the

tubes are able to absorb most of the heat passes through the convection

section.

9.2.6. STACK

The stack is a vertical duct that carries flue gases high into the atmosphere.

The stack also creates a natural draft in the fired heater.

9.3. FIRED HEATER OPERATION

Before beginning this section, you will see a slide tape program on fired

heaters. The program will show some examples of refinery fired heaters, and

explain how they work.

Figure 28 illustrates a typical horizontal coil fired heater. We will use this

example to explain the principle of operation of fired heaters. Vertical coil

fired heaters operate in a similar way.


Figure 28 – Crude Oil Heater Fired Heater

Crude oil flows back and forth through the tube coil, from the convection

tubes, through the shock tubes and radiant tubes.

9.4. PRODUCT FLOW

The fired heater in figure 28 heats crude oil before it enters a distillation

column.

The crude oil enters near the top of the fired heater. It first passes through

the convection tubes, which absorb heat from the flue gases rising through

the fired heater. The heated oil then passes through the shock tubes, where

more heat is absorbed by convection and radiation. Finally, the hot oil passes

through the radiant tubes. The hot oil leaves the fired heater near the bottom

of the firebox.

9.5. COMBUSTION CONTROL

To maintain fired heater temperatures, an instrument loop automatically

controls the fuel gas flow. This loop increases or decreases fuel flow as

necessary to meet the temperature requirements set for the fired heater.


Changes in load (the amount of crude oil to be heated) will also affect the

temperature. The instrument loop will adjust the fuel flow to maintain the set

temperature whenever the load changes.

Enough oxygen must be present to allow all the fuel coming from the burner

to burn. This called complete combustion. Air is the source of oxygen for the

fired heater. To produce complete combustion, a balance between air and

fuel must be maintained at all times. Complete combustion occurs when the

flue gas contains 3% oxygen.

The flow of fuel to the fired heater is automatically controlled. The airflow in

most process operations fired heaters, however, is controlled by dampers.

The dampers are locally adjusted by the operators to ensure the correct

balance of air and gas.

The correct damper adjustment is important to ensure proper combustion.

Too much air causes the heat to be carried through the fired heater too

quickly. This heat is lost up the stack, wasting valuable fuel gas.

Too little air will prevent the fuel from burning fully, causing smoke to form in

the fired heater. This smoke prevents good radiant heat transfer and also

puts a black deposit, called soot, on the convection and shock tubes. Soot

acts as insulation and reduces heat transfer.

9.6. DRAFT

Draft refers to the flow of gases through a fired heater. As the gases in the

fired heater are heated, they rise rapidly through the stack. The rising gases

create a partial vacuum in the fired heater in the fired heater, sucking outside

air into the firebox. In the process of combustion, this air is changed to

heated flue gas. This heated gas will rise rapidly into the stack, causing more

cool air to enter the firebox. The action becomes continuous, and constant

flow through the fired heater and up the stack is established.

Natural daft is produced by stacks alone. In a natural draft system, the rising

flue gas creates a low enough pressure to draw in the fresh air needed for

combustion.


In large systems, however, it is often necessary to use fans above the

convection section of a fired heater to provide enough draft. These fans are

called induced-draft fans.

Figure 29 – Fired heater Draft Systems

Different systems are used in fired heaters to provide the draft needed for

complete combustion. The type of system used depends on the design and

capacity of the fired heater.

Fans can also be mounted around the burners to push air into the combustion

chamber. These fans are called forced draft fans.

A common system on large fired heaters and boilers is the combustion of

forced draft and induced draft fans. This is called a balanced draft system.


9.7. COKING

Coking is the carbon layer found inside the fired heater tubes when the flame

touches the exterior of the tubes. A flame touching the outside of the tube is

called flame impingement. Flame impingement will overheat the tube. The

high temperature breaks down the hydrocarbons flowing through the tube,

creating solid coke. A coke layer inside the fired heater tubes reduces the

conduction of heat to the oil. It can also create an obstruction in the tubes,

which will reduce flow.

Flame impingement can prevented by proper control of the flame in the

firebox.


10. OPERATION OF A STEAM BOILER

We will discuss the operation of a water tube steam boiler. The operating

principles o the two types of boilers are generally the same.

10.1. DEAERATING

Dissolved oxygen and carbon dioxide in boiler feedwater can cause pitting

and scaling in the boiler and boiler tubes. A deaerator is used to drive these

dissolved gases out of the boiler feedwater. The steam removes most of the

oxygen and carbon dioxide from the feedwater.

10.2. CHEMICAL DOSING

Chemicals are added to boiler feedwater and to the boiler system to reduce

damage due to unwanted impurities.

Sulfite is a chemical that is injected into the feedwater to remove traces of

oxygen. Oxygen causes corrosion.

Acidity also causes corrosion. Caustic is a chemical that is injected into

feedwater to reduce acidity. Boiler water has a tendency to become acidic.

Caustic balances the PH of the water by neutralizing the acid. PH is an index

of the acidity or alkalinity of water.

Phosphate is a chemical that is injected into the mud drum to keep impurities

suspended in the water. This helps prevent the formation of scale.

10.3. WATER TUBE BOILER OPERATION

Feedwater that has been deaerated and chemically dosed enters the

economiser.

Hot gases rising from the combustion chamber heat the water in the

economizer by convection.


Warm feedwater enters the steam drum. Here the feedwater picks up more

heat from the water already in the steam drum.

Feedwater flows through the combustion chamber in a tube called

downcomer. The feedwater picks up more heat by radiation.

Hot feedwater flows into the mud drum. The water temperature is very close

to the boiling point. It is almost steam. Impurities in the water cannot turn

into steam, so they remain behind in the mud drum.

Hot water flows out of the mud drum in a tube called a riser. The water in the

riser is immediately exposed to the radiant heat of the burner. The water

turns to steam.

Steam flows upward through the riser into the steam drum.

The steam in the steam drum flows through a tube into the superheater. The

superheater is a small diameter tube coil, which is located above the burner

flame.

In the superheater, the steam temperature is increased to 700F, vaporizing

all water droplets in the steam. Steam pressure increases to 625 psig. At the

temperature and pressure, it is impossible for water to exist in its liquid form.

The super heated dry steam leaves the boiler for use in the plant.

Figure 30 – Operation of a Water Tube Boiler


This figure 30 shows the operation of a typical water tube boiler. Refer to the

graphic as you read the description of boiler operation.

The temperature controls for a boiler are similar to those for fired heaters.

The correct balance between fuel and air must always be maintained. An

incorrect balance of fuel and air in boilers causes the same problems as in

fired heaters. It is the operator’s responsibility to check the conditions in the

combustion chamber to ensure a correct balance between fuel gas and air.


11. GLOSSARY

Condensate: Liquid obtained from vapour. For example,

water obtained from steam

Corrosion: Usually, the eating of a metal by an acid

Intermittent: Periodic. Something that happens from time to

time

Saturated: Full of moisture. Thoroughly wet.

Caustic: An alkaline chemical agent used to neutralize

acidic boiler feedwater.

Chemical Dosing: Injecting chemicals into feedwater flow to

remove impurities

Blowdown: To drain liquid under pressure to reduce the

level of impurities in feedwater

Deaerator: A device in the system generation system that

removes oxygen and other undesired gases

from the feedwater

Draft: A slight pressure difference that produced the

flow of air through the fired heater

Firebox: The open area around fired heater burners

Flue: A channel for conveying hot gas or smoke to

the atmosphere.

Natural Draft: The flow of colder air from outside the fired

heater through the combustion chamber to the

hotter air in the stack. This flow pulls the air

through the combustion chamber and up

through the stack, with out the use of fans.


12. REFRIGERATION

12.1. INTRODUCTION

Refrigeration is the process of lowering the temperature of a substance to

below that of its surroundings. The chemical industry is a major user of

refrigeration facilities. Often, refrigeration is carried out at a central point and

the low temperature produced is then transferred to different areas of the

plant via coolants within a ring main. It is often classified as one of the

services available to the plant.

This first lesson looks at some of the more common uses and also

investigates the basic principles used to produce the low temperatures

required.

12.2. USES OF REFRIGERATION

Refrigeration is the process of lowering the temperature of a substance to

below that of the ambient surroundings. Refrigeration of materials is carried

out for the following reasons, which are in effect, its main uses:

a) To reduce the rate of reaction

b) To store flammable material

c) To condense and store the vapour of low boiling point liquids

d) Air conditioning

e) Freeze drying

These main uses are now considered in more detail.


To Reduce the Rate of Reaction

Some chemical reactions take place so rapidly that they can only be kept

under control if the reactants are cooled to a low temperature before and

during the reaction process. This is because the reaction rate is slowed by

reducing the temperature.

An example is the decomposition of food. In this case, the rate of

decomposition (reaction with air caused by bacteria) is usually slow.

However, the rate can be slowed even further, by reducing the temperature.

For example, consider meat. If it is left in the kitchen at 20°C, it will probably

go bad in a day or two. If kept in a domestic refrigerator, which lowers the

temperature to about 5°C, meat will probably be useable for about a week. If

placed in a deep freeze at -25°C, it will be useable for at least six months.

There are many similar examples to be found in the chemical industry e.g.

storage of drugs. Refrigeration can also be used for materials whose reaction

rate is very much faster. For example, if strong acid is to be added to strong

alkali, the heat produced by the fast reaction may cause boiling and splashing

of the liquids, thus giving rise to a safety hazard. If, however, both are cooled

to a low temperature and kept cool during the reaction, the reaction is slower

and less likely to create a hazard.

To Store Flammable Materials

Some liquids are flammable. A measure of their flammability (ease of

catching fire) is known as the flash point. This is the lowest temperature at

which the material gives off a vapour that will ignite when exposed to a flame.

There is a second higher temperature known as the auto-ignition

temperature. This is the lowest temperature at which the material will

spontaneously ignite without any external source of ignition.

Liquids that have flash points below the normal ambient temperature are

likely to explode if a spark is present. This may mean they need to be cooled

to a temperature below ambient to store them safely below their flash point.


To Condense and Store the Vapour of Low Boiling Point Liquids

Normal butane has a boiling point of -0·5°C and is a gas at normal

atmospheric temperatures. As a gas, it occupies a volume over 1,000 times

more than that of the same mass of liquid butane. Storage space for butane

will be smaller if it is converted to a liquid by lowering its temperature to below

-0·5°C using refrigeration.

An illustration of this (in reverse) is that in cold countries, on cold winter days,

butane cylinders cannot be used if left outside because the low temperatures

prevent the liquid butane from turning into gas. In winter, propane cylinders

should be used as the liquid propane boils at -42°C under atmospheric

conditions and will boil even on very cold winter days!

Another way of condensing the butane is to increase the pressure, which in

turn increases its boiling point. A pressure of 5 bar (500 kPa) gives butane a

boiling point of 42°C. This method is utilised to produce the liquid butane used

to fill the storage cylinders. However, some gases at normal atmospheric

temperatures will not condense, no matter what pressure is applied to them.

They will condense, however, if cooled to a specific temperature. This

temperature is known as the critical temperature for the gas concerned.

Critical temperature is defined as: the temperature above which a gas will not

liquefy, irrespective of the pressure applied. Even at the critical temperature,

large amounts of pressure may be needed to liquefy the gas.

Associated with the critical temperature is critical pressure, which is defined

as: the pressure required to change a gas to a liquid at the critical

temperature.

Some typical critical temperatures and pressures are given in the table below.


Gas Critical Pressure

(mPa)

Critical Temperature

(oC)

Nitrogen 3.4 - 147

Oxygen 5.0 - 118

Methance 4.6 - 83

Ethane 4.9 32

Propane 4.25 96

Table 1

In order to condense the first three gases in the above table, they must be

cooled to very low temperatures as well as being pressurised (5 mPa is

approximately 700 p.s.i.). This process of cooling below critical temperature

and compressing to condense gases is known as liquefaction and the

process is used to produce liquid air. This, in turn, is used to obtain liquid

oxygen and liquid nitrogen. The process is also used to produce Liquified

Natural Gas (LNG).

Air Conditioning

In very hot weather we feel uncomfortable because of the heat and the

amount of water in the atmosphere (humidity). To prevent this in a working

environment where uncomfortable conditions may increase the risk of

accidents and reduce the efficiency of the workers, air conditioning is used.

Basically, this consists of a refrigeration system that cools the air and

removes some of the water vapour by condensation. This reduces the

temperature and lowers the humidity of the air, making conditions more

comfortable.

Freeze Drying

The normal method used to dry materials is to heat them to evaporate the

moisture. This cannot be used if the material is sensitive to heat, as the

process of drying may harm it. To overcome this difficulty, freeze drying is

used.


The material to be dried is pre-frozen to around -25°C and is then placed

inside a chamber. The temperature is slowly increased and the ice sublimes

(turns directly from a solid to a gas) causing drying of the material

12.2.1. BASIC PRINCIPLES USED IN REFRIGERATION

When petrol or other volatile (low boiling point) material is accidentally

splashed over the hands, there is a cooling effect as the material evaporates.

This is because the change of state occurring (from liquid to gas) requires

heat (latent heat) which the liquid takes from its surroundings (in this case the

hands). Water has the same effect, which is less noticeable because the rate

of evaporation is slower.

12.2.2. ADIABATIC CHANGE

If a volatile liquid is placed in a highly insulated vessel so that heat can

neither enter nor leave the system, evaporation of the liquid causes cooling of

the liquid itself. The sensible heat removed from the liquid is now latent heat

in the gas. This change of state is an example of an adiabatic change - known

as adiabatic evaporation.

An adiabatic change is a process that occurs without heat entering or leaving

a system. In general, an adiabatic change involves a fall or rise in

temperature of the system.

12.2.3. ISOTHERMAL CHANGE

A true adiabatic change is very difficult to achieve in practice. When heat is

added to the evaporating liquid to keep the temperature constant, the process

is called isothermal.

An isothermal change is a change that occurs at constant temperature by the

change taking heat in from, or giving heat out to, the surroundings.

Once again, a true isothermal change is difficult to achieve in practice.


Boiling pure water at exactly 100°C at atmospheric pressure by adding heat is

an example of an isothermal process. Adiabatic and isothermal processes are

shown in diagrammatic form in Figure 31.

Figure 31

In theory, low temperatures can be achieved by using the adiabatic

evaporation principle. As the liquid cools however, the rate of evaporation will

slow and less cooling will take place. Eventually a low temperature limit is

reached where no evaporation occurs.

The lower the boiling point of the material, the lower is the temperature

produced by its evaporation. Remember, petrol (which starts to boil at 40°C)

has a greater cooling effect on the hands than water (boiling point 100°C).

This is because, in general, the lower the boiling point, the more evaporation

will occur at a given temperature.

A lowering of the pressure above the liquid will cause higher rates of

evaporation at a given temperature due to the lower resistance present above

the liquid.

The evaporation of a volatile liquid is one of the principles used in

refrigeration. The sensible heat removed from the liquid is converted to latent

heat in the gas.


Adiabatic and Isothermal Compression

If a gas is compressed within a highly insulated vessel so that heat cannot

enter or leave the vessel, the temperature of the gas rises. This is called

adiabatic compression.

If the heat of compression is removed to produce a pressurised gas at the

same temperature as the original un-pressurised gas, this is called isothermal

compression.

Adiabatic and Isothermal Expansion

If the pressure on the gas is now reduced without any addition of heat from

the surroundings, the temperature will fall. This is called adiabatic expansion.

This is because the energy required to expand the gas must come from within

the gas itself since energy can neither be created nor destroyed. Therefore

the temperature of the gas is reduced as energy is provided for expansion.

If heat is supplied from outside the system in order to keep the temperature

constant, this is called isothermal expansion.

12.2.4. THE JOULE-THOMSON EFFECT

Consider what may happen if a gas, which has a low condensing point at

atmospheric pressure, is compressed to a higher pressure but kept at the

same temperature, i.e. is subjected to isothermal compression.

Pressure affects the boiling point of a liquid; the higher the pressure, then the

higher is the boiling point. This is, in effect, the same as saying that an

increase in pressure on a gas will raise the temperature at which the gas will

condense and form a liquid. So, if a gas is subjected to isothermal

compression, it may condense and form a pressurised liquid.

However, this will be the case only if the constant temperature chosen is

below the critical temperature of the gas. The cooling system must remove

the latent heat present in the gas as well as any heat of compression to

condense the gas. Thus large amounts of heat are removed from the system

to keep the temperature constant.


If the pressure on the liquid is now reduced, the liquid will boil. For this to

happen, latent heat has to be supplied. If the process is carried out so that

heat cannot be taken from the surroundings (adiabatic change), then the

latent heat can only be supplied by the liquid itself losing sensible heat.

The liquid left will therefore cool down until a temperature is reached at which

the liquid is below its boiling point for the new pressure. This cooling caused

by the adiabatic expansion of a gas or liquid is called the Joule-Thomson

effect. This process of compressing a gas, cooling to its original temperature

to form a liquid and then allowing the pressure on the liquid to drop causing

the liquid to vaporise and cool forms the basis of the main type of refrigeration

system used by industry – the Vapour Compression System. This is the

subject of the next lesson.

12.3. SUMMARY

Refrigeration is a service supplied to many chemical plants. The principle

uses of refrigeration are:

to reduce the rate of reaction

to store flammable liquids

to dry-heat sensitive materials

to condense and store vapour from low boiling point liquids for

air conditioning

A gas can only be turned into a liquid by the application of pressure if it is at a

temperature below its critical temperature. The process of cooling a gas to

below its critical temperature and compressing to form a liquid is called

liquefaction.

An adiabatic change is one where heat is neither added to, nor removed from

the surroundings by the change. An isothermal change is one where heat is

added or removed by the surroundings to keep the temperature constant.

The Joule-Thomson effect is the cooling caused by the adiabatic expansion of

a gas or liquid. This principle is used in creating the low temperatures used in

refrigeration.


13. VAPOUR COMPRESSION

13.1. INTRODUCTION

Although the size of refrigeration plant may vary for different applications, the

basic principle of operation remains unaltered. The principle used in most

refrigeration systems is the Joule-Thomson effect. This lesson considers how

this principle is used industrially in the vapour compression refrigeration cycle

to achieve low temperatures and cooling.

13.2. THE VAPOUR COMPRESSION REFRIGERATION CYCLE

The purpose of a refrigeration system is to maintain a material at a

temperature below that of the surroundings. If this is achieved, then heat will

flow from the warmer surroundings to the colder material. If the material has

to be kept at the low temperature, then the heat flowing from the surroundings

must be continually removed.

As energy cannot be destroyed, this heat will build up somewhere in the

system. It must be removed - but to where? The readily available place is

back to the surroundings, but this appears to be impossible. How can the

same material take heat from the surroundings i.e. it must be colder and then

give heat back to the same surroundings, unless it is hotter. This, however,

does occur!

The principles used in the vapour compression refrigeration system are:

a) As a gas is compressed, its temperature rises. If the gas is cooled to

remove the heat of compression, then the gas may now be at its

condensation point (an increase in pressure causes the condensing or

boiling point to rise). If the latent heat is now removed, the gas will

condense and form a liquid.


b) If a liquid is expanded adiabatically, it will boil. This causes the liquid to

cool as it loses some of its sensible heat, which is used to provide the

latent heat energy required for the change to gas.

The vapour compression refrigeration system combines these principles to

produce a situation where heat can be both added and removed from the

same temperature surroundings.

The vapour compression system requires:

a compression stage to compress the gas

a cooler (condenser) to remove the heat of compression and

latent heat (condensing the gas)

an expansion device to reduce the pressure causing boiling of

the liquid

a heat exchanger where the cold liquid produced is used to

cool down a second material (the material to be cooled).

The gas formed in the expansion system is then recycled to the compression

stage where the process is repeated. The gas/liquid that is recycled in this

system is known as the primary refrigerant.

The vapour compression refrigeration system is illustrated in Figure 32. Refer

to this diagram as each stage is now studied in more detail.


Figure 32

The vapour compression refrigeration cycle can be broken down into the

following five stages, which are studied in turn. The number of each stage is

included on Figure 32.

1. Compression

2. Condensation

3. Expansion

4. Evaporation

5. Re-circulation


13.2.1. COMPRESSION

A compressor is used to pressurise the primary refrigerant gas to a pressure

such that, on cooling the gas back down to atmospheric temperatures

(compressing a gas raises its temperature), it will condense and form a liquid.

The pressure required will depend on how much the condensing point of the

gas is affected by pressure (an increase in pressure raises the condensing

point of a gas). The compressor used generally operates on one of two main

principles:

a) Reciprocating Motion

This consists of a piston inside a cylinder that is moved backwards

and forwards by a driver, sucking gas in on the backstroke and

forcing it out on the forward stroke.

The driver may be outside the main compressor housing (the open

type, reciprocating compressor) or the motor and compressor are

connected by a common shaft and totally enclosed in the same

housing (the hermetic type, reciprocating compressor). The latter type

eliminates leaks but is more difficult to service.

Both types will require lubrication of the piston to reduce friction and to

help seal the piston within the cylinder. This can lead to oil

contamination of the refrigerant, which is undesirable as it reduces the

efficiency of the refrigerant. If oil contamination is likely to occur, then

an oil separation stage is included. This consists of a device that uses

centrifugal force to throw the heavier oil particles to the outside of a

vessel from where they can be removed. Another method is by the

impingement of oil droplets onto baffles so that the droplets form into

large drops, which sink to the bottom of the vessel from where they

are removed.

If a totally oil-free system is required (e.g. if the oil reacts with the

refrigerant chosen) then a diaphragm pump is often used. This type

may also be used if the refrigerant chosen is extremely corrosive.

Usually, reciprocating compressors are used on very low temperature

duties because of their ability to produce high pressures on the


delivery side and high vacuum on the suction side. However, they are

less likely to be used on very large installations as they cannot handle

large amounts of material unless they themselves are extremely large.

This would make them very expensive due to the high cost of the

precision engineering required.

b) Centrifugal Action

These are multistage centrifugal fans where the high speed created by

the impeller is converted into pressure energy. These have the

advantages of low cost (no precision engineering), they can handle

large quantities of gas and do not require lubrication of parts in contact

with the refrigerant. However, they have a tendency to leak refrigerant

unless the shaft seal is well maintained. Also, they cannot produce the

large suction pressures possible with a reciprocating compressor.

Rotary compressors are rarely used in refrigeration systems. This is

due to the need for lubrication between the moving parts (e.g. the

gears) in contact with the refrigerant and other operating conditions

beyond the scope of this course.

13.2.2. CONDENSATION

The hot compressed gas from the compressor needs to be cooled to remove

the heat of compression. Also, the latent heat needs to be removed at the

condensing point to enable the gas to turn into liquid at the higher pressure.

In small-scale refrigeration systems such as domestic refrigerators, the

condenser consists of a pipe designed to give the maximum heat transfer

surface exposed to the air surrounding it. Using air as the coolant is cheap as

the air costs nothing and does not need to be supplied from an external

source. The condenser on the back of domestic refrigerators can be easily

identified. The major problems with this system are that:


1. Air is a poor conductor of heat, has a low specific heat capacity and as

it is a gas, has a relatively low density. It cannot therefore, remove

very much heat per kilogram (1 kg of air is about 700 litres in volume).

2. The air in contact with the condenser will heat up and cease to cause

cooling. Movement of the hot air away from the pipe relies on natural

convection currents. These cause the hot air to rise pulling cold air in

behind it. The convection currents can be hindered by placing the

refrigerator underneath a work surface without proper venting at the

top (to allow the hot air out) and the bottom and sides (to allow the

cold air in).

In this situation, the efficiency of the condenser is reduced and the gas

passing back to the compressor becomes hotter and hotter.

Then, the compressor has to develop higher pressures than normal, which

can lead to premature failure of the refrigerator. The modern built-in kitchen

has reduced the 'life' of refrigerators from an average of over 20 years in 1969

to 14�5 years in 1988.

On larger industrial systems, where natural convection currents are not very

effective, a fan may be used to force air over the condenser surfaces. The

design of the condenser can now become more compact, i.e. the pipes and

the fins are tightly packed (like a car radiator) as the pressure developed by

the fan can push the air through smaller spaces.

On very large systems, air-cooling is replaced by water-cooling. Water is a

better conductor and has a higher specific heat capacity than air and will

cause larger amounts of cooling. This means that for a specific duty, the

water-cooled exchanger is much smaller and less expensive. Also, the

provision of water to a process becomes progressively cheaper the more that

is supplied.

One water-cooled condenser design consists of water cascading over the

outside of the pipes containing the hot gas (a cheap system of low efficiency).

Another uses a counter-current shell and tube heat exchanger where the hot

gas is passed into the shell and water passes through the tubes (a more

expensive and effective system). See Heat Exchange, Module 3 for revision

of this type of vessel.


The cooled liquid at high pressure leaving the condenser then passes to a

storage vessel (the receiver) ready for the next stage.

13.2.3. EXPANSION

The cold pressurised liquid then passes through an expansion (or 'let-down')

valve. The expansion valve is a pressure reducer that automatically controls

the low pressure at a set value. In addition, it can be thermostatically

controlled so that it only allows liquid to pass through if the temperature in the

refrigerator system is above that required.

The expansion valve allows the liquid to pass from the high-pressure region

to a low pressure region. The reduction in pressure causes the liquid

refrigerant to boil. The latent heat required by the liquid to turn into a gas is

provided by the liquid itself, which therefore cools until a temperature is

reached where it stops boiling (the boiling point of the liquid at the controlled

low pressure). The cooled liquid then passes to an evaporator whilst the gas

formed passes to the suction of the compressor.

13.2.4. EVAPORATION

The evaporator takes the cooled liquid from the expansion valve and adds

heat to cause further boiling of the primary refrigerant. The design of this

varies according to the size and duty of the refrigeration system. In the

domestic refrigerator, the evaporator is the metal icebox at the top, which

absorbs heat from the rest of the contents of the fridge, thereby cooling them.

In a deep freeze, the evaporator forms one or more internal sides of the

freezer.

Note that the icebox and some sides of fridges and freezers are hollow and

contain the primary refrigerant. This is why sharp tools should not be used to

remove ice built-up.

In industry the evaporator may be simply a pipe or coil containing a fluid to be

cooled inside a vessel containing the primary refrigerant or may be a more

complex shell and tube design of heat exchanger.


The heat required by the evaporator is provided either by the surroundings or

by a process fluid known as the secondary refrigerant which, in turn, cools

down.

The heat taken in by the evaporator causes the primary refrigerant liquid to

boil by providing the latent heat required. The temperature remains at the low

value since the liquid is at its boiling point and cannot be heated above this

without the pressure increasing (the pressure is controlled by the expansion

valve!) or until all the liquid has boiled away. Thus the evaporator gives a

controlled low temperature. The net effect is that sensible heat is taken from

the surroundings or secondary refrigerant and is given to the primary

refrigerant as latent heat.

13.2.5. RE-CIRCULATION

The cold gas produced in the evaporator mixes with that from the expansion

valve and passes to the inlet of the compressor where the whole process is

repeated. Thus the primary refrigerant is recycled. Since the main duty of the

refrigeration system is to create cooling in the expansion/evaporator section,

the cold gas is kept as cold as possible as it passes to the compressor.

As it is at a temperature below that of its surroundings, the pipe-work is

usually lagged to prevent loss of the cold by the addition of heat from a part of

the surroundings that does not require cooling. It is also usual to lag all parts

of the expansion/evaporator section that are exposed to higher temperature

material that does not require cooling. For example, a domestic fridge has

lagging in the sides, top and bottom to keep the whole of the fridge from

gaining heat from the kitchen, but the icebox is not lagged allowing the 'cold'

to spread from the box through to all internal parts of the fridge.


Figure 32

Note

The icebox in a domestic fridge is cold enough to freeze water but the main

part of the fridge will not. This is because the icebox is the coldest part due to

the evaporation taking place there, whilst this 'cold' has to spread through the

rest of the fridge. Compare this to a fire, which is the hottest part of a heating

system. The greater the distance from the fire, the lower the air temperature

becomes).

With the domestic refrigerator, the circulating LP vapour (5) is compressed (1)

and then condensed in an air-cooled finned condenser (2). The liquid formed

passes to a receiver from where it is forced through the piping to an

expansion valve (3) where the pressure is reduced. This causes evaporation

of the liquid in the icebox (4), creating low temperatures (-6°C).

This low temperature spreads throughout the rest of the fridge but due to

leaks of heat into the system, the temperature in the rest of the fridge is

above that of the icebox. The temperature within the fridge is controlled by an


adjustable thermostat, which switches the compressor on (if temperature

above set point) and off (if temperature below set point).

In the industrial refrigerator, the hot compressed gas is cooled and

condensed in a water condenser. The liquid then passes via a reservoir to the

expansion valve, which reduces the pressure. The evaporating liquid creates

cooling of a brine solution (the secondary refrigerant) surrounding the

evaporator.

This cooled brine solution is then pumped through a system of pipes within

the vessel to be cooled. This helps to spread the cold from a single point (the

evaporator) throughout the whole of the vessel to be cooled. (This can be

compared to a fire in one room trying to heat up a whole house compared to a

central heating boiler in one room with pipes and radiators in other rooms.) In

this case the low temperature in the vessel is controlled by both the

temperature in the evaporator and the rate of circulation of the brine.

13.3. SUMMARY

The vapour compression refrigeration system consists of five stages:

a primary refrigerant gas is compressed by a compressor

(usually of the reciprocating or centrifugal type). Any oil

contamination of the primary refrigerant must be removed at

this stage

cooling the pressurised gas to ambient temperature and

condensing: it into a liquid in a heat exchanger (either a

finned air cooled type or a water cooled, shell and tube heat

exchanger)

reducing the pressure on the liquid by passing it through an

expansion valve (a pressure reducing valve which may be

thermostatically controlled) causing cooling of the liquid

evaporating the liquid to form a gas in an evaporator by

adding heat from the atmosphere or from a secondary

refrigerant


re-cycling the gas back to the compressor using a system of

lagged pipework


14. REFRIGERANTS

14.1. INTRODUCTION

In the vapour compression refrigeration cycle, the refrigerant is the fluid

medium that transfers heat from one part of the circuit to another. The choice

of refrigerant is based on the required duty. In this lesson, refrigerants are

classified, the 'desirable properties' of refrigerants are described and the

characteristics of several common refrigerants are listed.

14.2. PRIMARY REFRIGERANTS

In the vapour compression cycle a gas is compressed, cooled and condensed

to a liquid. This is then expanded to reduce the pressure (causing the liquid to

boil and cool) producing a gas, which is recycled back to the compressor. The

gas/liquid circulating in the vapour compression refrigeration cycle is known

as the primary refrigerant.

The desirable properties of primary refrigerants are:

a) low boiling point

b) low freezing point

c) high latent heat

d) high critical temperature

e) low critical pressure

f) non - corrosive

g) non - toxic, non - flammable

h) non - reactive with and easily separated from oil.

The primary refrigerant should also be cheap.


14.2.1. LOW BOILING POINT

The primary refrigerant has to be a gas at normal atmospheric temperature

and pressure. If not, the system will have to operate (on the recycle side) at a

pressure below atmospheric to evaporate the liquid and form a gas. This

could lead to air leaking into the system. Any moisture within the air will

freeze at the expansion valve, causing blockage of the evaporator. To prevent

this, in most refrigeration systems, the primary refrigerant usually has a

boiling point within the range -100°C to + 10°C (typically -30°C to - 40°C).

14.2.2. LOW FREEZING POINT

The freezing point of the material must be well below that of its boiling point

so that any cooling that occurs as the liquid is evaporated does not cause the

remaining liquid to freeze.

A frozen refrigerant will block the piping system, reduce the efficiency of the

compression system and will eventually damage the compressor.

14.2.3. HIGH LATENT HEAT

Latent heat is the heat required to change the state of a material. The liquid

refrigerant should require a large amount of heat to cause it to change its

state from liquid to gas i.e. a high specific latent heat of vaporisation.

If it possesses a high latent heat, it will remove a lot of sensible heat from

itself or the surroundings, giving large amounts of cooling per kilogram of

liquid evaporated. This will mean that the amount of refrigerant circulating in

the compression cycle can be kept small, thus keeping costs down.


14.2.4. HIGH CRITICAL TEMPERATURE

Critical temperature is the temperature above which a gas will not form a

liquid on compression. In the vapour compression system, a gas is

compressed to form a liquid. Its critical temperature must therefore be above

normal operating temperatures within the system.

14.2.5. LOW CRITICAL PRESSURE

Critical pressure is the pressure required to turn a gas into a liquid at its

critical temperature. This should be low so that the pressures required to

convert the gas to a liquid in the vapour compression system are low. The

compressor used will then only require a small amount of energy to produce

the low compression pressure, saving on running costs. Also, because of

lower pressures, the strength of the construction needed to contain the

refrigerant can be lower.

14.2.6. NON – CORROSIVE

If the chosen refrigerant is corrosive, the materials of construction would need

to be capable of withstanding the corrosion. Corrosion resistant materials

(e.g. stainless steel) are usually of higher cost than 'normal' materials such as

mild steel. Also, if corrosion did occur, refrigerant that could be hazardous to

personnel and other equipment might leak.

14.2.7. NON - TOXIC, NON – FLAMMABLE

The vapour compression system operates under pressure throughout most of

the system, therefore any leaks will lead to refrigerant gas escaping into the

surroundings. If the gas is toxic or flammable, then a loss of containment

could be very hazardous.


14.2.8. NON - REACTIVE WITH AND EASILY SEPARATED FROM OIL

The compressor used in the vapour compression system may need

lubricating and the oil used may come into contact with the refrigerant. If it

does, it should not react as this will affect the properties of the refrigerant and

lead to lower efficiency. The oil should also be easily removed from the

refrigerant for the same reasons.

So far, no single substance has been found which ideally fits all these

properties and it is therefore necessary to compromise in order to select the

best refrigerant for each particular duty required. It may be that after

considering the above points, several refrigerants will be suitable.

The last point to consider is the price of the alternatives. Far too often, cost is

put at the top of a list of priorities!

14.3. COMMON PRIMARY REFRIGERANTS

14.3.1. AMMONIA

Ammonia is a colourless, toxic gas with a characteristically pungent smell.

It is a good refrigerant in many ways, with fairly low boiling and freezing points

and it is cheap. It also gives a large amount of refrigeration for a given

compressor size, which keeps costs down. It is non - flammable, but

explosions have occurred when the gas has come into contact with welding

torches.

Against these advantages, however, must be set the fact that it is highly toxic

and irritant, and absorbs water readily. When it has absorbed water it is

severely corrosive to copper alloys.

It is generally used in large-scale industrial refrigeration plants, such as cold

storage plants and breweries. Leak testing for ammonia gas is by means of

hydrochloric acid, which forms a dense white smoke of ammonium chloride in

the presence of ammonia.


14.3.2. CHLORO-METHANE (METHYL CHLORIDE)

Chloromethane is a colourless gas with a faintly sweetish odour. It has a

relatively low operating pressure, high latent heat capacity and boils at -24°C.

If chloromethane absorbs water, it becomes very corrosive to aluminium,

magnesium and zinc alloys. Better refrigerants are now available and chloro-

methane is little used. Leak testing is done with a special alcohol burning

torch whose flame exhibits a colour change when chloro-methane is present.

14.3.3. SULPHUR DIOXIDE

Sulphur dioxide is a colorless gas with a pungent smell and is highly toxic. It

is non-explosive and non-flammable, but when mixed with water, forms

sulphurous acid which is highly corrosive to metals. It has good latent heat

capacity and is capable of achieving low refrigeration temperatures.

The toxicity and corrosive properties of sulphur dioxide have led to its

replacement by more modern refrigerants. Leak tracing can be done using an

ammonia swab, which will give white fumes in the presence of sulphur

dioxide.

14.3.4. PROPANE

Propane is a highly flammable gas whose boiling point is - 42°C. It is a good

refrigerant in many ways, its main disadvantage being its high flammability.

So, it should not be used if there is the possibility of escaped refrigerant

coming into contact with a naked flame.

14.3.5. HALOCARBON REFRIGERANTS (OFTEN CALLED CFCS)

The defects of the refrigerants generally available led to much research in the

late 1930s aimed at producing a more satisfactory material. Many substances

were investigated and in the 1940s a range of refrigerants were developed


based mainly on two hydrocarbon chemicals, ethane and methane. These are

the principal refrigerants in use today, usually under trade names such as

'Arcton' and 'Freon'.

Typical refrigerants in this group are dichlorodifluoromethane and

trichlorotrifluoroethane.

The name 'halocarbon' is derived from the fact that these refrigerants are

chemical combinations of halogens (the group of elements containing chlorine

and fluorine) with hydrocarbons.

All these refrigerants are non-flammable, have low toxicity and are non -

irritant at normal temperatures. They have good latent heat capacities and a

wide range of physical and thermodynamic properties which enable them to

be 'tailored' to meet the requirements of most refrigerator systems.

Leak detection is usually carried out using an electronic leak detector.

Although the halocarbon refrigerants are not health hazards at normal

temperatures, at high temperatures, such as those encountered in welding or

in a fire on the plant, they can break down chemically, forming very irritating

and toxic compounds.

Modern research has shown that halocarbons (or CFCs) affect the ozone

layer, which protects the earth from the harmful ultraviolet radiation emitted by

the sun and contribute to global warming. ‘Environmentally friendly’

substances are gradually replacing halocarbons, which are used in air

conditioning as well as refrigeration systems. Typically, these hydrocarbon-

based substances have an atmospheric lifetime of less than 1 year

(compared to the atmospheric life-time of 130 years for CFCs).

In industrial applications, refrigerants are not known by their actual chemical

name but are given a numerical designation. Some of these numerical

designations together with the corresponding chemical name are given in the

table in the Appendix (for information purposes only).

In the vapour compression system the evaporation of the primary refrigerant

creates cooling. In the domestic refrigerator this occurs in the icebox which

then cools down the rest of the fridge. However, the rest of the fridge is never

as cold as the icebox.

Heat is transferred by conduction, convection and radiation. In a cool fridge,

radiation will not occur. The fridge has mainly air inside, which is a poor


conductor of heat, so conduction will be low. This will mean that the air will

cool down very slowly. For convection there must be a temperature difference

to cause hot air to rise and colder air to fall. Thus the 'cold' will not easily

transfer from the icebox to the rest of the fridge.

In industrial systems, the 'fridge' is much larger so the cold needs to be

forcibly spread throughout the space if it is to be made cold. To do this, a

second refrigerant is used to take the cold from the evaporator and spread it

through the fridge. This refrigerant is known as the secondary refrigerant.

14.4. SECONDARY REFRIGERANTS

These are materials that are circulated through the evaporator, cooled and

then passed to other parts of the plant to spread the cold. In the process of

cooling the other parts, the secondary refrigerants are heated. The heated

material is then recycled back to the evaporator to be re-cooled. This process

was illustrated in the previous lesson which showed an industrial cold storage

system.

Note: primary and secondary refrigerants never come into direct contact with

each other; they are always separated by a heat exchange surface.

Secondary refrigerants, like primary refrigerants, should possess certain

properties:

a) very low freezing point

b) low viscosity, i.e. easily pumped

c) cheapness

d) non-toxic

e) non-flammable

f) non-corrosive.


14.4.1. VERY LOW FREEZING POINT

The secondary refrigerant passes through the evaporator, which is the

coldest part of the vapour compression system. It must not freeze as it would

block the piping system and not spread the cold. Boiling point is not so

important except that it should be above normal working temperatures to

maintain the refrigerant as a liquid. In other words, it should not easily boil or

evaporate at working temperatures.

14.4.2. LOW VISCOSITY

The refrigerant should have a low viscosity so that it is easily pumped through

the piping system. As temperature decreases, viscosity increases. This

means that the liquid will become more viscous during its passage through

the evaporator. The change of viscosity with temperature must be taken into

account when choosing the refrigerant.

14.4.3. CHEAPNESS

Large amounts of the secondary refrigerant are required to spread the cold

around the plant system. If the material is expensive, it would involve a high

cost. However, as before, the cost should only be a factor when the

properties mentioned earlier are met.

Water meets most of the above properties (especially cost) with one

exception i.e. it is unsuitable because it freezes at low temperatures. Most

secondary refrigerants however, are based on water with additives to reduce

the possibility of freezing.

A secondary refrigerant must also be non – toxic, non – flammable and non –

corrosive for the same reasons as primary refrigerants.


14.5. COMMON SECONDARY REFRIGERANTS

Secondary refrigerants can be formed:

14.5.1. BY THE ADDITION OF A SOLUTE TO WATER

Common salt (sodium chloride) when dissolved in water will lower its freezing

point. This is why roads are salted when the temperature is likely to fall below

freezing point (0°C). The stronger the solution made by adding salt, the lower

the temperature at which the water will freeze. A typical solution strength

used is 25% which freezes at around -20°C. This solution is often called

brine.

The major problem with salt solution is that it is very corrosive especially to

steel (e.g. it causes cars to rust). Other problems are:

the stronger the solution, the more difficult it is to pump

the maximum strength possible (saturated solution) limits the

lowest temperature to about -25°C.

Calcium chloride solution, which acts in a similar way to sodium chloride, can

be used instead. Although less corrosive, it is more expensive.

14.5.2. BY THE ADDITION OF A LIQUID TO WATER

Methanol and ethylene glycol have low freezing points (methanol -98°C and

ethylene glycol -16°C). When added to water, the mixture produced has a

lower freezing point than water alone. These are commonly used in car

radiators in winter to stop the cooling system from freezing and are known as

antifreeze. The strength of the mixture will once again determine the freezing

point.

Methanol is relatively cheap, but at normal temperatures its rate of

evaporation is high and the strength of the mixture will therefore change with

time giving less protection from low temperatures. Glycol is more expensive


and more corrosive but will not evaporate. Even though ethylene glycol has a

higher freezing point than methanol, when mixed with water it has a greater

effect on depressing the freezing point. A 50% mixture of ethylene

glycol/water freezes at -49°C whilst a 50% methanol/water mixture freezes at

a higher temperature.

In both these cases, a corrosion inhibitor is usually added to the mixtures to

reduce the amount of corrosion. Also, it is important that any loss of liquid is

replaced using the correct strength solution and not simply topped up with

water. Regular checks of refrigerant strength should also be carried out to

ensure that it gives the necessary protection.

Other secondary refrigerants used in industry, together with some of their

important properties, are:

air - cheap, does not freeze but has a low capacity for

removing heat and is a poor conductor

carbon dioxide - can be converted into a solid (dry ice) and

used to give portable 'cold' transfer

oils - these have low freezing points and are non-corrosive

but tend to be very viscous

process fluid - sometimes it is the process fluid that is cooled

by the refrigeration system so its properties cannot be listed

14.6. SUMMARY

Primary refrigerants are the materials circulating in the vapour compression

refrigeration cycle, which produce the cooling effect as they change from

liquid to gas.

They must possess certain properties, the most important of which is that

they must easily change from a liquid to a gas.

The most commonly used primary refrigerants in modern industrial systems

are the halocarbons (Freons and Arctons).

Secondary refrigerants are materials used to spread the cold to different parts

of the refrigeration system.


These must possess certain properties, the most important of which is that

they must not freeze.

Typical secondary refrigerants are water mixed with substances that depress

its freezing point, eg. salt, antifreeze.


15. SAFE & EFFICIENT OPERATION

15.1. INTRODUCTION

In the previous chapter, it was seen that there are many refrigerants, some of

which have hazardous properties. It is important therefore, that a refrigeration

system is run in a safe manner. Also, as with all processes, refrigeration

systems must be run efficiently to keep costs at a reasonable level.

In this chapter, some of the factors that affect the safe and efficient operation

of refrigeration systems are considered.

15.2. REFRIGERATOR PERFORMANCE

The system shown in Figure 33 is of a vapour compression refrigerator. When

operating at a steady state, the total energy within this system will be constant

(energy can neither be created nor destroyed, only changed from one form to

another).

Figure 33


Electrical energy is supplied to the electric motor that drives the compressor.

Heat energy enters the system from the secondary refrigerant or process

fluid, which cools down (the main reason for refrigeration). It may also enter

from the surroundings. Heat energy is removed by the cooling water that is

supplied to the condenser.

Any measure of a refrigerator’s performance must include how much cooling

it achieves. It should also include how much energy is put into the system to

produce the cooling.

There are three commonly used methods of measuring the performance of a

refrigerator in industry. These are:

a) Refrigeration Capacity

b) Refrigeration Effect

c) Coefficient of Performance

15.2.1. REFRIGERATION CAPACITY

This is simply the amount of heat removed from the secondary refrigerant (the

amount of cooling) per second. It is expressed in kilowatts (kW).

Refrigeration Capacity is not a measure of efficiency but simply a measure of

the refrigerator's capacity to create cooling. It will depend to a great extent on

the size of the refrigerator system, i.e. the bigger the system, the more

cooling it is likely to produce.

15.2.2. REFRIGERATION EFFECT

This is the quantity of heat that each kilogram of primary refrigerant removes

as it passes through the evaporator. It is expressed as kilojoules per kilogram

(kJ kg -1).

Again, this is not a measure of efficiency but a measure of how good the

primary refrigerant is at removing heat. It will depend to a large extent on the

specific latent heat of evaporation of the refrigerant.


15.2.3. COEFFICIENT OF PERFORMANCE (C.O.P.)

This compares the Refrigeration Effect (the cooling produced) with the

electrical energy (the driving force for refrigeration) put into the system per

kilogram of refrigerant compressed, i.e.

Refrigeration Effect

C.O.P. = Energy Input (per kg)

C.O.P. has no units because they cancel out. The higher its value, the better

is the refrigeration system. It can have a value greater than 1, i.e. a greater

amount of heat energy can be removed from the secondary refrigerant than

the amount of energy supplied to the compressor. It compares the cooling

produced with the power input (both of these are in effect energy inputs to the

system) and so is not a measure of efficiency. Heat energy is removed from

the system by the cooling/condenser, which must be included in efficiency

measurements.

C.O.P. does, however, enable direct comparisons of performance of both

refrigerants (since refrigeration effect measures this) and refrigeration

systems (since power input is included) to be made. It is therefore, the most

common way of expressing the performance of a refrigeration system.

The main factors that affect the value of the C.O.P. are:

choice of primary refrigerant

correct expansion valve setting

moisture control

use of insulation

control of heat exchangers

Choice of Primary Refrigerant


In an earlier lesson, some of the many different primary refrigerants and their

properties were studied. It is important, therefore, that the correct refrigerant

is chosen for a particular duty.

For example, if a very low temperature is required (say -25°C), then the

refrigerant should have a slightly lower boiling point (say -35°C) so that the

condensing and evaporating pressures can be kept close together. This will

avoid large compression costs. Also, if the refrigerant has a high specific

latent heat of evaporation, it will produce more cooling per kilogram than one

with a low latent heat value.

Correct Expansion Valve Setting

The expansion valve setting controls the pressure and flow of primary

refrigerant to the evaporator. If the pressure is too high, then the liquid may

not evaporate at the required rate, leading to insufficient cooling. If the

pressure is too low, excessive evaporation may occur leading to very low

temperatures. These may cause the secondary refrigerant to freeze and block

the system. Also, the low temperature and pressure gaseous primary

refrigerant produced will require a large input of energy at the compressor to

pressurize it back to a liquid.

If the flow rate is wrong, then too much liquid may enter the evaporator and

cause flooding so that evaporation does not occur. Alternatively, too little

liquid may enter, starving the evaporator and giving little cooling.


Moisture Control

If moisture is present within the primary refrigerant it may freeze at the

temperatures produced by the refrigerator. The ice formed may block pipes

and lead to non-circulation of refrigerant.

This will obviously reduce the efficiency of the system as the circulation of

refrigerant creates the cooling.

Also, if the air on the outside of plant equipment is humid, this may freeze on

the equipment. The presence of ice means that the cooling effect produced

by the refrigerator is being wasted on ice production. The ice may also act as

an insulation layer on surfaces where insulation is not required. This can be

illustrated with reference to a domestic fridge. Insulation is provided in the

sides, door, top and bottom of the fridge to prevent the ingress of heat from

the surroundings. Inside the fridge, the icebox (evaporator) cools the rest of

the fridge.

Notice how ice builds up on the icebox more quickly if the door is opened

often rather than when it is kept closed.

This is due to the air (and any moisture it contains) within the fridge being

replaced every time it is opened. The moisture in the air freezes on the

icebox. This build-up of ice prevents the cold from spreading out by acting as

an insulation layer. The ice should be removed at regular intervals to maintain

the efficient operation of the fridge.

Most modern fridges do this automatically to maintain the good performance

expected from modern devices. If not automatically defrosted, the ice should

be removed by manual defrosting but do not use a sharp instrument!

The reason is that the icebox is the evaporator. It is hollow and contains the

primary refrigerant. A sharp instrument may penetrate the evaporator and

cause leakage of the primary refrigerant.

From the earlier discussion, it is obvious that the presence of moisture within

the system should be kept to a minimum. This can be done by:

operating at a positive pressure throughout the system so that

any leakage is from the system to the surroundings

allowing air to contact cooled surfaces as little as possible.


Use of Insulation

For effective cooling it is important that any heat removed by the system

should come from the secondary refrigerant. This means that all piping

systems and equipment where cold surfaces may be exposed to warmer

surroundings should be insulated to prevent heat gain from the surroundings.

Control of Heat Exchangers

There are two heat exchangers in the vapour compression system, both of

which will affect the value of the coefficient of performance.

Heat is removed from the compressed primary refrigerant in the condenser to

cool and condense it. This is the only place where heat is removed from the

vapour compression system. It is important that the heat is removed in an

effective way, ensuring that all gaseous refrigerant is condensed and cooled

to the required temperature. If not, then energy is wasted in compressing gas

that is not condensed.

Also the evaporator has less liquid to evaporate thus reducing the

refrigeration effect.

The cooling water system should, therefore, have a flow indicator/controller

(to ensure the correct amount of water is flowing). Temperature indicators on

inlet and outlet water flows will enable the operator to see if the cooling water

is at the correct temperature and if it is performing its duty. A similar control

system would be required if the condenser were air-cooled.

The second heat exchanger is the evaporator. Its purpose is to remove heat

from the secondary refrigerant to evaporate the primary refrigerant. Once

again the operation of this will affect the value of the C.O.P.

For example, if too little secondary refrigerant is passed into the evaporator,

then the temperature of the secondary refrigerant will become closer to that of

the primary refrigerant. The closer the temperature difference becomes, the

lower is the amount of heat transferred. It is important that a control system is

used on the evaporator heat exchanger to ensure efficient operation and a

high value of the coefficient of performance.


15.3. HAZARDS ASSOCIATED WITH REFRIGERATION SYSTEMS

The three main hazards associated with vapour compression refrigeration

systems are:

the nature of the refrigerant materials

low temperatures

mechanical noise

15.3.1. THE NATURE OF THE REFRIGERANT MATERIALS

Refrigerants are classed into three basic Groups 1, 2, 3, depending on their

health and safety characteristics. These groups are:

Refrigerants in Group 1 are non-flammable and may be used

in systems where the total charge in the refrigerator would

cause no undue hazard if accidentally released into a space

occupied by humans.

Refrigerants in Group 2 have toxicity as a major

characteristic. If a typical refrigerator charge accidentally

leaked into a space occupied by humans, the concentration

would be toxic. A few of the refrigerants in this group are

flammable and when these are used, additional restrictions

are imposed by the authorities.

Refrigerants in Group 3 are generally non-toxic, but are more

flammable and explosive than the materials in Group 2. They

are particularly suited to some specialist requirements but

special restrictions are imposed when they are used.

There is a large volume of regulations and instructions associated with the

various types of refrigerants and reference should always be made to them

when compiling operator instructions. It is also good policy to consult the

manufacturer of the refrigerant for advice on any hazards caused by the


material. It is obvious that, if the refrigerants are in any way hazardous, the

correct choice of construction materials must be made.

15.3.2. LOW TEMPERATURES

Physical contact with very cold metal surfaces can produce painful injuries,

which are very similar to burns caused by heat. Accordingly, these injuries are

often called 'freeze' or 'cold burns'. Every effort should be made to fit and

maintain lagging on pipes and other equipment that carry the primary and

secondary refrigerants through areas where operators are likely to come into

contact with them.

This will prevent direct contact between the cold surfaces and the operator.

Furthermore, the lagging will also increase the efficiency of the refrigeration

system. If an operator is to enter the cooled space e.g. a cold store, then he

should be provided with and wear protective clothing (especially insulated

gloves).

15.3.3. MECHANICAL NOISE

The vapour compression system uses a compressor to compress the primary

refrigerant. Most compressors (especially reciprocating types) are extremely

noisy (well above the danger level in some cases). To reduce this hazard,

large or numerous compressors should be housed in a soundproof building. If

entry to the building is required, ear protection in the form of plugs and/or

muffs, must be worn.

15.4. SUMMARY

The performance of a refrigerator can be expressed in the following terms:

Refrigeration Capacity

Refrigeration Effect

Coefficient of Performance


Many factors will affect the performance of the refrigeration system including

correct choice of refrigerant

correct expansion valve setting

moisture control

use of insulation

control of heat exchangers

The hazards associated with vapour compression refrigeration systems are:

hazardous refrigerant materials

low temperatures

mechanical noise

Heat Exchangers

Documents

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