Part I Refrigeration Chapter 1

8/13/2019 Part I Refrigeration Chapter 1

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Part I Refrigeration1. Basic concepts of Refrigeration

1.1 Introduction

Refrigeration:Defined as the process of achieving and maintaining a temperature below that of thesurroundings, the aim being to cool some product or space to the required temperature.

Important Applications:

Preservation of perishable food products by storing them at low temperatures

Providing thermal comfort to human beings by means of air conditioning. Chemical and process industries

Special Applications such as cold treatment of metals, medical, construction, iceskating etc.

The availability of refrigerants, the prime movers and the developments in compressors andthe methods of refrigeration all are a part of it.

1.2 Types of refrigeration systems:

i.Natural Refrigeration:

Use of Ice

Evaporative cooling

Cooling By Salt


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Contd..II. Artificial Refrigeration:

Vapour Compression Refrigeration Systems:

Vapour Absorption Refrigeration Systems:

Solar energy based refrigeration system:

Gas Cycle Refrigeration:

Steam Jet Refrigeration System:

1.2 Reversed Carnot Cycle and its Limitation:

1.2.1: Difference between heat engine refrigerator and heat pump

A. HEAT ENGINE:

In a heat engine, the energy is transferred from a higher temperature to a lower temperature

level called sink. During the process, we get the output as work. The higher temperature is

known as source and the lower temperature is known as sink. The figure given below Shows

the energy transfer in a heat engine. The Coefficient Of Performance (COP) value of a heat

engine will be always less than 1.
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ContdB. REFRIGERATOR:

A refrigeratoris a reversed heat enginewhich cools and maintains the temperature of a body

lower than the atmospheric temperature.

This is done by the process of extracting heat from the cold body and then delivers it to a hot

body. In the figure, Q1is the energy taken from the cold body and Q2is the energy given to

T2. Since T2>T1, a work should be done to the system in order to make the process feasible.

T2will be equal to the atmospheric temperature.

COP may be greater than, equal to or less than 1.

The product is cold volume.

T1


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ContdHEAT PUMP:

There is no difference between a heat pump and a refrigeratorin the case of its cycle of

operation. The main difference between the heat pump and refrigerator is its operating

temperatures. The working temperatures of a refrigerator are cold temperature T1 andatmospheric temperature Ta. Where as in the case of a heat pump, the working

temperatures are atmospheric temperature and hot body temperature T2.

Here,

T1= Ta

COP always greater than 1. Hot volume is the product

T2>Ta
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Contd..1.2.2Air Cycle refrigeration system:

Belong to the general class of gas cycle refrigeration systems

Gas is used as the working fluid.

The gas does not undergo any phase change during the cycle,consequently, all the internal heat transfer processes are sensible heattransfer processes.

Air cycle refrigeration system analysis is considerably simplified if one makes

the following assumptionsi. The working fluid is a fixed mass of air that behaves as an ideal gas

ii. The cycle is assumed to be a closed loop cycle with all inlet andexhaust processes of open loop cycles being replaced by heat transfer

processes to or from the environment

iii. All the processes within the cycle are reversible, i.e., the cycle isinternally reversible

iv. The specific heat of air remains constant throughout the cycle


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Contd1.2.3 Basic concepts

The temperature of an ideal gas can be reduced either by making the gas to do work in an

isentropic process or by sensible heat exchange with a cooler environment. When the gas

does adiabatic work in a closed system by say, expanding against a piston, its internal energydrops. Since the internal energy of the ideal gas depends only on its temperature, the

temperature of the gas also drops during the process, i.e.,

where m is the mass of the gas, u1 and u2are the initial and final internal energies of the gas,T1

and T2are the initial and final temperatures and cv

is the specific heat at constant volume.

If the expansion is reversible and adiabatic, by using the ideal gas equation (PV=RT)and the

equation for isentropic process (P1V1=P2V2

) the final temperature is related to the initial

temperature (T1) and initial and final pressures (P1and P2) by the equation:


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Contd Isentropic expansion of the gas can also be carried out in a steady flow in a turbine

which gives a net work output. Neglecting potential and kinetic energy changes, the

work output of the turbine is given by:

1.2.4 Refrigeration Cycle

Heat flows in direction of decreasing temperature, i.e., from high-temperature to low

temperature regions. The transfer of heat from a low-temperature to high-temperature

requires a refrigerator and/or heat pump.

Refrigerators and heat pumps are essentially the same device; they only differ in their

objectives.

The performance of refrigerators and heat pumps is expressed in terms of coefficient of

performance (COP):


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Contd Reversed Carnot cycle employing a gas:Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat

source and heat sinks. Reversing the Carnot cycle does reverse the directions of heat andwork interactions. A refrigerator or heat pump that operates on the reversed Carnot cycle is

called a Carnot refrigerator or a Carnot heat pump.

Figures below show the schematic of a reversed Carnot refrigeration system using a gas as

the working fluid along with the cycle diagram on T-s and P-v coordinates.

Fig. a T-s diagram and major components for Carnot refrigerator


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Contd

Fig. b. P-V diagram of a reversed Carnot refrigerator

The reversed Carnot cycle is the most efficient refrigeration cycle operatingbetween two specified temperature levels. It sets the highest theoretical COP. Thecoefficient of performance for Carnot refrigerators and heat pumps are:

The COP of a Carnot system only dependson temperatures of the refrigeration (T1)and heat rejection (Th)


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Contd Process 1-2: Reversible, adiabatic compression in a compressor

Process 2-3: Reversible, isothermal heat rejection in a condenser

Process 3-4: Reversible, adiabatic expansion in a turbine

Process 4-1: Reversible, isothermal heat absorption in a turbine

The heat transferred during isothermal processes 2-3 and 4-1 are given by:

Applying first law of thermodynamics to the closed cycle,

The work of isentropic expansion, w3-4exactly matches the work of isentropic compression

w1-2.


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Limitations of Carnot cycle:Carnot cycle is an idealization and it suffers from several practical limitations.

One of the main difficulties with Carnot cycle employing a gas is the difficulty of

achieving isothermal heat transfer during processes 2-3 and 4-1. For a gas to have heat

transfer isothermally, it is essential to carry out work transfer from or to the system whenheat is transferred to the system (process 4-1) or from the system (process 2-3). This is

difficult to achieve in practice.

In addition, the volumetric refrigeration capacity of the Carnot system is very small

leading to large compressor displacement, which gives rise to large frictional effects.

All actual processes are irreversible, hence completely reversible cycles are idealizations

only.The Carnot cycle cannot be approximated in an actual cycle, because:

1. executing Carnot cycle requires a compressor that can handle two-phases

2. also process 4-1 involves expansion of two-phase flow in a turbine.

Seminar on (reading)

1. Ideal reverse Brayton cycle

2. Actual reverse Brayton cycle:


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Example:

1. A Carnot refrigerator extracts 150 kJ of heat per minute from a space which is

maintained at -20C and is discharged to atmosphere at 45C. Find the work

required to run the unit.2. A cold storage plant is required to store 50 tons of fish.

The temperature at which fish was supplied = 35C

Storage temperature of fish = -10C

Cpof fish above freezing point = 2.94kJ/kgC

Cpof fish below freezing point = 1.26 kJ/kgC

Freezing point of fish = -5C

Latent heat of fish = 250 kJ/kg

If the cooling is achieved within half of a day, find:

a) Capacity of the refrigerating plant

b) Carnot COP

c) If actual COP = Carnot COP/2.5 find the power required to run the plant.


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1.3 Actual Refrigeration System:The actual compression processes for most compressors and the actual expansion processes for

most expanders are irreversible polytropic processes. And there are temperature difference and

pressure losses in the actual heat transfer process.

These factors make the performance of the actual cycle different from the theoretical one. Theanalysis of the actual cycle is based on the following assumptions:

The air is an ideal gas.

There are no pressure losses in the heat exchangers.

The temperature difference in the actual heat transfer process is taken into account in the

exit temperature of heat Exchangers.Then the actual cycle is depicted on a T-s (temperature -specific entropy) diagram in the

following Figure as 1-2s-3-4s-1. For simplicity, the overall isentropic efficiency, which is a

widely used efficiency index for refrigeration compressors and expanders, is used to model the

compressor and the expander for the cycle.


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Contd

The actual work per kg of air input to the compressor for the process 1-2 is calculated as,

= 2 1

Where cis the isentropic efficiency of the compressor

The actual work developed per kg of air for the expansion process 3-4 is given by,

= 3 4 e

Where eis the isentropic efficiency of the expander (turbine)

Net work input to the air cycle refrigeration system is calculated as

= = 2 1

3 4 e

Net refrigerating effect per kg of air is given by,

= 1 = Cp(T1 T4s)

The coefficient of performance (COP) of this cycle can be calculated as


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Contd

The relationship between temperature and pressure for isentropic compression

process 1-2 is:

For the isentropic expansion process 3-4, The relationship between temperature

and pressure is

COP of this cycle can be calculated as


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1.4Vapour compression cycle and i ts equipment' s

Vapour compression refrigeration systems are the most commonly used amongall refrigeration systems.

As the name implies, these systems belong to the general class of Vapour cycles,

wherein the working fluid (refrigerant) undergoes phase change at least during

one process.

The input to the system is in the form of mechanical energy required to run thecompressor.

The actual Vapour compression cycle is based on Evans-Perkins cycle, which is

also called as reverse Rankine cycle.

Before the actual cycle is discussed and analyzed, it is essential to find the upper

limit of performance of Vapour compression cycles. This limit is set by acompletely reversible cycle.


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The Ideal VaporCompression Refrigeration Cycle The vapor-compression refrigeration is the most widely used cycle for

refrigerators, air conditioners, and heat pumps.

Assumptions for ideal vapor-compression cycle:

Irreversibilities within the evaporator, condenser and compressor are ignored

No frictional pressure drops Refrigerant flows at constant pressure through the two heat exchangers

(evaporator and condenser)

Heat losses to the surroundings are ignored

Compression process is isentropic


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Contd.

Figure: T-s andP-h diagrams for an ideal vapor-compression refrigeration cycle.

1-2 : A reversible, adiabatic (isentropic) compression of the refrigerant. The

saturated vapor at state 1 is superheated to state 2.

wc=h2 h12-3: An internally, reversible, constant pressure heat rejection in which the

working substance is de-superheated and then condensed to a saturated liquid at 3.

During this process, the working substance rejects most of its energy to the

condenser cooling water.

qH= h2 h3


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3-4 : An irreversible throttling process in which the temperature and pressure decrease at

constant enthalpy. The refrigerant enters the evaporator at state 4 as a low-quality saturated

mixture.

h3= h44-1: An internally, reversible, constant pressure heat interaction in which the refrigerant(two-phase mixture) is evaporated to a saturated vapor at state point 1. The latent enthalpy

necessary for evaporation is supplied by the refrigerated space surrounding the evaporator.

The amount of heat transferred to the working fluid in the evaporator is called the

refrigeration load. qL= h1h4

Notes:

The ideal compression refrigeration cycle is not an internally reversible cycle, since it

involves throttling which is an irreversible process. If the expansion valve (throttling

device) were replaced by an isentropic turbine, the refrigerant would enter the evaporator

at state 4s.

As a result the refrigeration capacity would increase (area under 4-4s) and the net work

input would decrease (turbine will produce some work). However; replacing the

expansion valve by a turbine is not practical due to the added cost and complexity.

The COP improves by 2 to 4% for each C the evaporating temperature is raised or the

condensing temperature is lowered.

1 4

2 1

/

/

e e

i i

q Q m h hCOP

w W m h h


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Th A l V C i f i i l


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The Actual Vapour Compression refrigeration cycle

Fig. below shows an actual vapor compression cycle compared with a basic

cycle. There are several differences between them.

And at the end of the actual heat rejection process in the condenser (process 2-3)the liquid is subcooled, not saturated.

The actual compression process is

irreversible (not isentropic) and goes in

the direction of increase of entropy

(S2>S1 ).

The isentropic efficiency of thecompressor is used to evaluate the

performance of the compressor and

define enthalpy at the exit of the actual

compressor (point 2).


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Actual VaporCompression Refrigeration Cycle contd..

Fig. T-s diagram for actual vapor-compression cycle.

Most of the differences between the ideal and the actual cycles are because of

the irreversibilities in various components which are:

1. In practice, the refrigerant enters the compressor at state 1, slightly superheated

vapor, instead of saturated vapor in the ideal cycle.

2. The suction line (the line connecting the evaporator to the compressor) is very long.

Thus pressure drop and heat transfer to the surroundings can be significant, process

6-1.


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Contd.

3. The compressor is not internally reversible in practice, which

increase entropy. However, using a multi-stage compressor with

intercooler, or cooling the refrigerant during the compression

process, will result in lower entropy, state 2.

4. In reality, the refrigerant leaves condenser as sub-cooled liquid.

The sub-cooling process is shown by 3-4 in Fig above. Sub-cooling

increases the cooling capacity and will prevent entering any vapor(bubbles) to the expansion valve.

5. Heat rejection and addition in the condenser and evaporator do not

occur in constant pressure (and temperature) as a result of pressure

drop in the refrigerant.

E i ' f V i f i i l


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Equipment's of Vapour compression refrigeration cycle

1. COMPRESSORS

There are different types of compressors that generally used in industry are,

(a) Reciprocating compressor(b) Centrifugal compressor

(c) Rotary compressor

(d) Screw compressor

(e) Scroll compressor

The reciprocating and screw compressors are best suited for use withrefrigerants which require a relatively small displacement and condenseat relatively high pressure, such as R-12, R-22, Ammonia, etc.

The centrifugal compressors are suitable for handling refrigerants thatrequire large displacement and operate at low condensing pressure, suchas R-11, R-113, etc.

The rotary compressor is most suited for pumping refrigerants havingmoderate or low condensing pressures, such as R-21 and R-114; this ismainly used in domestic refrigerators.


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2. CONDENSERS

The functions of the condenser are to desuperheat the high pressure gas, condense it and

also sub- cool the liquid.

Heat from the hot refrigerant gas is rejected in the condenser to the condensingmedium-air or water. Air and water are chosen because they are naturally

available. Their normal temperature range is satisfactory for condensing

refrigerants.

Like the evaporator, the condenser is also heat-exchange equipment.

There are three types of condensers, viz.(a) Air- cooled,

(b) Water-cooled and

(c) Evaporative.

As their names imply, air-cooled condensers use air as the cooling medium, water-

cooled condensers use water as the medium and the evaporative condenser is a

combination of the above, i.e. uses both water and air.

3 EVAPORATORS


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3. EVAPORATORS

The process of heat removal from the substance to be cooled or refrigerated is

done in the evaporator. The liquid refrigerant is vaporized inside the evaporator

(coil or shell) in order to remove heat from a fluid such as air, water etc.

Evaporators are manufactured in different shapes, types and designs to suit adiverse nature of cooling requirements. Thus, we have a variety of types of

evaporators, such as

a) prime surface types,

b) finned tube or extended surface type,

c) shell and tube liquid chillers, etc.

4. EXPANSION DEVICESThere are different types of expansion or throttling devices. The most commonly used are:

a) Capillary tube,

b) Float valves,

c) Thermostatic expansion valve.


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Multi stage (Cascade ) Vapour compression refrigeration cycle

Systems that have 2 (or more) refrigeration cycles operating in series.

Fig. A 2-stage cascade Vapour compression cycle


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Contd

Cascade cycle is used where a very wide range of temperature between TL and

TH is required. As shown in Fig. 5-5, the condenser for the low temperature

refrigerator is used as the evaporator for the high temperature refrigerator.

Cascading improves the COP of a refrigeration cycle. Moreover, the refrigerants

can be selected to have reasonable evaporator and condenser pressures in the two

or more temperature ranges.

Fig. T-S diagram for a 2 stage cascade Vapour compression cycle

Contd


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Contd

The two cycles are connected through the heat exchanger in the middle, which

serves as evaporator (cycle A) and condenser (cycle B). One can write:

Figure above shows the increase in refrigeration capacity (area under 4-7) anddecrease in compressor work (2-2-6-5).


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Multistage Compression Refrigeration system

Two-stage expansion system with a flash cooler

A two-stage system is a refrigeration system working with a two-stage

compression and mostly also with a two-stage expansion.

A schematic system layout and the corresponding process in a log p-h diagram

are shown in the following figure.

Flash gas is separated from liquid refrigerant in an intermediate receiver between the

two expansion valves.

The high-stage compressor will then remove the flash gas, as shown in Fig.6.13


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Fig Two stage expansion system with a flash cooler The basic components of the system, with two compressors in series and

two expansion valves are shown in Fig above, and the states of therefrigerant around the cycle are shown on pressure-enthalpy coordinates inFig above.


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Contd Vapor refrigerant at point 1 enters the low-stage compressor at the dry saturated

state.

It is compressed to the interstage pressure pi at point 2 and mixes withevaporated vapor refrigerant from the flash cooler, often called an economizer.

The mixture then enters the high-stage compressor at point 3. Hot gas,

compressed to condensing pressure pcon, leaves the high-stage compressor at

point 4.

It is then discharged to the condenser, in which the hot gas is desuperheated,condensed, and to liquid state at point 5 .

After the condensing process, the subcooled liquid refrigerant flows through a

throttling device, such as a float valve, at the high-pressure side.

A small portion of the liquid refrigerant flashes into vapor in the flash cooler at

point 7, and this latent heat of vaporization cools the remaining liquidrefrigerant to the saturation temperature corresponding to the interstage pressure

at point 8.

Inside the flash cooler, the mixture of vapor and liquid refrigerant is at point 6.

C td


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Contd

Liquid refrigerant then flows through another throttling device, a small portion

is flashed at point 9, and the liquid-vapor mixture enters the evaporator.

The remaining liquid refrigerant is vaporized at point 1 in the evaporator.

The vapor then flows to the inlet of the low-stage compressor and completes

the cycle.

This is the overall process of two stage expansion system with a flash cooler.

1) Fraction of Evaporated Refrigerant in Flash Cooler

In the flash cooler, out of 1 unit of refrigerant flowing through the condenser, xunit of it cools down the remaining portion of liquid refrigerant (1 - x) unit tosaturated temperature T8at interstage pressure pi.

Because h5is the enthalpy of the liquid refrigerant entering the flash cooler, h6is the enthalpy of the mixture of vapor and liquid refrigerant after the throttlingdevice, for a throttling process, h5= h6.

Enthalpies h7 and h8 are the enthalpies of the saturated vapor and saturatedliquid, respectively, at the interstage pressure, and h9 is the enthalpy of themixture of vapor and liquid refrigerant leaving the flash cooler after the low-

pressure side throttling device.

Again, for a throttling process, h8= h9

The fraction of liquid refrigerant evaporated in the flash cooler x is given as

C td


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Contd

2. Enthalpy of Vapor Mixture Entering high-Stage Compressor

Ignoring the heat loss from mixing point 3 to the surroundings, we see that the

mixing of the gaseous refrigerant discharged from the low-stage compressor at

point 2 and the vaporized refrigerant from the flash cooler at point 7 is an

adiabatic process. The state 3 at entry to the high-stage compressor is found by applying the

energy equation to the mixing of streams 7 and 2.

So the specific enthalpy of state 3 given by

87

85

hh

hhx

723 )1( xhhxh


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(3) Coefficient of Performance

For 1 unit of refrigerant flowing through the condenser, the amount of

refrigerant flowing through the evaporator is (1 -x) . The specific refrigeration effect can be expressed as

Total work input to the compressor (including the low and high-stage

compressor) ,Wi is

The coefficient of performance of the two-stage expansion system with a

flash cooler COP is

1 9(1 )( )

eq x h h

2 1 4 3(1 )( ) ( )iw x h h h h

1 9

2 1 4 3

(1 )( )

(1 )( ) ( )

e

i

q x h hCOP

W x h h h h

F t ff ti l f


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Factors affecting cycle performance

(a) Sub-cooling of Liquids:

In the below of simple vapor compression cycle, condensation process CD resulted

in the liquid at saturated state D. If it was possible to further cool down the liquid tosome lower value say up to D,then the net refrigeration effect will be increased as

(hBhA) > (hB- hA).

Hence, the sub cooling of the liquid increases the refrigerating effect without

increasing the work requirement. Thus COP is improved.

Fig. Effect of sub-cooling on cycle performance

Contd


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Contd

The sub cooling may be achieved by any of the following methods:

I. By passing the liquid refrigerant from condenser through a heat exchanger

through which the cold vapor at suction from the evaporator is allowed to flow

in the reversed direction. This process subcools the liquid but superheats the

vapor. Thus, COP is not improved though refrigeration effect is increased.

II. By making use of enough quantity of cooling water so that the liquid

refrigerant is further cooled below the temperature of saturation. In some cases,

a separate subcooler is also made use of for this purpose. In this case, COP is

improved.

b. Superheating of Vapor:

If the vapor at the compressor entry is in the superheated state B, which is

produced due to higher heat absorption in the evaporator, then the refrigerating

effect is increased as (hB

- hA

) > (hB

hA

). However, COP may increase,

decrease or remain unchanged depending upon the range of pressure of the

cycle.

C Change in suction pressure (PS):


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C. Change in suction pressure (PS):

Fig. Effects of change in t evaporator and condenser pressure.

Let the suction pressure or the evaporating pressure in a simple refrigerationcycle be reduced from PS to PS. It will be clear from the figure that:

The refrigerating effect is reduced to: (hB- h

A) < (h

B- h

A)

The work of compression is increased to: (hC - hB ) > (hC- hB)

Hence, the decrease in suction pressure decreases the refrigeration effect and atthe same time increases the work of compression. But, both the effects tend todecrease the COP.

D. Change in discharge pressure (Pd):


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D. Change in discharge pressure (Pd):

In Fig. above, let us assume that the pressure at the discharge or the condensing

pressure is increased from Pdto Pd. It will have effects as follows:

The compressor work requirement is increased to: (hC - hB) > (hC- hB)

The refrigerating effect is reduced to: (hB- hA ) < hB- hA)

Therefore, the increase in discharge pressure resul ts in lower COP. Hence, the

discharge pressure should be kept as low as possibledepending upon the

temperature of the cooling medium available.

GAS CYCLE REFREGIRATION


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GAS CYCLE REFREGIRATION

Ideal Gas refrigeration cycle:

The power cycles can be used as refrigeration cycles by simply reversing them.

Of these, the reversed Brayton cycle, which is also known as the gas

refr igeration cycle, is used to cool aircraft and to obtain very low (cryogenic)

temperatures after it is modified with regeneration.

Contd


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Contd.... The work output of the turbine can be used to reduce the work input

requirements to the compressor. Thus, the COP of a gas refrigeration cycle is:

The energy equations (neglecting kinetic and potential energy effects) are as

follows:

Seminar


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Seminar

Types of refrigerants

Selection of refrigerants

Read, report and present on seminar

Documents

Part I Refrigeration Chapter 1