RAC Lecture

8/10/2019 RAC Lecture

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Lesson

12Mu lti-Sta ge Va pou r

Compression

Refr igera t ion Sys tem s
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The objectives of this lesson are to:

1. Discuss limitations of single stage vapour compression refrigeration systems (Section12.1)

2. Classify multi-stage systems (Section 12.1)3. Discuss the conceptof flash gas removalusing flash tank (Section 12.2)4. Discuss the conceptof intercooling in multi-stage vapour compression refrigeration

systems (Section 12.3)5. Discuss multi-stage vapour compression refrigeration systems with flash gas removal

and intercooling (Section 12.4)6. Discuss the use of flash tank for flash gas removalonly (Section 12.5)7. Discuss the use of flash tank for intercooling only (Section 12.6)

Atthe end of the lesson, the studentshould be able to:

1. Justify the selection of single or multi-stage systems based on operating temperaturerange

2. Classify multi-stagesystems3. Applying mass and energy balance equations, evaluate the performance of multi-stage

vapour compression refrigeration systems with:a) Flash gas removalb) Intercoolingc) Flash gas removal using flash tank and intercooling using flash tank and/or

externalintercoolerd) Flash tank for flash gas removalonlye) Flash tank for intercooling only, and

f) A combination of any of the above

12.1. Introduction

A single stage vapour compression refrigeration system has one low side pressure

(evaporator pressure) and one high side pressure (condenser pressure). The performance ofsingle stage systems shows that these systems are adequate as long as the temperaturedifference between evaporator and condenser (temperature lift) is small. However, there aremany applications where the temperature lift can be quite high. The temperature lift can

become large either due to the requirementof very low evaporator temperatures and/or due to

the requirementof very high condensing temperatures. For example, in frozen food industries

the required evaporator can be as low as 40oC, while in chemicalindustries temperatures aslow as 150

oC may be required for liquefaction of gases. On the high temperature side the

required condensing temperatures can be very high if the refrigeration system is used as aheat pump for heating applications such as process heating, drying etc. However, as the

temperature lift increases the single stage systems become inefficient and impractical. Forexample, Fig. 12.1 shows the effectof decreasing evaporator temperatures on T s and P hdiagrams. It can be seen from the T s diagrams that for a given condenser temperature, asevaporator temperature decreases:
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i. Throttling losses increaseii. Superheatlosses increaseiii. Compressor discharge temperature increasesiv. Quality of the vapour atthe inletto the evaporator increasesv. Specific volume atthe inletto the compressor increases

As a resultof this, the refrigeration effectdecreases and work of compression increases asshown in the P h diagram. The volumic refrigeration effect also decreases rapidly as thespecific volume increases with decreasing evaporator temperature. Similar effects willoccur,though not in the same proportion when the condenser temperature increases for a givenevaporator temperature. Due to these drawbacks, single stage systems are notrecommended

when the evaporator temperature becomes very low and/or when the condenser temperaturebecomes high. In such cases multi-stage systems are used in practice. Generally, forfluorocarbon and ammonia based refrigeration systems a single stage system is used upto anevaporator temperature of 30

oC. A two-stage system is used upto 60

oC and a three-stage

system is used for temperatures below 60oC.

Apart from high temperature lift applications, multi-stage systems are also used inapplications requiring refrigeration at different temperatures. For example, in a dairy plantrefrigeration may be required at30

oC for making ice cream and at2

oC for chilling milk. In

such cases itmay be advantageous to use a multi-evaporator system with the low temperatureevaporator operating at30

oC and the high temperature evaporator operating at2oC
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Ss

3

4

4

4

1

1

1

22

2

T

h

1

1

1

2 2 2

P

3

4

4

4

Fig.12.1(a): Effectofevaporator temperature on cycle performance (T-s diagram)

Fig.12.1(b): Effectofevaporator temperature on cycle performance (P-h diagram)

.


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A multi-stage system is a refrigerationsystem with two or more low-side pressures. Multi-stage systems can be classified into:

a) Multi-compression systemsb) Multi-evaporator systems

c) Cascade systems, etc.

Two concepts which are normally integral to multi-pressure systems are, i) flash gasremoval, and ii) intercooling. Hence these concepts willbe discussed first.

12.2. Flash gas removalusing flash tank

Itis mentioned above thatone of the problems with high temperature liftapplicationsis the high quality of vapour at the inlet to the evaporator. This vapour called as flash gasdevelops during the throttling process. The flash gas has to be compressed to condenserpressure, it does not contribute to the refrigeration effect as it is already in the form of

vapour, and itincreases the pressure drop in the evaporator. Itis possible to improve the COPof the system if the flash gas is removed as soon as it is formed and recompressed tocondenser pressure. However, continuous removalof flash gas as soon as it is formed andrecompressing itimmediately is difficultin practice. One way of improving the performanceof the system is to remove the flash gas atan intermediate pressure using a flash tank. Figure

12.2 shows the schematic of a flash tank and Fig.12.3 shows the expansion processemploying flash tank. A flash tank is a pressure vessel, wherein the refrigerant liquid andvapour are separated at an intermediate pressure. The refrigerant from condenser is firstexpanded to an intermediate pressure corresponding to the pressure of flash tank, Piusing alow side floatvalve (process 6-7). The floatvalve also maintains a constantliquid levelin the

flash tank. In the flash tank, the refrigerant liquid and vapour are separated. The saturated

liquid atpoint8 is fed to the evaporator after throttling itto the required evaporator pressure,Pe(point9) using an expansion valve. Depending upon the type of the system, the saturatedvapour in the flash tank (point3) is either compressed to the condenser pressure or throttled

to the evaporator pressure. In the absence of flash tank, the refrigerantcondition atthe inlettothe evaporator would have been point 9, which has a considerably high vapour qualitycompared to point9. Asmentioned, the refrigerant liquid and vapour mustgetseparated inthe flash tank. This is possible when the upward velocity of the refrigerantvapour in the flashtank is low enough ( < 1 m/s) for the refrigerantliquid droplets to fallback into the flash tank

due to gravity. Thus the surface area of liquid in the flash tank can be obtained from thevolumetricflow rateofrefrigerantvapourand therequired low refrigerantvelocity.
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From

condenserTo compressor

To evaporator

Expansionvalve

Flashtank

6

7

8

3

9

Fig.12.2(a): Working principle ofa flash tank

P

12.3. Intercooling in multi-stage compression

The specific work input, w in reversible, polytropic compression of refrigerantvapouris given by:

n/)1n(

1

211

2

1 P

P1vP

1n

ndP.vw (12.1)

h

9

6

8 3

Pc

Pi

Pe

Fig.12.3: Expansion process using aflash tank on P-h diagram
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where P and P are the inletand exitpressures of the compressor, v1 2 1is the specific volumeof the refrigerantvapour atthe inletto the compressor and n is the polytropic exponent. From

the above expression, itcan be seen thatspecific work inputreduces as specific volume, v1isreduced. At a given pressure, the specific volume can be reduced by reducing thetemperature. This is the principle behind intercooling in multi-stage compression. Figures

12.4 (a) and (b) show the process of intercooling in two-stage compression on Pressure-specific volume (P-v) and P-h diagrams.

h

P

P

1

1

2

23 3

2

2

4

4

v

Savings in

sp. work

Fig.12.4(a) & (b): Intercooling in two-stage compression

As shown in the figures, in stead of compressing the vapour in a single stage from state 1 tostate 2, if the refrigerant is compressed from state 1 to an intermediate pressure, state 2,intercooled from 2 to 3 and then compressed to the required pressure (state 4), reduction in

work input results. If the processes are reversible, then the savings in specific work is givenby the shaded area 2-3-4-2on P-v diagram. The savings in work inputcan also be verifiedfrom the P-h diagram. On P-h diagram, lines 1-2-2and 3-4 represent isentropes. Since theslope of isentropes on P-h diagram reduces (lines become flatter) as they move away from thesaturated vapour line,

(h4-h

3) < (h -h

2

2) (h

2-h

1)+(h

4-h

3) < (h -h

2

1) (12.2)

Intercooling of the vapour may be achieved by using either a water-cooled heatexchanger or by the refrigerant in the flash tank. Figures 12.5(a) and (b) show these twosystems. Intercooling may notbe always possible using water-cooled heatexchangers as itdepends on the availability of sufficiently cold water to which the refrigerantfrom low stagecompressor can rejectheat. Moreover, with water cooling the refrigerant at the inlet to the

high stage compressor may not be saturated. Water cooling is commonly used in aircompressors. Intercooling notonly reduces the work inputbut also reduces the compressordischarge temperature leading to better lubrication and longer compressor life.
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Low-stage

compressor

High-stagecompressor

Flash

tank

Refrigerant

liquidfrom

condenser

1

2

3

Fig.12.5(a): Intercooling using liquid refrigerantinflash tank

4

Low-stage

Compressor

Water-cooled

heatexchanger

High-stage

Compressor

Water inWater out4

1

2

3

Fig.12.5(b): Intercooling using externalwater cooled heatexchanger

Intercooling using liquid refrigerantfrom condenser in the flash tank may or may notreduce the power inputto the system, as itdepends upon the nature of the refrigerant. This isdue to the fact that the heat rejected by the refrigerant during intercooling generatesadditionalvapour in the flash tank, which has to be compressed by the high stage compressor.

Thus the mass flow rate of refrigerant through the high stage compressor willbe more thanthatof the low stage compressor. Whether totalpower input to the system decreases or notdepends on whether the increased power consumption due to higher mass flow rate is


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compensated by reduction in specific work of compression or not. For ammonia, the powerinputusually decreases with intercooling by liquid refrigerant, however, for refrigerants such

as R12, R22, the power inputmarginally increases. Thus intercooling using liquid refrigerantis noteffective for R12 and R22. However, as mentioned one benefitof intercooling is thereduction in compressor discharge temperature, which leads to better compressor lubrication

and its longer life.

Itis also possible to intercoolthe refrigerantvapour by a combination of water-cooledheatexchanger and the refrigerant liquid in the flash tank. As a resultof using both water-cooling and flash-tank, the amount of refrigerant vapour handled by the high-stagecompressor reduces leading to lower power consumption. However, the possibility of this

again depends on the availability of cooling water atrequired temperature.

One of the design issues in multi-stage compression is the selection of suitableintermediate pressure. For air compressors with intercooling to the initial temperature, thetheoreticalwork inputto the system willbe minimum when the pressure ratios are equalfor

allstages. This also results in equalcompressor discharge temperatures for allcompressors.Thus for a two-stage air compressor with intercooling, the optimum intermediate pressure,Pi,optis:

highlowopt,i P.PP (12.3)

where Plowand Phighare the inletpressure to the low-stage compressor and exitpressure fromthe high-stage compressor, respectively. The above relation is found to hold good for idealgases. For refrigerants, correction factors to the above equation are suggested, for exampleone such relation for refrigerants is given by:

e

cceopt,i

T

TP.PP (12.4)

where Peand Pcare the evaporator and condenser pressures, and Tcand Teare condenser andevaporator temperatures (in K).

Severalcombinations of multi-stage systems are used in practice. Some of them arediscussed below.

12.4. Multi-stage system with flash gas removaland intercooling

Figures 12.6(a) and (b) show a two-stage vapour compression refrigeration systemwith flash gas removalusing a flash tank, and intercooling of refrigerantvapour by a water-

cooled heat exchanger and flash tank. The superheated vapour from the water cooled heatexchanger bubbles through the refrigerant liquid in the flash tank. It is assumed that in thisprocess the superheated refrigerantvapour gets completely de-superheated and emerges outas a saturated vapour atstate 4. However, in practice complete de-superheating may notbepossible. As mentioned the use of combination of water cooling with flash tank for

intercooling reduces the vapour generated in the flash tank. The performance of this systemcan be obtained easily by applying mass and energy balance equations to the individualcomponents. It is assumed that the flash tank is perfectly insulated and the potential andkinetic energy changes of refrigerantacross each componentare negligible.
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From mass and energy balance of the flash tank:

4

.

8

.

3

.

7

.

mmmm (12.5)

44

.

88

.

33

.

77

.

hmhmhmhm (12.6)

Evaporator

9

8Compressor - I1

Condenser

6

7

Compressor - II

Water intercoolerFlash

chamber

2

3

4

5

Qe

Qi

WI

WII

Qc

9

h

1

P

6

8 72

5

4 3

Pc

Pi

Pe

Fig.126(a): Two-stage vapour compression refrigeration system withflash gas removalusing aflash tank and intercooling

Fig.126(b): Two-stage vapour compression refrigeration system withflash gas removalusing aflash tank and intercoolingP-h diagram
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From mass and energy balance across expansion valve,

9

.

8

.

mm (12.7)

98 hh (12.8)

From mass and energy balance across evaporator:

1

.

9

.

mm (12.9)

)hh(mQ 911.

e (12.10)

From mass and energy balance across low-stage compressor, Compressor-I:

I

.

1

.

9

.

mmm (12.11)

)hh(mW 12I.

I (12.12)

where is the mass flow rate of refrigerantthrough Compressor-I.I.

m

From mass and energy balance across water-cooled intercooler:

I

.

3

.

2

.

mmm (12.13)

)hh(mQ 32I.

I (12.14)

where QIis the heattransferred by the refrigerantto the cooling water in the intercooler.

From mass and energy balance across high-stage compressor, Compressor-II:

II

.

5

.

4

.

mmm (12.15)

)hh(mW 45II.

II (12.16)

where is the mass flow rate of refrigerantthrough Compressor-II.II.

m

Finally, from mass and energy balance across condenser:

II

.

6

.

5

.

mmm (12.17)

)hh(mQ 65II.

c (12.18)

Finally, from mass and energy balance across the floatvalve:

II

.

7

.

6

.

mmm (12.19)

76 hh (12.20)

From the above setof equations, itcan be easily shown thatfor the flash tank:

II

.

4

.

7

.

mmm (12.21)

I

.

8

.

3

.

mmm (12.22)

74

83I

.

II

.

hh

hhmm (12.23)
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12/21

Itcan be seen from the above expression that the refrigerant flow through the high-

stage compression can be reduced by reducing the enthalpy of refrigerantvapour entering

into the flash tank, hII

.

m

from the water-cooled intercooler.3

The amountof additionalvapour generated due to de-superheating of the refrigerant

vapour from the water-cooled intercooler is given by:

84

43I

.

gen

.

hh

hhmm (12.24)

Thus the vapour generated will be zero, if the refrigerant vapour is completely de-

superheated in the water-cooled intercooler itself. However, this may not be possible in

practice.

gen

.

m

For the above system, the COPis given by:

)hh(m)hh(m

)hh(m

WW

QCOP

45II

.

12I

.

91I

.

III

e (12.25)

The above system offers severaladvantages,

a) Quality of refrigerant entering the evaporator reduces thus giving rise to higherrefrigerating effect, lower pressure drop and better heattransfer in the evaporator

b) Throttling losses are reduced as vapour generated during throttling from Pc to Pi is

separated in the flash tank and recompressed by Compressor-II.c) Volumetric efficiency of compressors willbe high due to reduced pressure ratiosd) Compressor discharge temperature is reduced considerably.

However, one disadvantage of the above system is thatsince refrigerantliquid in the flashtank is saturated, there is a possibility of liquid flashing ahead of the expansion valve due to

pressure drop or heat transfer in the pipelines connecting the flash tank to the expansiondevice. Sometimes this problem is tackled by using a system with a liquid subcooler. As

shown in Fig.12.7, in a liquid subcooler the refrigerant liquid from the condenser issubcooled by exchanging heatwith the refrigerantliquid in the flash tank. As a result, a smallamountof refrigerantvapour is generated in the flash tank, which needs to be compressed in

the high-stage compressor. Compared to the earlier system, the temperature of refrigerantliquid from the subcooler willbe higher than the saturated refrigeranttemperature in the flashtank due to indirect contact heat transfer. However, since the refrigerant at the inlet to theexpansion valve is athigh pressure and is subcooled, there is less chance of flashing of liquidahead of expansion valve.
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13/21

12.5. Use of flash tank for flash gas removal

Intercooling of refrigerant vapour using water-cooled heat exchangers is possible inammonia systems due to high discharge temperature of ammonia. However, this is generallynotpossible in systems using refrigerants such as R 12 or R 134a due to their low discharge

temperatures. In these systems, in stead of passing the refrigerantvapour from the low-stagecompressor through the flash tank, vapour from the flash tank is mixed with the vapourcoming from the low-stage compressor. As a result, the inlet condition to the high-stagecompressor willbe slightly superheated. A two-stage compression system with flash tank forflash gas removalfor refrigerants such as R 134a is shown in Fig. 12.8 (a). Figure 12.8 (b)shows the corresponding P-h diagram.

3

6

8

7

6From

condenser To high-stage

compressor

To

evaporator

Expansionvalve

Liquid

subcooler

Fig.12.7: Refrigeration system with liquid subcooler
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14/21

Evaporator

9

8 Compressor - I1

7

Condenser

6

Compressor - II

Flash

chamber

2

3

5

4

9

h

1

P

8 7

6

4 23

5

Fig.12.8: A two-stage compression system withflash tank forflash gas removalonly

(a) System schematic; (b) Cycle on P-h diagram

12.6. Use of flash tank for intercooling only

Sometimes the flash tank is used for intercooling of the refrigerantvapour betweenthe low and high-stage compressors. Itis notused for flash gas removal. Figures 12.9 (a) and

(b) show the system schematic and P-h diagram of a two-stage compression system where theflash tank is used for intercooling only.
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15/21

Evaporator

7

Compressor - I1

6

5Condenser

Compressor - II

Flash

chamber

2

3

4

7

h

1

P

6

5

4

3

2

2'

Fig.12.9: A two-stage compression system with theflash tank usedfor intercooling only

(a) System schematic (b) Cycle on P-h diagram
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16/21

Questions:

1. When the temperature lift of a single stage vapour compression refrigeration systemincreases:

a) Refrigeration effectincreasesb) Work of compression increasesc) Compressor discharge temperature decreasesd) Volumetric efficiency of compressor increases

Ans.: b)

2. Multi-stage vapour compression refrigeration systems are used when:

a) Required temperature liftincreasesb) Required temperature liftdecreases

c) Refrigeration is required atdifferenttemperaturesd) Required refrigeration capacity is large

Ans.: a) andc)

3. Using a flash tank:

a) Flash gas formed during expansion can be removed atan intermediate pressureb) Quality of refrigerantatthe evaporator inletcan be increasedc) Temperature of refrigerantvapour atthe inletto higher stage compressor can be reduced

d) Pressure drop in evaporator can be reduced

Ans.: a) , c) andd)

4. Using intercooling in multi-stage compression systems:

a) Refrigeration effectcan be increasedb) Work of compression in higher stage compressor can be reducedc) Maximum cycle temperature can be increased

d) Allof the above

Ans.: b)

5. Externalintercooling of refrigerantvapour:

a) Is feasible for ammonia based systemsb) Commonly used in air compressorsc) Commonly used for halocarbon refrigerants

d) Depends on availability of cold externalwater

Ans.: a) andb)
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17/21

6. Assuming the refrigerantvapour tobehave as an idealgas and with perfect intercooling,the optimum intermediate pressure of arefrigeration system thatoperates between 4 bar and

16 bar is equalto:

a) 10 bar

b) 8 barc) 6 bard) 12 bar

Ans.: b)

7. Refrigeration system with liquid subcooler is used to:

a) Preventthe entry of liquid into compressorb) Preventflashing of refrigerantliquid ahead of low stage expansion devicec) Reduce work of compression

d) Allof the above

Ans. b)

8. In two-stage compression system with flash gas removal:

a) Refrigerantmass flow rates in both low and high stage compressors are equalb) Refrigerant mass flow rates in high stage compressors is greater than that in low stagecompressor

c) Refrigerant mass flow rates in high stage compressors is smaller than that in low stagecompressord) Mass flow rates in low and high stage compressors are equal if the pressure ratios areequal

Ans.: b)

9. Use of flash tank for intercooling:

a)Always improves system COPb) COPincreases or decreases depends on the refrigerantused

c) Maximum compressor discharge temperature always decreasesd) Power inputto the system always decreases

Ans.: b) andc)
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18/21

10. The required refrigeration capacityof a vapourcompression refrigeration system(with R-22 as refrigerant) is 100 kW at 30oC evaporator temperature. Initially the system wassingle-stage with a single compressorcompressing the refrigerant vapourfromevaporatortoa condenseroperating at 1500 kPa pressure. Laterthe systemwasmodified to a two-stagesystemoperating on the cycle shown below. At the intermediate pressure of 600 kPa there isintercooling but no removalof flash gas. Find a)Power requirement of the originalsingle-

stage system; b)Totalpowerrequirement of the two compressors in the revised two-stagesystem. Assume that the state of refrigerant at the exit of evaporator, condenser andintercoolerissaturated, and the compression processesare isentropic.

-18oC

1500 kPa

intercooler

evaporator(100 kW)

1

condenser

2

nd

stagecompressor

ststagempressorco

600 kPa

st1 stagecompressor

Ans.:

Fromrefrigerant propertydata, the following valuesare obtained forR22:

PointTemp.,

oC Pressure,kPa

Drynessfraction

Density,kg/m3 Enthalpy,

kJ/kgEntropy,kJ/kg.K

-30 163.9 1.0 7.379 392.7 1.8021

39.1 1500 0.0 - 248.4 -3

1500 - - 449.9 1.8022 76.93

1500 - - 429.6 1.7422 53.55

5.86 600 1.0 - 407.2 1.7422

600 - - 424.4 1.8022 28.94
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19/21

a)Single stage system:

h

1

23

P

4

Required refrigerant massflowrate, mrisgiven by:

)= 100/(392.7 248.4)= 0.693 kg/shmr= Q /(he 1 4

Powerinput to compressor, Wcisgiven by:

)= 0.693(449.9 392.7)= 39.64 kWhWc= mr(h2 1COPof the single stage systemisgiven by:

COP= Qe/Wc= 100/39.64 = 2.523

Compressordischarge temperature = 76.93 oC(frompropertydata)

Two-stage systemwithflashtank forintercoolingonly:

Required refrigerant mass flow rate through evaporator and 1sth

P

4

1

22

2 23

4

stage compressor(mr,1)issame asthat of single stage system, i.e.,

mr,1= 0.693 kg/s
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20/21

Powerinput to 1ststage compressor, W isgiven by:c,1

)=0.693(424.4 392.7)= 21.97 kWhWc,1= m (hr,1 2 1

The mass flow rate of refrigerant vapour through 2nd stage compressor (mr,2) is

obtained fromenergybalance acrossintercooler:

mm .hr,2 2 = m .h + (m ).hr,1 2 r,2 r,1 4

Substituting the valuesof enthalpyand massflowrate through 1ststage compressor:

= 0.768 kg/smr,2

Powerinput to 2ndstage compressor, W isgiven by:c,2

Wc,2= mr,2(h2 h2 )= 0.768(429.6 407.2)= 17.2 kW

Therefore, totalpowerinput, Wcisgiven by:

Wc= Wc,1+Wc,2= 21.97+17.2 = 39.17 kW

COPof the two-stage systemisgiven by:

COP= Qe/(Wc,1+Wc,2)= 100/39.17 = 2.553

st ndFrom property data, the discharge temperatures at the exit of 1 and 2 stagecompressorsare given, respectivelyby:

T2= 28.94oC

T2= 53.55oC

Comments:

It is observed from the above example that for the given input data, thoughthe use of a two-stage system with intercooling in place of a single stage system

doesnot increase the COPsignificantly( 1.2 %), there isa significant reduction in

the maximum compressor discharge temperature ( 24oC). The results would bedifferent if the operating conditionsand/orthe refrigerant used isdifferent.
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RAC Lecture

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Transcript of RAC Lecture