exergy analysis on a vapor compression refrigerating system using

8/2/2019 Exergy Analysis on a Vapor Compression Refrigerating System Using

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Purdue University

Purdue e-Pubs

International Refrigeration and Air ConditioningConference

School of Mechanical Engineering

1992

Exergy Analysis on a Vapor CompressionRefrigerating System Using R12, R134a and R290

as RefrigerantsX. XuEcole des Mines de Paris; France

D. ClodicEcole des Mines de Paris; France

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Xu, X. and Clodic, D., "Exergy Analysis on a Vapor Compression Refrigerating System Using R12, R134a and R290 as Refrigerants"(1992).International Refrigeration and Air Conditioning Conference. Paper 160.http://docs.lib.purdue.edu/iracc/160
http://docs.lib.purdue.edu/http://docs.lib.purdue.edu/iracchttp://docs.lib.purdue.edu/iracchttp://docs.lib.purdue.edu/mehttps://engineering.purdue.edu/Herrick/Events/orderlit.htmlhttps://engineering.purdue.edu/Herrick/Events/orderlit.htmlhttps://engineering.purdue.edu/Herrick/Events/orderlit.htmlhttps://engineering.purdue.edu/Herrick/Events/orderlit.htmlhttp://docs.lib.purdue.edu/mehttp://docs.lib.purdue.edu/iracchttp://docs.lib.purdue.edu/iracchttp://docs.lib.purdue.edu/


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\EXERGYANALYSISONAVAPOR COMPRESSION REFRIGERATING SYSTEMUSINGR12, ANDR290 ASREFRIGERANTS

X.XUan d D. C LO D IC Centre d 'E nergetique, Ecole des M ines dePari s,

60, Bd StM ic hel. 75006 Paris, F ran ce A B ST R A C T

This . paper d esc ribes an exergetic analysis on a vapor compression refrigerating sy stem .The exergetic re la tions have been defined. A mathematicalmodel for carryingout exergyanalysis by computer has been developed. An experiment an exergy ana lys is on 3refrigerators/freezers using R12, R134a and R290 have been carried out to illustrate th e various exergy losses occurring in th e d ifferen t components, and to display th e potential improvements .

COP coefficient ofe specific excrf(y (kf kg-1)E aergy (k f!h specific enthalpy (kf kg I) m mass flow (kg sec-1)Q heat (k /)s specific entropy (k/ kgI Kl) T temperature (K)w work (kf kg1) W work (kj)Greek.1 exergy loss11cx exergy efficiellcyS11bscriptsair or surrounding

condenser

NOMENCLATUREcampeJr

Ism,71

p

v!ISX01...10

IN T R O D U C T IO N

compressor evaporator lulricanl oildefrostin g capil/arvliquid in suhwolermotorCOT!Ilecti ng pipecompression heat emission in airsubwolersuperheating in suction pipevapor in subcoolerdcSllpahcatillg in dischargereference statestate points

A ccordin g to the M ONTR EAL PR OTO COL, the refrigeran t Rl2w il l beabandonedby1995,due to i ts high ODP and GWP le vel. For domestic refrigeration, R134a and R290 arepossib le substi tutes known. Their interest is that th ey can be appli ed by some simplealtera tions of materials since they have near thermodynamic properties with R12.Therefore , a comparisonof performance withinR12, R134a and R290 has been carried outwith th e helpof exergeti c analysis method in th is paper. Exerg eti c analy sis has showeda g rea t in terest in recen tyears due to th e fact that it com binesth e application of th e fir st and the second law of thermodynamic. T h is an aly s is helps to understand clearly irreversibil ity influences in thermodynamic process. It permit s to iden tify andcalcula te th e va r iou s exergy lo sses in d ifferen t components , and h en ce lead s toimprove thermodynamic effic ie ncy. T he aim of this paper is to in troduce an exergetic analysis methodon vapor compressionre frig era ting system in order to compare the performancebetween 3 refrigerators/freezers

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using Rl2, R134a andR290 as the refrigerants, hence to find an appropriatesubsti tu te ofR l2. In this paper, an exergetic 11nalysis model bymicro-computer has been developed .Experimentw ith 3 refrigerators /freezers has beencarried out.Theexergy analysis resultatsare reported. Themeans for reducing the various exergy losses in different components arealsodiscussed.

REVIEWOFEXERGYCONCEPTSExergy defiuitionExergy at a givenstateis equal to the maximum work that can beob ta ined when opera tingreversibly between the given state and the reference state:

e=(h-ho)-T 0(s-so) (1 )

ForaheatQataconstant temperatureT, exergycanbealsocalculatedby

(2)

The reference state is often the su rrounding of system .E:rergy lo ss ande:rergyeffic itm cy

Figure 1 Exergy balanceAccording to the second law of thermodynam ic, irreversibilities in process cause alwayssome thermodynamic losses. Therefore, for a steady process w ith amechanical inputw(Figure 1), theexergy input flow is always greater than the exergyoutflow .Theexergy lossof thisprocesscanbecalculatedbyexergybalance:

(3)

In particular, for a reversible process(4)

Since w>w,, th e irreversib ili ties absorbe a portion of mechanical energy input. forcompensating the exergy losses. For apprec ia ting the perfecti on degree of an irreversibleprocess in comparing its correspondent reversible process , we introduce an exe rgy efficiency:

Exergyefficiency workrequested bycorrespondent reversible process

workconsummated byirreversib leprocess

(5 )

Theexergyeffidency is equal to 1when process is reversible, andis less than 1in elsecase.

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EXERGY ANALYSIS O F VAfOR C O M P R E SSIO N SYSTEM

The refrigerator system consists of a compressor, a condenser, a subcooler, a capillary andan evaporator (Figure 2). For the freezer system, after com pression the refrigerant encirclesthe joint of fr eezer door for defrosting (Figure3).

Condenser 4

Figure 2 Refrige rator circuit

Condense r 4

Defrosti ng6

Frgure 3 Freezer circu it

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1 analysis ofcomponents In condenser and in subcooler, exergy lossesarecaused by hea t transfer between differenttempera tu re levels.Condenser

(6)

Subcooler(7)

Evapora to r .In evaporator,exergy loss is caused by different tempera tu re levels andpressureloss

(8)

Capillary In capilla ry,exerg y loss is caused byirreversib leexpansion.

(9)

Compressor

The exergy analysis of motor-compressor is Taking electric effic iency of mo tor (60%-80%) into account, a considerablepart ofelectricpoweris transform ed into heat flowabsorbed by lubricant oil. A part of this hea t flow is used to superheat refr igeran t vaporbetween the compressor inle t and the suctio n of compressor piston. The rest is get free inthe surrounding. For reason of simplification , the compress ion is assumed adiabatic .Between piston discharge and compressor outlet, refrigerant is desuperheatedbyyieldin gheat flow into lubricantoil, finally into th e surrounding . By thesameways above,we carryou tan exergyanalysis in compressor.LossofmotorIn the motor , a part of electric power is transformed in to hea t fl ow by magnetic andmechanical losses. According to the second law of thermodynam ic , the quality of electricenergyandof heatis different. Hence, theexergy losscanbecalculatedby

To Toi . 0 ThJWm"'ThWm - (10)

With hea t flowSuperlreati11g i11 suction pipe

(11)

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With superheating flow W v=(hl-h JO)m Heat emission in surro1mdiu g

W r=W m -W vTo ToLl.Er=Ehr-Ear=O- Th)W ,-0- Tal3)W r (1 2)

Compression The irreversibility of compress10n causes an increase of entropy, hence an exergy loss:

(13)

superhea ting iu dis charge pipe

(14)

The to tal exergy loss ofcom pressor is(15 )

For freezer system , we have to take into account the exergy losses in desuperhea ting and in defrosting upstream from condenser.Desuperhea ting ill couuectiug pipe

(16)

Defrosting

(17)

2 Exergybalanceof system A fte r exergy analys is of each component, we can make an exergy balance of th is refrigeration system . For a close system , the exergy ga ins a re ega! to the sum of exergy losses :

(1 8)Considering the above exergy equa tions .of each com ponen t, we can finally obta in the exergy balance of this refrigerationsyste m:

W=(Ll.Em+t.Ev+Ll.E,+t.Ep+Ll.Ex+6E;,+Ll.E;+tJ.Ec:+Ll.EJ+Ll.E 5 +t.Ecl (19)+(


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(i) Raise th e exergy ot sys tem from the system sou rce a t a low temperature (coldroom) to the sys tem sink at a hig h tem perature (surround ing ).

(ii) C om pensa te to ta l exergy lo sses due to ir reversibilities in system .

3 COPandex ergy efficiencyThe coeffici ent of perform ance is calc ulated by

PrCOP=wW here Pr is th e re frigera ting capacity of syste m .

(20)

(22)

In order to calculate th exergy ef ficien cy given by equation (5), we have to calculate at firstthe reversible power of system. Therefore, we determine an equiva lent averagetemperature fo r the sou rce cf. J and for th e s ink (T;;) from their exergy. Here T, is theaverage air temperature in ClJld room and T;; is the average air temperature of sys tem surrounding. In th is w ay, the C O P of CA RNOT and th e reversil(l e powerof system can becalculated :

T,C O Pca rnot=- . -Tk -T,

w .- coPc." ''"tthe exergy coefficient is

W r COPTtc, = W - COPcarnot

(23)

(24)

(25)

T he exergy effi ciency shows clearly thermodynamic perfec t ion degree of re fr_igerationsystem .In this paper, a model of vapor com pression systen:t fo r carry ing out exergy analysis bycom pu te r has beendeveloped .

EXPE RIM ENTSIn anair -conditioned cabin et, 3 re fr ige ra to rs/freezers each using one flu ide of R12, R134aand R290 have been tested at a surrounding temperature of 32 "C accord ing to ISO 8187.The circu it is the sam e. But e s ter oi l is used fo r R134a com presso r lubrication . For R290refrigerator, the volume of compressor is a little reduced because of it s too highcom pressor load and greater volum etric refrigera t ing capacity. In th is te st, temperatureregulation is canceled in system and refrigerating capacity of system is contro led constan tby an ele ctr ic resistance for achieving steady operat ing state. 27 tem peratures ; 2 p ressu res

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and 2 ele ctric power have been mea surel1 a t steady s ta te on each cycle lor carry ing outcare fu lly an exergy analysis .

Parameter s R12 R134A R290T cond c 57 .4 54.1 48.3P cond bar 14.4 14.6 16.5 Tin evap "C -21 .3 -17 .7 -17.8Pm evap bar 1.43 1 .46 2 .62 Tout evap 'C -19 .6 -17.1 -14 :9 Pout evap bar 1.42 1 .3 5 2 .43

Table 1 Operating parametersof 3 refrigera torsR12- R134a R290

exerFwJ'oss % % r .al loss %Condenser 4.60 6 .57 3.67 5 .62 2.76 3.78Subcooler 2.75 3 .92 2.08 2.97 2.30 3.28Cap illary 0.06 0.08 0.03 0.04 0.05 0.07Evaporator 6.67 9 .52 5.37 7 .70 5 .43 7 .43 Com press1on 55.98 79.90 58.65 84.03 62 .44 85.43To ta l 70 .05 100 69.79 100 73.08 100

Table 2 Exergy losses in refrigera torcomponentsR12 Rl34a R290Ts oc 3.1 1.3 3 .85 Tk oc 38.6 39.1 38.7Pf w 61 .59 57.30 51 .27w w 79 .50 79.30 81.13Wr w 7 .93 7 .89 6.45COP 0.775 0.721 0.63COPcarnot 7.7 71 7 .253 7.9520.0997 0.0995 0.0795

Table 3 COPand exergy effic iencyof 3re frigerato rs R12 R134a R290exerFwJ'oss % 11oss % loss %Com press1on 0.16 0.3 4.77 8.1 4.68 7 .49 motor 4i.n 76.4 40.79 69.6 44.70 71.6Superheating 0.89 1.6 0.82 1.4 0.98 1.6 Heat em ission 5.82 10.4 5 .43 9 .3 5 .37 8.6Desuperheating 6 .33 11 .3 6.84 11.7 6.71 10 .7 Total 55.98 100 58.65 100 62.44 100

Table 4 Exergy losses in refrigera torcompressors

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Param eters R12 R134A R290T.cond c 44 .5 4 6 .6 46.4Pcond bar 10.7 - 12.1 15.8 Tin evap c -30 -25 .7 -30 .7 Pin evap bar 1.00 1.03 1 .6 2Tout evap c -2 2 .6 -18 .4 -20.1 Pout evap bar 0 .9 0 .8 1 .2 4

Table 5 Operating_parameters of 3 freezersR12 R134a R29 0

exerpx ]loss % xerrJ lossw] % loss %1:33 1.5 9 1.82 2 .3 0 1 .5 3 1 .4 6Subcooler 2.83 3 .39 2.35 2 .9 7 4.-68 4 .4 8Capillary O.Q2 O.Q2 0 .0 3 0 .04 0 .0 9 0 .08

Evapora tor 2 .89 3 .4 6 3.41 4.31 6 .4 2 6 .14 Compression 73.9 2 8 8 .42 6 9 .0 7 8 7 .27 84 .12 80.46Desuperheating 0 .5 3 0.63 0.38 0.49 0.9 0 .8 6Defros ting 2 .08 2 .48 2 .0 8 2.63 6 .80 6.51Total 83 .59 1 00 7 9 .1 4 100 1 04 .55 100

Table 6 Exergy losses in freezer-components

R12 R134 a R290Ts c -23.9 -20.9 -22.3 i'k oc 35 .6 34.9 31 .4 Pf w 62 .45 59 .26 7 5 .36 w w 9 9 .5 0 9 3 .0 0 120.50Wr w 14 .91 13.11 16 .15 COP 0.628 0 .637 0 .62 5 COPcarnot 4 .1 87 4.521 4 .6 7'lex 0 .1 50 0.141 0.134

Table 7 COP and exergy effic ie ncy of 3 freezersR12 R134a R290

exer(U;11oss % xer?i!,Joss % loss %Compress ion 0 .23 0.3 3 .66 5.3 0.48 0 .6m otor 59 .30 80.2 51.38 7 4 .4 63.95. 76 .0 Superheating 0 .8 5 1.2 0.88 1.3 1.7 7 2.1Heat emission 7 .3 7 10.0 6 .0 5 8.76 9 .2 8 11.0 Desuperheating 6 .17 8 .4 7 .0 9 10.3 8 .6 3 10.3.Total 73 .92 100 6 9 .0 7 1 00 84.12 1 00

Table 8 Exergy losses in freezercompressors

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REFERENCES

(1) S. KUMAR, M. PREVOST, R. BOUGAREL "Exergy analysis of a compressiO n refrigera tion system ", Hea tRecovery Systems& .CHPVo1.9,No.2, pp.151-157, 1989.

(2) Lucien BOREL "Theorie generale del'exergieetapp lica tion pratique", ENTROPIE N85,janv ie r-Fev rie r 1979.(3] B.PETERSSON,H.THORSELL, "Comparison o f th e refr igeran ts HFC 134a andCFC 12'',Int. ). Refrig eration , Vol 13, 1'990.(4] Q.SYUAN,J.C BLAISE,M. DUMINIL, "Analyse exerge tique etapplication aux pompesa chaleur",Rapportde la D irecti on des E tudes et Recherches de l'EDF, 1988.

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exergy analysis on a vapor compression refrigerating system using

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