ABSTRACT 1. INTRODUCTION

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1 Analysis of Hat Pump Cyl Using CO /DME Mixtur Rfrigrant Yoji ONAKA, Akio MIYARA *, Koutaro TSUBAKI, Shigru KOYAMA 3 Saga Univrsity, Graduat shool of nginring, Honjomahi, Saga-shi, 8-85, Japan Saga Univrsity, Dpartmnt of Mhanial Enginring, Honjomahi, Saga-shi, 8-85, Japan Phon: (8) , Fax: (+8) , miyara@m.saga-u.a.jp 3 Kyushu Univrsity, Intrdisiplinary Graduat shool Enginring Sin, 6- Kasuga-kon, Kasuga, , Japan 9, Pag ABSTRACT In this study, th prforman analysis of hat pump yl using arbon dioxid (CO ) and dimthyl thr (DME) zotropi mixtur has bn arrid out by yl alulation in ordr to larify th haratristis and offiint of prforman (COP) of CO /DME hat pump yl. Th alulation onditions wr stablishd as a hot-watr supply systm and th alulations wr ondutd by onsidring th hat transfr btwn th rfrigrant and hat sour/sink watr. Th hat pump yl is formd around th ritial point for high CO onntration mixturs and it is formd undr th ritial point for low CO onntration mixturs. Th COP has th maximum at a rtain prssur for ah mixtur and th COPs of th mixtur and pur DME ar highr than that of pur CO. Oprating prssur and rfrigrant mass flow rat drass with inras of DME onntration. Th ffts of onntration on haratristis of th yl hav also bn disussd.. INTRODUCTION Natural rfrigrants whih hav zro ozon dpltion potntial (ODP) and low global warming potntial (GWP) suh as arbon dioxid (CO ), ammonia (NH 3 ), and hydroarbons (HCs) hav attratd attntion as altrnativ to fluoroarbons (CFCs, HCFCs, HFCs). Sin CO is nonflammabl and nontoxi, it is spially fousd as rfrigrant of hat pump watr hatr and ar air-onditionr. In th as of hat pump watr hatr, CO shows highr offiint of prforman (COP) than othr rfrigrants baus th CO yl forms around th ritial point, alld as trans-ritial yl, and th watr is hatd fftivly by th supr-ritial fluid, whr th hat xhangr is rfrrd as gas oolr. Howvr, th oprating prssur of hat pump using CO boms vry high, ovr th ritial prssur 7.3MPa. Th COP of th trans-ritial yl as an air-onditionr is lowr than that of th yl opratd undr th ritial point, whih is th onvntional yl using fluoroarbons and is alld as onvntional yl in this papr. On th othr hand, dimthyl thr (DME) has som good thrmal proprtis, suh as highr thrmal ondutivity and larg latnt hat, and it is aptabl to th nvironmntal problms. Th DME has zro ODP and low GWP. Th dfts of DME ar flammability and low prssur. So mixing DME with CO may rdu th dfts of th high oprating prssur of CO and th flammability of DME. Koyama t al. (7) ompard xprimntally th systm prforman of pur CO and CO /DME mixtur. Thy showd th CO /DME systm ould rdu th oprating prssur than pur CO with sam systm COP lvl. Afroz t al. (8) rportd on hat transfr offiints and prssur drops during in-tub ondnsation of CO /DME mixtur. Kim t al. (7) ompard th systm prforman of CO and CO /propan mixtur xprimntally and showd that CO /propan systm ould rdu th oprating prssur. Th COP and yl haratristis of hat pump using zotropi mixtur rfrigrant hang with onntration of rfrigrant. In xprimnt, howvr, it is diffiult to larify th haratristis and COP ovr a wid rang of onntration. An analysis by yl alulations is onvnin in this as. Although many invstigations on th yl alulation wr arrid out, most of studis using zotropi mixtur rfrigrants ar fousd on th onvntional hat pump yl. Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

2 9, Pag In this study, th alulation mthod on th trans-ritial and th onvntional yl using CO /DME mixtur has bn disussd and th haratristis and COP of CO /DME mixtur hat pump yl hav bn larifid by th yl alulation.. CALCULATION METHOD Th yl alulation is takn into aount hat xhangs btwn hat sink/sour watr and rfrigrant at hat xhangrs. By onsidring a hat pomp watr hatr, hat sink/sour watr onditions, suh as inlt/outlt tmpraturs, t in, t out, t in, t out, and hating rat Q, ar rquird as alulation onditions. Th othr alulation onditions ar th CO onntration y B, B omprssor disharg prssur P, ovrall hat transfr offiint of th hat xhangr K, and hat transfr ara of th hat xhangr A. Th hat pump yl is assumd to apply watr hating, and th oprating onditions as shown in tabl hav bn givn. In addition th abov mntiond onditions, th following assumptions ar givn for simpliity. () Rfrigrant is saturatd vapor at inlt of omprssor () Comprssion pross is isntropi pross (3) Expansion pross at xpansion valv is isnthalpi pross () Prssur drops ar ngltd at gas oolr/ondnsr and vaporator Tabl : Hat pump yl oprating ondition B ~ y B [mass%] Q [kw] 3. KA[kW/K]. t in [ºC] t out [ºC] 65 t in [ºC] t out [ºC] 5 Figur shows a trans-ritial yl of CO /DME mixtur rfrigrant on T-s diagram. Th numbr of,, 3, and on this diagram man stat points of rfrigrant at a omprssor outlt, a gas oolr/ondnsr outlt, an xpansion valv outlt, and an vaporator outlt, rsptivly. CO /DME mixtur rfrigrant is zotropi mixtur with larg gliding tmpratur du to larg diffrnt boiling points. Th boiling points at atm ar -78.ºC for CO and -5.ºC for DME. A saturation tmpratur at vaporator (3-) hangs as shown in figur. In trans-ritial yl, a rfrigrant in gas oolr (-) boms suprritial fluid. Sin th spifi hat of th suprritial fluid hangs signifiantly with tmpratur, th tmpratur variation in ooling pross is not linar as shown in figur. In this as, applying th logarithmi-man tmpratur diffrn (LMTD) to th total hat xhangr is not appropriat baus th LMTD was drivd for onstant spifi hat ondition. A similar problm is indiatd by Utamura t al. (7). Consquntly th hat xhangr was dividd into substions from inlt to outlt in this alulation as shown in figur. Thrfor th problm was avoidd. Th division numbr is larg nough to anl th fft of spifi hat variation on th alulatd COP. Th parnthti numbrs in figur man th substion numbrs. T (), T (), tw (), tw (), T (), T (), tw (), and tw () in figur ar qual to T, T, t out, t in, T 3, T, t out, and t in in figur, rsptivly. Thus, th following rlations ar workd out in th yl alulation. At gas oolr/ondnsation pross (-) Q Q W ( h h ) () R W C ( t tin ) () p out Q Q (3) Q W [ h ( i ) h ] () R Q W C [ tw ( i ) tw ] (5) p Q KA( i) T (6) m Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

3 [ T ( i ) tw ( i )] [ T tw ] Tm (7) T ( i ) tw ( i ) ln[ ] T tw At vaporation pross (3-) Q Q W ( h h3) (8) R W C t t ) (9) p ( in out Q Q () Q W [ h h ( i )] () R Q W C [ tw ( i ) tw ] () p Q KA( i) T (3) m [ tw T ] [ tw ( i ) T ( i )] Tm () tw T ln[ ] tw ( i ) T ( i ) At omprssion pross (-) s s (5) At xpansion pross (-3) h3 h (6) 9, Pag 3 Whr W R is mass flow rat of rfrigrant, W is mass flow rat of hat sink watr, C p is spifi hat at onstant prssur hat sink watr,t m is LMTD for ah substion in gas oolr/ondnsr, W is mass flow rat of hat sour, C p is spifi hat at onstant prssur of hat sour, T m is LMTD for ah substion in vaporator, h is Tmpratur t in t out 3 Q Q t out t in Tmpratur T () Rfrigrant T () Hat sour & Hat sink T () T T (i-) (i) Q () T (99) Q () T () Q (i) tw () Q () tw () tw () tw (i-) tw (i) tw (99) tw () tw () tw () Q () T () T () tw (i-) Q (i) tw (i) T T (i) (i-) tw (99) Q () T (99) tw () T () Spifi ntropy Figur : T-s diagram of trans-ritial CO /DME Spifi nthalpy Figur : Tmpratur profil at hat Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

4 9, Pag rfrigrant spifi nthalpy, s is rfrigrant spifi ntropy. Thrmal proprtis of rfrigrant ar alulatd from Bndit-Wb-Rubin quation of stat (BWR EoS) (Bndit t al., 9, Miyara t al., 7). Th COP of hat pump is ( COP) H h h (7) h h Th Irrvrsibl loss L at gas oolr/ondnsr is givn by {( T ( ) ( )) ( ( ) ( ))}( ( ) ( )) i tw i T i tw i s i s i L (8) i Th irrvrsibl loss L at vaporator is givn by {( tw ( ) ( )) ( ( ) ( ))}( ( ) ( )) i T i tw i T i s i s i L (9) i Th irrvrsibl loss L p at xpansion valv is givn by L p ( T3 T )( s3 s) () 3. RESULT AND DISCUSSION Figur 3 shows th variation of (COP) H with omprssor disharg prssur. For ah onntration, (COP) H shows th maximum at a rtain disharg prssur. For trans-ritial yl, it is wll known that thr is a optimum prssur whr th (COP) H boms maximum. In th as of high CO onntration mixtur rfrigrant, th yl is a trans-ritial yl. On th othr hand, th yl of low CO onntration rfrigrant, whih is lss than 85mass%CO, boms a onvntional yl. Th alulation was arrid out in th sam way for both th transritial yl and onvntional yl. Undr th prsnt alulation mthod, th (COP) H of onvntional yl also shows th pak valu. Figur shows th yls of 7mass%CO on P-h diagram. Not only th rfrigrant saturation tmpratur but also th subooling tmpratur at ondnsr outlt hang with th disharg prssur. For low prssur ondition, th stat point of ondnsr outlt ntrs two phas rgion. Inrasing th prssur from 5.MPa, th hating apaity (h -h ) shows largr inras than omprssion work (h -h ). Howvr, th inrasing rat of (h -h ) boms small in high prssur rgion. This is th rason that (COP) H shows th maximum. (COP) H 5.5 mass%co 9mass%CO 7mass%CO P [MPa] P=5.MPa P=5.MPa P=5.8MPa P=6.MPa P=6.6MPa P [MPa] Figur3: Variation of (COP) H h [kj/kg] Figur : P-h diagram of onvntional yl of 7mass% of CO Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

5 9, Pag 5 Figur 5 shows th variation of optimum prssur with th DME onntration. Th optimum prssur drass with inras of th DME onntration. Drasing rat is mor rmarkabl in low DME onntration rgion. Figur 6 shows th variation of mass flow rat of rfrigrant at optimum prssur with th DME onntration. Th mass flow rat of rfrigrant also drass with inras of th DME onntration. Th dras of mass flow rat is ausd by inras of hating apaity (h -h ). Ths rsults show fftivnss of adding DME into CO. 5 Optimum P [MPa] Figur 5: Optimum P vrsus DME onntration W R [kg/h] Figur 6: Mass flow rat W R vrsus DME onntration Figur 7 shows th variation of th maximum (COP) H with DME onntration. Inrasing onntration of DME tnds to inras (COP) H. Howvr, in th low DME onntration rgion, whr th yl is trans-ritial, (COP) H has a pak at mass%dme. It is rdud slightly from mass%dme to 3mass%DME. Th yl of 3mass%DME is a onvntional yl. 5.5 (COP) H Figur 7: (COP) H vrsus DME onntration Figur 8 shows th variations of irrvrsibl loss ratio with DME onntration. Th Irrvrsibl loss ratio dfind as th ratio of irrvrsibl loss to th hating apaity (h -h ). In figur 9, th variation of irrvrsibl loss is shown. Sin th irrvrsibl loss affts on th (COP) H and drasing th irrvrsibl loss givs highr (COP) H, it is important to invstigat th irrvrsibl loss in ah omponnt. As shown in figur 8, total irrvrsibl loss ratio L/(h -h ) has a minimum valu at mass%dme whr (COP) H has a pak valu and has th maximum at 3mass%DME whr (COP) H has a minimum pak. On th othr hand, th irrvrsibl loss shown in figur 9 has no pak val at mass%dme. On th whol, th irrvrsibl loss of gas oolr/ondnsr is lagr than that of vaporator and xpansion valv though th loss of xpansion valv is largst for pur CO. Th major aus of th drmnt of total irrvrsibl loss ratio and th inrmnt of (COP) H with DME onntration is drmnt of irrvrsibl loss ratio at xpansion valv. Th drmnt is ausd by th inrmnt of latnt hat at vaporating prssur, whih maks largr hating apaity (h -h ). Figur 9 shows th rlation of th Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

6 9, Pag 6 hating apaity and latnt hat at vaporating prssur with DME onntration for th yl whih has th maximum (COP) H of ah onntration. L/(h -h ).5..5 Gas Coolr or Condnsr Evaporator Expansion Valv Total 6 8 Figur 8: Variation of irrvrsibl loss ratio with DME onntration Irrvrsibl Loss [kj/kg] Gas Coolr or Condnsr Evaporator Expansion Valv Total 6 8 Figur 9: Variation of irrvrsibl loss with DME onntration hating apaity & Latnt hat [kj/kg] hating apaity latnt hat 6 8 Figur : Variation of hating apaity and latnt hat at vaporating prssur with DME onntration In ordr to invstigat th dtails of irrvrsibl losss, T-s diagrams at, 3, 8, and mass%dme, ar shown in figur (a)~(d), rsptivly. Aras of rgion A and B in ths figurs indiat th irrvrsibl losss ausd by hat transfr in gas oolr/ondnsr and vaporator. Ara C shows th irrvrsibl loss at th xpansion valv. In th as of mass%dme, whih has a pak of th (COP) H, tmpratur variations of rfrigrant and hat sour watr ar almost sam and onstant tmpratur diffrn is fairly kpt. Th ara of C is smallr in omparison with that of pur CO though it is not shown. For 3mass%DME, whih has a minimum pak of th (COP) H, th tmpratur diffrn btwn rfrigrant and hat sour watr in vaporator is larg at vaporator inlt of rfrigrant (point 3) and th irrvrsibl loss boms largr. Although th tmpratur diffrn in ondnsr is kpt onstant in th ondnsing rgion, it is larg at ondnsr inlt (point ). In th as of 8mass%DME, whih has highr (COP) H than and 3mass%DME, th tmpratur diffrn is almost onstant in th vaporator and it is middlingly onstant. Th suprhat at th ondnsr inlt is small and th irrvrsibl loss boms small. For pur DME, tmpratur diffrn is larg at th nd point of ondnsation. Baus th ondnsing tmpratur is onstant and hat sink watr is hatd with larg tmpratur ris to mak hot watr, th tmpratur diffrn is largst at this point. On th othr hand, th tmpratur diffrn in th vaporator is not so larg in spit of th onstant vaporating tmpratur baus th tmpratur drop of hat sour watr is small. Th irrvrsibl loss at xpansion valv of pur DME is spially small. Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

7 9, Pag 7 T [] Rfrigrant Hat sour & Hat sink A 3 C B 3 s [kj/kgk] (a) mass%dme T [] Rfrigrant Hat sour & Hat sink A 3 C B 3 s [kj/kgk] (b) 3mass%DME T [] Rfrigrant Hat sour & Hat sink 3 A C 3 s [kj/kgk] () 8mass%DME B T [] A Rfrigrant Hat sour &Hat sink 3 B C 3 s [kj/kgk] (d) mass%dme Figur : Irrvrsibl losss on T-s diagram. CONCLUSION Th yl alulation onsidrd hat transfr has bn arrid out on a hat pump opratd with CO /DME mixtur rfrigrant whr th yl of rih CO mixtur is formd around th ritial point, alld trans-ritial yl, and that of rih DME mixtur is undr th ritial point. In th as of supr ritial fluid, th spifi hat apaity varis signifiantly during th hat transfr pross and th logarithmi man tmpratur diffrn is not appliabl to th whol of hat xhangr. A mthod that th hat xhangr is dividd into suffiintly small part and th hat transfr of th small part is alulatd has bn proposd. Cyl alulations hav bn ondutd undr a hot-watr supply ondition and following rsults hav bn obtaind. () Th COP of th mixtur and pur rfrigrants of CO and DME has th maximum valu at a rtain prssur for ah onntration. () Th maximum COP varid with th onntration. On th whol, it inrass with inras of DME onntration though it has a pak valu at mass%dme. (3) Th optimum prssur, whr th COP boms th maximum, an b rdud by adding DME into CO. From th analysis of irrvrsibl loss at th gas oolr/ondnsr, vaporator, and xpansion val, following rsults hav also bn obtaind. () Th irrvrsibl loss at gas oolr/ondnsr is lagr than othr losss xpt th loss at xpansion valv of pur CO. (5) Th irrvrsibl loss at xpansion valv is dras with dras of DME onntration. Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

8 9, Pag 8 REFERENCES Afroz, H.M.M., Miyara, A., Tsubaki, K., 8, Hat transfr offiints and prssur drops during in-tub ondnsation of CO /DME mixtur rfrigrant, Int. J. Rfrig., doi:.6/j.ijrfrig.8..9 (in prss). Bndit, M., Wbb, G. B., Rubin, L. C., J. Chm. Phys., 9, 8, p-33. Kim, J.H., Cho J.M., L, I.H., L, J.S., Kim, M.S., 7, Cirulation onntration of CO /propan mixturs and th fft of thir harg on th ooling prforman in an air-onditioning systm, Int. J. Rfrig., Vol.3:p.3-9. Koyama, S., Jin, D., Xu, J., Takata, N., Kuwahara, K., Miyara, A., 7, Exprimntal study on th prforman of a CO /DME, Pro.7 Intrnational Congrss of Rfrigration, ICR7-E-986. Miyara, A., Afroz H.M.M., 7. Prdition of vapor-liquid quilibrium data of CO /DME mixtur, Pro. 7 JSRAE Annual Conf., p.c- - C-. Utamura, M., Nikitin, K., Kato, Y., 7, Gnralization of logarithmi man tmpratur diffrn mthod for hat xhangr prforman analysis, Thrmal Sin & nginring, Vol. 5, no. 3:p (in Japans). Intrnational Rfrigration and Air Conditioning Confrn at Purdu, July -7, 8

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