Volume 4, Issue 1 (2016) ISSN International Journal of Advance Research and Innovation

Transcription

1 Effcts of Variation in Evaporator and Condnsr Tmpratur on Cascad Condnsr Tmpratur, COP and Scond Law Efficincy of a Cascad Rfrigration Systm Akhilsh Arora Dpartmnt of Mchanical Enginring, Dlhi Collg of Enginring, Dlhi, India Articl Info Articl history: Rcivd 02 April 2016 Rcivd in rvisd form 20 May 2016 Accptd 28 May 2016 Availabl onlin 15 Jun 2016 Kywords Cascad Rfrigration Systm; Exrgy; Optimum Tmpratur In Cascad Condnsr; Natural Rfrigrants; NH3/CO2; C3H6 /CO2 Nomnclatur A COP E ED (k) EDR EF EP h m P Approach Abstract Exrgy rat of fluid (k) Exrgy dstruction rat Exrgy dstruction ratio Exrgy rat of ful (k) Exrgy rat of product (k) Enthalpy (kj/kg) Mass flow rat (kg/s) Prssur (kpa) Rat of hat transfr (k) s Entropy (kj/kg K) T Tmpratur (C) Tmc Condnsation tmpratur in cascad condnsr Tm Evaporation tmpratur in cascad condnsr Powr () Subscript c Condnsr cc Cascad condnsr comp Comprssor crs Cascad rfrigration systm Corrsponding Author, addrss: aroraakhilsh@yahoocom All rights rsrvd: In th prsnt study, th ffcts of variation in vaporator and condnsr tmpraturs on first and scond law fficincy of th cascad systm using NH3/CO2 (Ammonia-Carbon dioxid) and C3H6 /CO2 (Propyln -Carbon dioxid) pairs hav bn carrid out Th optimum tmpratur in cascad condnsr tmpratur corrsponding to maximum xrgtic fficincy is also dtrmind undr for ths conditions It is obsrvd that maximum COP and maximum xrgtic fficincy occur at th sam cascad condnsr tmpratur It is obsrvd that optimum cascad tmpratur incrass with incras in vaporator tmpratur, condnsr tmpratur and approach in cascad condnsr Th optimum tmpratur in cascad condnsr for C3H6/CO2 pair is highr than that for NH3/CO2 pair NH3/CO2 pair offrs bttr xrgtic fficincis at optimum cascad condnsr tmpratur than C3H6/CO2 pair This also mans that ovrall xrgy dstruction in NH3/CO2 pair is lss than C3H6/CO2 pair Evaporator x Exrgtic htc High tmpratur circuit ltc Low tmpratur circuit opt Optimum r Rfrigrant, room rr Rvrsibl rfrigrator s Isntropic rtv, t Rfrigrant throttl valv total Total 0 Dad stat Grk lttrs ΔTsb,c Sub-cooling in condnsr ΔTsh, Suprhating in vaporator Efficincy δ Efficincy dfct 1 Introduction Low tmpratur rfrigration systms ar normally rquird in th tmpratur rang from -30 C to -100 C in various industris such as pharmacutical, food, chmical, blast frzing and liqufaction of gass Th application of multi-stag vapour comprssion rfrigrating systms is not dsirabl for attaining vry low tmpraturs du to th solidification tmpratur of th rfrigrant, low vaporator prssur, normously larg spcific volum and difficultis ncountrd in th opration of mchanical quipmnt such as comprssor with th us of a singl rfrigrant Ths problms ar usually ovrcom by adopting a cascad rfrigration systm whr two or mor indpndnt vapour comprssion systms ar cascadd Gupta [1] has numrically optimizd th cascadd rfrigration-hat pump systm for maximum ovrall COP and minimum oprating costs with rfrigrants R-12 in high tmpratur circuit and R-13 in low tmpratur circuit Kanoğlu [2] accomplishd th xrgy analysis of th multistag cascad rfrigration cycl usd for natural gas 261

2 liqufaction Th rlations for th total xrgy dstruction, xrgtic fficincy and minimum work rquirmnt for th liqufaction of natural gas in th cycl ar dvlopd It was shown that th minimum work dpnds only on th proprtis of th incoming and outgoing strams of natural gas, and it incrass with dcrasing liqufaction tmpratur Ratts and Brown [3] prformd th cascading of an idal vapour comprssion cycl for dtrmining th optimal intrmdiat tmpraturs basd on th ntropy gnration minimization mthod Agnw and Amli [4] optimizd two stag cascad rfrigration systm for minimum powr consumption and a givn rfrigration rat using finit tim thrmodynamics approach for rfrigrants R717 and R508b in high tmpratur circuit and low tmpratur circuit rspctivly This pair was found to xhibit bttr prformanc in comparison to R12 and R13 pair Nicola t al [5] carrid out th first law prformanc of a cascad rfrigration cycl, oprating with ammonia in high tmpratur circuit and blnds of CO2 and HFCs in low tmpratur circuit, for thos applications whr tmpraturs blow tripl point of CO2 (21658 K) ar ndd Thir rsults show that th R744 blnds ar an attractiv option for th low-tmpratur circuit of cascad systms oprating at tmpraturs approaching 200 K Bhattacharyya t al [6] carrid out th analysis of a cascad rfrigration systm for simultanous hating and cooling with a CO2 basd high tmpratur cycl and C3H8 (Propan) basd low tmpratur cycl Thy prdictd th optimum prformanc of th systm with variation in th dsign paramtrs and oprating variabls This cascadd systm can oprat simultanously btwn rfrigrating spac tmpratur of -40 C and a hating output tmpratur of about 120 C Morovr, propan vindicats itslf as a bttr rfrigrant than ammonia du to its nontoxic natur Howvr, its flammability rmains a concrn L t al [7] optimisd condnsing tmpratur of a two stag cascad rfrigration systm for ammonia and carbon dioxid for maximization of COP and minimization of xrgy loss It was dducd that optimal condnsing tmpratur incrasd with condnsation and vaporation tmpraturs Th ffcts of sub-cooling and suprhating wr not takn into th considration Th computation of xrgtic fficincy was also not prformd Krus and Rüssmann [8] invstigatd th COP of a cascad rfrigration systm using N2O (Nitrous oxid) as rfrigrant for th low tmpratur cascad stag and various natural rfrigrants lik NH3, C3H8, propn, CO2 and N2O itslf for th high tmpratur stag Thy compard its rsult with a convntional R23/HFC134a cascad rfrigration systm for hat rjction tmpraturs btwn 25 to 55 C Thy concludd that by substituting th lowr stag rfrigrant R23 by N2O practically achivd th sam nrgtic prformanc with high stag fluids R134a, ammonia and hydrocarbons Niu and Zhang [9] carrid out th xprimntal study of a cascad rfrigration systm with R290 in high tmpratur circuit and a blnd of R744/R290 in low tmpratur circuit Th prformanc of th blnd was compard with R13 in low tmpratur circuit Th blnd showd good cycl prformanc compard with R13 and is considrd as a promising altrnativ rfrigrant to R13 whn th vaporator tmpratur is highr than 201 K Gtu and Bansal [10] carrid out th nrgy analysis of a carbon dioxid ammonia (R744/R717) cascad rfrigration systm Thir study involvd th xamination of th ffcts of vaporating, condnsing and cascad condnsr tmpraturs, sub-cooling and suprhating in both high and low tmpratur circuits on optimum COP Thy mployd a multi-linar rgrssion analysis and dvlopd mathmatical xprssions for maximum COP, th optimum vaporating tmpratur of R717 and th optimum mass flow ratio of R717 to that of R744 in th cascad systm Thir study did not includ th xrgy analysis approach to achiv maximum xrgtic fficincy Bhattacharyya t al [11] carrid out th analysis of an ndorvrsibl twostag cascad cycl and analytically obtaind th optimum intrmdiat tmpratur for maximum xrgy and rfrigration ffct Thy also dvlopd a comprhnsiv numrical modl of a trans-critical CO2/C3H8 cascad systm and vrifid th thortical rsults Bansal and Jain, [12] rviwd th litratur on cascad rfrigration systm Thy rportd that a cascad rfrigration systm is normally rquird for producing low tmpraturs ranging from (-)30 C to (-)100 C for various industris such as pharmacutical, food, chmical, blast frzing and liqufaction of gass Th rfrigrants spcifid for us in high tmpratur circuit ar HCFC22, HFC134a, R507A, ammonia, propan, and propyln whras carbon dioxid, HFC23 and R508B ar suitabl for us in low tmpratur circuit Th rfrigrant pairs that hav rcivd th most attntion in rcnt yars ar R717/R744 (ammonia / carbondioxid) and R1270/R744 (propyln / carbon-dioxid) for applications down to (-) 54 C Dopazo t al [13] carrid out thortical analysis of a NH3/CO2 cascad rfrigration systm for cooling applications at low tmpraturs Th rsults hav bn prsntd for optimization of cofficint of prformanc in th vaporation tmpratur rang (- )55 C to (-)30 C in low tmpratur circuit, 25 to 50 C condnsation tmpratur in high tmpratur circuit and (- )25 to 5 C in cascad condnsr Th approach tmpratur was varid btwn 3-6 C Th ffct of comprssor isntropic fficincy on systm COP is also xamind Th rsults show that, whn following both xrgy analysis and nrgy optimization mthods, an optimum valu of cascad condnsr tmpratur is achivd Howvr in this study, ffct of sub-cooling and suprhating for dtrmining th optimum cascad condnsr tmpraturs is not includd Thus from litratur rviw it is obvious that natural rfrigrants ar attracting th intrst of scintists and a lot of work is bing don in this ara Th rfrigrant pairs which hav garnrd th attntion ar NH3 (R717) and R508b, R717 and R744, R744 and R290 and R717 and blnds of R744 and HFCs and R717 and C3H6 (R1270) Morovr, th studis citd abov prtain to nrgy analysis and in vry fw studis th xrgy analysis of cascad systms has bn prsntd In th studis prtaining to cascad systm, analysis of R1270/R744 is not prsntd Hnc in th prsnt study th ffct of variation in vaporator and condnsr tmpraturs on COP and scond law fficincy of a cascad rfrigration systm is invstigatd using rfrigrant pairs NH3/CO2 (R717/R744) and C3H6/CO2 (R1270/ R744) 262

3 2 Dscription Of Cascad Rfrigration Systm Fig 1(a) schmatically rprsnts a two stag cascad systm and Fig 1(b) prsnts th corrsponding prssur nthalpy diagram This rfrigration systm compriss two sparat rfrigration circuits- th high-tmpratur circuit (htc) and th low-tmpratur circuit (ltc) Each circuit has a diffrnt rfrigrant suitabl for that tmpratur with lowr tmpratur units progrssivly using lowr boiling point rfrigrants Th lowr boiling point rfrigrant will hav highr saturation prssur at low tmpraturs that kps th ingrss of air undr control and rquirs a smallr comprssor for th sam rfrigrating ffct du to highr dnsity of suction vapours Th circuits ar thrmally connctd to ach othr through a cascad-condnsr, which acts as an vaporator for th htc and a condnsr for th ltc Fig 1(a) indicats that th condnsr in this cascad rfrigration systm rjcts hat C from th condnsr at condnsing tmpratur Tc to its warm coolant or nvironmnt at tmpratur T0 Th vaporator of th cascad systm absorbs a rfrigratd load from th cold rfrigratd spac at Tr to th vaporating tmpratur T Th hat absorbd by th vaporator of th ltc plus th work input to th ltc comprssor quals th hat absorbd by th vaporator of th htc Tmc and Tm rprsnt th condnsing and vaporating tmpraturs of th cascad condnsr, rspctivly Approach is dsignatd as A and it rprsnts th diffrnc btwn th condnsing tmpratur (Tmc) of ltc and th vaporating tmpratur (Tm) of htc Th vaporating tmpratur (T), th condnsing tmpratur (Tc), and th tmpratur diffrnc in th cascadcondnsr (A) ar thr important dsign paramtrs of a cascad rfrigration systm 3 Thrmodynamic Analysis Of Cascad Rfrigration Systm Th thrmodynamic analysis of th two stag cascad rfrigration systm involvs th application of principls of mass consrvation, nrgy consrvation and xrgy balanc 31 Mass balanc Th mass flow rats ar htc rspctivly 32 Enrgy balanc ṁ ṁ ltc and htc in ltc and Th nrgy balanc across vaporator is givn by: m ltc ( h h ) 4 1 (1) Enrgy balanc across cascad condnsr is givn by: m ltc ( h2 h3 ) mhtc( h5 h8 ) (2) Powr rquird to oprat th comprssors is givn by: comp _ total comp _ ltc comp _ htc m ltc ( h (3) Cofficint of prformanc of cascad rfrigration systm is givn by: COP 33 Exrgy Balanc (4) comp _ ltc comp _ htc Th scond law of thrmodynamics drivs th concpt of xrgy, which always dcrass du to thrmodynamic irrvrsibility Exrgy [14] is dfind th masur of usfulnss, quality or potntial of a stram to caus chang and an ffctiv masur of th potntial of a substanc to impact th nvironmnt hn th kintic and potntial nrgis ar nglctd, spcific xrgy of a fluid stram [15] can b dfind as: (5) ( h ho ) To ( s so ) whr is th spcific xrgy of th fluid at tmpratur T Th trms h and s ar th nthalpy and ntropy of th fluid, whras, ho and so ar th nthalpy and ntropy of th fluid at nvironmntal tmpratur (or dad stat tmpratur) To (is in all cass absolut tmpratur is usd in K) According to Bjan t al [16], th xrgy balanc applid to a fixd control volum is givn by th quation (6) T0 mi m 1 ED 0 (6) i T or T0 E E 1 ED 0 (7) i T Th first two trms ar xrgy input and output rats of th flow, rspctivly Th third trm is th xrgy associatd with hat transfr, which is positiv if it is ntring into th systm It is can also b rgardd as work obtaind by Carnot ngin oprating btwn T and T0, and is thrfor qual to maximum rvrsibl work that can b obtaind from hat nrgy is th mchanical work transfr to or from th systm, and th last trm ( ) is xrgy dstroyd du to th intrnal irrvrsibilitis Th principl xrgy dstruction factors in a procss ar friction, hat transfr undr tmpratur diffrnc and unrstrictd xpansion Using quation (6) and (7), total xrgy dstruction in systm componnts hav bn calculatd 331 Exrgtic Efficincy ED Th xrgtic fficincy is th ratio btwn xrgy in product to th xrgy in ful and is givn by quation (16): η x Exrgy in product Exrgy of ful EP EF 1 T comp _ ltc 0 (8) T r comp _ htc h ) m 1 htc ( h

4 1 T0 Tr COP or crs η x (9) COPrr comp _ total whr COPcrs is cofficint of prformanc of cascad rfrigration systm and COPrr is th cofficint of prformanc of rvrsibl rfrigrator oprating btwn dad stat tmpratur T0 and Tr 4 Rsults And Discussion A computational modl is dvlopd for carrying out th nrgtic and xrgtic analysis of th cascad systm using Enginring Equation Solvr softwar (Klin and Alvarado[19]) Th input data spcifid blow, for th computation of rsults shown in figurs (2) through (9) is rfrrd from Bansal and Jain [12]: 1 Rfrigration capacity ( : 1 TR 2 Sub cooling of rfrigrant laving htc condnsr ( T sb,c ) : 5 C 3 Suprhating of suction vapour in ltc vaporator ( T sh ) : 10 C, 4 Isntropic fficincy of comprssors ( comp ) : 70 % 5 Diffrnc btwn vaporator and spac tmpratur : (Tr-T) = 10 C 6 Evaporator tmpratur ( : -55 C to -15 C 7 Condnsr tmpratur ( ) T T c ) (in stps of 5 C) ) (in stps of 10 C) : 30 C to 60 C 8 Dad stat tmpratur (T0) and prssur (P0) ar 25 C and bar rspctivly 9 Rfrnc nthalpy (ho) and ntropy (so) of th working fluids hav bn calculatd corrsponding to th dad-stat tmpratur (T0) of 25 C 10 Hat losss and prssur drops in conncting lins and various componnts ar nglctd Figs 2(a) and (b) rprsnt th comparison of prsnt rsults obtaind using th computr cod dvlopd for prformanc analysis of two stag cascad systm with rsarch of Bansal and Jain [12] In Fig 2(a) th variation of COP vrsus approach in cascad condnsr is shown and in Fig 2(b) variation of COP vrsus tmpratur in cascad condnsr (Tcc) is prsntd It is obsrvd that rsults calculatd using th prsnt modl ar in agrmnt with th thortical rsults rportd by Bansal and Jain [12] for ammonia / carbon dioxid pair and th diffrnc in rsults is lss than 05% Th incras in approach causs a drop in COP bcaus of incras in cascad condnsr tmpratur and hnc prssur ratio across comprssor in ltc incrass thrby incrasing th comprssion work in ltc This nhancmnt of comprssion work causs a rduction in COP in ltc and COP of th cascad systm also Th variation of COP with tmpratur in cascad condnsr shows that thr xists a maximum valu of COP corrsponding to which cascad condnsr tmpratur is optimum This happns bcaus th prssur ratio of ltc comprssor incrass with incras in cascad condnsr tmpratur causing a rduction in COP in ltc bcaus of incras in comprssor work in ltc whras th rvrs happns in htc and hnc thr xists an optimum cascad condnsr tmpratur Tcc_opt corrsponding to which total comprssion work is minimum and hnc COP is maximum Th rsults of propyln/carbon-dioxid ar also shown in this figur and it can b obsrvd that th COP curv is idntical for this pair of rfrigrants howvr th COP offrd is lowr in comparison to ammonia/ carbondioxid pair 41 Effct of Cascad Condnsr Tmpratur Fig 3 prsnts th variation of xrgtic fficincy and total xrgy dstruction vrsus cascad condnsr tmpratur It is obsrvd that total xrgy dstruction dcrass up to crtain cascad condnsr tmpratur and furthr incrass with incras in cascad condnsr tmpratur Th xrgtic fficincy shows a rvrs trnd in comparison to total xrgy dstruction Th rason for such a bhaviour of xrgtic fficincy can b xplaind on th basis of total comprssor powr rquird in ltc and htc Th total comprssor powr rquird is lowst at a particular cascad condnsr tmpratur Th rfrigration T capacity ( 1 0 is also r ) is constant and th trm T constant sinc both dad stat tmpratur (T0) and cold room tmpratur (Tr) ar constants Hnc th input xrgy givn by T 0 Tr xrgtic fficincy givn by 1 is a constant valu Thus η x 0 1 T r is comp _ total highst at a spcific cascad condnsr tmpratur corrsponding to which total comprssor powr rquird is lowst This spcific tmpratur, corrsponding to which th xrgtic fficincy is highst, is optimum cascad condnsr tmpratur It is obsrvd from Figs (2) and (3) that th optimum cascad condnsr tmpratur corrsponding to maximum COP and maximum xrgtic fficincy is idntical Fig (3) also dpicts that R717/R744 shows bttr xrgtic fficincy as compard to R1270/R744 Th Influnc of Various Dsign Paramtrs which affct th optimum Cascad condnsr tmpratur, maximum COP and maximum xrgtic fficincy ar (i) Evaporator tmpratur (ii) Condnsr tmpratur (iii) Approach in cascad condnsr (iv) Isntropic fficincis of comprssors in ltc and htc (v) Sub-cooling of rfrigrant xiting condnsr in htc and (vi) Suprhating in vaporator in ltc In th prsnt study th ffct of vaporator and condnsr tmpratur is considrd T 264

5 Tabl: 1 Comparison of optimum cascad condnsr tmpratur, COPmax and maximum xrgtic fficincy of R717/744 and R1270/R744 pairs A=0 C, Tc = 50 C, T= -45 C η_comp Tcc ( C) COPmax η x_max Ltc htc R717/ R744 R1270/ R744 R717/ R744 R1270/ R744 R717/ R744 R1270/ R (-76%) (-78%) (-89%) (-898%) (-69%) (-68%) (-84%) (-813%) (-78%) (-839%) Surroundings (T 0) Condnsr (Tc) m ht High tmpratur 6 rtv htc Cascad condnsr 5 Comp htc comp htc m lt Low tmpratur rtv ltc Evaporator (T) 2 Cold room (Tr) Comp ltc Fig1 Cascad Cycl 265

6 7 Tc 6 ṁhtc P 3 Tmc Tm A 8 5 ṁltc 4 T 1 h Fig: 1 (b) P-h diagram of cascad rfrigration systm 266

7 267

8 268

9 269

10 270

11 271

12 42 Effct of Evaporator Tmpratur Fig (4) shows th variation of optimum cascad condnsr tmpratur and maximum COP with vaporator tmpratur and th influnc of isntropic fficincy of ltc and htc comprssors on optimum tmpratur in cascad condnsr and maximum COP It is vidnt from this Fig that th incras in vaporator tmpratur incrass th optimum tmpratur in cascad condnsr and maximum COP Th incras in optimum cascad condnsr tmpratur is attributd to dcras in ovrall working tmpratur rang and it also rducs th prssur ratio in ltc and htc Hnc comprssor works dcrass and COP of th cascad systm incrass It is obsrvd that dcras in isntropic fficincy of th ltc comprssor from 1 to 08 (kping th isntropic fficincy of comprssor in htc = 1) causs th optimum cascad condnsr tmpratur to dcras whras th rvrs happns in cas whn isntropic fficincy of th htc comprssor dcrass to 08 from 1 (kping th isntropic fficincy of th ltc comprssor =1) This ffct is narly compnsatd whn th fficincis of both ltc and htc comprssors rduc from 1 to 08 and th optimum cascad condnsr tmpratur obtaind in this particular cas is vry nar (but lowr) to th optimum cascad condnsr whn assuming isntropic fficincis of both th comprssors ar takn as 1 On mor obsrvation that is important to highlight hr is that 20% rduction in th fficincy of ltc comprssor causs about 10K drop in optimum cascad condnsr tmpratur as compard to about 8K ris in cascad condnsr tmpratur for th sam dcras in fficincy of htc comprssor On th othr hand 20% drop in isntropic fficincy of ltc comprssor causs th systm COP to drop by 73% to 89% as compard to a drop of 13-14% in COP for 20% dcras in isntropic fficincy of htc comprssor Th variation in EDRmin and maximum xrgtic fficincy is rprsntd in Fig (5) Two main charactristics of this Fig ar dcrasing trnd of maximum xrgtic fficincy with incras in vaporator tmpratur and dcras in maximum valu of xrgtic fficincy with dcras in isntropic fficincy of ithr of th comprssors In this cas also, it is crucial to mphasiz that th rduction in isntropic fficincy of htc comprssor by 20% causs about 13-14% rduction in maximum valu of xrgtic fficincy as compard to 73-9% rduction whn th isntropic fficincy of ltc comprssor rducs by sam amount Thus onc again it is confirmd that lowring of isntropic fficincy of comprssor in htc (i comprssor for ammonia) has mor damaging ffct on systm prformanc as compard to carbon dioxid comprssor Th trnds of curvs of minimum EDR ar just opposit to maximum xrgtic fficincy curvs This fact is also highlightd in quation (15) givn abov Fig (6) illustrats th ffct of variation in vaporator tmpratur on optimum tmpratur in cascad condnsr, maximum COP, maximum xrgtic fficincy and minimum xrgy dstruction ratio for R1270/R744 pair Fig7 shows th ffct of variation in vaporator tmpratur on minimum EDR and maximum xrgtic fficincy Th trnds for optimum cascad condnsr tmpratur, maximum COP, maximum xrgtic fficincy and minimum EDR ar similar to th trnds of ths paramtrs prsntd in Figs (4) and (5) for R717/R744 Th 20% rduction in isntropic fficincy of ltc comprssor (i carbon dioxid comprssor) is accountabl for lowring both maximum COP and maximum xrgtic fficincy by 87% and 102% corrsponding to vaporator tmpraturs of -35 C and -55 C rspctivly Similar to abov, th rduction in maximum valus of COP and xrgtic fficincy is 131% and 126% for idntical tmpratur conditions whn isntropic fficincy of htc comprssor rducs by 20% Tabl 1 prsnts th comparison of th two pairs of rfrigrants considrd for various conditions of isntropic fficincis of ltc and htc comprssors It is obsrvd that R717/R744 rfrigrant pair offrs bttr prformanc in trms of maximum COP and maximum xrgtic fficincy as spcifid in th tabl Th valus givn in bracs show th prcntag diffrnc by which th valus of maximum COP and maximum xrgtic fficincy for rfrigrant pair R1270/R744 ar lowr than th corrsponding valus for R717/R Effct of Condnsr Tmpratur Figs (8) and (9) dpict th ffct of condnsr tmpratur on optimum cascad condnsr tmpratur, maximum COP, minimum EDR and maximum xrgtic fficincy rspctivly for R717/R744 Simultanously ths Figs also prsnt th ffct of isntropic fficincis ltc and htc comprssors on abov mntiond paramtrs Th optimum tmpratur incrass with incras in condnsr tmpratur This happns bcaus of incras in ovrall working tmpratur rang Th maximum COP of th systm rducs sinc th incras in condnsr tmpratur causs th prssur ratios of th ltc and htc comprssors to incras and hnc th powr input incrass thrby rducing th maximum COP Th ffct of isntropic fficincis of comprssors on optimum tmpratur in cascad condnsr and maximum COP ar similar to th trnds obsrvd in th cas of variation of vaporator tmpratur dpictd in Fig (4) and xplaind in corrsponding para Fig (9) shows that maximum xrgtic fficincy rducs with incras in condnsr tmpratur This dcrasing trnd of xrgtic fficincy is achivd bcaus of incras in input xrgy i total comprssor powr rquird for th sam output xrgy of th corrsponding to a constant cooling capacity Th ffct of isntropic fficincis of th comprssors is similar to th trnds that wr achivd in cas whn vaporator tmpratur was varid (Rfr Fig (5)) Th variation of optimum tmpratur in cascad condnsr and maximum COP for R1270/R744 is illustratd in Fig(10) Th variation in maximum xrgtic fficincy and minimum EDR ar prsntd in Fig(11) Th xplanation of trnds of ths curvs is similar as has bn discussd for R717/R744 Th comparison of th optimum tmpraturs, maximum COP and maximum xrgtic fficincis for a particular st of data is alrady prsntd for ths two pairs of rfrigrants in Tabl (1) 5 Conclusions In this study, a dtaild nrgy and xrgy analysis of a two stag cascad rfrigration systm has bn carrid out for R717/R744 and R1270/R744 pairs of th rfrigrants for th computation of optimum cascad condnsr 272

13 tmpratur Th ffcts of various paramtrs ar also computd Th following conclusions can b drawn from th abov analysis:- 1 Th COP and xrgtic fficincy incras with incras in cascad condnsr tmpratur, achiv maximum valus at a particular cascad condnsr tmpratur and dcras with furthr incras in cascad condnsr tmpratur This spcific cascad condnsr tmpratur, corrsponding to which both COP and xrgtic fficincy ar maximum, is dsignatd as optimum cascad condnsr tmpratur Th optimum cascad condnsr tmpratur also corrsponds to th condition of minimum total xrgy dstruction in th systm 2 Th optimum valu of cascad condnsr tmpratur incrass with incras in vaporator and condnsr tmpraturs Th dcras in isntropic fficincy of th ltc comprssor causs th optimum cascad condnsr tmpratur to rduc whras optimum cascad condnsr tmpratur incrass with dcras in isntropic fficincy of th htc comprssor Th maximum COP incrass with incras in vaporator tmpratur whras rvrs happns whn condnsr tmpratur incrass Th maximum xrgtic fficincy rducs with incras in vaporator and condnsr tmpraturs Th dcras in valus of maximum COP and maximum xrgtic fficincy is highr whn th isntropic fficincy of htc comprssor is rducd as compard to th idntical dcras in th valu of isntropic fficincy of ltc comprssor 3 Th valus of maximum COP and maximum xrgtic fficincy of R717/R744 ar 7-9% highr than R1270/R744 pair Rfrncs [1] Gupta V K Numrical optimization of multi-stag cascadd rfrigration-hat pump systm Hat Rcovry Systms 1985; 5(4): [2] Kanoğlu M Exrgy analysis of multistag cascad rfrigration cycl usd for natural gas liqufaction Intrnational Journal of Enrgy Rsarch 2002; 26: [3] Ratts EB and Brown J S A gnralizd analysis for cascading singl fluid vapour comprssion rfrigration cycls using an ntropy gnration minimization mthod Intrnational Journal of Rfrigration 2000; 23: [4] Agnw B and Amli SM A finit tim analysis of a cascad rfrigration systm using altrnativ rfrigrants Applid Thrmal Enginring 2004; 24: [5] Nicola G D, Giuliani, G, Polonara, F and Stryjk, R Blnds of carbon dioxid and HFCs as working fluids for th low-tmpratur circuit in cascad rfrigrating systms Intrnational Journal of Rfrigration 2005; 28: [6] Bhattacharyya, S, Mukhopadhyay, S, Kumar A, Khurana RK and Sarkar J Optimization of a CO2/C3H8 cascad systm for rfrigration and hating Intrnational Journal of Rfrigration 2005; 28: [7] L T S, Liu C H and Chn T Thrmodynamic analysis of optimal condnsing tmpratur of cascadcondnsr in CO2/NH3 cascad rfrigration systms Intrnational Journal of Rfrigration 2006; 29: [8] Krus, H and Rüssmann, H Th natural fluid nitrous oxid-an option as substitut for low tmpratur synthtic rfrigrants Intrnational Journal of Rfrigration 2006; 29: [9] Niu, B and Zhang, Y Exprimntal study of th rfrigration cycl prformanc for th R744/R290 mixturs Intrnational Journal of Rfrigration 2007; 30: [10] Gtu, HM, Bansal PK Thrmodynamic analysis of an R744/R717 cascad rfrigration systm Intrnational Journal of Rfrigration 2008; 31(1): [11] Bhattacharyya, S, Bos, S and Sarkar, J Exrgy maximization of cascad rfrigration cycls and its numrical vrification for a transcritical CO2/C3H8 systm Intrnational Journal of Rfrigration 2007; 30: [12] Bansal, P K and Jain, S, Cascad systms: past, prsnt, and futur ASHRAE Transactions 2007; 113(1): [13] Dopazo J A, Frnándz-Sara J, Sirs J, Uhía F J Thortical analysis of a CO2 NH3 cascad rfrigration systm for cooling applications at low tmpraturs Applid Thrmal Enginring 2009; 29(8-9): [14] Dincr, I Rfrigration Systms and Applications 2003ily, UK, 26 [15] Sözn A Effct of hat xchangrs on prformanc of absorption rfrigration systms Enrgy Convrsion Managmnt 2001; 42: [16] Bjan, A, Tsatsaronis, G, Moran, MThrmal Dsign and Optimization John ily and Sons 1996, USA, pp [17] Said SA and Ismail B Exrgtic assssmnt of th coolants HCFC123, HFC134a, CFC11 and CFC12 Enrgy 1994; 19(11): [18] Kotas, TJ Th Exrgy Mthod of Thrmal Plant Analysis 1985, Buttrworths, London [19] Klin, SA and Alvarado, F (2005) Enginring Equation Solvr, Vrsion 7441, F Chart Softwar, Middlton, I 273