A THEOREITICAL ANALYSIS OF THE APPLICATION OF CO 2 THANSCRITICAL HEAT PUMP TO THE DRYING OF SEEDS

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1 A HEOREIICAL ANALYSIS OF HE APPLICAION OF CO 2 HANSCRIICAL HEA PUMP O HE DRYING OF SEEDS Wei Dong Ma Yitai Lixin Li Weiceng Su Lv Canren Intitute of ermal Energy of ianjin Univerity, ianjin, Cina ABSRAC A new eat pump drying metod of applying CO 2 trancritical proce to te drying of biotecnology material (eed) i originally propoed in te paper. e metod make full ue of te advantage of eat recovery in te cycle, and eat rejection proce a a good temperature matcing wit drying medium (air). e paper preent a contrative analyi of te coefficient of performance (COP) and te pecific moiture extraction rate (SMER) of drying eat pump wit CO 2 and tat wit conventional working fluid R134a, NH 3 and R12 under te ame drying condition. e reult ow tat eat pump drying wit CO 2 trancritical proce i uperior to tat wit conventional working fluid, and i an environment-protecting, energy-aving and igly efficient metod of drying eed. 1. INRODUCION Scientit worldwide are being engaged in earcing for refrigerant alternative, a CFC and HCFC, widely ued in refrigeration and eat pump indutry, ave a greenoue effect and do arm to te ozone layer. u, looking for te efficient and pollution-free working fluid become an international ot topic. Since 1990, reearc in te ue of CO 2, a natural refrigerant, in replacement of man-made working fluid a been given pecial attention, and reearcer in Norway, Germany, Denmark ave made many acievement (Lorentzen G. 1994) (Petteren J. et al. 1995). CO 2, woe ODP=0, GWP=1, i of great ignificance to environment protection. Moreover, it come from indutrial wate gae, and i nonflammable. For tee reaon, it a obviou advantage in term of economy, afety, and environment protection. Conequently, CO 2 i more and more widely ued a a working fluid in refrigeration, air condition and eating ytem. Heat pump drying i one of tee ue. Heat pump drying can freely control te temperature of drying medium. e eat pump dryer (HPD) i, terefore, uitable for biotecnological material, epecially for te drying of eed. e drying of eed i different from tat of grain, fruit, vegetable and oter crop in tat eed are more enitive to te temperature and umidity of te drying atmopere. e proce ould take into account te ditinct ape and inner tructure of eed; oterwie, improper drying will reult in eed deterioration (Jialing. and Guangua Z. 1991). o enure te eed vigor, keep teir biological feature and ereditary caracteritic, and acieve te aim of environment protection and energy aving, CO 2 trancritical eat pump drying i an ideal drying metod. i paper originally preent te feaibility of uing CO 2 a a refrigerant in te 713

2 drying of eed. e eat rejection proce in te cycle can ave a good temperature matcing wit drying medium (air), wic lead to te reult tat te COP of te ytem i over 6.0. So ti HPD a good potential in practical ue. 2. AN ANALYSIS OF HE HERMODYNAMIC BEHAVIOUR OF CO 2 RANSCRIICAL CYCLE e critical temperature of CO 2 i low, only 31.1C; it critical preure i 7.38MPa. Wen ued a a working fluid in vapor compreion refrigeration cycle, te COP and refrigeration capacity are under a direct influence of te temperature of te cooling medium and tat of te ambience. If te trancritical cycle i ued, teir influence can be avoided. From te - diagram and lgp- diagram of te CO 2 trancritical cycle (Lorentzen G. 1994), we can ee tat eat aborption and rejection of te cycle occur in ubcritical and upercritical area repectively. Moreover, te eat rejection proce i one of temperature cange, wic mean it a a great temperature glide. i temperature glide, matcing te eat reource of variable temperature needed, i a pecial Lorenz cycle, and, wen ued in eat recovery, a a ig efficiency. i i one of it advantage. Anoter advantage i tat under upercritical preure, CO 2 a no aturated tate, and it temperature and preure i independent. Wen te evaporation temperature and outlet temperature of te ga cooler remain uncanged, wit te cange of te eat rejection preure, te COP of te cycle a a maximum (woe correponding preure i called optimal eat rejection preure), tu economizing compreor work. Furtermore, a it a te inerent 2 Compreor 1 3 rottle Valve Ga Cooler Evaporator Auxiliary eat Condenate excanger 6 7 Seed drying camber Outide Figure 1. e Flow Cart of Cloed Heat Pump Drying Sytem wit Auxiliary Heat Excanger Applying CO 2 rancritical Cycle. propertie of a upercritical fluid, CO 2 a better eat tranfer caracteritic and le flow reitance tan CFC. A it i le vicoity, we can promote eat tranfer rate by increaing te flowing peed, and te volume of te wole equipment will be greatly reduced. 714

3 3. PRINCIPLES AND FEAURES OF A CO 2 RANSCRIICAL DRYING HEA PUMP 3.1 A brief introduction to te proce Drying eat pump cycle, according to te cycle of te drying medium, can be furter divided into te open ytem, te partially cloed ytem and completed cloed ytem. For te drying of organic ubtance uc a eed or wood, cloed-loop proce i uually ued. For one ting, it a little interference from te ambience and a a wide cope of application; for anoter, becaue of te recycle of te drying medium (air), deumidification can be acieved by evaporator, o tat eat can be recovered, tu economizing energy. Ruiter (Ruiter J. P. et al. 1978) analye everal drying eat pump, and te reult of i experiment ow tat te cloed ytem can ave energy by 40%, compared wit te open electric eating ytem. Gopalnarayanan (Gopalnarayanan S., Radermacer R. 1997) believe tat adding an auxiliary eat excanger in front of te entrance of te evaporator can enure a complete ue of te latent eat of te refrigerant in condening te team in te umid air, wic can improve deumidification efficiency. i completely cloed ytem wit auxiliary eat excanger i ued in my reearc, and it flow cart can be een from figure 1. i ytem include an inner cycle and an outer one. e former i te cycle of te refrigerant in te eat pump, and te latter, tat of te drying air in te paage. e cycle of te refrigerant differ from conventional cycle in tat te eat rejection device i a ga cooler 6 intead of a condener, and te refrigerant undergoe a continuou cange of temperature, but ave not pae cange. A 7 for te air cycle, it tate cange are own in figure 2. e air of low temperature and low umidity from te evaporator i eated in =1 te ga cooler, enter te eed-drying 8 camber, ten, after a umidity excange 5 6 = 7 wit eed, te exaut from drying camber re-enter te evaporator to be deumidified and recycled. e auxiliary eat excanger in d 6 d 7 d front of te entrance of te evaporator can Figure 2. Entalpy-Water Content Diagram of Air Cycle reject te eat of te dried air into te ambience, and improve deumidification efficiency. 3.2 e drying condition of eed e equilibrated water content of eed i mainly decided by te relative umidity of te drying atmopere. For te dryer, wic cannot do deumidification, te relative umidity can only be reduced by raiing te temperature. However, a ig temperature will caue termal denature of protein, and reduce vigor of eed. If a HPD i ued, deumidification occur in te evaporator, o te air of low temperature and low umidity can be ued in drying witout doing arm to eed. e drying temperature of open-field crop eed i uually lower tan 50C, and te relative umidity of te drying atmopere, conidering te equilibrated water content, ould be about 17%. So, baed on tee condition, and uppoing te umidity of te exaut 715

4 from te drying camber to be 55%, te tate variable of te air at ignificant point in te drying proce, given in table 1, are determined according to te figure 2. e property value of umid air and refrigerant are found from EES (Klein, S. and Alvarado, F. 1996). e exaut temperature from te drying camber i 34.7C by calculation, wic atified te requirement of eat rejection into te ambience. able 1. ermodynamic Parameter of Humid Air at Significant Point in e Drying Proce (Ambient preure i 100kPa) [C] [%] d [g/kg air] A COMPARISION OF DRYING EFFICIENCY BEWEEN CO 2 AND CONVENIONAL REFRIGERAIONS e pecific temperature of te refrigerant cycle i determined according to te property value of te drying air, calculated in te above part of ti paper. Suppoe te temperature difference of te pinc point between air and refrigerant to be 5C, and te working fluid in te evaporator to ave a upereating of 5C, te performance and te drying effect of two ytem are compared CO 2 cycle Air cycle Ientropic line 6 2 c 3' CO 2 cycle Air cycle Ientropic line 2 2' ' 1 7 e ' 1 7 Figure 3. S Diagram of CO 2 ran-critical Drying Proce. Figure 4. S Diagram of Subcritical Cycle of Conventional Working Fluid Heat Pump Drying Proce. 716

5 4.1 e trancritical CO 2 drying proce e - diagram of te CO 2 drying ytem i own in Figure 4. e proce 1-2 i te irreverible compreion in te compreor; proce 2-3, upercritical iobaric cooling of CO 2 in ga cooler, wic i interrelated wit 5-6, te io-umid eating proce of te air. e proce 3-4, trottling in te trottling valve; te proce 4-1, evaporation of CO 2 in evaporator, wic i interrelated wit te proce 8-5, te cooling and deumidification of te drying air; proce 6-7, ioentalpic umidification in te eed-drying camber; proce 7-8, eat rejection to te ambience by auxiliary eat excanger. e ientropic efficiency of te compreor i uppoed to be 0.75, and te ambient preure i 100 kpa. In calculation, te influence of bot te optimal eat rejection preure of te CO 2 trancritical cycle and te temperature difference of te pinc point in te eat excanger i taken into account. e termodynamic parameter of te CO 2 cycle at te ignificant point are own in table 2. able 2. ermodynamic Parameter of CO 2 rancritical Cycle [C] p [kpa] ' e ubcritical drying proce of conventional working fluid Analyzing te working fluid R134a, NH 3, and R12 in te ubcritical cycle, one can find tat tey differ from te CO 2 trancritical cycle in tat teir eat rejection proce include te deeating proce 2-2, te iotermal condenation 2-3 and te ubcooling proce 3-3. e ientropic efficiency of te compreor remain 0.75 (in fact, a te preure ratio of conventional working fluid i iger tan tat of CO 2, it ientropic efficiency i lower tan CO 2 trancritical cycle). able 3~5 are repectively termodynamic parameter of R134a, NH 3, and R12 cycle. able 3. ermodynamic Parameter of R134a Subcritical Cycle [C] p [kpa] ' ' '

6 able 4. ermodynamic Parameter of NH 3 Subcritical Cycle [C] p [kpa] ' ' ' able 5. ermodynamic Parameter of R12 Subcritical Cycle [C] p [kpa] ' ' ' Analyi of te reult ere are mainly two criterion to evaluate a HPD: COP, te performance of te eat pump cycle, and SMER, te moiture extraction efficiency of te HPD, woe definition are: [2] [3] COP (1) [2] [1] ma d[7] d[6] SMER m (2) R [2] [1] cd Were m A and m are repectively te ma flow rate of air and refrigerant. According R to te equation of eat equilibration: m ( [6] g[5]) m ( [2] [3]) A R we obtain: m m A R cd [2] [3] [6] [5] (3) Similarly, 718

7 m m A R e [1] [4] [8] [5] (3') e ubcript of cd and e repreent condenation and evaporation, repectively. Due to calculation error, tere i a ligt difference between tee two number, and te value of te condenation i adopted in ti paper. e reult of te calculation are own in table 6. able 6. Comparion of Performance and drying Effect Between HPDS Applying CO 2 Conventional Working Fluid. and CO 2 R134a NH 3 R12 m A /m R COP SMER From ti table we can ee tat CO 2 trancritical HPD i uperior to conventional working fluid ubcritical HPD in bot te performance of te eat pump and drying efficiency. If te difference of te ientropic efficiency i taken into account, it advantage in energy aving are even more obviou. e main diadvantage of CO 2 trancritical drying eat pump i tat te working preure i ig, wic raie iger requirement for te ealing of te ytem and manufacture tecnology of it part. Moreover, te compreor and te eat excanger ave to be re-deigned to enure te afety and reliability of te ytem. Fortunately, all tee problem ave a good olution now: te prototype of CO 2 trancritical eat pump ave come into being in Germany. In Cina, te firt trancritical water-water eat pump, made by Intitute of ermal Energy of ianjin Univerity, a been uccefully experimented on, wic pave te way for future application and reearc. 5. CONCLUIONS 1) CO 2 i an environment-protecting working fluid, woe ODP=0, GWP=1, and wic can be obtained from indutrial wate gae, o it a obviou advantage in term of environment protection and economy. 2) e analyi ow tat CO 2 drying eat pump meet te requirement of drying eed. 3) CO 2 HPD ue trancritical eat rejection proce intead of latent eat excange, wic make te eat tranfer proce ave a good temperature matcing wit drying medium, and lead a ig eat tranfer rate. 4) CO 2 HPD a an obviou effect in energy aving: COP i a ig a 6.878, iger tan tat of te conventional working fluid R134a, NH 3, and R12 HPD. 5) CO 2 drying eat pump can increae te deumidification efficiency of te drying ytem, and te SMER of te drying eed, i iger tan tat of conventional refrigerant. 719

8 NOMENCLAURE COP coefficient of performance p preure (kpa) d water content (kg/kg air, g/kg air) SMER pecific moiture extraction rate (kg/kw) pecific entalpy (kj/kg) pecific entropy (kj/kg K) m A ma flow rate of air (kg/) emperature (K) m R ma flow rate of refrigerant (kg/) Relative umidity(%) ACKNOWLEDGEMEN i work i upported by te project (No ) of Cinee National Science Foundation and te project (No ) of ianjin Science Foundation. REFERENCES Gopalnarayanan S., Radermacer R., 1997, Heat pump aited dryer uing refrigerant mixture-batc mode drying, ASHRAE ran., 103(1), pp Jialing. and Guangua Z., 1991, Seed Vigor, Cinee National Science Publier, Beijing, Cina. Klein, S. and Alvarado, F. 1996, Engineering Equation Solver F-Cart Software, Middleton, WI. Lorentzen G., 1994, Revival of carbon dioxide a a refrigerant, International Journal of Refrigeration, Vol. 17, No.5, pp Petteren J., Neka P., Neje O. M., Sciefloe P. A. and Rektad H., 1995, Recent advance in CO 2 Refrigeration, International Intitute of Refrigeration eme 4, HR Commiion B2, Neterland. Ruiter J. P., Leentvaar G., Zeyltra A. H., 1978, umbler dryer wit eat pump Electrotecnik 56(4), pp Scmidt E.L., Klocker K., Flacke N. and Steimle F., 1998, Applying te trancritical proce to a drying eat pump. International Journal of Refrigeration, Vol. 21, No. 3, pp