DEFROST CONTROL METHOD FOR AIR HEAT PUMP BASED ON AVERAGE HEATING CAPACITY

Transcription

1 DEFROS CONROL MEHOD FOR AIR HEA PUMP BASED ON AVERAGE HEAING CAPACIY LI Chun-Ln, Maer, heral Engneerng Deparen, nghua Unery, Bejng , Chna LI Jun-Mng, Aocae Profeor, heral Engneerng Deparen, nghua Unery, Bejng , Chna Wang Ru-Xang, Aocae Profeor, Urban Conrucon Deparen, Bejng Inue of Cl Engneerng and Archecure, Bejng , chna Abrac he rong ye hould operae ely for an ar hea pup a lower eperaure. Inegaon hae ndcaed ha he perod for rong one of he poran facor ha affec he operang characerc of he ar hea pup. o frae he conrol ehod, eeral apec hould be uded, ncludng fro growh proce, ye operang pon araon and he conrol ehod. In h paper, ye-operang characerc were analyzed and a rong conrol ehod preened baed on aerage heang capacy [1]. 1 INRODUCION In cold and waery clae, he eaporaor of ar-ource hea pup operae under frong condon for par of he heang eaon. Fro accuulaon on he eaporaor can reduce he heang capacy of he hea pup, o ha he eaporaor need o be roed perodcally n order o anan adequae perforance under frong condon. he lo of capacy durng he ro operaon uually ade up ung auxlary hea, ofen n he for of elecrc heang. Aong he eeral rong ehod for he eaporaor of ar-ource un, he o effece ay be pang refrgeran n uperheaed apor ae hrough he eaporaor ube. here are wo ehod for h [2] : (a) Ho ga by-pa, ha ean he uperheaed refrgeran apor fro he copreor paed no he eaporaor, by-pang he condener and he expanon dece. (b) Reere cycle, he noral heang operaon reered by ung a 4-way ale o ha he oudoor col becoe he condener and he ndoor col he eaporaor. he reere cycle ehod generally ued n reerble ar-o-ar ye degned o prode boh heang and coolng. Recenly here are eeral ro conrol ehod [3] ued for ar-coolng hea pup and oher refrgeraon facle runnng n low eperaure enronen, ncludng: (a) Fxed e ro conrol ehod. h ehod would no be uable o arable enronen condon. (b) e-eperaure or e-preure conrol ehod. A fro layer grow, ye eaporang eperaure wll fall down correpondngly. So ha he eaporang eperaure or eaporang preure could be ued a a creron of ro. (c) Ar preure dfferenal conrol ehod. In fro condon fro wll bloc ar flow channel and ae ar preure dfferenal larger han 577

2 whch n noral condon. (d) Voce-operaed conrol, ge fro layer hcne by noe agaor reonan frequency nalled n eaporaor. (e) Aerage heang capacy conrol ehod [1], ned by: 1 0 ( ) d (1) Analy ndcae ha could reach axu when and confred uable. h concepon good a one dea bu here are any rouble o go no pracce. No aer whch rong projec wll be choen, a good conrol ehod hould be degned o deerne boh he arup and he e lengh of rong acon. o plan h, eeral facor u be uded: (a) Fro foraon and deelopen a a funcon of e, enronen condon and eaporang eperaure. (b) Eaporang and condenaon eperaure change caued by fro depong. (c) Coeffcen of hea ranfer beween oudoor ar and refrgeran. (d) uany change of ar flow hrough he eaporaor. 2 CALCULAION OF FROS HERMAL CONDUCIVIY AND FROS DEVELOPMEN he heral conducy of fro layer play an poran par n rucure and rae of foraon. he fro conan ar and cryal of ce, he ar-ce heral conducy of fro e hould be oewhere beween he heral conducy of ar and ce. he heral conducy of ar gen by [4] and ha of ce by [4] a 1/ [ ] (2) 12/ 1 (245/ ) / (3) Reearche denoe ha radaon effece conducy and enlaon enhalpy rae er can be negleced. he heral conducy due o he waer apor laen hea flux by [4] d L LDB P L 1 2 ( d / dx) (1 ) R R can be ned (4) e (5) Snce he waer apor dffuon occur only n he ar poron of he fro, Bgura and Wenzel [5] ugge ha f he effece ar heral conducy nead of he rue ar heral conducy ued, beer reul can be expeced. he effece ar heral conducy can be obaned fro he relaon 578

3 he propoed odel n [4] reference ade he followng aupon abou he fro rucure, a hown n fgure 1 and 2. A low fro deny or a hgh poroy, wo ype of fro rucure predonae. One he ce cylnder creaed by he dffuon of waer ono he ce, whch reul n a parallel conduce hea ranfer. he oher poron he ce phere creaed by nucleaon of waer apor or waer drople, reulng n a uch lower conduce hea ranfer. Specfc analy and calculaon pleae refer o reference [4]. Splfcaon expreon of : eff DP L P L ar a a P P R R _ 1 (6) 2 B Fgure 1. Fro rucure odel: rando xure of ce cylnder and ce phere a hgh poroe or low fro dene Fgure 2. Fro rucure odel: rando xure of ce plane and bubble a low poroe or hgh fro dene u l (7) 2 u b B b B c 1 (8) 1 a 1 a 1 2B 1 B (9) 2 a 2 a a eff (10) _ ar c l 1 B B eff _ ar (11) B p B 1 (12) a 1 2Ba 1 3 B 3 (13) a 579

4 p eff _ ar 1 B B eff _ ar (14) o decrbe araon of wh e, cure of fro growh hould be porrayed. In h paper an approxae way wa ued. Analogy coneconal a ranfer o hea ranfer, quany of fro depoon ganed. Baed on fro layer deny uually 0.5 g 3 c, fro conducy can be wor ou o be abou 1.2 W K, wh equaon (7)~(14). Ignore fro pecfc hea, eperaure nde fro layer ha a lnear drbuon. he oal calculaon procee are gen by 15.5V (15) ax 2 / 3 (16) D C p Le D,, (17) X (18) X w f H (19) D,, A calculaon reul gen a an exaple. Aue ha aben eperaure 5 and relae hudy 50%, nal eaporang eperaure 15, fgure 3 ge ou cure of fro hcne, urface eperaure, hea exchanged and wall eperaure. 3 HEA PUMP RUNNING SAES DURING FROSING CONDIIONS o porray araon of, an exaple wa gen n h paper. A ZR84KCE-F5/FD copreor choen. Fgure 4 ge ou perforance cure. Baed on calculaon reul aboe, refer o pecfc copreor perforance cure, we can porray araon cure of heang capacy wh e n fgure 5. Fgure 5 how he ey facor o degn rong conrol ehod, whch porrayed baed on daa fro Fgure 4. Proce fro pon O o A nae ep, heang capacy begn o coebac. Proce fro A o B eady ep. Sye run on no fro age. Fro B o D fro ep. In h age, heang capacy and COP deerorae anly for ar flow decreang. Fro D on, eaporaor col badly froed, no ar flow pa hrough he hea exchanger hen. Eaporaor ranfer hea wh free conecon ehod. 580

5 Fgure3. Fro layer and Hea ranfer araon by e Fgure 4. Perforance cure of copreor Fgure 5. Heang capacy araon wh e Ung h ehod, heang capacy change wh fro deelop can be wor ou under dfferen enronen condon and pecfc facle. Hea pup operang pon are clearly 581

6 paned by fro foraon proce, hen eaer o udy ro conrol ehod appled n pecfc urroundng. 4 DEFROS CONROL MEHOD USING AVERAGE HEAING CAPACIY Aerage heang capacy could be a creron o judge wheher rong ye hould be ared or no, when rong proce end, heang capacy need e o reue. If roer ac ery frequenly, heang capacy can no coe bac o a good leel, f ro acon ar oo lae, fro wll deelop unboundedly and heang capacy wll fall down badly due o fro heral reance and flux of ar. So hea pup aerage heang capacy could reach he be leel f he rong perod are approprae. ha ean heang capacy wll hae enough e o reue afer rong, whle frong-growh proce wll ll be conrolled. Purpoe of ung hea pup prodng hea, one of he o poran perforance ealuang ndcaor heang capacy bede COP (coeffcen of perforance). So ung aerage heang capacy a a creron of ro conrol hae nruconal gnfcance. o wor ou he conrol ehod, wo facor hould be confred. One, e lengh of heang durng one perod, he oher, rong e lengh. When hea pup run under frong condon, fro foraon can ncreae conducon heral reance, reduce flow paage of ar hrough eaporaor and enhance flow reng force. Fnally quany of ar flow recedng nduced. oal quany of exchanged hea fall down, ye run n bad wor condon. Runnng n pecfc urroundng, fro deelopen a funcon of e. X ( ) fro layer hcne, aung ha fro ha a unfor drbuon on he eaporaor urface. ( ) a funcon of e, ye worng ae and urroundng condon, bu all of hee can be expreed ung e. Fgure wll be ued o expre relaon of ( ) and. Durng, hea pup run n heang ae, fro depo on he eaporaor, hcne X (), oal a expreed a () be reoed. ha ean : ( ) L q So porrayed n fgure 5.. hen rong acon ar up. In e, fro hould (20) 1 d f ( ) (21) 0 Chooe a pon C, ang area of OEFO and FACF equal. When heang perod e 582

7 exceed G, aerage heang capacy n heang perod wll reduce. And rong e wll be longer oo, o oal heang capacy u be le. If a hor gen, rong acon ar up frequenly. When rong proce end, heang capacy need e o reue, heang capacy could no coe bac o a good leel. Durng rong wor ae, houe need auxlary hea, ofen n he for of elecrc reance heang. If ro acon ar oo lae, fro wll deelop unboundedly and heang capacy wll fall down badly due o fro heral reance and decreaed ar flux. So hea pup aerage heang capacy could be bad eher hor or long. An approprae can ae o be axu. Defne and correcly ean heang capacy wll hae enough e o reue afer rong, whle frong-growh proce wll ll be conrolled. 5 DISCUSSION In engneerng applcaon, when rong acon end, f dehydraon do no coplee, waer wll clng o eaporaor urface. hen hea pup go no heang operang ode. Eaporaor urface eperaure wll be low enough o ae he waer ce up. Afer eeral perod, hng go wore and wore. h dangerou for ye perforance. So ro ehod u conder how o conan eaporaor urface a no-ce ae. A long dehydraon perodc can be eboded n conrol projec o enure afer a few rong perod, he la rong cycle can dehudfy he eaporaor urface aboluely. 6 CONCLUSION Baed on he aboe analy, concluon can be drawn a below: (1) COP declne caued by fro deelop do no only due o conducon heral reance or rae of ar flow change, bu alo due o decreang ar flow nduced by ncreang flow reng force. Copared he wo facor, ar flow arable ee he an rgger. (2) Aerage heang capacy conrol ehod can be appled. A o pecfc wheher record n pecfc area, nuercal ulaon can ge ou rough auoaon facor. Applcaon of prograable er and prograable conroller can enure hea pup COP eep n a good leel. ACKNOWLEDGEMENS he projec fnancally uppored by he Naonal Key Projec of Fundaenal R&D of Chna wh gran nuber G

8 NOMERCLAURE hea pup heang capacy axu heang capacy e e lengh of heang durng one perod e lengh of rong durng one perod a heral conducy of ar heral conducy of ce e waer apor heral conducy heral conducy of fro effece heral conducy of he cobned hea conducy proporon of ar and ce D ordnary dffuon coeffcen, D P 256 B fro poroy, f a 1.81 [6] P aben preure P waer apor preure R L D waer apor ga conan fro eperaure laen hea of ce ublaon conecon hea ranfer coeffcen conecon a ranfer coeffcen a ar deny C p Le effece pecfc hea of forced ar flow n fro layer Lew Nuber a elocy of fro foraon 584

9 f fro deny, apor deny on fro urface, aben apor deny X hcne of fro layer aben ar eperaure fro urface eperaure w L q eaporaor urface eperaure fro el laen hea rong hea flux rong effcency REFERENCE 1. Chen Ru-Dong Sudy on ar hea pup ro conrol ehod. Flud Machnery, 1999b (2): 55~57 2. D. H. Neder Defrong of ar un n cenral ye, ASHRAE ran. 81, 581~ Huang Hu Analy on ro conrol ehod of ar-coolng hea pup. Buldng energy & enronen, 1999(3), 38~40 4. Mar A. Deenberger Generalzed correlaon of he waer fro heral conducy, In J Hea Ma ranfer Vol.26 No.4 pp.607~ G. Bgura and L. A. Wenzel Meaureen and correlaon of waer fro heral conducy and deny, I&EC Fundaenal 9, 129~ Sa SM, Dong Nuercal predcon of fro foraon on cooled hea exchanger. Inernaonal Councaon of Hea Ma ranfer, 1988, 15:81~