Split heat pipe heat recovery system

Transcription

1 Split hat pip hat rovry systm E. Azad Solar Enrgy Lab., Iranian Rsarh Organization for Sin & Thnology (IROST), 7 Forsat Av. Frdousi sq., Thran-Iran [email protected] Abstrat This papr dsribs a thortial analysis of a split hat pip hat rovry systm. Th analysis is basd on an Efftivnss-NTU approah to ddu its hat transfr haratristis. In this study th variation of ovrall fftivnss of hat rovry with th numbr of transfr units ar prsntd. Kywords fftivnss; fins; hat pip; hat rovry; NTU Nomnlatur Symbol Dsription A Total hat transfr ara (m 2 ) A f Finnd surfa ara (m 2 ) A o Total xtrnal surfa ara (m 2 ) Flow-stram apaity rat (m. p ) (W/K) Flow-stram apaity rat of old-sid fluid (m. p ) (W/K) Flow-stram apaity rat of hot-sid fluid (m. p ) (W/K) L Hat pip working fluid apaity (W/K) min Minimum of or max Maximum of or p Spifi hat at onstant prssur (J/kg K) sf onstant E o Ovrall xhangr fftivnss (dimnsionlss) g Alration du to gravity (m/s 2 ) g Proportionality fator in Nwton s sond law (kg m/ns 2 ) G Exhangr flow-stram mass vloity (kg/m 2 s) h Hat transfr offiint (W/m 2 K) J St Pr 2/3 k Thrmal ondutivity(w/mk) L Latnt hat (KJ/kg) m Fin fftivnss paramtr (dimnsionlss) ṁ mass flow rat, kg/s M Molular wight n Numbr of tubs in dirtion of flow (dimnsionlss) NTU Numbr of hat transfr units of an xhangr (dimnsionlss), AU min Intrnational Journal of Low arbon Thnologis 3/3

2 92 E. Azad N u Nusslt numbr (dimnsionlss) hx. k P Prssur (N/m 2 ) P r Prandtl numbr (dimnsionlss) R Gas onstant for vapour R R o /M R Thrmal rsistans (K/W) R o Univrsal gas onstant (834 J/mol K) R v Hat pip vapour thrmal rsistan (K/W) R Rynolds numbr (dimnsionlss) S l Tub spaing in dirtion of flow (m) St Stanton numbr (dimnsionlss) T Tmpratur (K) T s Wall tmpratur (K) t Thiknss (m) U Ovrall thrmal ondutan (W/m 2 K) h Effiiny (dimnsionlss) Efftivnss (dimnsionlss) s Surfa tnsion of liquid-vapour intrfa (N/m) Volum fration of solid phas m Visosity (N s/m 2 ) s Surfa tnsion (N/m) Subsripts ondnsr Evaporator f Fin i Insid l Liquid n For n row in dirtion of flow o Outsid p Pip wall s Solid w Wall w Saturatd wik. Introdution It is in th fild of hat rovry from xhaust gas that th largst bnfit from invstmnt in nrgy onsrvation quipmnt an b ralizd. Thr ar a onsidrabl numbr of uss to whih this wast hat from xhaust gas an b put, and ths dpnd to a larg xtnt on th tmpratur and ondition of th xhaust gass or air, th hat rovry quipmnt usd, and th onomi assssmnt of th ovrall systm prforman []. Intrnational Journal of Low arbon Thnologis 3/3

3 Split hat pip hat rovry systm 93 It is possibl to atgoriz four main appliation aras for wast hat rovry quipmnt. (i) Gas-to-gas (ii) Gas-to-liquid (iii) Liquid-to-gas (iv) Liquid-to-liquid Th appliation of hat pip for hat rovry is not nw. D.A.Ray [] has rportd a rviw of gas-to-gas hat rovry systms. Azad and Goola [2] invstigatd th variation of ovrall fftivnss with / for diffrnt valus of tub spaing normal to th dirtion of flow; numbr of fins pr mtr; fin thiknss and vaporator lngths. Prtz and Horbaniu [3] applid th fftivnss-ntu mthod to study th influn of gomtry on hat pip hat rovry (HPHR) opration. Shao t al. [4] hav prsntd a study of prssur loss and hat rovry ffiiny of hat pip units for natural vntilation, using both xprimntal and omputational approahs. It is found in thir rsarh that hat rovry ffiiny drasd with inrasing air vloity. Th fftivnss-ntu modl for gravityassistd air-to-air HPHR was applid by Azad and Goola [2]. As in th log man tmpratur diffrn mthod, this rquirs th total thrmal rsistan of th HPHR. For intrnal rsistans, th Rohsnow orrlation insid th vaporator was usd. For th intrior ondnsr rsistan, Azad and Goola [2] dvlopd a nw orrlation for ondnsing watr vapour on vrtial arbon-stl. Azad t al. [5] xtndd th mthodology of Azad and Goola [2] to modl watr-to-air hat pip hat xhangrs. Azad t al. [6] and Azad and Gibbs [7] furthr xtndd this mthod to modl o-axial air-to-watr hat pip hat xhangr and watr-to-air hat xhangr. Rntly, Lamfon t.al [8] applid two-phas thrmosyphon for xtrating wast hat from th gas turbin himny and dlivring this nrgy to th gnrator of an aqua-ammonia absorption hillr. Riffat t al. [9] hav applid hat pip for thrmoltri rfrigration. Furthrmor, substitut of th onvntional hat sink systm with an napsulatd phas hang matrial was found to improv th prforman of th thrmoltri rfrigration systm. So ylmz [0] studid thrmoonomi fasibility analysis yilding a simpl algbrai optimization formula for stimating th optimum lngth of a finnd pip usd for wast hat rovry. Li t al. [] prsntd th prforman analysis of a multifuntion hat pip typ adsorption i makr with ativatd arbon al2 as ompound adsorbnt and ammonia as rfrigrant. Filippshi [2] proposd partiular passiv wiklss two-phas loop dvis that ar abl to oprat with or against gravity. Thrfor, du to thir onsidrabl advantags, sin th 970s, hat pip hat xhangrs hav bn xtnsivly applid in many industris suh as nrgy nginring, hmial nginring and mtallurgial nginring as wast hat rovry systms. Ong and Haidr-E-Alalhi [3] dsribd work on th hystrsis fft during start-up and ooling down of thrmosyphon lmnts in an HPHE, using R-22, R-34a and watr as working fluids. Intrnational Journal of Low arbon Thnologis 3/3

4 94 E. Azad Th hat transfr in th systm is basd on th ontinuous yl of th vaporization and ondnsation pross. Th hat pip an transfr a larg quantity of hat with a rlativly small tmpratur drop by th vaporation of a part of th fluid. Th vapour flows to th ondnsr, whr th fluid ondnss whil giving off its latnt hat, ausd by ooling from th outsid. Aftr ondnsation at th ondnsr stion th working fluid rturns to th hatd stion along th wall by gravitation or apillarity, whih loss th yl. Th hat pip an b usd to promot hat transfr btwn two gas strams. Th hat pip advantags ar high hat rovry fftivnss, ompatnss, no moving parts, light wight, rlativ onomy, no xtrnal powr rquirmnts, prssur tightnss, no ross-ontamination btwn strams and rliability. In sltion of th working fluid, th latnt hat of th fluid is an important paramtr. Th highr th latnt hat of a fluid, th highr th transfr of hat is at a lowr prssur. Th hat pip is rprsntd shmatially in Fig.. 2. Split Hat Pip Hat Rovry (SHPHR) systm dsription Th basi onfiguration of th Split Hat Pip Hat Rovry (SHPHR) is shown in Fig.2. Th SHPHR and onvntional hat pip hav th sam prinipal funtion of transfrring hat nrgy from on loation to anothr loation whr hat an b rjtd. It ssntially ompriss four omponnts; vaporator, vapour transport lin, liquid rturn lin, and ondnsr. In th SHPHR th vaporator is onntd to th ondnsr through a piping systm. Th ondnsr stion is loatd abov th vaporator so that th ondnsat is rturnd by gravity. Th vapour flows through a tub (vapour lin) to th ondnsr and ondnsat rturns to th vaporator through a tub (liquid lin) by gravity. Th vapour lin is onnting th top of th vaporator to th top of th ondnsr, and th liquid lin is onnting th lowr part of th ondnsr to th lowr part of th vaporator. Th vaporator stion of th hat pip onsists of on bank of xtrnally finnd hat pips, th vaporator stion is lind with apillary wik strutur Figur. Hat pip. Intrnational Journal of Low arbon Thnologis 3/3

5 Split hat pip hat rovry systm 95 ondnsr Fins Liquid lin Vapor lin Hat pip Evaporator Figur 2. Split hat pip hat rovry systm. to prott th liquid against th shar strss and to kp th surfa of th vaporator wt. Th SHPHR posssss all th main advantags of a onvntional HPHR. Th only diffrn btwn th SHPHR and onvntional hat pip hat rovry lis mainly in th sparation of th vaporator and ondnsr. In a SHPHR th vaporator and ondnsr an b loatd in diffrnt loations. 3. Thortial analysis In th following analysis th watr hat pips ar onsidrd to b in a staggrd arrangmnt with ontinuous aluminium finnd irular tubing and only th Intrnational Journal of Low arbon Thnologis 3/3

6 96 E. Azad innr wall of th vaporator stion of th hat pip is lind with th apillary strutur. Assuming th axial hat ondution through th hat pip walls is ngligibl, th following quations for stady-stat opration of th hat rovry an b writtn. 3. Hat pip vaporator xtrnal surfa rsistan R o Th alulation of thrmal rsistans is basd on analysis prsntd by Riffat [4]. Th thrmal rsistan R o xists btwn th air and hat pip vaporator xtrnal surfa. Th total thrmal rsistan R o is givn by: R o η h o o () 3.2 Hat pip vaporator tub wall rsistan R p Th thrmal rsistan aross th tub wall thiknss in th vaporator is as follows: R p tpao (2) k A p i 3.3 Hat pip vaporator wik rsistan R w Th thrmal ondutivity of saturatd wik in k w is givn by Dunn and Ray [5] as follows: k w whr β β kl β+ ( + km k ) l ( km k ) Th rsistan is thn: l (3) R w twao (4) k A w i 3.4 Hat pip vaporator thrmal rsistan R i Th thrmal rsistan du to vaporation an b givn by: Ao Ri (5) hi Ai whr rsistans ar basd on outsid total ara of vaporator stions. Intrnational Journal of Low arbon Thnologis 3/3

7 Split hat pip hat rovry systm Hat pip vapour thrmal rsistan R v Thr is a prssur drop to maintain th vapour flow from th hat pip vaporator to its ondnsr. A tmpratur diffrn thrfor xists and givs ris to a thrmal rsistan: RTv Pv Rv (5) QLP l l v R v an b normally ngltd but may b important in th starting up of gas-loadd hat pips [5] Thrmal rsistan du to ondnsation within th hat pip R i Th thrmal rsistan at th ondnsr is similar to th vaporator: R i Ao (6) h A i i 3.7 Hat pip ondnsr wall rsistan R p tp Ao Rp (7) k A p i 3.8 Hat pip ondnsr xtrnal surfa rsistan R o Ro ηoho whr rsistan is basd on outsid total ara of ondnsr stions. Th ovrall hat transfr offiints of th vaporator and ondnsr ar: U tpao twao Ao η h k A k A h A o o p i w i i i (8) (9) tpao Ao U + + (0) ηoho kpai hi Ai h o and h o in Equations and 8 ar stimatd from orrlations dvlopd by Rih [6] for ontinuous fin with irular tubing. Th orrlations ar: GSl RL µ Whr J St.Pr 2/3 035 J 0. 95R L. (2) Th total surfa tmpratur fftivnss h o stimatd as follows (for vaporator stions): Af ηo ( ηf ) (3) Ao h f an b alulatd from quation 4: () Intrnational Journal of Low arbon Thnologis 3/3

8 98 E. Azad Evaporator stion η f tanh( ml f ) ml f (4) m ho (5) kt 2 2 f f Th sam quations an b writtn for th ondnsr stions (i.. h o ). Th hat transfr offiint h i an b dtrmind from rlation givn by Rohsnow [7] as follows: pl ( Tw Ts ) h i( T w T s ) gσ sf L ll g ( l v) µ ρ ρ Pr. (6) In quation 6, all proprtis ar valuatd at th saturation tmpratur T s, and th valu of offiint sf for a varity of fluid-surfa ombinations an b obtaind from Rohsnow [7] and Tong [8]. Th valu of intrnal hat transfr offiint in th ondnsr stion, h i, for watr vapour is alulatd from hapman [9]. For R v 50,000 Nu i 3 3 v 503. ( R ) Pr (7) For R v 50,000 Nu i ( R ) Pr v 08 3 (8) 4. Hat transfr fftivnss In th vaporator stions of a singl row hat pip hat rovry, th hot fluid is in rossflow with th vapour flow insid th hat pips. Howvr, sin th vapour insid a hat pip is almost at onstant tmpratur, its spifi hat, p, and apaity rat, L, bom by dfinition, qual to infinity and as a rsult / L / L 0. Thrfor, th fftivnss-ntu quations for singl row hat pip hat rovry ar as follows [20]. For vaporator stions: xp ( NTU ) (9) For ondnsr stions: xp ( ) (20) NTU Now for a hat pip hat rovry with n rows of hat pips in th dirtion of flow, th fftivnss-ntu quations ar as follows [2]. Intrnational Journal of Low arbon Thnologis 3/3

9 Split hat pip hat rovry systm 99 Intrnational Journal of Low arbon Thnologis 3/3 For vaporator stions: Ε n L n L n L (2) For ondnsr stions: Ε n L n L n L (22) For / L 0 and / L 0 quations 2 and 22 rdu to: Ε n n ( ) (23) And Ε n n ( ) (24) Th ovrall fftivnss of th hat pip hat rovry, E o, may b obtaind from Azad and Goola [2] as follows: For > Ε Ε Ε o n n + (25) For > Ε Ε Ε o n n + (26)

10 200 E. Azad 5. Rsults and disussion Th thortial analysis prsntd in this study an b usd to prdit th SHPHR prforman. In gnral, hat xhang data may b prsntd by hat xhangr fftivnss whih dpnds upon flow onfiguration, apaity rat (ṁ p ) of th two strams fluids and th NTU. Th following variations of paramtrs wr hosn: numbr of rows in dirtion of flow to 4; / from to 3; and numbr of hat transfr units from to 5. Fig. 3 shows th ovrall hat transfr fftivnss as a funtion of numbr of transfr units and numbr of rows; /. From this figur it an b obsrvd that an inras in valu of numbr of rows from to 4, at /, th ovrall fftivnss inrass from 32% to 48.25% (inrasd by 50.8%) for NTU. For n th ovrall fftivnss varis sharply up to NTU 2 and thn inrass gradually. This ris is lss in n 2 3 and in n 4 almost rmain onstant. Howvr, it is important to not that th influn of n > 4 at / on ovrall fftivnss is not obvious from Fig. 3. Th ovrall fftivnss as a funtion of NTU for / 2 is shown in Fig. 4. Th fftivnss for n > 2 and NTU 3 rahs to 67% and in Fig. 5 at / 3 for n > 2 and NTU 2 maximum fftivnss is 74%. 6. onlusions This papr dsribs how hat pips an b usd in a split hat pip hat rovry systm. -Th thortial modl basd on -NTU (fftivnss-numbr of Transfr Units) mthod has bn dvlopd to prdit th prforman of th split hat pip hat rovry systm. Figur 3. Ovrall fftivnss as a funtion of NTU for /. Intrnational Journal of Low arbon Thnologis 3/3

11 Split hat pip hat rovry systm 20 Figur 4. Ovrall fftivnss as a funtion of NTU for / 2. Figur 5. Ovrall fftivnss as a funtion of NTU for / Inrasing th ratio of / and numbr of rows of hat pips within th normal dsign rangs an improv th prforman of th split hat pip hat rovry systm. Rfrns [] D. A. Ray, A rviw of gas-gas hat rovry systms, Hat Rovry Systms, (980), 3 4. [2] E. Azad and F. Goola, A dsign produr for gravity-assistd hat pip hat xhangr, Hat Rovry Systms, 4 (984), 0. [3] R. Prtz and B. Horbaniu Optimal hat pip hat xhangr dsign, Hat Rovry Systms, 4 (984), Intrnational Journal of Low arbon Thnologis 3/3

12 202 E. Azad [4] L. Shao, S. B. Riffat and G. Gan, Hat rovry with low prssur loss for natural vntilation, Enrgy and Buildings, 28(2) (998), [5] E. Azad, F. Bahar and F. Moztarzadh, Dsign of watr-to-air gravity-assistd hat pip hat xhangr Journal of Hat Rovry Systms, 5 (985), [6] E. Azad and F. Moztarzadh, Dsign of air-to-watr o-axial hat pip hat xhangr Journal of Hat Rovry Systms, 5 (985), [7] E. Azad, B. M. Gibbs, Analysis of air-to-watr hat pip hat xhangr, Journal of Hat Rovry Systms and HP, 7 (987), [8] N. J. Lamfon, Y. S. H. Najjar and M. Akyurt, Modlling and simulation of ombind gas turbin ngin and hat pip systm for wast hat rovry and utilization, Enrgy onvrsion and Managmnt, 39 (998), [9] S. B. Riffat, S. A. Omr and X. L. Ma, A novl thrmoltri rfrigration systm mploying hat pips and a phas hang matrial: an xprimntal invstigation, Rnwabl Enrgy, 23 (200), [0] M. S. So ylmz, Optimum lngth of finnd pip for wast hat rovry, Enrgy onvrsion and Managmnt, 49 (2008), [] T. X. Li, R. Z. Wang, L. W. Wang, Z. S. Lu and. J. hn, Prforman study of a high ffiint multifuntion hat pip typ adsorption i making systm with novl mass and hat rovry prosss, Intrnational Journal of Thrmal Sins. [2] S. Filippshi, On priodi two-phas thrmosyphon oprating against gravity Intrnational Journal of Thrmal Sins, 45 (2006), [3] K. S. Ong, Md. Haidr-E-Alalhi, Exprimntal invstigation on th hystrsis fft in vrtial two-phas losd thrmosyphons Applid Thrmal Enginring, 9 (999), [4] S. B. Riffat and J. Zhu Mathmatial modl of indirt vaporativ oolr using porous rami and hat pip Applid Thrmal Enginring, 24 (2004), [5] P. D. Dunn and D. A. Ray, Hat pips, third d., Prgamon Prss, Oxford, 982. [6] D. G. Rih, Th ffts of fin spaing on th hat transfr and frition prforman of multi-row smooth plat fin-and-tub hat xhangr ASHRAE Trans.79 (973), [7] M. W. Rohsnow and H. Y. hoi, Hat, mass and momntum transfr John Wily, Nw York (965). [8] L. S. Tong, Boiling hat transfr and two phas flow Nw York (965). [9] A. J. hapman, Hat transfr 3 rd dition, Mamillan, Nw York (974). [20] M. W. Kays, A. L. London ompat hat xhangr M Graw-Hill Book o. (964). [2] Y. L, A. Bdrossian Th haratristis of hat xhangrs using hat pips or thrmosyphons Int. J. Hat and Mass transfr, 2 (978), Intrnational Journal of Low arbon Thnologis 3/3