Fins Geometry

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COMPACT HEAT AND MASS EXCHANGERS OF THEPLATE FIN TYPE IN THERMAL SORPTION SYSTEMS

Application in an absorption heat pumpwith the working pair CH30H-LiBr/ZnBr2

H. BeckerTR diss1698


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H. Becker


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CI P - D A T A K O N I N K L I J K E BI BL I O T H E E K , D E N H A A GBecker, HarryCompact heat and mass exchangers of the plate fin type in thermal sorption systems: appiication in anabsorption heat pump with the working pair CH30H-LiBr/ZnBr2Harry Becker. - Delft: Delft University of Technology, Mechanical Engineering DepartmentThesis Delft. - With ref. - With summary in DutchISBN 90-370-0022-3SISO 653.3 UDC 621.577(043.3)Subject heading: absorption heat pumps.

Copyright 1989, Faculty of Mechanical Engineering and Marine EngineeringDelft University of Technology

All rights reserved.This report, or parts thereof, may not be reproduced in any form without permission of the publisher.Any use or appiication of data, methods and/or results etc, occuring in this report will be at user 's own risk.The Delft University of Technology, Faculty of Mechanical Engineering and Marine Engineering accepts noliability for damages suffered from the use or appiication.


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PROEFSCHRIFT

Ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,op gezag van de Rector Magnificus, Prof.drs. P.A. Schenckin het openbaar te verdedigen ten overstaan vaneen commissie aangewezen door het College van Dekanenop dinsdag 7 februari 1989 om 14.00 uur.

door

HARRY BECKERgeboren te AmersfoortWerktuigkundig ingenieur

TR diss1698


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Dit proefschrift is goedgekeurd do or de pro mo torprof.ir. A.L. Stolk


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CONTENTSSUMMARYSAMENVATTING

CHAPTER 1. THE ABSORPTION HEAT PUMP1.1 I nt r oducti on1. 2 Wor ki ng pr i nci pl e1. 3 The l og P - l / T di agr am1. 4 The heat r at i o, t he cof f i ci nt of per f or mance andthe ci r cul at i on rat i o1. 5 Resear ch wor k on absor pt i on heat pumps ( AHP)1. 5. 1 Worki ng pai r s f or an AHP1. 5. 2 Syst em conf i gur at i ons1. 5. 3 Heat and mass t r ansf er1. 6 Thi s r esear ch wor k1. 6. 1 Goal s1. 6. 2 Tool s1. 6. 3 Set up

CHAPTER 2. COMPACT HEAT AND MASS EXCHANGERS

page

113466810

111212

2. 4

1 Def i ni t i on2 Compact and enhanced t r ansf er sur f ace3 Appl i cat i on2. 3. 1 Fi el d of appl i cat i on2. 3. 2 Cor r ugat i ons2. 3. 3 Const r uct i on mat er i al sChar acter i zat i on2. 4. 1 I dent i f i cat i on2. 4. 2 Anal yt i cal sol ut i ons2. 4. 3 Exper i ment al r esul t s2. 4. 3. 1 I nt r oduct i on2. 4. 3. 2 The wor k of Kays and London

2. 4. 3. 3 Rect angul ar of f sets t r i p f i n sur f acesDi scussi onAppl i cat i on i n sor pt i on systemsCHAPTER 3. THE ABSORPTION HEAT PUMP (AHP) TEST PLANT

3. 1 I nt r oducti on3. 2 Choi ce of t he worki ng pai r3. 3 Type of compact heat and/ or mass t r ansf er sur f ace3. 4 Over vi ew of t he t est pl ant3. 5 The component s3. 5. 1 The absor ber3. 5. 2 The condenser and evapor at or3. 5. 3 The m xt ur e- m xt ur e heat exchanger3. 5. 4 The generat or3. 5. 5 Di st r i but i on of l i qui d f l ows3. 5. 6 The m xt ure pump

131315151616171818182122

25262627293031313132


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3.6 The heat i ng and cool i ng ci r cui t s3. 6. 1 The cool i ng system of t he absorber andcondenser6. 2 The heat i ng syst em of t he evapor at or6. 3 The heat i ng syst em of t he generat or6. 4 General3.7

333Cont r ol33337. 1 Mass f l ows7. 2 Temper at ur es7. 3 Wei ght f r act i on7. 4 Pr essur es3. 8 Measur ement3.8.1 Mass f l ows3.8.23.8.33.8.43.8 .53.8.6

Mi xture densi t yTemper at ur esPressuresAccur acy of measur ement sDat a r egi s t r at i on3. 9 Const r uct i on mat er i al s and cor r osi onCHAPTER 4. THE WORKING PAIR : SELECTION AND DATA

4.1 I nt r oduct i on4. 2 Exper i ment s4. 3 Si mul at i on4. 4 Li t er at ur e sur vey on wor ki ng pai r s4. 5 Sel ect i on of a new wor ki ng pai r4. 6 Consequences f or t he t est pl ant4. 7 Consequences f or t he si mul at i on pr ogr am4. 8 Compar i son R123a - DTG wi t h CH30H - L i Br / ZnBr 24. 9 Concl usi ons

323232323333333434343434353636

393939394041414143

CHAPTER 5. COMPUTER SIMULATION MODEL AND PROGRAM5. 1 I nt r oduct i on 455. 2 St ar t i ng poi nt 455. 3 Devel opment of a new s i mul at i on model and pr ogr am 465. 4 Fi nal s i mul at i on pr ogr am5. 4. 1 St r uct ur e 475. 4. 2 Heat and/ or mass t r ansf er cor r el at i ons 485. 4. 3 Comput er subr out i nes 505. 5 A si mpl i f i ed s i mul at i on pr ogr am 515. 6 Appl i cat i on of the si mul at i on pr ogr am Some exampl es 525. 7 Concl usi ons 55

CHAPTER 6. RESULTS OF THE EXPERIMENTS6. 1 I nt r oduct i on 576. 2 Experi ment al r esul t s of t he AHP and i t s component s

6. 2. 1 The absor pt i on heat pump 60


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6. 2. 2 The component s6. 2. 2. 1 The6. 2. 2. 2 The6. 2. 2. 3 The6. 2. 2. 4 The6. 2. 2. 5 The

gener at orcondenserevapor at orabsor berm xt. - m xt . heat exchanger

60616365706.3 Concl usi ons f r om the exper i ment s 71

CHAPTER 7. INTERPRETATION OF AND DISCUSSIONON THE EXPERIMENTAL RESULTS7.1 I nt r oducti on 737. 2 A qual i t at i ve i nt erpretat i on 737.3 A quant i t at i ve i nt erpr et at i on7.3. 1 I nt r oducti on 747. 3. 2 The m xt ur e- m xt ur e heat exchanger 757. 3. 3 The absor ber 777. 3. 4 Di scussi on and compar i son 797. 4 Detai l ed absor ber si mul at i on model7.4. 1 I nt roduct i on 817. 4. 2 Former r esear cher s 827. 4. 3 Resul t s f r om t hi s r esear ch wor k 837. 4. 4 Di scussi on on cor r ect i on f actors 847.5 Compar i son SAT absor ber / generat or and the CHME absor ber7.5. 1 I nt roduct i on 857. 5. 2 Compact ness ( ar ea densi t y) 85

7. 5. 3 Heat t r ansf er ( heat f l ow densi t y) 867.5.4 Di scussi on and concl usi on 887.6 Concl usi ons 88

CHAPTER 8. GENERAL CONCLUSIONS8.1 Ret r ospect i ve vi ew8.2 Concl usi ons8.3 Consi der at i ons8. 4 Recommendat i ons

91919293

APPENDIX A - HEAT AND MASS TRANSFER CORRELATIONSOF THE SIMULATION PROGRAMAPPENDIX B - CALCULATION SCHEMES OF THE AHP COMPONENTSREFERENCESNOMEMCLATURE

9799103109

CURRI CULUM VITAE 113


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SUMMARY

Thi s di sser t at i on cover s a t heor et i cal and exper i ment al st udy i nt o t heposs i bl e appl i cat i on of compact heat and mass exchanger s ( CHME) i n a gasf i r ed absor pt i on heat pump ( AHP) f or domest i c heat i ng.The f r amewor k of t he study i s def i ned by di scussi ng t he gener al pr i nci pl esand t he r esear ch f i el ds of t he AHP. I n addi t i on t he goal s, t he means and theset up of t hi s r esear ch work ar e expl ai ned. ( Chapt er 1. )The above- ment i oned heat and mass exchanger s ar e of t he pl at e t ype. Thespace bet ween the par al l el and pl ai n pl at es i s f i l l ed up wi t h cor r ugat edpl at es of a cer t ai n hei ght , t he cor r ugat i on or f i nned pl at e.The pl ai n and f i nned pl at es ar e st acked and wel ded t oget her . Thi s gi ves aheat and mass exchanger whi ch i s ver y compact expr essed by a hi gh ar eadens i t y ( m2/ m3) . Thi s l eads t o heat and mass t r ansf er pr ocesses wi t h smal lt emperat ur e and concent r at i on di f f er ences. ( Chapt er 2. )For t est i ng pur poses a pi l ot pl ant was bui l t usi ng t he above t ype ofcomponent s i n order t o t est t hei r heat and/ or mass t r ansf er per f or mance.Onl y t he gener at or i s of t he shel l and t ube ( SAT) t ype.As t he wor ki ng pai r CH30H - L i Br / ZnBr 2 was chosen, wi t h t he al cohol as t hesol vent and t he sal t m xt ur e as t he absor bent . Thi s l eads t o sub- at mospher i cworki ng pressur es wi t h onl y sol vent i n t he vapour phase. ( Chapt er 3. )A l i t er at ur e sur vey has been conduct ed on worki ng pai r s f or s or pt i on syst emsi n order t o updat e the knowl edge i n t hi s f i el d and t o sel ect a new worki ngpai r f or t he pi l ot p l ant . Thi s of f er s t he poss i bi l i t y t o ver i f y and val i dat et he si mul at i on and the exper i ment al r esul t s.As poss i bl e new wor ki ng pai r R123a - DTG was sel ect ed and t est ed i n t hesi mul at i on pr ogr am For pr act i cal r easons exper i ment s i n t he pi l ot pl ant areout si de t he scope of t hi s r esear ch wor k. ( Chapt er 4. )At t he same t i me a comput er pr ogr am has been devel oped t o si mul at e t he t estpl ant , based on heat and mass t r ansf er cor r el at i ons f ound i n t he l i t er at ur e,l at er t o be r epl aced by cor r el at i ons based on exper i ment al r esul t s.The pr ogr am consi st s of t hr ee par t s. The mai n pr ogr am cover s t he over al lcal cul at i on and i t er at i on pr ocedur es. The component pr ogr am cont ai ns t hesubr out i nes of t he separ at e component s, whi l e t he pr oper t y pr ogr am cont ai nst he t hermodynam c and physi cal pr opert y dat a of sever al wor ki ng pai r s andcool i ng/ heat i ng medi a. ( Chapt er 5. )Thr ee seri es of experi ment s have been car r i ed out , dur i ng whi ch t he i nputpar amet er s wer e var i ed over a cer t ai n r ange.The heat i ng t emper at ur e of t he evapor at or was bet ween 5 and 10C, of t hegenerat or most l y 125C due t o t he i nst abi l i t y of t he met hanol i n t he m xt ur eabove t hat t emper at ur e) .The cool i ng t emper at ur e of t he absor ber and condenser was var i ed bet ween30 - 60" C over a m xt ur e mass f l ow r ange of 25 - 125 g/ s.( Chapt er 6. )


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Di f f er ent appr oaches have been adopt ed t o i nt er pr et and expl ai n t heexper i ment al r esul t s, t he emphasi s bei ng on t he absor ber as t he mosti mpor t ant and i nt erest i ng component ( si mul t aneous heat and mass t r ansf er i na f i l m f l ow) .The r esul t s have been used t o f i nd mat chi ng ( heat ) t r ansf er cor r el at i ons andt o ver i f y the f i l m ( heat t r ansf er ) and penet r at i on ( mass t r ansf er ) t heor y asadopt ed i n t he si mul at i on pr ogr am t o mat ch t hem by means of corr ect i onf ac to r s . Al so t he SAT gener at or / absor ber and t he CHME absor ber have beencompared f or t hei r compactness ( ar ea densi t y) and t hei r heat t r ansf er ( heatf l o w dens i t y) . ( Chapt er 7. )Concl usi ons have been dr awn concerni ng t he possi bl e appl i cat i on of t hef i nned pl at e compact heat and/ or mass exchanger s i n t her mal sor pt i onsys t ems, whi l e r ecommendat i ons have been gi ven f or f urt her r esear ch wor k.( Chapt er 8. )


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SAMENVATTING

Di t pr oef schr i f t doet ver sl ag van het t heor et i sche en experi ment el eonder zoek naar de mogel i j ke t oepass i ng van zgn. " compact heat and massexchangers ( CHME) " i n een gas- gest ookt e absor pt i e- war m epomp ( AWP) voori ndi vi duel e woni ngver warm ng.Het kader wor dt aangegeven m ddel s een beschr i j vi ng van de wer ki ngspri nci pes en van het hui di ge onder zoeksvel d bet r ef f ende de AWP. Ook wor dende doel st el l i ng, de m ddel en en de aanpak van het onder zoek ui t eengezet .( Hoof dst uk 1. )De bovengenoemde war mt e- en st of wi ssel aar s z i j n van het pl aat - t ype. Der ui m e t ussen de par al l el l e en vl akke pl at en i s " gevul d" met een ver vor mdepl aat , de zgn. " cor r ugat i on" of gev i nde pl aat .De vl akke en gevi nde pl at en wor den gest apel d en tot een pakket gesol deer d.Di t geef t een war mt e- en st of wi ssel aar di e zeer compact i s en heef t een hogeopper v l aktedi cht hei d ( m2 / m3) . Di t geef t over dr acht spr ocessen van war mt e enst of met k l ei ne t emper at uur - en concent r at i ever schi l l en. ( Hoof dst uk 2. )Voor bepr oevi ng hi ervan i s een t est opst el l i ng gebouwd met di t t ypecomponent en om de warm e- en st of overdr acht t e onder zoeken. Al l een degenerat or i s van het " shel l and t ube (SAT)" type.Al s s t of paar i s CH30H - Li Br / ZnBr 2 gekozen met de al cohol al s hetopl osm ddel ( sol vent ) en het zout mengsel al s de absor bent . Di t geef t sub-at mosf er i sche wer kdr ukken met al l een sol vent i n de dampf ase. ( Hoof dst uk 3. )Er i s een l i t er at uur st udi e ver r i cht naar s t of par en voor sorpt i esyst emen. Di tom de kenni s op di t gebi ed bi j t e houden en om een ni euw st of paar voor det es t ops t el l i ng t e sel ec t er en. Di t l aat s te bi edt de mogel i j khei d de s i mul at i een de exper i ment el e resul t at en t e ver i f i r en. Al s mogel i j k ni euw st of paar i sR123a - DTG gesel ect eerd en beproef d i n het si mul at i epr ogr amma. Exper i ment eni n de t est opst el l i ng val l en hel aas om pr act i sche redenen bui t en di tonder zoek. ( Hoof dst uk 4. )Tegel i j ker t i j d i s een comput erprogr amma ont wi kkel d dat de t est opst el l i ngsi mul eer t , werkend met war mt e- en st of over dr acht sr el at i es bekend ui t del i t erat uur . Lat er zul l en deze ver vangen worden door r el at i es komende ui t deexper i ment en.Het progr amma best aat ui t dr i e del en. Het hoof d- pr ogr amma z or gt voor de" overal l " berekeni ngen en i t er at i epr ocedures. Het component en- pr ogr ammabevat de subr out i nes van de onder schei den component en, t er wi j l het st of -pr ogr amma de t her modynam sche en f ysi sche st of gegevens van meer der est of paren en ver war m ngs/ koel medi a bevat . ( Hoof dst uk 5. )Er hebben dr i e ser i es van exper i ment en pl aat sgevonden waar bi j een aant ali nvoer par amet er over hun wer kgebi ed gevari eer d z i j n.De ver war m ngst emper at uur van de verdamper l ag t ussen de 5 en 10C, di e vande gener at or was meest al 125 C i . v . m de i ns t abi l i t ei t van de al cohol i n hetmengsel boven deze t emper at uur . De koel wat er t emper at uur ( abs. / cond. ) wer dgevar i eer d t ussen de 30 en 60 C, de mengsel massast r oom t ussen de 25 en 125g/ s . ( Hoof dst uk 6. )


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Ver schi l l ende benaderi ngen hebben pl aat sgevonden om de exper i ment el er esul t at en t e i nt er pr et er en. De nadr uk l ag daar bi j op de absorber al s demeest bel angr i j ke en i nt eressant e component ( si mul t ane war m e- en st of over -dracht i n een f i l mst r om ng) .De r esul t at en zi j n gebr ui kt om cor r el at i es voor de war m eover dr acht t evi nden, om de j ui st hei d van het t oepassen van de f i l m heor i e voor dewar m eover dr acht en van de penet r at i et heor i e voor de st of over dr acht t eonder zoeken en om cor r ect i ef act or en voor deze t heor i en t e vi nden.Ook i s een vergel i j ki ng gemaakt t ussen de SAT gener at or / absor ber en de CHMEabsor ber . Ver gel eken zi j n zowel de compact hei d ( opper vl akt edi cht hei d m2/ m3)al s de war m eover dr acht ( war m est r oomdi cht hei d W/ m 2 ) . ( Hoof dst uk 7. )Concl usi es zi j n get r okken voor de mogel i j ke t oepassi ng van de gevi nde pl aatCHME' s voor t herm sche sor pt i esyst emen. Ook zi j n een aant al aanbevel i ngen enr i cht i ngen voor verder onderzoek aangegeven. ( Hoof dst uk 8. )


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CHAPTER 1. THE ABSORPTI ON HEAT PUMP

1. 1 I nt r oduct i onThi s chapt er i s meant as an i nt r oduct i on t o t he absor pt i on heat pump ( AHP)i n gener al and f ocuses on i t s most i mpor t ant aspect s. Thi s subj ect wi l l bedeal t wi t h mor e spec i f i cal l y i n t he sect i ons f ol l owi ng thi s i nt r oduct i on.I n Sect i on 1. 2 an expl anati on i s gi ven of t he wor ki ng pr i nci pl e of t heAHP, i nt r oduci ng t he di f f er ent heat and mass f l ows, t he def i ni t i on of t heconcent r at i on of a m xture and the need f or r ect i f i cat i on.I n Sect i on 1. 3 t he l og P - l / T di agr am wi t h t he concent r at i on as apar amet er, of an absorpt i on wor ki ng pai r ( m xture / pur e subst ances) i sexpl ai ned f r om a t heor et i cal basi s . The di f f er ent pr ocesses i n t he AHP ar epr esent ed i n t he corr espondi ng di agr amSect i on 1. 4 def i nes some r at i os concer ni ng t he per f or mance of t he AHP.These ar e t he heat r at i o, t he cof f i c i nt of per f or mance and the c i r cul at i onr at i o.Sect i on 1. 5 deal s wi t h the r esear ch f i el d on AHP' s. Apar t f r om adi s t i nct i on i nt o t heor et i cal and exper i ment al r esear ch wor k , a di s t i nct i onhas been made i nt o t hr ee di f f er ent f i el ds of r esear ch. Those f i el ds ar er esearch r egardi ng new worki ng pai r s, new syst em conf i gur at i ons and heat andmass t r ansf er i mpr ovement i n component s.The l ast sect i on, Sect i on 1. 6, descr i bes i n t he f r amewor k of t he ot hersect i ons ment i oned above t he r esear ch work r epor t ed i n t hi s di sser t at i on. I tf ocuses on t he goal s, t he means and t he set up of t hi s AHP pr oj ect .1. 2 Wor ki ng pr i nci pl eAl most al l handbooks on absor pt i on and on heat pumps cont ai n a det ai l eddescr i pt i on of t he basi c pr i nc i pl e of t he AHP, i t s component s, t he chosenwor ki nq pai r s and t he pr ocesses of f l ow, heat and mass t r ansf er ( Ki r n [ KI ] ,Ber gmahs [ B I ] , St ol k [ S I ] ) .The f i gur e on t he next page shows a si mpl e scheme of an AHP, wi t h i t scomponent s, t he cor r espondi ng heat f l ows and t emper at ur e l evel s and t hedi f f er ent mass f l ows ( Fi gur e 1 . 1) .I n an AHP one i s deal i ng wi t h a worki ng pai r , t hat i s a sol vent and anabsor bent . The f i r st i s the vol at i l e component , t he second the l ess ( ornon- ) vol at i l e component . The concent r at i on of a m xt ure, vapour or l i qui d,i s def i ned as t he wei ght f r act i on w, t hat i s

w = kg sol vent / kg m xt ur e ( 1)I n the AHP t her e ar e t wo mass f l ows ( l i qui d/ vapour ) , and bot h can be am xt ur e of bot h component s . That i s f i r s t t he f l ui d t hat i s c i r cul at i ng i nt he cycl e of t he absor ber , t he heat exchanger and t he gener at or , andsecondl y t he f l ui d/ vapour t hat i s f l owi ng t hr ough t he condenser and t heevapor at or , and t hen absor bed i n t he absor ber and gener at ed i n t he gener at or( absor bed i n and gener at ed f r om t he f i r st f l u i d ) .Dependi ng on the boi l i ng poi nt di f f er ence AT bet ween t he sol vent andt he absor bent , t he vapour generat ed i n t he gener at or can cont ai n a cer t ai namount of t he absor bent , t he l ess vol at i l e component .

1


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Q c , T c Mv. WvCOND RECT

Q e. T e

GENE

Qr. Tr

Q g . T gMp. Wp _

H.E.

Ph tPIEVAP

Mr, WrPpH _ J _ABSO Qa. Ta

Fi gur e 1. 1 Scheme of an absor pt i on heat pumpI f s o , a r ect i f i cat i on col umn bet ween t he gener at or and t he condenser i snecessary t o ensure a wei ght f r act i on w of al most 1. 0 i n t he vapour goi ng t ot he condenser . The r emai ni ng absor bent i n t he vapour , wi t h or wi t houtr ec t i f i c at i on, wi l l r emai n i n t h e evapor at or and must be t ransported t o t heabsor ber t o mai nt ai n a steady state i n t h e process.I n pr ac t i ce, a boi l i ng poi nt di f f er ence AT l ar ger t han 200 K wi l lensure a vapour wei ght f r act i on w c l ose t o 1. 0.So the f i r s t f l u i d i s a r el at i vel y " r i ch" or " poor " ( sol vent ) m xt ur eof bot h c omponent s , t he second f l ui d/ vapour has a wei ght f r act i on w of 1. 0or ver y c l ose t o t hat , because, i f r ect i f i cat i on i s necessary, i t wi l l neverbe a compl et e r ect i f i cat i on of t he vapour .Let us now f o l l o w t he pr ocess cycl e, s t ar t i ng i n t h e absorber (seeF i gur e 1. 1) .A r i ch m xt ur e, com ng f rom t he absorber,pressure, through a heat exchanger , wher ethe generator. By addi ng a heat f l ow Q atvapour i s generated from t he r i ch m xture , whi ch, now a poor m xtur e,returns vi a t he heat exchanger , wher e i t i s l ower ed i n t emper at ur e, and anexpansi on val ve, where i t i s l ower ed i n pressure, t o t h e absorber. Now, t hevapour ent er s, i f necessary, t he r ect i f i cat i on col umn wher e i t i s cool ed byr emovi ng a heat f l ow Q at a t emper at ur e T . The now r ect i f i ed vapour passest o t he condenser and t he r emai ni ng " ver y poor" m xt ur e ret ur ns t o t hegenerator. The vapour , r ej ect i ng a heat f l ow Q at a " medi um" t emper at ur el evel T by condensat i on i n t he condenser , r et ur ns by anot her expansi onval ve, Feduci ng t he pressure, t o t he evapor at or wher e a heat f l ow Q i sadapted at a " l ow" t emper at ur e l evel T by evapor at i on. Then t he vapour( pur e sol vent ) f l ows t o th e absor ber and i s absorbed by t he poor m xture ,r ej ect i ng a heat f l ow Q at a " medi um" t emper at ur e l evel T . I n t hef ol l owi ng f i gur e t he di f f er ent t emper at ur e and pr essur e l vel s ( Fi gur e 1. 2)can be f ound wi t h:

i s pumped, causi ng a r i s e ofi t i s r ai sed i n t emper at ur e, t oa " hi gh" t emper at ur e l evel T ,

2


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T = t emper at ur e [ K]P = press ur e [ Pa]w = wei ght f r act i on [ kg/ kg]Q = heat f l ow [ W

Fi gur e 1. 2Scheme of t he AHP i na P - T di agr am

Ph!P

P IQe

* QmQcf f QT

Qal

Qg

Te T Tm Tg

Under most condi t i ons t he heat f l ows i n t he absor ber , i n t he condenser andi n t he rect i f i cat i on col umn ar e r ej ect ed at t he same t emper at ur e l evel T_so ' mVVV TmThe total heat f l ow out put Qm i smQ = Qm wa + QC +

(2)(3)

I n t he case of our wor ki ng pai r met hanol - l i t hi um br om de / z i nc br om de( CH30H - L i Br / ZnBr 2 ( 2 :1 mol ) ) , t he met hanol i s t he sol vent . For t hi sworki ng pai r AT 200 K, sal t has a negl i gi bl e vapour pr essur ehol ds f or t he wel l - known wor k i ng pai r l i t hi um br om de/ wat er ( H20So i n bot h cases r ect i f i cat i on i s not necessar y .The m xt ur e of our worki ng pai r has a wei ght f r act i on w bet weenand 0. 40. I n t he case of t he al so wel l - known wor ki ng pai r NH3 - H20,sol vent i s ammoni a, AT = 135 K, so r ect i f i cat i on i s a necessi t y.

The same- Li Br ) .0. 28t he

1. 3 The l og P - l / T di agr amWhen deal i ng wi t h a wor ki ng pai r , one has one ext r a var i abl e, namel y t hewei ght f r act i on w. I n ot her wor ds, t he l i qui d m xt ur e possesses t wo degr eesof f r eedom at l i qui d- vapour equi l i br i um so w = w( P , T) , P = P( w, T) and T =T( w, P) .Der i ved f r om t he wel l - known Cl ausi us - Cl apeyron l aw, t he f ol l owi ng r el at i onbet ween t he absol ut e vapour pr essur e P and t he absol ut e t emper at ur e T, knownas t he Ant oi ne equati on, can be wr i t t enf or t he pur e sol vent l og P = a - b r / T ( 4)and f or a m xt ur e l og P = a' - b' ( r + 1) / T ( 5)

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wi t h r = ent hal py of evapor at i on/ condensat i on [ J / kg]1 = ent hal py of absor pt i on/ desor pt i on [ J / kg]a, a' = const antb, b' = const antW t h t hi s i n hand one can draw f or t hi s type of m x t u r e a l og P - l / Tdi agr am wi t h t he wei ght f r act i on w as par amet er . One shoul d keep i n m ndt hat ever y poi nt i n t hi s d i agr am descr i bes an equi l i br i um s i t uat i on, so aprocess can not be dr awn i n t he di agr am onl y t he t wo poi nt s bet ween whi chi t t akes pl ace and t he di r ec t i on. The "processes" i n t he AHP are dr awn i nt he f ol l owi ng l o g P - l / T di agr a m ( F i gur e 1. 3) .

Figure 1.3The log P - l/T diagramfo r an AHP w it h the weightfraction w as parameter

In P

1/TI n t hi s di agr am t wo i mport ant si mpl i f i cat i ons have been made, namel y t hatt he ent hal py of evaporat i on/ condensati on and t he ent hal py of absorpt i on/gener at i on ar e i ndependent of t emper at ur e and wei ght f r act i on. Never t hel esst hi s di agr am gi ves a good over vi ew and pr esent at i on o f t h e pr ocesses t aki ngpl ace i n t he AHP.1. 4. The heat r at i o, t he c of f i c i nt of per f or mance and t he ci r c ul at i onr a t i oI f one consi der s t he AHP as a r ever s i bl e one, one can, wi t h t he i ngoi ngout goi ng heat f l ows at t hr ee di f f er ent t emper at ur e l evel s ( F i gur e 1. 2) ,def i ne a so cal l ed heat r at i o , based on t he Car not ef f i c i ency:

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e - [ ( r + l ) + r ] / ( r + 1) ( 6a)and wi t h# - 1 / r ( 7)C - ( 2 + # ) / ( l + # ) ( 6b)

Most of t he wor ki ng pai r s have a posi t i ve 0, so f or a ( si ngl e st age) AHP t heheat r at i o w l l never exceed 2.As one can see t he heat r at i o i s a pur e t heor et i cal paramet er , al so someassumpt i ons had t o be made, as wel l as some si mpl i f i cat i ons.A more pract i cal par amet er , most l y based on t he out put of exper i ment s,i s t he so- cal l ed cof f i ci nt of per f or mance, t he COP, whi ch i s def i ned asf ol l ows :COP = ener gy out put / energy i nput ( 8)

For t he AHP t hat l eads t owi t h: COP = ( Qa + Qc + Qr ) / ( Qg + Pp ) ( 8a)Q = heat f l ow f r om absor ber [ WQ = heat f l ow f r om condenser [ WQ = heat f l ow t o generat or [ WQ = heat f l ow f r om r ect i f . col umn [ WP = pumpi ng ener gy t o pump [ WSi nce P Q , t he pumpi ng ener gy P i s of t en l ef t out f r om Equat i on ( 8a) .Thi s cof f i c i nt w l l be used l at er on i n t he exper i ment s wi t h thecomput er si mul at i on model and progr am and wi t h t he t est pl ant as a mai nparamet er f or t he perf ormance of t he AHP.Anot her pr acti cal par amet er i s the ci r cul at i on rat i o f , t he mass f l owr at i on of t he r i ch m xt ur e and t he vapour l eavi ng t he r ect i f i cat i on col umn:

f = Mr / Ms ( 9)wi t h: f = ci rcul at i on rat i o [ kg/ kg]M = mass f l ow [ kg/ s]r = r i ch, p = poor , s = sol ventBy usi ng t wo mass bal ances, of t he t ot al mass and of t he mass of t hesol vent , t he ci r cul at i on r at i o can be expr essed i n t er ms of t he wei ghtf r acti on w:t ot al mass bal ance M + M = M ( 10)mass bal ance sol vent w M + M = w M ( 11)l eadi ng t o: f ( 1 - W) / ( wr - w ) ( 9a)Toget her , t he t ot al heat pr oduct i on Q , t he cof f i ci nt of per f or mance COPand the ci r cul at i on rat i o f f orm t he f l a i nout put paramet ers of t he AHP.

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1. 5 Resear ch wor k on absor pt i on heat pumpsThe ongoi ng r esearch act i vi t i es and t hose whi ch t ook pl ace i n t he past i nt he f i el d of t he AHP can, gener al l y speaki ng, be di v i ded i nt o t hr ee r esear chcat egor i es :

a. r esearch on ( new) wor ki ng pai r s,b. r esear ch on new sor pt i on syst em conf i gur at i ons andc. r esear ch on heat and mass t r ansf er ( component s) .W t hi n these f i el ds one can al so make a di s t i nct i on i n t heor et i cal andexper i ment al r esear ch act i v i t i es .I t i s not a sur pr i se t hat t her e al ways i s one mai n goal behi nd t hedi r ect goal s of t he resear ch work: savi ng ener gy.Looki ng at t he l ast f i ve year s, i t seems t hat mor e and mor e t her esear ch wor k on sor pt i on syst em i s f ocussed on new syst em conf i gur at i ons.Most l y by means of a t heoret i cal appr oach, t hat i s by usi ng comput ers i mul at i on t echni ques . I t i s i n t hi s r esear ch f i el d t hat r esear cher s expectt he most pr om si ng r esul t s concer ni ng ener gy savi ng.I n t he r esearch work on ( new) worki ng pai r s f or sor pt i on syst ems, onl ya very f ew pr om si ng ones have been f ound up unt i l now and t hey ar e sti l l i nt he experi ment al st age. The wel l - known worki ng pai r s NH3- H20 and H20- L i Brare s t i l l f avour abl e because of t he exper i ence and r el i abi l i t y o f t hesewor k i ng pai r s , al so t aki ng t hei r d i sadvant ages i nt o account .Research wor k on heat t r ansf er , wi t h or wi t hout change of phase, t akes pl aceext ens i vel y . Thi s i s not so when al so mass t r ansf er t akes p l ace.So i n component s wi t h si mul t aneous heat and mass t r ansf er , whi ch i s t hebasi c pr i nci pl e of sorpt i on, not so much r esul t s can be f ound.Al t hough t hi s r esearch wor k i s mai nl y f ocussed on component s, onexchanger s, f or s i mul t aneous heat and mass t r ansf er, f or heat t r ansf er wi t hor wi t hout a change of phase, al l t hr ee resear ch f i el ds wi l l be i nt r oducedbr i ef l y.1. 5. 1 Wor ki ng pai r s f or an absor pt i on heat pumpThe wel l - known wor ki ng pai r s NH3 - H20 and H20 - Li Br are t he most appl i edworki ng pai r s i n an AHP, but a wor l d- wi de resear ch t ook pl ace at t echni caluni ver s i t i es and r esearch i ns t i t ut es t o f i nd bet t er wor k i ng pa i r s , o rwor ki ng pai r s t hat coul d meet t he di sadvant ages of t he above ment i onedwor k i ng pai r s .I n an AHP f or exampl e, t he wor ki ng pai r NH3 - H20 has hi gh wor ki ngpr essur es and t he necess i t y of r ec t i f i cat i on. L i m t at i ons i n t he use of t hewor ki ng pai r H20 - L i Br are t he sol ubi l i t y ( danger of c r y s ta l l i zat i on) andt he evapor at i on t emper at ure ( danger of f r eez i ng) .The f ol l owi ng l i st gi ves an overvi ew of t he most i mpor t ant sel ect i oncr i t er i a f or a worki ng pai r f or an AHP. Of cause some cr i t er i a ar e morei mpor t ant t han ot her s or are i n a way dependi ng on one or mor e ot herc r i t e r i a .1. t he heat of evapor at i on r [ J / kg] o f t he sol vent ,2. t he maxi mum worki ng pr essur e P, [ bar. kPa] of t he sol vent ,3. t he c i r cul at i on rat i o f [ kg/ kg] ' oT t he mass f l ows ,4. t he ener gy consumpt i on P [ W of t he m xt ur e pump,5. t he heat capac i t y c [ J / \ kg. K) ] of t he m xt ur e ( sens i bl e heat ) ,

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6. t he cri t i cal t emper at ur e T [ K] of t he sol vent and t he absor bent ,7. t he boi l i ng poi nt di f f er ent e AT [ K] bet ween t he sol vent and t heabsorbent,8. t he sol ubi l i t y i n t he des i red oper at i on f i el d (no c r ys t al l i zat i on) ,9. t he chem cal st abi l i t y at t he maxi mum desi r ed t emper at ur e,10. t he cor r osi on t o const r uct i on mat er i al s,11. t he t oxi t y and12. t he avai l abi l i t y and the cost s .At t he Techni cal Uni ver si t y of Ess en, West Ger many, a gr eat ef f ort has beenmade i n t he measur ement of t he t her modynam c proper t i es of and t hecomposi t i on of t he P - T and h - w di agr am f or pr om si ng worki ng pai r s.Based on t he l i st above, t hey sel ected the f ol l owi ng cr i t er i a:1. t he heat of evaporat i on at 0" C, r 0 [ J / k g ] ,2. t he pr essure of t he sol vent at 50 C, P [ b a r ] ,3. t he ci r c ul at i on r at i o f [ kg / kg ] ,4. t he pumpi ng energy f act or N :

M p - ( f AP) / ( r Q . p r ) ( 1 2 ) ,wi t h: AP = pr essur e di f f er ence over t he pump [ kPa]p - dens i t y [ k g/ m3]

5. t he heat exchange f act or N. :Nh = ( ( f - D c p AT) / r Q ( 1 3 ) ,

wi t h: AT = t emper at ur e di f f er ence absor ber and gener at or [ K]6. t he boi l i ng poi nt di f f er ence AT [ K] , t he necess i t y of r ec t i f i cat i on and7. t he t oxi t yI n t he f undament al exper i ment al r esear ch t o new worki ng pai r s, t hat i s t hedeter m nat i on of t he t her modynam c and physi cal pr oper t i es, a gr eat ef f or ti s put over t he l ast t went y year s t o meet t he di sadvant ages of and r epl acet he wel l - known wor ki ng pai r s l i ke NH3 - H20 and H20 - L i Br . Al t houghsuccesses ar e made i n t hi s di r ect i on, t he di sadvant ages of t he new wor ki ngpai r s and t he advant ages of t he convent i onal worki ng pai r s, l eads t o at endency t o choose f or r el i abi l i t y and exper i ence, so t o convent i onalwor k i ng pai r s.Onl y a ver y f ew new worki ng pai r s have been successf ul l y, t hat i s, ar eappl i ed i n exper i ment al t est pl ant s of a sor pt i on syst em But t heconvent i onal wor ki ng pai r s ar e st i l l f avour ed i n i ndust r i al appl i cat i ons ,t hey ar e t he onl y syst ems i n pr act i cal use t oday.The pr obl em i s that most of t he new and pr om si ng worki ng pai r s f ai l onone or mor e cr i t er i a. I n most cases cor r osi on and/ or e chem cal st abi l i t y i st he bot t l eneck, f ur t hermore t he t oxi t y seems t o pl ay an i mport ant r ol e.

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A r o u n d 1 9 8 0 me t h a n o l ( C H3 0 H ) s e e me d t o be t h e s o l v e nt of t h e f u t u r e , i nc o mb i n a t i o n wi t h L i B r or a mi x t u r e of L i B r a nd Z n B r 2 as t h e a b s o r b e n t .U n f o r t u n a t e l y t h e me t h a n o l i t s e l f a nd t h e me t h a n o l / s a l t mi x t u r e s h o we d to bec h e mi c a l l y u n s t a b l e a b o v e 1 1 0 - 1 3 0 C , as d e t e c t e d by K o eb e l [ K 2 ] .T h a t me a n t t h a t o n l y a ma x i mu m t e mp e r a t u r e ( i n t h e g e n e r a t o r ) of a b o u t 1 2 0 " Ci s a l l o we d a n d , wi t h an e v a p o r a t i o n t e mp e r a t u r e of a b o u t 0 C , t h e u s e f u lh e a t c a n o n l y be p r o d u c e d at a b o u t 40 - 5 0 C . T h i s d r a s t i c a l l y d i mi n i s h e st h e c h a n c e s f o r a d i r e c t g a s - f i r e d d o me s t i c A HP h e a t i n g s y s t e m.Un t i l n o w, o n e o f t h e mo s t p r o mi s i n g s o l v e n t s ( wo r k i n g f l u i d s ) s e e ms t ob e t h e 2 , 2 , 2 - t r i f l u o r e t h a n o l ( T F E , t r i f l u o r e t h a n o l , C F 3 C H 2 0 H ) . Thet h e r mo d y n a mi c p r o p e r t i e s a r e e x t e n s i v e l y i n v e s t i g a t e d by B o k e l ma n n ( T U-E s s e n ) [ B 2 , B 3 ] a n d Gi r s b e r g e r ( T U- B e r n ) [ Gl , G 2 ] .Wo r k i n g p a i r s wi t h T F E a s t h e s o l v e n t h a v e a l r e a d y b e e n t e s t e d i n s o m e A H Po r A HT p i l o t p l a n t s . B e r g h ma n s [ B 4 ] t e s t e d t h e wo r k i n g p a i r T F E - C h i n o l i n e( C 9 H7 N ) i n an a b s o r p t i o n h e a t t r a n s f o r me r ( A HT ) wi t h a h e a t o u t p u t of 275k W. N a k a y a ma [ NI ] h a s d e v e l o p e d a s o r p t i o n s y s t e m ( A HT ) wi t h t h e wo r k i n gp a i r T F E - N- me t h y l 2 - P y r r o l i d o n ( NMP , C 5 H 9 N 0 ) .B o k e l ma n n [ B 3 ] i n v e s t i g a t e d t h e p o s s i b i l i t i e s of s e v er a l wo r k i n g p a i r wi t hT F E of wh i c h t h e T F E - N M P s h o w e d t o be t h e mo s t p r o mi s i n g o n e . S i n c e wi t hN M P t h e r e i s t h e n e e d of r e c t i f i c a t i o n , he l a t e r c h a n ge d t o 2 - P y r r o l i d o n( P y r , C 4 H 7 N 0 ) [ B 5 ] .T h i s l a s t wo r k i n g p a i r , TFE - 2 - P y r r o l i d o n , wi l l n o w be t e s t e d i n anA H T t e s t p l a n t ( h e a t o u t p u t 10 k W) at t h e L a b o r a t o r y of R e f r i g e r a t i o nE n g i n e e r i n g at t h e T ec h n i c a l U n i v e r s i t y of D e l f t , t h e Ne t h e r l a n d s [ Wl ] .1 . 5 . 2 S y s t e m c o n f i q u r a t i o n sA s t h e a b s o r p t i o n h e a t p u mp AH P i s i n f a c t d e v e l o p e d f r o m t h e a b s o r p t i o nc o o l i n g ma c h i n e A CM, t h e a b s o r p t i o n h e a t t r a n s f o r me r A HT i s d e v e l o p e d f r o mt h e A H P . T h e f o l l o wi n g f i g u r e ( F i g u r e 1 . 4 ) s h o ws t h e d i f f e r e n t t e mp e r a t u r el e v e l s of t h e s e t h r e e b a s i c s o r p t i o n s y s t e ms .Ne w c o n f i g u r a t i o n of s o r p t i o n s y s t e ms a r e mo r e or l e s s b a s e d on t h e s et h r e e s y s t e m s .

Figure 1.4Temperature levels >in the ACM, theA H P and t h e A H T ACM AHP AHTT h e f i r s t t o me n t i o n i s t h e t wo o r mu l t i - s t a g e c o n f i g u r a t i o n s t o a c h i e v eh i g h e r t e mp e r a t u r e d i f f e r e n c e s a n d / o r a b e t t e r p e r f o r ma n c e ( C OP ) ( Ho b l i n g[ H l ] , A l e f e l d [ A l ] , Z i e g l e r [ Z l ] . A l t h o u g h s o me e x p e r i me n t a l r e s e a r c h hast a k e n p l a c e wi t h t e s t p l a n t s , mo s t i s d o n e by m e a n s of c o mp u t e r s i mu l a t i o nb e c a u s e of t h e h i g h i n v e s t me n t c o s t s of t e s t p l a n t s .

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For the ACM and the AHP a higher temper ature lift (doub le- li ft) or ahigher COP (doubl effect) can be achieved. For the AHT a higher COP at asmaller temperatur e li ft (doubl ef fec t) or a higher temperature lift at alower COP (doubl li ft) can be achieved [ Z l ] . Fig ur e 1.5 shows a two- stageAHP system.

double-lift(temperature) Ore. Tre

dg. Tg

Ode. TdePpa-

Qa. Ta*

b . doubl effect(COP)Qg. Tg

Qc, Tc

Figure 1.5A two-stage absorptionheat pump system Qe, Te

In practice it seemed that more than two-stage is not yet feasible. Forindustrial application more and more two-stage configurations are foundbecause of the inc reasing possibil ities to reduc e the in vestment c osts.An also possib le con fi gu ration is the resorp tion heat pump or heattransf orm er (RHP / RHT). In the AHP / AHT the evapo rato r and the con den sercir cui t is replaced by a solution ci rcu it with a desorber (a gen erator atlow pressur e and temp eratur e) and a resor ber (an absorb er at high pr essureand t e m p e r a t u r e ) . In fact the comp lete n ame should be the absorp tion /resorption heat pump. So the "evaporator" and the "condenser" are nowworkin g w ith glidi ng temperature di ff erenc es. Figure 1.6 is showing aresorption heat pump system.The choi ce b etween an absorption or a resorp tion system is not easy(Westra [ W2] , v/d Ree [ Rl] , Baehr [ B 6 ] ) , because the differences inper for man ce are small . The tenden cy is that the pr ofits fr om the gli di ngtemperatures are smaller than the losses because of the extra investmentcosts.

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Al so i n vi ew of t he ener gy ef f i ci ency t he absor pt i on syst em seems to bef avour abl e. For t he adapt abi l i t y t o changi ng pr ocess t emper at ur es, ar esorpt i on syst em seems t o be better .Ore. Tr e

Fi gur e 1. 6A ( absor pt i on/ ) r esor pt i onheat pump syst em Qde. Tde

RESO - -

1 .E.rn

RFNF

L K E a - JP p r Pht PI

DESO(JTABSO

O Tg

PpaQa. Ta

An ot her newconf i gur at i on i s a combi nat i on of t he RHP / RHT and t hecompressi on cool i ng machi ne CCM and t he compr essi on heat pump CHP. That i st he compr essi on heat pump w t h sol ut i on ci r cui t . I nst ead of t he evapor at orand t he condenser a sol ut i on c i r cui t wi t h an absorber and a gener at or i sused. Fi gur e 1. 7 shows such a systemOre. Tre RESO

Fi gur e 1. 7 KE.A compr ess i on heatpump syst em wi t h asol ut i on ci r cui t Qde. Tde

Pp PhPI

Pcomp

DESO

As wi t h t he r esor pt i on syst em t her e i s t he possi bi l i t y t o wor k wi t h gl i di ngt emperat ur e di f f erences (Ahl by [ A2] ) t o achi eve a hi gher COP. Anot her namefor such a conf i gur at i on i s a compr essi on/ absor pt i on syst em1. 5. 3 Heat and mass t r ansf erI n sor pt i on ( absor pt i on, r esor pt i on, desor pt i on, et c. ) t he bas i c pr i nc i pl ei s t hat i n t he component s f or t hese pr ocesses, si mul t aneous heat and masstransfer i s t aki ng pl ace, i ndependent of the type of component ( bubbl e,f i l m spr ay, e t c ) .On macr o- scal e t he measur ement of t he t r ansf er r ed amount of heat andmass i s not t oo d i f f i c u l t by means of t he di f f er ent heat and mass bal ances.

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But on mi cr o-scal e, where one is interested in the temperatur e and weig htfraction distrib ution (boundary l a y e r s ) , it is very diff ic ul t, if notimp ossible, to measure temperature and weig ht fr action.To meet thi s, simulation m odels are developed based on the di ff erenttheories on heat and mass transfer (film theory, penetration theory, etc.)li ke v/d Wekken c.s. [W3] and Grossman [G3] di d. But for a betterun derstandi ng and impr oving of the sorption pr ocesses, this simpl ifi cationis necessary.In this research work there wer e used to pr edic t the rig ht surf aceconf ig ur ation for the di fferent pr ocesses. Key word in this pr ocedur e is thearea density 0, the transfer surf ace area [ m 2] per volume [m 3] of thecomponent.

1.6 This research work1.6.1 GoalsThe scope of the researc h r eported here is a theoretic al and exp eri mentalstudy into the possib il ities of the appl ic ation of so call ed comp act heatand mass exchangers (CHME) in gas fired AHP's for domestic heating.This heat and mass exchanger, the CHME, is of the plate type. The spacebetween the parallel and plain pl ates are filled with cor ru gated pl ates ofsome heig ht. This is what is call ed the cor ru gati on. The plain andcorru gated plates are stacked and, with headers and p ip es, brazed or weldedtogether . This qiv es a heat and mass exc hang er whi ch is very c ompac t and hasmore than 700 m 2 transfer surface per m 3. This leads to heat and masstransfer wi th small temperature and conc entration di ff eren ces.Besides this more quali tativ e aspects, more qu antitative inf ormation isneeded to say more about the behaviour and performance of this type of heatand mass exc hang er. This leads to a theoretical and exp erim ental researc hinto the application of this type as a component of an AHP and well as:- absorber and generator: simultaneous heat and mass transfer- evapor ator and cond enser: heat transfer w ith change of phase- mi xtu re-m ix tur e heat exchanger : heat transf erNext to that, the AHP as a whole - its behav iou r and its per for man ce at alldesired worki ng cond itions - will be subject of this research.In that frame work an AHP with heat and/or mass transfer componentsbuilt as compact heat and/or mass exchangers (except for the generator,whic h is of the shell and tube ty pe) was tested wi th the wor ki ng pairlithium br omide LiBr / zinc b romid e ZnBr 2 (2:1) and methanol CH3OH, so asalt / alcohol mi xtu re with alcohol as the solv ent. The therm ody nam icpr operties of this worki ng pair are well known and it is extensivel y testedin an earl ier AHP test pl ant by Iedema [II] and Saurw alt [S2] .Theref ore this wor ki ng pair is suited to test the comp act heat and /ormass exc hang er com ponen ts in a new AHP pi lot pl ant, although one of the mostimportant and li miting di sadvantages of this workin g pair is the fact thatthe methanol in the mi xtu r e is not stabl e above tem per atu res of 120C (393K)and is breaking up, a pr ocess whic h is not reversi bl e.

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1. 6. 2 Tool sThe st art i ng poi nt of t hi s r esear ch st udy was t he resear ch wor k of I edema,espec i al l y hi s di sser t at i on "The Absor pt i on Heat Pump" [ I I ] .I n t hi s one can f i nd t he f i r st set up f or t he devel opment of an AHP f ordomest i c heat i ng.Based on t he t her modynam c pr oper t i es, deri ved exper i ment al l y andt heor et i cal l y by t he f undament al equat i ons of st at e, o f t he unt i l t hat t i meknown worki ng pai r s, t he most sui t abl e one was chosen, Li Br / ZnBr 2 ( 2: 1) -CH3OH.W t h t hat i n hand a comput er si mul at i on model and pr ogr am was devel opedby Bakker [ B7] t o si mul at e an AHP f or domest i c heat i ng wi t h t hi s worki ngpai r . I n t hi s pr ogr am al l t he component s wer e put i n separat ed subr out i nesand wer e of t he shel l and t ube t ype, wi t h st r ai ght or wound t ubes. Onl y t hem xt ur e heat exchanger was of t he pl at e t ype.

The compl et e pr ogr am was run t hr ough unt i l al l t he component s i t sel fand t he AHP as a whol e wer e i n bal ance. The mai n i nput var i abl es wer e t heambi ent t emper at ur e and t he chosen cont r ol st r at egy ( m xt ur e mass f l ow andaddi t i onal heat i ng) .Fur t her mor e by means of a l i t er at ur e st udy on f l ow hydr odynam cs, heatand/ or mass t r ans f er , al l concer ni ng t he f al l i ng f i l m f l ow ar ound tubes , at heor et i cal model was devel oped f or t he absor ber , consi st i ng of r ows ofpar al l el t ubes, and f or t he occur r i ng pr ocesses of f l ow, heat and masst r ansf er . I t shoul d be emphasi zed t hat t hi s di ssert at i on was mai nl yconcent r at i ng on t he absor ber as t he most i mpor t ant and l i m t i ng componento f the AHP.Fi nal l y an AHP pi l ot pl ant was bui l t t o t est t he absorber and t he heatpump as a whol e. Al so here t he component s wer e of t he shel l and t ube t ype,onl y t he m xt ur e heat exchanger was of t he pl ate t ype.For a bet t er heat and mass t r ansf er and a mor e compact const r uct i on( domest i c heat i ng ! ) t he st ep i s made t o t he above ment i oned compact heatand mass exc hanger s , CHME.So f or t hi s r esear ch, t here ar e t hr ee st ar t i ng poi nt s :a. t he di sser t at i on of I edema [ I I ] , mai nl y t he chapt er s 3 unt i l 7deal i ng wi t h heat and/ or mass t r ansf er , t he absor ber model and t hesi mul at i on of and t he exper i ment s wi t h an AHP,b. t he comput er si mul at i on model and pr ogr am of an AHP f or domest i cheat i ng wi t h a sal t / al cohol m xture [ B7] ,c. an AHP pi l ot pl ant wi t h component s bui l t wi t h so cal l ed compact heatand/ or mass exchangers , onl y the generat or i s st i l l of t he shel l an dt ube t ype.1. 6. 3 Set upThi s wor k, wi t h t he above ment i oned t hr ee st ar t i ng poi nt s, has al so t hr eebasi c e l ement s, namel y s i mul at i on/ model l i ng, exper i ment s and l i t er atur e.The next chapt er s wi l l cont ai n t he r esul t s of t hi s r esear ch work,r eal i zed al ong t he above ment i oned l i nes.

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CHAPTER 2. COMPACT HEAT AND MASS EXCHANGERS

2. 1 Def i ni t i onCompact heat and/ or mass exchangers have a compact t r ansf er sur f ace, wi t h aar ea densi t y gr eat er t han 700 n r / m3, a somewhat arbi t r ary val ue. Heat and/ ormass exchanger st ands f or t he transfer surface of heat and mass, whi l ecompact st ands f or not onl y compact i t sel f , but al so f or enhanced.In short , a compact heat and/ or mass exchanger i s an exchanger f or heatand/ or mass of a compact const r uct i on w t h many " ext r a" t r ansf er sur f ace anda r el at i vel y smal l vol ume.2. 2 Compact and enhanced t r ansf er sur f aceConcer ni ng t he heat t r ansf er , one can def i ne t he amount of heat whi ch i stransferred from one si de of t he exchanger t o t he ot her si de as f ol l ows:

Q = K A AT ( 14)wi t h: Q = heat transfer rate [ WK = overal l heat t ransf er cof f i c i nt [ W m2. K]A = t ot al heat t r ansf er surf ace ar ea [ m2]AT= mean t emper at ure di f f er ence [ K]Fur t her mor e one can def i ne f or t he heat exchanger

7 = Q / AT ( 15)w i t h : 7 = sp ec i f ic heat tra ns fe r rate [W/K]and 0 = A / V (16)wi t h: f} = ar ea densi t y [ m2/ m3]V = t ot al vol ume [ m3]For exchanger s of t he pl at e t ype, t hi s l eads t o

h = c / Vc r *h =h / Vh (16 a>wi t h: c = col d si de, h = hot si deand f or exchangers of t he shel l and t ube t ype, t hi s l eads t o

0 t = A t / V t ( 16b)wi t h: t = totalAn cl ear over vi ew of t he di f f er ent t ypes of exchangers concer ni ng thei r ar eadensi t y i s der i ved f r om Shah [ 9] and shown i n Fi gur e 2. 1.From ( 14) , (15) and (16) one can der i ve

7 = K fi V ( 15a)13


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-COK PAC TNE SS?" * KATTER OF DERf-Eo o o o o oo o o o o o

o o o o o o o o o o 1o o o o o o o o o o oo o o o o o o o o o I^ Q O _ o o o o o o o o y

\-"'''"'" " > " " > > "

K H-O O O-rO I O O ^Ga i T urb i n .R o t a r yi He ge nc c i t o i

Buun: ...DA u t o m o t i v rf U d i a t o r s M a t r i x T y p e s , H . r * S c r e e nS p h e r e B e d , C o r r u g a t e d S h e e t s

F o r X X , 1 . B 8 ,

SLS t r i p - F i n a n d L o u v ^ r e d - F i n H . E . E j p Jnd O O.B! ]

P U i n T u b u U r , S h e l l - a n d - T u b H . E . C O M P A C T S U R F A C E S

20 10l l yd r du l l c Di a a w t U DL

5 2T l l l i l i l200 500 1 (

Heat t ra ns lV i Brfac- *

I , II I I I I60 100 1 r2 0 0 0 I M U I . I I I .5000 10 2 1x10Fi gur e 2. 1 The ar ea densi t y /? f or di f f er ent t ypes of exchanger sf r om Shah [ 9]

Fi gur e 2. 2 A stack of paral l el pl ai n and cor r ugat ed pl ates

plain tr iangular f in plain rectangular f in wavy f in

offset str ip f in round perforated f in pin f insFi gur e 2. 3 Di f f er ent t ypes of cor r ugat ed f i nned pl at es ( cor r ugat i ons)f r om Shah [ 3]

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The goal i s to achi eve a gr eat spec i f i c heat t r ansf er r at e i n combi nat i onwi t h a smal l mass and vol ume of t he exchanger by means of t he use of compactsur f aces. Thi s most of t he t i me l eads al so t o a gr eat er over al l heatt r ans f er cof f i c i nt K whi ch i t sel f al so l eads t o a smal l er vol ume. Thi scompact const r uct i on i s st r ong and st abl e wi t h smal l wal ! t hi cknesses. Thi sal so r educes t he vol ume and t o a smal l er cont ent t he mass of t he exchanger .W t h enhanced sur f aces i s meant t o achi eve a great er over al l heatt r ansf er cof f i c i nt compared t o t he not enhanced pl ai n sur f aces. So i tdeal s wi t h t he f act or K A (= y ) , achi evi ng an enl argement of K and/ or A.Thi s can be done bya. addi ng ext ended surf ace t o t he pr i me surf ace ( K and A gr eat er )b. addi ng t ur bul ator s t o t he sur f ace ( K gr eat er )c. r educi ng t he f l ow passages ( hydr aul i c di am ) ( K and A gr eat er )As one can see, a. and c. al so i ncr ease t he t r ansf er surf ace ar ea. Compactand enhanced t r ansf er sur f aces char act er i zes mor e t he cat egor y of compactexchanger s t han t he ear l i er ment i oned area densi t y of 700 m2 / m3. Cl ear l y i sshown t hat i n most cases c ompact and enhanced appear t oget her .Thi s al l of f er s t he poss i bi l i t y t o appl y t hi s t ype of exchanger wi t h anext ensi ve choi ce i n t ype, geomet r y and ar ea densi t y of t he surf ace. So agr eat f l ex i bi l i t y i n t he choi ce of sur f ace on bot h s i des and a r educt i on i nt he t ot al mass and vol ume can be obt ai ned.Thi s al so of f er s t he poss i bi l i t y of aut omat ed pr oduct i on t echni ques,cer t ai nl y i n t he case of t he pl ate f i n t ype exchangers as used i n t hi sr esearch wor k and shown i n Fi gur e 2. 2. Thi s al l can l ead t o a compet i t i vepr i ce of t hi s t ype of exchanger .2. 3 Appl i cat i on2. 3. 1 Fi el d of appl i cat i onThe most i mport ant appl i cat i on of compact heat and/ or mass t r ansf er surf acesconcerni ng t he t ype of mass f l ow, i s on t he gas si de i n heat exchangi ngpr ocesses ( i n gas/ gas, gas/ l i qui d, gas/ condens i ng, evapor at i ng l i qui ds ) . To amuch smal l er ext end t hey are used f or appl i cat i ons f or t wo phase f l ow or ont he l i qui d s i de.I t i s obvi ous t hat f or a cer t ai n r at e of t r ansf er r ed heat a much l ar gert r ansf er surf ace i s needed on t he gas si de t han on t he l i qui d si de becauseof a nor mal l y 10 - 100 t i mes smal l er heat t r ansf er cof f i c i nt on t hat gass i de. Unf or t unat el y t he enhancement of t r ansf er sur f ace by means ofcor r ugat i ons or t ur bul at ors gi ves a much gr eat er pr essur e dr op on t he gass i de. Ther ef or e the addi ng of t hat t ype of t ur bul at or s i s l i m t ed.Appl i cat i on i n t he ai r craf t and aut omobi l e i ndust r y was t he st ar t i ngpoi nt of t he devel opment , f ol i owed by appl i cat i on as heat t r ansf ercomponent s i n i nst al l at i ons [ S4] . The appl i cat i on i n sor pt i on syst ems i sr at her new, up unt i l now har dl y no l i t er at ur e i s f ound on t hat , except f orMi nkus [ Ml ] .2. 3. 2 Corr ugat i onsI f one l i m t s onesel f t o compact heat and/ or mass exchangers consi st i ng of ast ack of par al l el pl ai n and cor r ugat ed pl at es, one can make t he f ol l owi ngc l as s i f i cat i on f or t he t ype of cor r ugat i on [ S3] ( Fi gure 2 . 3 ) :

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- p l a i n f i n s ( r e c t a n g u l a r , t r i a n g u l a r , s q u a r e )- wa v y f i n s- o f f s e t s t r i p f i n s- l o u v e r e d f i n s- p e r f o r a t e d f i n s- p i n f i n s2 . 3 . 3 C o n s t r u c t i o n ma t e r i a l sC o n s t r u c t i o n ma t e r i a l s a r e ma i n l y a l u mi n i u m a n d c o p p e r wh i l e s t a i n l e s s s t e e la n d o t h e r c o r r o s i o n r e s i s t a n t ma t e r i a l s a r e o n l y s c a r c e l y u s e d . T h i s i sma i n l y c a u s e d by t h e d i f f i c u l t ma n u f a c t u r i n g a n d t h e b r a z i n g o f t h ec o r r u g a t e d p l a t e s o f t h i s ma t e r i a l s ( A KG [ A 3 ] , T r a n e C o mp a n y [ T l ] ) .

2 . 4 C h a r a c t e r i z a t i o n2 . 4 . 1 I d e n t i f i c a t i o nT o i d e n t i f y t h e p e r f o r ma n c e a n d q u a l i t y o f e a c h t y p e o f s u r f a c e ( h e r ec o r r u g a t e d p l a t e ) t wo c h a r a c t e r i s t i c p r o p e r t i e s a r e u s e d , o n e f o r t h e h e a tt r a n s f e r , j , a l s o c a l l e d t h e Co l b u r n f a c t o r , a n d o n e f o r t h e f r i c t i o n , f ,a l s o c a l l e d t h e F a n n i n g f r i c t i o n f a c t o r [ K 3 ] . B o t h a r e n o n - d i me n s i o n a l a n dd e f i n e d a s f o l l o ws :

j = S t P r 2 / 3 ( 1 7 )f = 2 t o / ( p v 2 ) ( 1 8 )

wi t h : S t = S t a n t o n n u mb e r ( S t = N u / ( R e P r ) )P r = P r a nd t l n u mb e rNu = Nu s s e l t n u mb e rR e = R e y n o l d s n u mb e rt o = s u r f a c e s h e a r s t r e s s [ k g / m. s 2 ]p = ma s s d e n s i t y [ k g / m3 ]v = v e l o c i t y [ m/ s ]Wh e n t h e t h e r mo d y n a mi c p r o p e r t i e s o f t h e f l o w me d i u m a r e k n o wn , o n e c a nd e f i n e j a n d f f o r a g i v e n g e o me t r y o f t h e c o r r u g a t i o n a s a f u n c t i o n o f t h eR e y n o l d s n u mb e r R e .A n o t h e r f o r m o f p r e s e n t a t i o n wh i c h i s a l s o c o mmo n l y u s e d i n l i t e r a t u r ei s t h e f o l l o wi n g o n e :and Nu = a R e

b P r c ( h e r e c =l / 3 ) ( 1 9 )A P = f d 1/ 2 p v 2 ( 2 0 )

wi t h : a , b , c , d = f o r m f a c t o r s ( c o n s t a n t ) [ - ]A P = p r e s s u r e d r o p [ N / m2 ]S o i n t h i s wa y t h e f a c t o r s j a n d f , o r N u a n d A P , a r e c o mmo n l y u s e d t oe x p r e s s t h e p e r f o r ma n c e o f t h e t y p e o f t r a n s f e r s u r f a c e ( c o r r u ga t e d p l a t e ) .

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2.4.2 Analvtical solutionsIn Shah an d London [S5] one ca n find a great amount of analytical solutions,that is for simple geometrical confi gur ations lik e triang ul ar, rectangu larand circular channels. It is restricted to full y thermal an d hydro-dynamically developed laminar flow, that i s fully developed temperature an dvelocity pr ofil es. This giv es a constant Nusselt number, independent of theReynolds an d Prandtl num ber, only dependi ng on the geometry.For three sets of thermal b oundary conditions the Nusselt numbers ar e given:

1. con stant wall temp eratu re,2 . constant wall temperature an d heat flow i n the axial direction an d3. constant heat flow in the axial an d peripheral direction.The characteristic length i n the Nusselt n um ber an d i n the earlier mentionedReynolds number is the so called hydraulic diameter d

d h a 4 A r 1 h' (21)with: d, = hydraulic diameter [ m ]A f f flow cross sectional area [ m 2]A = total heat transfer area [ m 2]1 = flow length [ m ]For most o f th e geometries one ca n write:

h = - Q - fwith: 0 = wetted perimeter [ m ]

(21a)

For a better u nderstandin g o f the above mentioned form ul as, this means f o r arectangular channel (see Figure 2.4) :d h - i A r 1 _ 4 (w hl 1 2 w hA 2 (w + h) 1 w + h (21b)

with:w = flow passage wid th [ m ]h = flow passage height [ m ]1 = flow passage lenght [ m ]If w h than d h 2

Figure 2.4Offset strip f i ncorrugated platet = fin thickness [ m ]x = uninterrupted f i n length [ m ]

f l ow di r e ct i on

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2.4.3 Experimental results2.4.3.1 I n t r o du c t i o nT h e t y p e of h e a t a n d / o r ma s s e x c h a n g e r s t h a t i s u s e d i n t h e e a r l i e rme n t i o n e d a b s o r p t i o n h e a t p u mp ( A HP ) p i l o t p l a n t i s of t h e p l a t e f i n t y p e .T h e t r a n s f e r s u r f a c e i s c o r r u g a t e d a n d of t h e o f f s e t s t r i p f i n t y p e .F i g u r e 2. 4 s h o w s an e x a mp l e of t h i s t y pe of c o r r u g a t i o n .I n t h e f o l l o wi n g t h e l i mi t a t i o n i s m a d e t o h e a t e x c h a n g e r s of t h e p l a t et y p e a n d t o h e a t t r a n s f e r . T h i s b e c a u s e mo s t of t h e a v a i l a b l e l i t e r a t u r ed e a l s wi t h h e a t t r a n s f e r , a n d t h e e x c h a n g e r s i n t h e p i l o t p l a n t a r e o f t h e( c o r r u g a t e d ) p l a t e t y p e as s h o wn F i g u r e 2 . 4 .2.4.3.2 T h e w o r k of K a v s and L o n d o nA S t a n d a r d r e f e r e n c e i n t h i s f i e l d i s t h e b o o k " C o m p a c t H e a t E x c h a n g e r s " ,w r i t t e n by K a y s and L o n d o n [ K 3 ] , I t g i v e s a t h o r o u g h and d e t a i l e d s u r v e y onc o mp a c t h e a t e x c h a n g e r s , n o t o n l y c o n c e r n i n g t h e g e o me t r y , b ut a l s o a v e r yl a r g e a mo u n t of e x p e r i me n t a l r e s u l t s .No t o n l y f o r t r a n s f e r s u r f a c e s of s i mp l e g e o me t r y , b u t a l s o f o r v e r yc o mp l i c a t e d g e o me t r i e s e x p e r i me nt a l r e s u l t s a r e g i v e n f o r t u b e s a nd p l a t e s .F o r a l l t h e s e t y p e s d a t a a r e g i v e n f o r t h e f a c t o r s j and f as we l l as ac l e a r i n d i c a t i o n of t h e g e o me t r y . A l mo s t a l l t h e e x p e r i me n t s t o o k p l a c e wi t ha n a i r f l o w t h r o u gh t h e c o r r u ga t e d p a s s a g e s a n d a ( c o n d e n s i n g ) s t e a m f l o wt h r o u g h t h e o t h e r p a s s a g e s . F o r t h e o v e r a l l h e a t t r a n s f e r c o f f i c i n t K i nE q u a t i o n ( 1 4) o n e c a n wr i t e :

K = [ 1 /a Q + d y ^ + 1 /a s V1 (22)wi t h : a- h e a t t r a n s f e r c o f f i c i n t on t h e a i r s i d e [ W/ m2 . K ]d = p l a t e t h i c k n e s s of t h e wa l l [ mX t h e r ma l c o n d u c t i v i t y of t h e wa l l [ W/ m. K ]a = h e a t t r a n s f e r c o f f i c i n t on t h e s t e a m s i d e [ W/ m2 . K ]F o r t h i s k i n d of h e a t e x c h a n g e i t i s a l l o we d t o s a y :

a a a n d X/d L s o : K * a ( 22a)s o ' o o2.4.3.3 R e c t a n q u l a r o f f s e t s t r i p f i n s u r f a c e sS i n c e t h e c o r r u g a t e d s u r f a c e s of t h e c o m p o n e n t s of t h e p i l o t p l a n t a r e oft h e r e c t a n g u l a r o f f s e t s t r i p f i n t y p e , t h e f o c u s wi l l be on t h a t t y p e ofc o r r u g a t e d s u r f a c e s .Wi e t i n g [ W4 ] h a s g a t h e r e d al l t h e e x p e r i me nt a l r e s u l t s f o r r e c t a n g u l a ro f f s e t s t r i p f i n c o n f i g u r a t i o n s f r o m [ K 3 ] and [ S 6 ] a n d c o r r e l a t e d t hee x p e r i me n t a l h e a t t r a n s f e r a nd f l o w f r i c t i o n d a t a o v e r t wo R e y n o l d s n u mb e rr a n g e s , t h a t i s f o r Re < 1 00 0 ( l a mi n a r ) and Re > 2 0 0 0 ( t u r b u l e n t ) .A c l e a r l y d e f i n e d t r a n s i t i o na l R e , R e * , wa s n o t f o u n d . So t o mi n i mi z et h e e f f e c t of t h e t r a ns i t i o na l R e , o c c u r r i n g b a s i c a l l y b e t we e n 1 0 0 0 < R e *


31/119

j = a ( x / d h ) b - ( t / d h ) c . ( w / h ) d - Ree (23)( a , b, c, d, e en d ar e unknown coef f i ci ent s. )The Reynol ds number i s based on t he hydr aul i c di amet er d. . The r anges of t hei ndi cat ed var i abl es ar e as f ol l ows: 0. 7 < x/ d. < 5. 60. 03 < t / d" < 0. 1660. 162 < w/ hn < 1. 1960. 65 < d. < 3. 41100 < R < 10000Fur t hermore, t he area densi t y p i s i n t he range 1000 < fi < 3000.Leavi ng out t he f l ow f r i cti on f act or , W et i ng f ound t he f ol l owi ngcor r el at i ons , based on the avai l abl e heat t r ansf er dat a of 22 of f set st r i pf i n sur f aces:

j = 0. 483 ( x / d , ) - - 162. ( w / h ) " 0 ' 1 8 4 . Re" 0 ' 53 5 ,f or Re < 1000 n (24)j . 0. 242 ( x / d . ) - - 322. ( t / d . ) 0 " 0 8 9 . Re " 0" 3 6 8 ,f or Re > 2000 n n (25)

Thi s wi t h j f r om Equat i on ( 17) andNu = ( a dh ) / X (26)Re - ( fi . v dh) / r\ (27)P r = ( t) . c ) / X (28)wi t h: r? = dynam c vi scosi t y cof f i ci nt [ kg/ m s]The over al l di scr epancy between t he cor r el at i ons and the exper i ment alr esul t s i s wi t hi n 10 %For t he t r ansi t i onal Re W et i ng der i ved f r om Equat i on ( 24) and ( 25) t hef ol l owi ng equat i on f or Re*:R e * = 61. 9 ( x / d . ) 0 " 9 5 2 . ( t / d . ) " 0 - 5 3 . ( w / h ) - 1 - 1 (29)f or 1000 < Re* Re* , t han use Equat i on( 25) i n t he t r ansi t i onal r ange.For any sur f ace geomet r y of t he cor r ugat ed pl at e one can w t h t hi scal cul at e t he Col bur n modul us j as a f unct i on of t he Reynol d number Re.I n t he same way W et i ng der i ved equat i ons f or t he f r i ct i on f act or f .I f one cor r el at es Equat i on ( 24) and ( 25) f or al l t he 22 of f set st r i p f i nsur f aces, one can f i nd an " aver age" corr el at i on f or t hi s t ype of sur f ace f oral l geomet r i es: n , .j = 0. 487 Re" u- DJ D f or Re < Re* ( 30)j = 0. 149 Re - 0 ' 3 6 8 f or Re > Re* ( 31)w t h Re* = 1185

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For t he use of t he Equat i on ( 24) and ( 25) and even mor e of t he Equat i on ( 30)and ( 31) one shoul d be car ef ul i n extr apol at i ng dat a f or st r i p f i ngeomet r i es t hat have geomet r i cal par amet er s out si de t he r ange of t hose f ort he cor r el at i ons. Thi s one i s st r i ct l y based on a l i m t ed amount of r epor t edt est dat a. Al so t hese cor r el at i ons may be appl i cabl e onl y f or ai r or gas ast he wor k i ng f l ui d (Pr 1) .Mochi zuki and Yagi [ M2] have done exper i ment al r esear ch wi t h al um ni umt est cor es of pl at e f i n t ype heat exchanger s wi t h ( of f set ) st r i p f i n s , al sousi ng ai r as t he t est f l ui d and condensi ng st eam as t he ot her medi umSeven t ypes of st r i p f i n sur f aces wer e t est ed, wi t h a const ant hei ght h( 10 mm) and f i n t hi ckness t ( 0. 2 mm) . They wer e mai nl y i nt er est ed i n t hei nf l uence of t he f i n spaci ng ( wi dt h) w and t he f i n l engt h x.The r ange of t he par amet er s i s here i n t he same range as t hose bel ongi ng t ot he dat a W et i ng [ W4] used i n Equat i on ( 23) , except f or t he hydr aul i c

di amet er dh, her e 3. 04 and 4. 35 mm The cor r espondi ng area densi t i es ,8 are700 and 1000 m2/ ni 3.They al so pr esent ed t hei r r esul t s i n t he f or m of t he Col bur n f act or jf or t he heat t r ansf er and the Fanni ng f act or f f or t he f l ow f r i ct i on as af unct i on of t he Reynol ds number Re, but i n a di f f er ent way t han W et i ng di d.Her e t he wal l sur f aces of t he f l ow passages are i nt er r upt ed i n t he f l owdi r ect i on and t he pr ocess i s r epeat i ng i t sel f s i nce t he ai r f l ows al ong t hever y shor t st r i p sur f ace and t hen separ at es at t he t r ai l i ng edge of t hes t r i p. Thus t he boundar y l ayer i s never abl e t o become t hi ck. Ther ef or e, i ft he pr essur e l oss i s consi der ed to consi st of t he t wo ef f ect s of f or m dr agof t he f i n and a f r i ct i on dr ag of t he f i n sur f ace owi ng to the v i scosi t y of

t he f l ui d, t hen t he ski n f r i ct i on f act or f may be expr essed i n t he f ol l owi ngf o r m Qf = a + b Re 1 , (32)and a s i m l ar expr ess i on f or t he Col bur n f actor j :

j = c + d Re " 1 , 0 (33)For each t ype of st r i p f i n sur f ace t hey def i ned a set of a, b, c and d.For compari son t he f ol l owi ng aver aged cor r el at i on f or t he Col bur n f act or wasder i ved f rom t hei r dat a:

j = 4. 72 10" 3+ 10. 06 Re" 1 - 0 f or 1000 < Re < 8000 ( 34)For compar i son wi t h the cor r el at i ons of W et i ng f or t he Reynol ds numberr ange 2000 - 8000, Equat i on ( 34) was t r ansf or med i nt o the f ol l owi ng one:

j * 0. 135 Re " 0 - 3 5 0 f or 2000 < Re < 8000 ( 35)W t hi n t hi s r ange t he maxi mum devi at i on i n j f r om Equat i on ( 31) i s l ess than6 % .

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2. 5 Di scuss i onShah and Webb [ S3] have di scussed bot h t he r es ul t s of W et i ng and Mochi zukiand Yagi .They st ar t ed wi t h a t heore t i cal sol ut i on. For t he heat t r ansf er t heytook t he Pohl hausen l am nar boundar y l ayer sol ut i on f or a f l at pl at e of" pl at e l engt h" x [ S7] :

j = 0. 664 Re 0 - 5 ( 36)and fo r the f r i c t i o n the mo d i f i ed B las iu s lam ina r bounda ry l ay e r so lu t i o nfo r a f l a t p l a t e us ing the fo rm drag asso c ia ted wi t h the lead ing b lu n t edgeo f t h e s t r i p f i n [ S 7 ] :

f =(C d to ) / ( 2 x) + 1.328 R e x 5 (37)wi t h: C. = f or m dr a g c of f i c i nt [ - ]In order t o i ndi cat e and compar e t he per f or mance of of f s et s t r i p f i nsur f aces , one can use t he factor j / f as an i ndi c at i on.Based on t he Reynol ds anal ogy f or f l ow over a f l at pl at e, i n t h e absence off or m dr ag ( C . = 0 ) , t he f ac to r j / f shoul d be 0. 5 ( f or P r * l ) . Si nce t hecont r i but i on of t he f or m dr ag i s of t he same or der of magni t ude as the s ki nf r i ct i on dr ag f or such an i nt er r upt ed sur f ace, t he f ac to r j / f wi l l be about0. 25. I f one takes t he c or r el at i ons of W et i ng of Equat i on ( 30) and ( 3 1 ) , onewi l l f i nd t hat f or t he ranges Re < 1000 and Re > 2000 t he f ac to r j / f i ssmal l er t han 0. 275. Even i n t he t r ans i t i onal r ange t he factor j / f does notexceed 0. 30.Mochi zuki and Yagi used t he factor j / f t o det er m ne t he i nf l uence oft he geomet r i cal f act or x/ w and t he Reynol ds number Re. They f ound, wi t h anegl i gi bl y smal l dependence on t he Reynol d number Re, f or t he factor j / fthat 0. 2 < j / f < 0. 6.Shah and Webb have t hei r doubt s about t he r es ul t s of Mochi zuki and Yagibecause t he factor j / f > 0. 3 and st at ed that " publ i shed data f or s t r i p f i n sar e quest i onabl e i f j / f > 0. 3. Al l t he meas ur ement s and possi bl e sour ces off l ow l eaks and heat l osses must be checked t hor oughl y f or al l t hose basi cdat a havi ng j / f > 0. 3 f or s t r i p f i n s . "Dubr ovski i and Fedot ova [ Dl ] , [ D2] i nvest i gat ed an o f f s e t s t r i p f i nsur f ace, not a r ect angul ar one but wi t h a sl i t f or m wi t h d, = 2. 9 mm and = 1280 m2 / m3. nThey f ound t he f ol l owi ng expr ess i ons f or j and f :

j = 0. 090 Re " 0 - 3 0 f or 800 < Re < 3250 (38)f = 1. 590 Re " 0 , 2 7 f or 1500 < Re < 3250 ( 39)

As one can s ee, i n t hi s r ange t he factor j / f i s al most i ndependent of thatReynol ds number and t he factor j / f = 0. 045. Compar ed wi t h ear l i er r esul t st h i s i s rather l ow.

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Also attempts have been made to predict the factors j and f numericallyby a fin ite di ff erenc e method for the offset strip fins consid erin g thelam i nar boilndary lay er on each strip fi n. In [ S3] Sparr ow [S8] and Patank ar[PI] are men tion ed. They comp ared the num erical resul ts with a strip finsurf ace from Kays and London [K3] and f ound a reasonab le agr eement f or thefac tor f, bu t the predi cted j factors were about 100 % hig her. The p redi ctedslopes of both j and f ver sus Re cu rv es were steeper than the test d ata.One of the major unp redic table factors, mention ed in both [ S3] and

[ K 3 ] , is the exi stenc e of small b ur rs at the leadi ng and trai lin g edges ofthe fin du ri ng its form ation by a shearin g op erati on. Fins of this type aregeneral ly constru cted by a machin e-cu tting process that inevitabl y l eaves aslightly bent and grazed fin edge that differs depending upon the finmaterial and character of the cutting tooi. These bur rs i nc rease theeffecti ve pl ate thick ness, causing incr eased form drag. This factor can notbe taken in to accoun t accu rately in the num erical solu tion s, nor can thein fl uen ce on the experi mental data be estimated ac cu rately . Theref ore, thepossibie exi stence of burrs causes uncertainty in the corr elations or in thecom pari son of pr edic ted val ues with data. Bri gg s and London [B8] have paidsome attention to this point.

So far the theoretic al, nu merical and experim ental inf ormation found onthe perf orman ce of rectangu lar offset strip fin surfaces in l iteratur e.

2.6 App li cation in sorption svstemsIn the last section of this chapter on compact heat and m ass exc hang ers, aglobal compari son will be mad e between the appli cation as describ ed in thesections bef ore and the appl ic ation in a sorption system, in par tic ul ar inthe AHP test pl ant. The main imp ortant d if fer enc es are pointed out.

First of all should be menti oned that one has to do with sim ul taneousheat and/ or mass transfer and/ or wi th a chang e of phase, not only w ith heattransf er. In literature only inf ormation is found c oncern ing pure heattra ns fe r.In the cases of the analytical soluti ons and the experi mental resul tsthe flow was in the di rec tion of the "most open" side of the cor ru gatedp assag es. In the exc hang ers of the AHP the flow s on both sid es - both si deshave cor ru gated passages - are in the di rec tion of the "most cl osed" side ofthe corrugated passages. Because the strip fins are offset, the passaqes are

not comp letely block ed.The worki ng fluid flowin g through the corru gated passages was air, withon the other side con densin g steam. In the AHP, in the secondary ci rc ui ts(for cooli ng or heatin g) water or liqui d methanol wer e used as medi a whi lein the primary ci rcu its lithium b rom id e/ zinc b romid e - methanol (theabsor ben t or m ix tu r e) and methanol (the sol ve nt), l i q u i d a n d / o r va po u r , we rethe media.To have a rough ind ication of the therm ody nam ic pr oper ties of thesemed ia, in com pari son wi th those of air , one can fin d in Figu re 2.5 thePrandtl num ber Pr, the kin ematic v iscosity v and the thermal cond uc tivi ty \over a cer tain temperatur e range for the di ff erent m edi a. By this one canestim ate the possib ie inf lu ences on the heat transfer , k eeping in min d theequ ations for the Nusselt numb er, the Reynol ds nu mb er and the Prandtl nu mb er(Equation (26), (27) and (28)) and the heat tran sfer co rr el ati on of Equ ation

( 1 9 ) .

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100 -

5 0 -

20-

10-

0.5-

0.2

PrandNX-10*(W/m-K]V-10* (*/$)

r-6020 20 r 80 100 120

- T ( C)

Figure 2.5 The Prandtl number Pr, the kinematic viscosity u an d thethermal conductivity A as a function o f t he temperature Tfor:air (A), water (B), methanol (C liquid, D vapour) an d theworking pair lithium bromide zinc bromide - methanol (E)

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I n t h i s t h e k i n e m a ti c v i s c o s i t y v i sV = v / P [ m 2 / s ] ( 4 0 )

T h e s e c o n d a r y p a s s a g e s o f t h e e x c h a n g e r s i n t h e A H P a r e c o m p l e t el y fi l l edw i t h t h e l i q u i d f l o w m e d i a , t h e p ri m a r y p a s s a g e s , e x c e p t f o r th e m i x t u r eh e a t e x c h a n g e r , a r e n o t . I n t h e a b s o r b e r t h e r e i s a ( l i q u i d ) f i l m f l o w o v e rt h e f i n s a n d a v a p o u r f l o w i n b e t w e e n . I n t h e e v a p o r a t o r a n d c o n d e n s e r t h e r ei s a l i q u i d a n d v a p o ur f l o w , t h a t i s a n e v a p o r a t i n g a n d c o n d e n s i n g f i l mf l o w . T h e g e n e r a t o r i s l e ft o u t , t h i s e x c h a n g e r i s n o t a C H M E .F o r h e at e x c h a n g e r s o p er a ti n g w i t h h i g h - d e n s i t y f l u i d s ( i n t h i s c a s el i q u i d s ) t h e f r i c t i o n p o w e r i s g e n e r a l l y s ma ll r e l a t i v e t o t h e h e a t t r a n s f e rr a t e , s o h a s n o t a g r e a t i n f l u e n c e . B ut f o r l o w - d e n s i t y f l u i d s l i k e a i r , a so n e c o u l d s ee i n th e s e c t i o n s b e f o r e , t h e f r i c t i o n p o w e r i s o f th e s a m eo r d e r a n d e v e n g r e a t e r t h a n t h e h e at t r a n s f e r r a t e . A s s ai d b e f o r e , t h e h e a tt r a n s f e r c o f f i c i n t o n t h e l i q u i d s i d e i s 10 - 1 0 0 t i m e s g r e a t e r t h an o nt h e v a p o u r / g a s s i d e . S o f o r t h e e x c h a n g e r s i n t h e t e s t p l a n t , n o t t h e f a c t o rj / f i s o f i m p o r t a n c e b u t t h e C o l b u r n f a c t o r j .T h e e x p e r i m e n t a l r es u lt c a m e a l w a y s f ro m a c r o s s - f l o w c o n f i g u r a t i o n t o k ee pt h e h e ad e r c o n s t r u c t i o n s i m p l e , i n t h e e x c h a n g e r s o f th e A H P a l w a y s c o u n t e r -f l o w t o o k p l a c e . O n l y t h e p o s i t i o n o f t h e h e a d e r s c o u l d g i v e v e r y a s m al lc r o s s - f l o w i n f l u e n c e .I n s h o r t , t h e t h e r m o d y n a m i c p r o p e r t i e s a n d t h e p h a s e o f t h e c h o s e n m e d i a asw e ll a s t h e f l o w an d c o r r u g a t i o n c o n f i g u r a t i o n o f t h e e x c h a n g e r s i n t h e A H Pt e st p l a n t f o rm t h e m a j o r d i f f e r e n c e s w i t h t h e i n f o r m a t i o n f o un d i nl i t e r a t u r e .T o f i n d o u t w h e t h e r o r n o t t h e c o r r e l a t i o n s f o u n d i n l i t e r a t u r e f o r h e a tt r a n s f e r a r e i n w h a t e v e r f o rm a p p l i c a b l e f o r th e h e at t r a n s f e r p r o c e s s e s i nt h e e x c h a n g e r s o f t h e t es t p l a n t , a c o m p u t e r s i m u l a t i o n m o d e l i s d e v e l o p e df o r a l i q u i d f l o w o v e r a o f f s e t s t r i p f i n s u r f a c e .I n c o m b i n a t i o n w i t h t h e r e s u l t s o f t h e e x p e r i m e n t s w i t h t h e t e st p l a n t h e a ta nd m a s s t r a n s f e r c o r r e l a t i o n s w i l l b e s e ar c h e d w h i c h c a n b e c o m p a r e d w i t ht h o s e f o u n d i n l i t e r a t u r e . B o th t h e e x p e r i m e n t s a n d t h e s i m u l a t i o n m o d elw i l l b e d i s c u s s e d l a t e r .

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CHAPTER 3. THE ABSORPTION HEAT PUMP TEST PLANT

3.1 IntroductionDurin g the research work of Iedema [II] an exp eri men tal absorp tion heat pump(AHP) test plant was built. Experiments took place mainly to test theabsorber. Furthermore to investigate the possibilities of the working pairCH3OH - LiBr / Z n Br 2.The absorber was a horiz ontal tube bun di e heat and m ass exc hanger witha liqu id film f lowing over and dri pp in g between the tubes (Figure 3.1).

Figu re 3.1 Film fl ow over a row of horiz ontal tub esThis type of fil m f low giv es a good heat and mass tran sfer in the absorp tionpr ocess because the flow is frequ ently in terrup ted. So the fil m flow, andwi th that the formation of the thermal and concen tration b ound ary layerswill be disturb ed. This breakin g-u p, when f alling from one tube andsplashing on the next one, gi ves a good m ixi ng of the liq uid film , leadingto a more homogeneous temperatur e and c oncen tration d istri bu tion over thefilm thickness.

So in this test plant, the absorber was the main subject ofin vestigation and exp eri men ts. The other com ponents in this plant wer e alsoconventional heat and/or mass exc hang ers: a shell and tube bath evap orator,a coaxiall y wound condenser and a electri call y heated b ath gener ator. Themi xtur e heat exchanger w as of the plate typ e, but with a rather badperformance.All these component were only to form an AHP cycle in which this typeof absorber could be tested. So the absorb er was the determ in ativ e andlimi ting comp onent in the cy cl e.The test pl ant was desig ned to have a total heat outp ut Q (fromabsorber and con denser ) of about 10 kW. The result of the exper im ents wi ththis absor ber and this AHP cy cl e as a whol e, can be foun d in [ II] and [ S2].In the present research wor k a next step is mad e to dev elop an AHP fordom estic heatin g as descr ib ed in Secti on 1.6.

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To t hi s , one mus t still keep i n m nd t hat t he goal i s t o devel op such a AHP,i n combi nat i on wi t h an addi t i onal heat i ng syst em t hat can meet t he requi r edheat out put and heat i ng t emper at ur e at a gi ven out door t emper at ur e i ncombi nat i on wi t h an accept abl e cof f i ci nt of perf ormance ( ener get i cef f i ci ency) and occupyi ng an accept abl e space i n t he house ( vol ume) .To meet t hese demands t o a cer t ai n l evel t he i dea was gener at ed t oi nvest i gat e t he possi bl e appl i cat i on of so cal l ed compact heat and/ or massexchanger s ( CHME) as component s i n an AHP.So an evapor at or , a condenser , an absor ber and a m xt ur e heat exchangerof t hi s t ype wer e i nst al l ed whi l e t he " ol d" absorber wi l l now be used as agenerator.3. 2 Choi ce of t he worki ng pai rBecause of t he knowl edge of and t he exper i ence with it, t he same wor ki ngpai r as used by I edema [ I I ] wi l l be used i n t he new t est pl ant :l i t hi um br om de L i Br / z i nc br om de ZnBr 2 ( 2: 1 mol ) and met hanol CH30H, soan al cohol / sal t m xt ur e. I n t he absor pt i on cycl e t he al cohol i s t he sol ventand a al cohol / sal t m xtur e the absorbent .W t h t hat wor ki ng pai r a good qual i t at i ve and quant i t at i ve anal ys i s canbe made of t he per f or mance of t hi s t ype of exchanger s i n an AHP by means ofexper i ment s.3. 3 Type of compact heat and/ or mass t r ansf er sur f aceAs ear l i er ment i oned t hi s t ype consi st s of a st ack of par al l el pl ai n andcor r ugat ed pl ates, br azed or wel ded t oget her . Thi s cor r ugat ed pl at e w l l becal l ed " t he cor r ugat i on" .So t he space bet ween the par al l el pl ai n pl at es i s f i l l ed wi t hcor r ugat i ons , i n t hi s case even on bot h si des and of a di f f erent t ype. Thet ype of t he cor r ugat i on i s mai nl y det er m ned by t he ki nd of medi um ( m xt ur e,wat er , met hanol , o i l ) , t he st at e of t hat medi um ( l i qui d, l i qui d f i l m vapouror a combi nat i on) and the t ype of pr ocess t aki ng pl ace t hat i s heat and/ ormass t r ansf er , somet i mes i n combi nat i on wi t h a change of st at e.I n al l t he CHME component s t he cor r ugat i ons ar e of t he of f set st r i p f i nt ype. Wher e t her e i s a l i qui d ( f i l m f l ow, somet i mes i n combi nat i on wi t h avapour f l ow, t hr ough t he cor r ugat ed pass ages, t he f l ow i s not i n t he " open"di r ec t i on of t he st r i p f i n but at r i ght angl es t o it. I n t hat conf i gur at i ont he heat and/ or mass t r ansf er wi l l be much bet t er . There wi l l be a mor et ur bul ent f l ow because of t he const ant change i n f l ow di r ect i on.Wher e a vapour f l ow i s t he mai n f l ow ( condenser and evaporator ) , t he f l ow i si n t he " open" di r ect i on. Thi s because pr essur e l osses ar e f ar mor e gr eat eri n t he vapour phase t han i n t he l i qui d phase.I n gener al t hr ee types of cor r ugat ed pl at e sur f aces of t he of f set st r i pf i n t ype ar e appl i ed:a. I n t he pr i mary c i r cui t s of t he absor ber ( l i qui d f i l m f l ow of m xture ) andof t he evapor at or ( vapour and l i qui d f i l m f l ow of met hanol ) , a t r apezi umt ype ( F i gur e 3. 2a) : m unFi gur e 3. 2a The t r apez i um t ype of f set s t r i p f i n

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b. I n t he secondar y ci r cui t s of t he condenser ( wat er ) and t he evaporator( met hanol ) , a wavy t ype ( Fi gur e 3 . 2 b ) :

Figure 3.2b The wavy t ype of f set st r i p f i nc. On bot h si des i n t he (mxture) heat exchanger, on t he cool i ng si de i n t h eabsorber and on t he condensat i on si de i n t he condenser, a r ect angui ar t ype( Fi gur e 3 . 2 c ) :

- 111 ,11J

111

111

Fi gur e 3. 2c The r ectangui ar t ype of f set st r i p f i n3. 4 Over vi ew of t he AHP t est pl antFor a bet t er under st andi ng of t he whol e AHP t est pl ant , an over vi ew o f t h et est pl ant i s shown by t he schemat i c f l ow sheet i n Fi gur e 3. 3.The desi gn quant i t i es of t he AHP t est pl ant ar e gi ven i n Tabl e 3. 1.

Tabl e 3. 1 Desi gn quant i t i esmaxi mum heat pr oduct i on Qmaxi mum r i ch m xt ur e mass f l ow Mmaxi mum sol vent mass f l ow Mheat i ng t emperat ur e generat or Tcool i ng t emper at ur e abs. / cond. fheat i ng t emperat ur e evaporat or T

10 kW0. 1 kg/ s3. 5 g/ s100 - 120 C30 - 50 C- 15 - +15 CGi ven t he chosen worki ng pai r , t hi s means a pressure P. i n t he generator-condenser sect i on of 20 - 70 kPa and a pressure P, i n t he absorber-evapor ator sect i on of 2 - 7 kPa .

Al so f rom thi s t he heat f l ows can be der i ved and, wi t h an est i mat i on oft he appeari ng t emper atur e di f f erences and t he over al l heat t r ansf ercof f i c i nt , t he r equi r ed t r ansf er sur f aces can be cal cul at ed r oughl y (seeTabl e 3. 2) .Tabl e 3. 2component

absor bergener at orcondenserevapor at orheat exch.

The r equi r edr equi r edheat f l owQ [ kW

664410

heat f l ows and t r ansf er sur f acesest i matedt emp. d i f f .AT [ K]

553310

est i mat ed heatt rans f er coef f .K [ J / kg. K]600300750750200

r equi r edtransfers u r f . [ m2]2. 04. 01. 81. 85. 0

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( ^ P u m p

e l e c . h r a l i n g

Figure 3.3 An overview of the AHP test plantIn the next section a description o f the components in the AHP test plant isgiven, such as the geometry o f t he transfer surfaces, the constructionmaterials, the processes o f heat and/or mass transfer taking place and thegeometry an d construction o f t he component itself.A more detailed descri ption c a n b e found i n v / d Welle [W5] .

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3.5 The components3.5.1 The absorberIn the absorber a process of simultaneous heat an d mass transfer takesplace, the vapour coming from the evaporator i s absorbed b y th e liquidmixture (poor to r i c h ) , rejecting the heat o f condensation an d mixing. Thatheat is removed by a water flow on the other side.The absorber (Figure 3.4) is a construction of stainless steel, thecorrugation ar e made of Inconel 600 and the rest of the absorber o f AISI3 1 6 . It consists of 10 primary passages (h = 8 m m) and 11 secondary passages(h = 3 m m ) , the plate thick ness is 1 mm . The plate transfer surface has alength o f 7 0 0 m m an d a width o f 240 m m .

poor mixture (in)

cooling water (in )

cuo l ing wate r (ou t )

vapour

rich mixture (out)Figure 3.4 The absorber

As one can see in Figure 3.4, in the primary passages only 160 m m o f thatwidth is wetted by the liquid, over the other 60 mm the corrugations ar eplaced in the opposite direction to get a better distribution of the vapourflow, giving a better contact with the mixture an d reducing the pressurelosses. Takin g this all into account, the absorber has a total (plate)transfer surface of about 2.2 m 2.29


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3. 5. 2 The condenser and the evapor at orThe super heat ed vapour f r om t he generat or i s f ed t o t he condenser . I t w l lf i r st cool down ti l l t he condensat i on t emper atur e i s r eached and t hencondense, f or m ng a ( gr owi ng) l i qui d f i l m f l ow. The r ej ect ed heat i s r emovedby a wat er f l ow on t he ot her si de of t he exchanger .The condenser i s a compl et e al um ni um const r uct i on. I t consi st s of 10pr i mar y passages (h = 10 mm) and 11 secondar y passages (h = 3 mm) . The pl at et r ansf er surf ace has a l engt h of 370 mm and a wi dt h of 250 mm gi vi ng at ot al ( pl at e) t r ansf er sur f ace of about 1. 8 m2 .I n t he evapor at or t he l i qui d sol vent ( met hanol ) com ng f r om t he condenservi a an expansi on val ve i s evaporat ed by addi ng a heat f l ow f r om t he ot hersi de of t he exchanger . I n t hat secondar y ci r cui t , al so met hanol i sci r cul at i ng t o be abl e t o achi eve evaporat i ng t emperat ur e bel ow 0 C ( 2 7 3 K ) .On t he si de of t he evaporat i ng methanol t he exchanger i s open, t here are noheader s. The whol e i s pl aced i n a gl ass/ st ai nl ess steel cont ai ner , wi t h adi st r i but i on t r ay on t op of t he exchanger . Thi s component i s al so made ofal um ni um and ha

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