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    P e r g a m o nI n t . C o m m . H e a t M a s s T r a n sf e r V o l . 2 5 , N o . 5 , p p . 7 1 1 - 7 2 1 , 1 9 9 8C o p y r i g h t @ 1 9 9 8 E l s e v i e r S c i e n c e L t dP r i n t e d i n t h e U S A . A l l r i g h t s r e s e r v e d0 7 3 5 - 1 9 3 3 / 9 8 1 9 . 0 0 + . 0 0

    P I I S 0 7 3 5 - 1 93 3 9 8 ) 0 0 0 5 8 - X

    C O M P A R I S O N B E T W E E N A M M O N I A - W A T E RA N D W A T E R - L I T H I U M B R O M I D E S O L U T I O N S IN

    V A P O R A B S O R P T I O N R E F R I G E R A T I O N S Y S T E M S

    I . H o r u zU n i v e r s i t y o f U l u d a gF a c u l t y o f E n g i n e e r i n g a n d A r c h i t e c tu r eD e p a r t m e n t o f M e c h a n i c a l E n g i n e e r in g

    1 6 0 59 G o r u k l e , B u r s aT u r k e y

    C o m m u n i c a t e d b y J .P . H a r tn e t t a n d W , J . M i n k o w y c z )

    A B S T R A C TThe study included an investigat ion to analyze the V apor Abs orpt ion Refr igerat ion VA R)systems using am mo nia-wa ter and water - l i th ium brom ide solut ions. A fundamental VA Rsystem is descr ibed and the operat ing sequence is explained. S ince the m ost com mon VA Rsystems use ammonia-water solut ion wi th ammonia as the ref r igerant and water - l i th iumbrom ide solut ion wi th w ater as the ref r igerant , the com par ison of the two is presented inr espec t o f t he coef f ic i en t o f pe r fo rmance COP) , t he coo l ing capac i ty and the m ax imum andminim um sy stem pressures. I t is concluded that the VA R system using water - l i th ium bromidesolut ion provided bet ter per formance than the system using ammonia-water solut ion.How ever , there are some points to be considered such as; the d anger o f crysta l l izat ion andimposs ib i l i ty o f opera t ing in ve ry low t em pera tu r es because o f t he use o f wa te r a s t her e f r i g e r a n t . 1 9 9 8 E l s e vi e r S c i e n ce L t d

    In t roduc t i on

    A Vap or Absorpt ion Refr igerat ion VA R) System is s imi lar to a Va por Compression Refr igerat ionVCR) System. In both systems the required ref r igerat ion is provided by ref r igerants vapor iz ing in the

    evaporator . Howev er , in the V AR System, a physico-che mical process replaces the m echanical process ofthe VC R sys t em and hea t r a the r t han a mechan ica l and e l ec t ri ca l ene rgy i s used . The advan tages o f th i ssystem l ie in the possibi l i ty o f u t i l iz ing of waste energy f rom indust r ia l p lants as wel l as of using solarenergy. 711

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    712 I . Horu z Vo l . 25 No . 5

    As F igu re 1 i llu s tr a tes the fundam en ta l V AR cyc le con ta ins fou r main com ponen ts ; a genera to ran abs o rber a condens e r and an evapora to r . The condens e r and the evapora to r func t ion in the s am e m ann era s t h e y d o i n t h e V C R c y c le .

    2

    Restrictor3

    Evaporator I

    E I G e n e r a t o r ]

    9 t :Restrictor on10FIG . 1

    T h e s c h e m a ti c i l l u s tr a t io n o f t h e f u n d a m e n t a l V A R c y c l e

    P r e s su r e -t e m p e r a tu r e a n d P r e s su r e - en t h a lp y d i a g r a m s o f t h e f u n d a m e n t a l V A R c y c l e a r e s h o w n i nFigu re 2 an d Fig ure 3 respectively .

    F I .~ It .~ Peon

    5 :il10 Pevap

    i

    Solution Temperature

    F IG . 2T h e p r e ss u r e -t e m p e r a tu r e d i a g r a m o f t h e f u n d a m e n t a l V A R c y c l e

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    Vol. 25, No. 5 VAPOR ABSORPTION REFRIGERATION 713

    f/1 / t J

    3

    I

    Pcvap

    Enthalpy

    PconT1

    FIG, 3The pressure-enthalpy diagram of the fundamental VAR cycle

    The combination of the generator, the absorber and solution pump may be thought of as a means oftransferring refrigerant vapor from the low pressure side of the cycle back to the high pressure side, thefunction performed by the compressor of the VCR system.

    The VAR cycle uses a refrigerant-absorbent solution rather than pure refrigerant as the workingfluid. The absorbent acts as a secondary fluid to absorb the primary fluid which is the refrigerant in itsvapor phase.

    The refrigerant-absorbent solution passing through the solution pump is referred to as strongsolution, being relatively rich in refrigerant. The solution returning from the generator to the absorbercontains only a little refrigerant and is therefore referred to as weak solution.

    In the absorber, weak solution meets refrigerant vapor from the evaporator. The solution absorbsthe vapor, giving out heat as it does so, until all the vapor is absorbed to produce strong solution. Theprocess occurring in the absorber is normally referred to as absorption, but it can also be thought of ascondensation of a binary mixture, where the vapor phase contains predominantly one component.

    Solution from the absorber 5) is transferred to the high pressure side of the circuit by the solutionpump. Between the solution pump and the generator is a solution heat exchanger SHE) where the cold

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    714 I . Horuz Vo l . 25 , No . 5

    s t rong s o lu tion from the abs o rber (6 ) i s in c oun te r f low to the ho t weak s o lu t ion coming f rom the genera to r(8). T he s t rong solut ion is thu s heated an d the w eak solut ion is cooled in th is process . Th e s t ron g solut ionen te r ing the genera to r (7 ) i s s t il l s ubcoo led a t the h igher p res s u res p resen t the re . T he h ea t s ou rce to thegenera to r m us t the re fo re f i rs t hea t i t to the s t ate where i t beg ins to bo i l. A s i t bo i l s , evo lv ing the r e f r ige ran tvapor wh ich pas s es to the condens e r , the s o lu t ion in the genera to r becomes p rog res s ive ly poore r inr e f r ige ran t and f ina l ly l eaves the gene ra to r a s weak s o lu t ion (8) . Af te r pas s ing th rough the SHE (9 ), thewea k s o lu t ion a t h igh p res s u re pas s es th rough an expans ion va lve back to the abs o rber (10 ), co mple t ing thecyc le . I t can be s een tha t the S HE i s impor tan t in tha t the more hea t the s t rong s o lu t ion r ecovers f rom theweak s o lu t ion l eav ing the genera to r , the l e ss i t needs f rom the ex te rna l s ou rces in the genera to r .

    T h e V A R c y c l e w i d e l y u se s a m m o n i a - w a t e r s o l u ti o n w i t h a m m o n i a a s t h e r e fr i g er a n t a n d w a t e r -l i th ium b romide s o lu t ion w i th w a te r a s the r e f rige ran t.

    T h e s t u d y w i ll c o n c e n t ra t e o n t h e c o m p a r i so n b e t w e e n a m m o n i a - w a t e r a n d w a t e r - li t h iu m b r o m i d esolut ions .

    The coef f i c ien t o f pe r fo rmance (C OP) i s a m eas u re o f a cyc le s ab i l i ty to t r ans fe r hea t be tw eenvar ious t empera tu re l eve ls . S ince the p r im ary us e o f the V AR s ys tems has been fo r r e f r ige ra t ion pu rpos es ,the c onven t iona l de f in i t ion o f the CO P i s:

    Hea t t ake n in a t low tem pera tu re ( in the evapora to r )C O P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

    Ne t energy s upp l ied ( in the genera to r )O

    I n th e V A R s y s te m , h e a t e n e r g y b e i n g r e du c e d i n te m p e r a t u r e f r o m T g e n ( i n t h e g e n e r a t o r) t o T a b s( in the abs o rbe r ) p rov ides the d r iv ing fo rce to l i f t hea t f rom Tevap to Tcon . Ca m ot cyc le opera t ing be tweenthes e tempe ra tu res s e ts an uppe r l imi t to the C OP o f ;

    T g e n - T a b s T e v a pC a m o t C O P . . . . . . . . . . . . . . . . . . . . . .. . .. . . .. . .. . ..

    T g e n T c o n - T e v a p(2)

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    V o l. 2 5 , N o . 5 V A P O R A B S O R P T I O N R E F R I G E R A T I O N 7 15

    C o m p a r i s o n B e t w e e n A m m o n i a W a t e r A n d W a t e r L i th i u m B r o m i d e S o lu t i o n s

    The a mm onia -w ate r V AR s ys tem has been us ed in la rge capac i ty indus t r i a l app l i ca t ions r equ i r inglow tempera tu res fo r p roces s coo l ing . The w a te r - l i th ium b romide VA R s ys tem i s u s ed to p roduce ho t wa te rfo r comfor t hea t ing , p roces s hea t ing and domes t i c pu rpos es , a s we l l a s fo r coo l ing . Th i s s ys tem can a l s o b eus ed to de l ive r hea t a t a t em pera tu re h igher than tha t o f the d r iv ing hea t s ou rce , bu t i t i s p r edomina n t ly fo ra i r - cond i t ion ing app l i ca tions .

    The ma in p rob lem w i th the wa te r - l i th ium b rom ide pa i r i s the pos s ib i l i ty o f s o l id fo rmat ion . S incethe r e f r ige ran t tu rns to i ce a t 0 C (32 F ) , the pa i r can no t be us ed fo r low tempera tu re r e fr ige ra t ion .Fur the rmore , l i th ium b romide c rys ta l l i zes a t modera te concen t r a t ions . When the abs o rber i s a i r coo led ,thes e con om tra t ions t end to b e r eached ; thus , the pa i r i s u s ua l ly l imi ted to app l i ca t ions in wh ich theabs o rbe r i s wa te r coo led . I t i s pos s ib le tha t the us e o f a c om bina t ion o f s a l ts a s the abs o rben t w i l l r educe thec rys ta l l iz ing t enden cy enough to pe rm i t a i r coo l ing . F igu re 4 s hows the c rys ta l l iza t ion t empera tu re o f thewate r - l i th ium b romide s o lu t ion aga ins t the L i th ium b romide concen t r a t ion . O ther d i s advan tages o f thewate r - l i th ium b romide pa i r a r e thos e as s oc ia ted w i th low p res s u re and w i th the h igh v i s cos i ty o f thes o lu tion . Thes e l a t t e r d is advan tages a r e l a rge ly overcome by p roper equ ipme n t design . The c om bina t iondoes have the advan tages o f h igh s a fe ty , h igh v o la t i l i ty ra t io , h igh a f f in i ty , h igh s t ab i l i ty and h igh l a ten theat .

    300 w250~ 200 ~,

    150 ~~ 1oo ~

    50 ~.~ 0 /NN 4 o ;

    , 100 [-150- 2 0 00 I 0 2 0 3 0 40 50 60 70 80

    LithiumBromideConcen~afion,weightpercent90 100

    FIG . 4The c rys ta l l iza t ion t empera tu re o f the w a te r - l i th ium b rom ide s o lu t ion

    a g a i n st t h e m a s s c o n c e n t r a t io n o f t h e L i t h i u m b r o m i d e

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    716 I. Horuz Vol. 25, No. 5

    Probably the major disadvantages of the ammonia-water system is the fact that water is reasonablyvolatile, so that ammonia vapor leaving the generator usually contains appreciable amount of water vapor,which, if allowed to pass through the condenser and go into the evaporator, will raise the evaporatortemperature and reduce the refrigerating effect by carrying unvaporised refrigerant out of the evaporator.For this reason, the efficiency of the ammonia-water VAR system can be improved by the use of ananalyser and a rectifier which function to remove the water vapor from the mixture leaving the generatorbefore it reaches the condenser. The analyser is essentially a distillation colunm that is attached to the top ofthe generator.

    The ammonia-water cycle must be more complex than the water-lithium bromide cycle to provideacceptable performance. Better heat recovery means is required and because of the cycle s complexity, thedesign for optimum performance, based on a set of design parameters, requires extensive calculations.

    Most water-lithium bromide VAR machines meet load variation and maintain chilled watertemperature control by varying the reconcentration rate of the absorbent solution. Because of the lowoperating pressures of these machines, atmospheric air can leak into units which are improperly operated ormaintained. Concentration control to avoid crystallization is required, however, they are comparativelytrouble-free and simple to operate.

    All calculations and graphs are based on the system presented in Figure 1. For these calculationsthe efficiency of the solution heat exchanger is chosen to be 0.6. The limitation caused by the crystallizationof the lithium bromide is also shown in the graphs. The operating range of the water-lithium bromide VARsystem is shown by solid lines.

    Figure 5 shows the COP of the VAR system against the condenser temperature. As can be seenfrom Figure 5, when the condenser temperature increases, the COP decreases. This is due to the fact that, ifthe condenser temperature gets higher, the condensing temperature increases and hence causes less heattransfer in the condenser. This results in an increase in the temperature and the enthalpy of the refrigerant atthe condenser outlet (state 2). Hence, the cooling capacity decreases as does the COP. The water-lithiumbromide VAR systems have better COP, but are limited by the crystallization.

    Figure 6 shows the COP system against the generator temperature. If the generator temperatureincreases, so also does the heat transfer to the solution in the generator and hence, the refrigerant mass flowrate increases. As the refrigerant mass flow rate increases, the cooling capacity increases causing anincrease in the COP.

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    V o l . 2 5 , N o . 5 V A P O R A B S O R P T I O N R E F R I G E R A T I O N 7 1 7

    65.54.5 ~-8

    ~ ~, ~ 2 . 5 -

    2 -

    8 1L- . . . . . ~ ~ _ - - =r.) 0.505 20 25 30 35

    Condenser Temperature, Tcon ( C )

    Tgen=75 C, Tevap=l 0 C-A-CO P(NH3-H 20) -~COP(H 20-LiBr) -o-COP(era not)

    40

    F I G . 5T h e C O P o f th e V A R s y s te m a g a i ns t th e c o n d e n se r t e m p e r a tu r e

    4F Tevap=10 C, Teon=30 C[- -*-COP(NH3-H20) -~C OP (H20 -LiB r) ~-COP(carnot).5

    3 ~-2 .5 L ,~~ j ~ ) ~

    1.5 -

    045 50 55 60 65 70 75 80 85 90 95 100 105 110 115Generator Temperature, Tgen ( C )

    120

    F I G . 6T h e C O P o f t h e V A R s y s t e m a g a in s t t h e g e n e r a to r t e m p e r a t u r e

    T h e e f f e c t o f t h e e v a p o r a t o r t e m p e r a t u r e o n t h e C O P s y s t e m i s s h o w n i n F i g u r e 7 . A s c a n b e s e e nf r o m F i g u r e 7 , w h e n t h is t e m p e r a t u r e i n c r e a se s , t h e C O P o f t h e V A R s y s t e m i n c r e a s e s. F i g u r e s 5 , 6 a n d 7a l s o s h o w t h e c a m o t C O P o f t h e V A R s y s t e m w h i c h i s f o r m u l a t e d m E q . 2 .

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    7 1 8 1 . H o r u z V o l . 2 5 N o . 5

    4~ 3 . 5 -8

    2 5 ~

    1 . 5Eo~ 1N :~ 0 .5

    0 4

    T g e n =7 5 C , T c o n =3 0 C~ - C O P ( N H 3 - H 2 0 ) ~ C O P ( H 2 0 - L i B r ) ~ -C O P ( c a rn o t )

    j ~YJ ~~ J

    5 6 7 8 9 10 11 12 13 14 15 16E v a p o r a t o r T e m p e r a t u r e , T e v a p ( C )

    FIG. 7The C O P o f t he V A R s y s t e m a g a i nst t he e v a po ra t or te m pe r a tur e

    Figure 8 sh ow s the coo l ing cap ac i ty against the condenser t emperature . A s F igure 8 i llus trate swh en the condenser temperature increases the coo l ing capac i ty decreases.

    3500 F~ 3000 ;

    2500 - -g 2 0 0 0 F-

    ~ 1 5 o o ~U~ 1000 ~@~ 500 ~

    O '15

    T g e n = 7 5 C , T e v a p = 1 0 C: ~ - Q e v a p ( N H 3 - I - I 2 0 ) - ~ Q e v a p ( I 4 . 2 0 - L iB r )

    A - - --A ~lt

    2 0 2 5 3 0 3 5 4 0C o n d e n s e r T e m p e r a t u r e , Teon ( C )

    FIG. 8The cool ing capac i ty against the condenser t emperature

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    V o l. 2 5, N o . 5 V A P O R A B S O R P T I O N R E F R I G E R A T I O N 7 19

    F i gu re s 9 a nd 10 show t he ma x i m um a nd mi n i mum a bso l u t e sys t em p re ssu re s. As F i gu re s 9 a nd1 0 i n d ic a te s th e o p e r a t in g p r e s su r e s o f t h e a m m o n i a - w a t e r V A R s y s te m a r e a l w a y s h i g h e r th a n t h e w a t e r -l i t h i um b romi de VAR sys t e m ' s ope ra t i ng p re s su re s .

    28002600 ~-24002200 -2000 i18001600E 1400 ~-1200 r1000800 ~-E 600 ~-~ 400200 L0 15

    I Tgen=75 C, Tevap=10 C i '[-* Pmax(NH3-U20) ~-Pmax(mO-LiBr) ]

    j J - ~JfY

    20 25 30 35 40 45 50 55Condenser Temperature, Tcon ( C )

    F IG. 9M a x i m um sys t e m p re s su re s a ga ins t t he c onde nse r t e mpe ra t u re

    Eo

    ._=

    1 0 0 0 | Tgen=75 C, Tevap=10 C900 t - ] - * - P m i n ( N H 3 - H 2 0 )mPmin(H20-LiBr) I800 ~_~,_~_~,~1/-A700 ~ _ ~ _ _ . ~ i ~ -600 -500400 V300 ~-200 I1 0 0 L

    0 4 5 6 7 8 9 10 I1 12 13 14 15Evaporator Temp erature, Tevap ( C ) 16

    FIG. 10M i n i mu m sys t e m p re ssu re s a ga ins t the e va pora t o r t e mpe ra t u re

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    720 I . Horuz Vo l . 25 , No . 5

    R e s u l t n d C o n c l u s i o n

    The VA R s ys tem us ing wa te r - l i th ium b romide s o lu t ion w i th wa te r a s the r e f r ige ran t , i sp r e d o m i n a n t l y f o r a i r c o n d i t i o n in g a p p li c a ti o ns . T h e V A R s y st e m u s i n g a m m o n i a - w a t e r w i t h a m m o n i a i sthe r e f r ige ran t , has been us ed in l a rge - tonnage indus t r i a l app l i ca t ions r equ i r ing low tempera tu res fo rprocess work.

    T h e a m m o n i a - w a t e r c yc l e m u s t b e m o r e c o m p l e x t h a n t h e w a t e r - l it h i u m b r o m i d e c y c l e t o p r o v id eaccep tab le pe r fo rmance . Mo re hea t r ecovery m eans a r e r equ i r ed and the r ec t i f ica t ion is neces sa ry .

    The V AR s ys tem us ing wa te r - l i th ium b romide s o lu t ion s howed be t t e r pe r fo rmance than the s ys temus ing amm onia -w ate r s o lu t ion . However , the re a r e s ome po in t s to be cons ide red s uch as ; the dange r o fc rys ta l l iza t ion and impos s ib i l i ty o f opera t ing in ve ry low tem pera tu res becaus e o f the us e o f wa te r a s theref r igerant .

    N o m e n c l a t u r e

    CO P Coef f i c ien t o f Pe r fo rmance - )P Abs o lu te P res s u re kPa)T Tem pera tu re C)

    S u b s c r i p t s

    a b s A b s o r b e rcon Condens erevap Evapora to rg e n G e n e r a t o rm a x M a x i m u mr a i n M i n i m u m

    Referen ces

    1. L . A . M c N e e l y , T h e r m o d y n a m i c P r o p er ti e s o f A q u e o u s S o l u t io n s o f L i th i u m B r o m i d e ,A S H R A E T r a n s a c t i o n s 8 _ _ 5 5No. 3 , 413 1979) .

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    Vo l. 2 5, No . 5 VA P OR AB S ORP T I ON RE F R I GE RA T I ON 7 21

    34

    56.

    7.

    8

    9.

    10.

    H. Perez-Blonco, Absorption Heat Pump Performance for Differ~t Types of Solut ions,Int. Journal o f Refrigeration 7, 115 1984).ASHRA E E quipment Handbook Thermodynamics and Refrigeration Cycles, 1.21 1977).ASHRA E E quipment Handbook Absorption Cooling, Heating and Refrigeration Equipment, 13.11988) .

    R. J. Dossat, Pnneiples o f Refrigeration John W iley and Sons, Canada, 1981).B. Zigler and C. Tr ~ p, Equation of State for Am monia-Water Mixtures, Int. Journal ofRefrigeration 7 No. 2, 101 March 1984).B. H. Je~nings, Th e Thermodynamic Properties o f Am monia-Water M ixtures: A Reassessment inTabular Format, ASHRAE Transactions 419 1979).K. G o m m ~ and G. Grossman, Performance A nalysis o f Stagecl Absorption H eat Pumps: W ater-Lithium bromide Systems, ASHRAE Transaction 96 Part 1, 1590 1990).R. M. To zer and R. W . James, Fundamental Thermodynamics o f Ideal Absorption Cycles, Int.Journal o f Refrigeration 20, No. 2, 120 1997).R. M . To zer and R. W . Jam es, A Review o f Absorption Refrigeration A pplications, CIBSENational Conf. Chartered Institute of Building Servwes Engineers Brighton, U. K, Voi. 1,pp. 161-172 1994).

    Rece i ved D ecem ber 15 1997