International Journal of Thermal Sciences

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1 Internationa Journa of Therma Sciences 50 (2011) 230e2310 Contents ists avaiabe at ScienceDirect Internationa Journa of Therma Sciences journa homepage: Contribution of an terna heat exchanger to the performance of a iquid desiccant dehumidifier operatg near freezg conditions Sergio M. Peda, Gerardo Diaz * Schoo of Engeerg, University of Caifornia Merced, 5200 North Lake Rd, Merced, CA 9533, USA artice fo abstract Artice history: Received 1 February 2011 Received revised form 1 June 2011 Accepted June 2011 Avaiabe one 13 Juy 2011 Keywords: Liquid desiccants Interna heat exchangers Refrigerated warehouse The aim of the present study is to anayze numericay the effect of an terna heat exchanger (IHX) a iquid-desiccant based dehumidification system operatg near freezg conditions that are typica of a refrigerated warehouse. The study is based on previous work done by the authors that showed reduced ice formation on the surface of a coog coi by dehumidifyg the air usg iquid desiccants. The resuts of the present study show that IHX effectiveness has a direct impact on the et temperature of the iquid desiccant eavg the absorber. High IHX effectiveness resuts high absorber effectiveness. However, IHX effectiveness ess than 60% eads to a desorption process where the iquid desiccant concentration creases, augmentg the humidity ratio of the air gog through the mass exchanger. Ó 2011 Esevier Masson SAS. A rights reserved. 1. Introduction The growth the demand for energy has been projected to crease by ab 50 percent from 2005 to 2030 [1]. The ma source of energy to match this demand wi come from fossi fues that are currenty facg creasg uncertaty with respect to their ong-term avaiabiity. They aso pay an important roe augmentg the emission of greenhouse gases to the atmosphere [2,3]. A feasibe option for matchg risg demand with creasg power generation is to improve energy efficiency residentia, commercia, and dustria appications. One exampe of an energy tensive dustria appication is the refrigerated warehouse sector. A refrigerated warehouse operates as a traditiona vapor-compression refrigeration cyce. The evaporator is staed side the cod storage and the compressor and coog towers are ocated side. Agricutura produce is brought and of the pre-coog rooms aowg hot and humid air to enter the cod storage. The quaity of the produce is mataed by keepg a high reative humidity side the room and the temperature remas near freezg conditions. These operatg conditions transate to a contuous formation of ice on the surface of the evaporator. Therefore, defrostg cyces are run between one to three times a day to remove the ice from the evaporator. Defrostg cyces are herenty efficient sce they raise the surface * Correspondg author. Te.: þ ; fax: þ E-mai address: gdiaz@ucmerced.edu (G. Diaz). temperature of the evaporator and vove the use of natura gas, propane, or water sprays to met the ice. Hybrid vapor-compression/iquid-desiccant coog systems have been anayzed the past as an aternative that reduces energy consumption air conditiong and refrigeration appications. Yadav [] suggested energy savgs of up to 80% when the atent oad constitutes 90% of the tota coog oad with a iquiddesiccant dehumidifier regenerated by soar energy. Peda and Diaz showed reduced energy and defrostg-water consumption cod storages where a iquid desiccant absorber was used to remove moisture from the air upstream of the evaporator [5e7]. Athough a number of researchers have anayzed dehumidification and coog systems that utiize iquid desiccants [8e13] the study of iquid desiccant systems operatg near freezg conditions has received itte attention. A iterature review of the theoretica and physica modes of a refrigerated warehouse, as we as, the comparisons between a traditiona vapor-compression cyce and a iquid desiccant refrigeration cyce are found [8]. Esayed et a. [1] anayzed the performance of an air cyce refrigerator combed with a desiccant rotor for an air conditiong appication. More recenty, the same authors numericay anayzed the performance of an air cyce refrigerator tegrated with a desiccant system for coog and dehumidifyg a warehouse [15]. They found that the coefficient of performance (COP) can crease more than one hundred percent with respect to a conventiona system. In genera, COP improvement vapor-compression systems usg iquid desiccants is shown to depend on three ma factors: the type of iquid desiccant used [16,17], the effectiveness of the /$ e see front matter Ó 2011 Esevier Masson SAS. A rights reserved. doi: /j.ijthermasci

2 S.M. Peda, G. Diaz / Internationa Journa of Therma Sciences 50 (2011) 230e Nomencature C water concentration desiccant (kg H2Okgsoution 1 ) COP coefficient of performance D diffusivity (m 2 s 1 ) h gf enthapy of condensation (J kg 1 ) H absorber height (m) k therma conductivity (W m 1 K 1 ) L absorber depth (m) p t tota pressure (Pa) p v vapor pressure (Pa) _Q heat transfer (W) T temperature ( C) u veocity x direction (m s 1 ) v veocity y direction (m s 1 ) x coordate x direction y coordate y direction z coordate z direction Greek etters a therma diffusivity (m 2 s 1 ) d fim thickness (m) e effectiveness m dynamic viscosity (kg m 1 s 1 ) u humidity ratio (kg H2Okgda 1) r density (kg m 3 ) Subscripts a moist air abs absorber da dry air i et condition t terface IHX terna heat exchanger iquid doma d iquid desiccant m moisture max maximum m mimum Superscripts eq equiibrium et condition et condition dehumidifier and regenerator [8,18], and the effectiveness of the terna heat exchanger [6]. However, the effect of the IHX on the performance of a iquid-desiccant system (LDS) has not been anayzed depth. Interna heat exchangers are used refrigeration and air conditiong systems to assure proper system operation and to improve performance [19,20]. Positive benefits terms of creased COP have been shown by a number of researchers [21e23]. For stance, Vijayan and Srivasan [23] found that cusion of an IHX a wdow air conditiong unit creased COP by 5.86% for refrigerant R22 and by 6.30% for R07C. However, an IHX can aso have a negative fuence on the overa performance of the refrigeration cyce. Navarro et a [19] anayzed a refrigeration cyce with and with an IHX for severa workg fuids detectg that the pressure drop troduced by this device is of significant importance. In genera, the adoption of an IHX depends on thermophysica properties of the workg fuid and the et temperatures to the compressor and evaporator. Typica appications of IHXs are seen CO 2 transcritica air conditiong systems and heat pumps [2,25]. This work focuses on the effect of the IHX utiized for therma preconditiong of the iquid desiccant order to enhance its absorption capabiities before it enters the absorber. Reduction iquid-desiccant vapor pressure is obtaed by reducg its temperature before enterg the absorber and thus, better air dehumidification performance is obtaed. The thermophysica properties used the deveopment of the modes have been obtaed from current pubished data on iquid desiccants [26,27]. door air conditions range between 0 and C and 80e100% RH [5]. The dehumidified air is sent directy to the cod storage evaporator, reducg the formation of ice at its surface. The cod and diuted iquid desiccant exits the absorber and is driven by gravity through the ow-pressure side of the IHX where it exchanges energy with the iquid desiccant stream on the high-pressure side, owerg its temperature and thus, enhancg its absorption capabiities. The iquid desiccant eavg the ow-pressure side of the IHX dras to the top of the storage tank. Liquid desiccant is pumped from the top of the storage tank to the regenerator where ow-reative-humidity ambient air regenerates the iquid desiccant creasg its concentration [7]. In the system beg anayzed, the IHX is used with the objective of owerg the temperature of the iquid desiccant and thus, augmentg its absorption properties before it enters the dehumidification device. Fig. 2 shows the IHX device (A) connected to the put and put of the absorber core (B). The exchange of heat between the high and ow-pressure sides of the IHX occurs counter fow configuration. 2. System description The operation of a iquid desiccant system at a refrigerated warehouse is depicted Fig. 1, that shows how the processes of absorption and regeneration take pace at different ocations. A concentrated soution of iquid desiccant is pumped at ambient temperature from the bottom of the reservoir tank to the highpressure side of the IHX and then to the absorber device. At the absorber, the desiccant fas down the was as a iquid fim that is contact with cod and humid air fowg across the absorber. The Fig. 1. Absorptioneregeneration cyce for the iquid desiccant system operatg at a refrigerated warehouse.

3 2306 S.M. Peda, G. Diaz / Internationa Journa of Therma Sciences 50 (2011) 230e2310 B A Fig. 2. Schematic of the connections between terna heat exchanger and absorber. 3. Numerica mode In this study a iquid desiccant absorber mode, aready deveoped and vaidated by the authors [5e7,28], is used combation with a mode of an terna heat exchanger to anayze the effect of the IHX on the dehumidifyg performance of the absorber core. The setup beg studied is shown Fig. 2. The foowg assumptions are used the anaysis: Lamar fow and fuy deveoped conditions are used. Heat osses to the surroundgs from the terna heat exchanger are negected. Based on the narrow temperature range studied, constant fuid properties are utiized. Steady state conditions are utiized. Counter-fow configuration the terna heat exchanger is considered. Negigibe therma resistance at was and foug factors. The pressure drop connectg pipees, hoses, and bends are assumed negigibe Absorber For the dehumidification device, a three-dimensiona cross-fow mode between a iquid-desiccant fim fag on parae pates and a horizonta humid-air stream was utiized [5,6], where the contuity equation, and equations of motion, energy, and mass transfer are considered the iquid doma as foows: vv vy ¼ 0; (1) v m 2 v vz 2 þ r g ¼ 0; (2) vt v v ¼ a 2 T vy ; (3) vz 2 vc v 2 C v ¼ D vy ; () vz 2 with boundary conditions y ¼ 0; 0 z d ; 0 x L; T ¼ T ;i ; C ¼ C i ; (5) z ¼ 0; 0 y H; 0 x L; v ¼ 0; vt ¼ 0; vc ¼ 0; (6) z ¼ d ; 0 < y H; 0 x L; vv ¼ 0; T ¼ T a : (7) Simiary, contuity, momentum, energy, and mass transfer equations for the humid-air region are considered as foows: vu a vx ¼ 0; (8)

4 S.M. Peda, G. Diaz / Internationa Journa of Therma Sciences 50 (2011) 230e vp a vx ¼ m v 2 u a a ; (9) vz 2 a vt a u a vx ¼ a v 2 T a a ; (10) vz 2 a vu u a vx ¼ D v 2 u a ; (11) vz 2 a with boundary conditions x ¼ 0; 0 y a H; 0 z a d a ; T a ¼ T a;i ; u ¼ u i ; (12) z a ¼ 0; 0 y a H; 0 x L; vu a ¼ 0; vt a ¼ 0; vu ¼ 0; (13) z a ¼ d a ; 0 y a H; 0 x L; u a ¼ 0; T ¼ T a ; u ¼ u t ; (1) where u t and p v are p v u t ¼ 0:62185 ðp t p v Þ ; (15) p v ¼ p v;h2o 1 0:828ð1 CÞ 1:96ð1 CÞ 2 þð1 CÞ ðt t 0Þ 350 (16) Additionay, energy and mass baances at the iquid air terface are aso considered as foows: k vt ¼ k a vt a þ r a D a h gf vu ; (17) r D vc ¼ r a D a vu (18) Fay, the effectiveness of the absorber is defed Eq. (19), Tabe 1 Operatg conditions for the simuations. Inet air temperature to absorber 2 C Inet air reative humidity to absorber 90 % Inet air veocity to absorber 1.6 m s 1 Inet iquid desiccant temperature to IHX 20 C Mass fow rate of iquid desiccant per unit depth kg m 1 s 1 Inet iquid desiccant concentration 35 % 3.3. Operatg conditions for the simuations The operatg conditions used for the heat and mass exchangers have been obtaed from measurements at an actua prototype iquid-desiccant system previousy anayzed by the authors [5,7].A typica pre-coog room that stores produce durg a number of hours operates at a range of temperatures between 0 and C and between 80 and 100% of reative humidity. The vaues utiized for the put parameters to the mode are summarized Tabe 1. Due to avaiabiity of experimenta test data obtaed previousy by the authors [7], Cacium Choride soution was used as iquid desiccant durg the simuations. Liquid desiccants absorb moisture from humid air may due to the difference vapor pressure between the air and the desiccant soution. The vapor pressure of a iquid desiccant tends to zero at ow temperatures. Thus, absorption capacity is creased by owerg the temperature of the desiccant soution or by creasg its concentration. It is important to operate the iquid desiccant system at conditions where crystaization is not reached. Fig. 3 shows the properties of cacium choride soution cudg the change vapor pressure from 8 to 3.5 mm Hg for cacium choride soution at 35% concentration for a change temperature from an et vaue of 20 Cto C. This process coud be obtaed utiizg an terna heat exchanger.. Resuts and discussion The effect of the IHX effectiveness on the absorber et air temperature and humidity ratio, and iquid-desiccant e abs ¼ u u u ; (19) ueq where u eq is the humidity ratio of the air, which is equiibrium with the desiccant soution at the oca soution temperature and concentration [29] Interna heat exchanger The therma effectiveness of the IHX is cacuated as foows: e IHX ¼ _ Q _Q max ¼ U H TIHX T d;abs U m TIHX T d;abs ; (20) where U ¼ _mc p, and TIHX is the et temperature to the highpressure side of the IHX, Td;abs is the et of the high-pressure side of the IHX which is the same as the et iquid-desiccant temperature to the absorber, and Td;abs is the et temperature to the ow-pressure side of the IHX which is the same as the et desiccant soution temperature of the absorber. As fuid properties are considered constant and the mass fow rate on the high and ow-pressure side of the IHX are approximatey the same, i.e. rate of moisture absorption is sma compared to the iquid desiccant fow rate [11,30], then the effectiveness of the terna heat exchanger depends on temperatures ony. Fig. 3. Vapor pressure change at constant iquid-desiccant concentration due to temperature change. This change temperature can be obtaed usg an terna heat exchanger.

5 2308 S.M. Peda, G. Diaz / Internationa Journa of Therma Sciences 50 (2011) 230e2310 Tabe 2 Inet simuation conditions: Ta e IHX ½%Š T d;abs ½CŠ ¼ 2 C, RH a u a ½kgm Š kg da ¼ 90%, C d ¼ 35%, and T IHX ¼ 20 C. T a ½CŠ T d;abs ½CŠ temperature and concentration is studied first. Considerg fixed et conditions given by Ta ¼ 2 C, RH ¼ 90%, Cd ¼ 35%, and TIHX ¼ 20 C, the combed modes of the absorber and IHX are utiized to obta the vaues of u a, Ta, Td;abs,andC based d on prescribed vaues of IHX effectiveness. The resuts of the simuations are presented Tabe 2. Thetabeshowsthatfor ahighihxeffectivenesssuchase IHX ¼ 90% the iquid desiccant temperature enterg the absorber, Td;abs,isreducedfrom20 C to 5.8 CbytheIHX.Thus,attheseconditionsthehumidityratio of the air is reduced from approximatey kg m /kg da (for Ta ¼ 2 C, RH a ¼ 90%) to kg m /kg da. Moisture from the air is absorbed by the iquid desiccant which owers its concentration from the itia 35% to 32.9%. The et temperatures of air and iquid desiccant at the absorber are.3 Cand.2 C, respectivey. As the effectiveness of the IHX decreases, Td;abs creases and the absorber becomes ess effective dehumidifyg the air. The reationship between IHX effectiveness and the et iquid desiccant temperature to the absorber is shown Fig.. WheneIHX is very ow, the iquid desiccant enters the absorber near the et temperature to the IHX (TIHX ¼ 20 C) and for conditions where eihx 50%, the absorber starts actg as a regenerator by creasg the air humidity ratio. This effect is observed Fig. 5 where the horizonta e denotes the et air humidity ratio to the absorber, u a,andtheethumidityratio,u a,isrepresented with diamonds. As the et iquid desiccant temperature to the absorber creases, the process changes from air dehumidification to iquid-desiccant regeneration. Outet humidity ratio C d Humidity ratio [kg m / kg da ] 5.5 x Absorption T d, abs [ o C] Desorption versus IHX effectiveness is seen Fig. 6 where at the simuated operatg conditions, the dehumidification process stops near IHX effectiveness of 50% or ower. As Td;abs creases, the absorber becomes ess effective dehumidifyg the air and eventuay, it starts addg moisture to the air. The et iquid desiccant becomes ess diuted and the concentration becomes higher than the itia vaue. Fig. 7 shows this effect, where the et iquid concentration, Cd, is shown as a contuous horizonta e and the et iquid-desiccant concentration as circes. It is observed that diuted desiccant with concentration near 32% is obtaed at ow Td;abs which occurs at high IHX effectiveness, but for Td;abs 16 C the mass exchanger acts as a regenerator creasg the soution concentration with respect to et conditions. Outet temperatures of air and iquid desiccant are depicted Fig. 8. As opposed to the sighty nonear behavior seen for u a and Cd with respect to Td;abs, the et air and iquid desiccant ω a ω a Fig. 5. Outet air humidity ratio as a function of absorber et temperature of the iquid desiccant. The contuous horizonta e represent the et humidity ratio and diamonds represent et humidity ratio. [ o C] T d, abs Simuations Humidity ratio [kg m / kg da ] 5.5 x Desorption Simuation ouet air Inet air Absorption IHX effectiveness [%] Fig.. Inet iquid desiccant temperature at the absorber as a function of IHX effectiveness. Absorber et conditions are: Ta ¼ 2 + C; RHa ¼ 90% and C d ¼ 35% IHX effectiveness [%] Fig. 6. Outet humidity ratio versus IHX effectiveness. The contuous horizonta e represent the et humidity ratio and circes represent et humidity ratio.

6 S.M. Peda, G. Diaz / Internationa Journa of Therma Sciences 50 (2011) 230e Absorber et CaC 2 concentration [%] C d C d T d, abs [ o C] Fig. 7. Outet cacium choride concentration versus et iquid desiccant temperature at the absorber. The contuous horizonta e represents the et soution concentration, circes represent et iquid-desiccant concentration. temperatures, Ta and Td;abs, show a ear behavior with a sight divergence as Td;abs creases. The effect of the IHX effectiveness on the et conditions of the air gog through the absorber are now shown by means of a Psychometric chart Fig. 9. The arrows the figure show the dehumidification (or humidification) of the air side the refrigerated warehouse as a function of IHX effectiveness. It is seen how a the processes end-up at approximatey the same et reative humidity, i.e. 55%, but with creasg dry-bub temperatures. However, as the et air dry-bub temperature creases, the itia negative sopes of the process change to positive dicatg that the air humidity ratio creases for ow eihx. Therefore, it is cear from the figure that a iquid-desiccant based dehumidification system operatg side a refrigerated warehouse with doors regeneration, requires an terna heat exchanger order to remove moisture from the air. If no IHX is used to ower the temperature of the concentrated iquid desiccant soution then the air gog through the mass exchanger absorbs moisture stead of rejectg it. Fig. 9. Absorption-desorption simuations for severa IHX effectiveness. Typica absorber et conditions are: Ta ¼ 2 + C; RHa ¼ 90% and C d ¼ 35%. Fay, the absorber effectiveness, given by Eq. (19), is potted as a function of eihx, as seen Fig. 10. The reference horizonta e at zero absorber effectiveness, e abs, is added to dicate the separation between absorption and desorption behavior of the mass exchanger. It is observed that for the absorption process, a higher IHX effectiveness impies a higher absorber effectiveness. In order to have high absorber effectiveness, i.e. e abs >50%, eihx needs to be higher than 85%, approximatey. The curve is extended to negative vaues of e abs ony to show the desorption process when eihx is ow. The dehumidification system is not tended to operate under such conditions. The fdgs obtaed this study dicate that under the operatg conditions simuated, an terna heat exchanger is deed required to dehumidify the air side a refrigerated warehouse with a iquid desiccant soution. Moreover, the effectiveness of the IHX needs to be higher than 60%, otherwise desorption occurs. Absorber et temperature [ o C] T a T d Absorber effectiveness [%] Desorption Simuations Reference Absorption T [ o C] d, abs Fig. 8. Outet temperature for air and iquid desiccant soution versus et iquid desiccant temperature at the absorber IHX effectiveness [%] Fig. 10. Absorber effectiveness versus IHX effectiveness. Absorption and desorption regions are depicted as a function of severa IHX effectiveness and for the foowg absorber et conditions: Ta ¼ 2 + C; RHa ¼ 90% and C d ¼ 35%.

7 2310 S.M. Peda, G. Diaz / Internationa Journa of Therma Sciences 50 (2011) 230e Concusions The effect on iquid desiccant absorber performance operatg near freezg conditions as a function of terna heat exchanger effectiveness is studied numericay. An terna heat exchanger effectiveness mode combation with a previousy vaidated mode of a cross-fow absorber with iquid desiccant fag as a fim through parae pates contact with humid air are used for the simuations. The operatg conditions utiized to run the simuations are typica of a cod storage. The resuts show a strong dependency of the absorber performance with respect to IHX effectiveness. Vaues of eihx greater than 60% are required to dehumidify the air. Low IHX effectiveness resuts creased humidity ratio of the air gog through the mass exchanger. The resuts aso show a significant effect of eihx the iquid desiccant temperature at the et of the absorber. 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