III E

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y i-M633 The Society shall not be responsible for statements or opinions advanced ii papers or thicussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion is printed only if the paper is published in an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not fairing within the fair use -provisions of the Copyright Act is granted.by ASME to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page is paid directly to the CCC. 27 Congress Street Salem MA 01970, Requests for special permission or bulk reproduction shotdd be addressed to the ASME Technical Publishing Department CopyrigM by ASME M Rights Reserved Printed in U.S.A HEAT AND MASS TRANSFER CHARACTERISTICS IN CONSOLIDATED SILICA GEL/WATER ADSORPTION COOLING SYSTEM Hideharu Yanagi Nobumi ho Mayekawa Mfg. Co. Ltd., 2000, Tatsuzawa, Moriya-Machi, Kitasooma-Gun, Ibaraki-Pref, Japan III E ABSTRACT NOMENCLATURE The heat and mass transfer on adsorption beds is decisive for the operational characteristics of adsorption refrigerators. To overcome its heat and mass transfer limitations a consolidated silica gel C. adsorption heat exchanger has been designed and successfully developed. It was made of composite as 4.3 wt% graphite,86.4 wt% silica gel and binders, which was pressed into a heat exchanger module and then dried under a temperature of 90 C for 24 hours. With use of this adsorption heat exchanger, we measured an overall heat transfer coefficient of 62.2 W/m2K at the beginning of desorption cycle and 61.5 W / m2k during desorption, respectively. M. Whereas corresponding values of a granular silica gel Mt bed were 44.8 W / m 2K and 25.9 W / m2k, P. respectively. This paper presents the operational characteristics of consolidated silica gel adsorption heat exchanger in refrigeration cycles, in addition to its isoster measurements for silica gel-water pair compared to that of the granular heat exchanger. A b Ceff Ct U Wo =Adsorption potential (1a/mol) =Mass of refrigerant desorbed per mass of adsorbent (kg/k) =Specific heat of adsorbent (kj/kg) =Effective specific heat of adsorption, metal and refrigerant (kj/kg K) =Specific heat of refrigerant (kj/kg K) =Specific heat of metal in heat exchanger (kj/kg K) =Characteristic energy of adsorption (10/mol K) =Heat of adsorption (kj/kg) =Mass of solid adsorbent per bed (kg) =Mass of metal of heat exchanger (kg) =Saturated vapor pressure (Pa) =Surface area of heat exchanger (m 2) =Overall heat transfer coefficient ovan2 =Volume of micropore (1/kg-Silica gel) =Limit volume of adsorption (1/kg-Silica gel) Presented at the ASME ASIA 1 97 Congress & Exhibition Singapore - September 30-October 2, 1997

2 a =Average refrigerant adsorption fraction (kg/kg-silica gel) INTRDUCTION In order to obtain an efficient cooling power and compact refrigerator, the adsorption / desorption cycle times must be shorter, so that the heat exchange between adsorption beds and external transfer fluid must be higher. Whereas conventional type of a powder or granular bed provides a not sufficient heat transfer, due to both the low effective thermal conductance of the bed and the high thermal contact resistance to the wall. Hence much attention was paid to improve heat transfer applying for a new consolidated silica gel heat exchanger. The effective thermal conductance of the bed was enhanced by adding graphite powder with a high thermal conductance, while the thermal contact resistance to the wall of aluminized finned copper tubes was decreased by forming the bed into a solid state from a porous one. For an application of zeolite, the consolidated zeolite in brick form has been studied (Cacciola et al. 1992) with consideration to both heat and mass transfer. Guilleminot et al. studied on a consolidated composite of zeolite with the insertion of metallic foam which provides substantially enhanced heat transfer coefficients. The compound was compressed and sintered at 1000 C for 3 hours (Guillrninot et al. 1992). Whereas in the present paper, a new method processed by a lower temperature without sintering is demonstrated with no degradation of surface area of silica gel. Since an exposing at temperature higher than 150 C causes a decrease of the surface area. The main attention will be focused on heat transfer between the adsorbent bed and the working fluid besides discussion of mass transfer in refrigerating cycles. HEAT EXCHANGER The adsorption heat exchanger was comprised of a number of copper tubes with aluminized plate fins and granular or consolidated silica gel adsorbent packed between the fins. Figure 1 shows a bare structure of the heat exchanger itself (230W X 230H)< 251). The adsorption beds were consolidated composite made of silica gel and graphite with binders. The mixed gel was first packed and compressed into the heat exchanger and then dried for fixing under a temperature of 90 C for 24 hours.!iiiiiv iitoutit It 11 ttli l iotialitli t Fig.1 The structure of heat exchanger 2

3 Table] Parameters of test model Granular Sol idified Sol idified No.1 (pressed) Si I icagel (g) Graphite _ (g) HEC _ (SP-200) (g) Aid-plus (8) (SP) 23 (SP) 28 Fin pitch (em) Bed density (kg/n?) or desorbed water on the heat exchanger module were directly measured in situ by an installed gravimetric meter with an accuracy of ± 0.5g. The parameters of tested adsorption beds are given in Table I. The bed density is specified only to the silica gel itself The binders were varied to investigate their effects on adsorption. EXPERIMENTAL SET UP ' An experimental set up utilized to test the heat exchanger module is schematically shown in Figure 2. It is consisted of an adsorber bed installed inside a vacuum chamber, connected to an evaporator and a condenser via two vapor valves. These four components are interconnected via working fluid, and are coupled to external heat sources comprised of the thermostatic bathes corresponding to the heat of the four operating temperatures. A horizontal copper fin tube heat exchanger was used for the condenser and evaporator. The temperatures were measured by Cu-Constantan thermocouples (0.2mm diameter) with an accuracy of ± 0.5K. Amounts of adsorbed (1) Adsorber Bed (2) Vacuum chamber (3) Condenser (4) Evaporator (5) Gray imetr i c meter (6) Chi I led water bath (7) Cool i ng water bath (8) Hot water bath (9) Flow meter (10) Vacumm pump adsorption cycle * desorption cycle Fig.2 Experimental setup for testing of heat exchanger modules Adsorption Measurement Isobaric adsorption curb measurements were carried out in order to investigate the influenced of binders on adsorption. The test bed is first connected to the condenser and to the hot water bath, and is then regenerated. During regenerating the vacuum chamber was being 3

4 evacuated for a through regeneration. Thereafter in switching to the adsorption cycle, the test bed is connected to the evaporator and to the cooling water bath. In keeping an evaporating temperature ( pressure ) constant, amounts of adsorption were measured step by step, over changing the adsorption temperature of the bed by controlling the cooling water bath. Obtained isobar adsorption data were plotted by the Dubinin plot, from which Dubinin-Astalchov equation, that is, the adsorption equilibrium equation was determined. The isosteric curb was then derived from the equation. Heat transfer coefficient measurement overall heat transfer coefficient measurements were carried out to investigate the effects of consolidated bed on heat and mass transfer characteristics, in particular in desorption cycles. Hence during the desorption cycle, the water in the adsorbent is decreasing so that the heat transfer is degrading. During the tests, the test modules were connected to the condenser, and then hot water was supplied to the bed and, from the time varying mass of desorption and adsorbent temperature were drawn the heat and mass transfer characteristics. All measurements were carried out under the nominal operation temperature conditions; inlet temperature of hot water = 75 C, cooling water = 29 C, and chilled water = 9 C. EXPERIMENTAL RESULTS Adsorption Measurement Adsorption equilibrium relation of water on silicagel is given by the D-A equation, which is generalized to the following form. W=Woexp[ (A/E)"] (1) In this expression the parameter n in the equation is integer value dependent on adsorption site, for silicagel ; n = 1.1, W is the volume of tnicropore filled by the adsorbate and adsorption potential, A, is defined as the difference in free energy between the adsorbed phase and the saturated liquid. A = RT In (p/ps) (2) The characteristic energy of adsorption, E, is obtained from adsorption potential A at W/W o = lin the Dubinin plot of ln W versus An. The limit of adsorption, W o is determined by the value of In W at the point of A" = 0 and is considered as the rnicropore volume of the adsorption. Table 2 illustrates test results of limit of adsorption, Wo and characteristics energy, E. It was found that the pair of HEC (SP-200) and aidplus (SP-20) used for the compressed and consolidated adsorbent was appropriate, Wo is not so much degraded in comparison with granular one while E is though slightly increased, that is, results in a slight increase of heat of adsorption. Table2 The results of limit of adsorption & characteristic energy of adsorption Granular Consolidated No.1 Consolidated (compressed) Wo (//kg silicagel) E (cal/mol-k)

5 Heat transfer coefficient measurement The mathematical model aimed at the determination of the overall heat transfer coefficient can be stated as follows. The energy balance equation is MC dt =US(To TH (3) eff di Where Ca. is effective specific heat of adsorbent, metal (tube, fin ) and refrigerant defined as ( Miles, D.J. 1986) ; Ceff =C +C t + Hb+ ot, (4) Ma The first term is specific heat of adsorbent, the second term that of heat exchanger, the third term that of adsorption and the last term that of refrigerant, respectively. H is refrigerant heat of adsorption, b mass of refrigerant desorbed per mass of adsorbent per degree K of temperature. The value of b is derived from the slope of isobaric adsorption curb at operating condenser pressure. The value M is the mass of the heat exchanger module, a is average refrigerant adsorption fraction and U and S respectively the overall heat transfer coefficient and the surface area of the test module; T o the hot water inlet temperature and T(t) the adsorbent temperature at time t. The boundary condition is : T(t=0)=T,d, a temperature of adsorbent at the end of adsorption cycle. The integration of equation (3 ) results in : US In(To TH= In(To T ad) ( 5) Ma Ceff By plotting In (To T(t) ) versus time in a semi-log scale, a strait line is obtained. Through the evaluation of its slope, the U value can be withdrawn. Figure 3 demonstrates the plotted curb on the No 1 consolidated bed. During the first part of the heating cycle, the bed must be pressurized from the evaporator to condenser pressure. No refrigerant is pumped to the condenser, so that the slope is rather steep. Namely the U value obtained from the first part slope corresponds to the adsorption heat exchanger itself The obtained overall heat transfer coefficients, U to the consolidated bed were 62.2 W/m 2 K at the beginning of desorption cycle and 61.5 W/m 2 K during desorption, respectively. Whereas corresponding values of the granular bed were 44.8 W/m2 K and 25.9 W/m2 K, respectively 'rf I- C J Time[min] Fig.3 Inlet hot water and adsorber temperature difference vs. time. Consolidated No.1 (Cycle time 10min) Notice that heat transfer is significantly increased. Figure 4 verifies this heat transfer enhancement, in the figure the bed temperature at the end of regeneration was by 5 degrees higher than that of granular and in addition its temperature at the end of adsorption was by 5 degrees lower than that of granular. While the adsorption amounts were 5

6 opposite down to 10 g as shown in the figure. This means that the mass transfer rate is decreased due to an increased packing density. 80 c o 70 c 60 o E 50 < -c , 0 a 30 ci. o E latorm ix!akaa 10111allklla nbansa ISE= r!" pra'71 NIMIW5: Time[min] Fig.4(a) Adsorption/desorption amounts vs. time Granular (Cycle time 5min) aral n linral A a l ran Is km 1: Time[min] 0 Adsorption/desorption amounts a Inlet hot&cooling water temp. 0 Adsorber temp. Fig.4(b) Adsorption/desorption amounts vs. time Consolidated No.! (Cycle time 5min) CONCLUSION The following conclusions were obtained from the present work. I.The consolidated plate silica gel bed was developed with no degradation of adsorption characteristics as well as no cracks on the surface. 2.The characteristics of its heat and mass transfer was clarified. Its U value reaches to higher value (approx. 60W/m2 K ) than that of granular bed ( approx. 25W/m 2 K). 3.The mass transfer was significantly defected by compressing the bed. FUTURE WORK Further study provided grooves on the bed to reduce such a drawback is necessary. The relation between a packing density and mass transfer should be clarified. ACKNOWLEDGMENTS The authors would like to thank Mr. S. Okabayashi of MIZUSAWA Industrial Chemicals Ltd. for the work of producing the consolidated silica gel samples and discussions. REFERENCES Cacciola, G., Camrnarata, A. Fichera, and G. Restuccia Advances on innovative heat exchangers in adsorption heat pumps. Proc. Solid Sorption Refrigeration Symposium, Paris, pp , 6

7 Guilleminot, J.J. and et al Amelioration des transferts thermiques dans les adsorbeurs a lits fixes consolides. Proc. Solid Sorption Refrigeration Symposium, Paris, pp Miles, D. J Analysis of a solid adsorption heat driven heat pump. ASME HTD ( Am Soc Mec Eng. Heat Transf Div) 65. pp