The Primary Research on Liquid Desiccant Dehumidifier With Cooling Capacity Using Compression Heat Pump System

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1 Purdue University Purdue e-pubs International Refrigeration and Air Conditiong Conference School of Mechanical Engeerg 2006 The Primary Research on Liquid Desiccant Dehumidifier With Coolg Capacity Usg Compression Heat Pump System Anm Wu Northwestern Polytechnical University Chunl Li Northwestern Polytechnical University Hefei Zhang Northwestern Polytechnical University Follow this and additional works at: Wu, Anm; Li, Chunl; and Zhang, Hefei, "The Primary Research on Liquid Desiccant Dehumidifier With Coolg Capacity Usg Compression Heat Pump System" (2006). International Refrigeration and Air Conditiong Conference. Paper This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional formation. Complete proceedgs may be acquired prt and on CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 R025, Page 1 The Primary Research on Liquid Desiccant Dehumidifier with Coolg Capacity Usg Compressible Heat Pump System AnM WU 1,ChunL LI 2,HeFei ZHANG 3 1 Institute of Air-Conditiong & Solar Energy, School of Dynamics and Energy, Northwesern Polytechnical University, Xi an, ShaanXi, ,Cha Phone: wuanm@mail.nwpu.edu.cn 2 Institute of Air-Conditiong & Solar Energy, School of Dynamics and Energy, Northwesern Polytechnical University, Xi an, ShaanXi, ,Cha Phone: lichunl@nwpu.edu.cn 3 Institute of Air-Conditiong & Solar Energy, School of Dynamics and Energy, Northwesern Polytechnical University, Xi an, ShaanXi, ,Cha Phone: zhfyu@nwpu.edu.cn ABSTRACT In this paper, a new kd of dehumidification system which consists of a dehumidifier, a generator, two heat exchangers and a mechanical vapor compressible heat pump was studied. When this system was operatg, the desiccant solution was evenly distributed over the evaporator by many spray heads. The cool from evaporator was utilized to cool the absorption processes and simultaneously cool the air at desired condition to the space. The heat from condenser was utilized to heat diluted desiccant solution for regeneratg. Both will enhance the capacity of dehumidification. Accordg to the experimental results and a simplified model, the performance of dehumidifier was discussed under the effects of variables such as flow rate, temperature, concentration of desiccant solution, temperature and humidity of air, the effectiveness of heat exchanger and coolg capacity. The results show that good system performance and energy savg could be achieved if proper values of these variables are selected. Keywords: liquid desiccant, dehumidification with coolg capacity, heat pump 1. INTRODUCTION Air dehumidification is an important operation process not only dustry but also comfort coolg. Although many methods are available for this operation, liquid desiccant system is more attractive due to its flexibility operation along with an added benefit of absorbg organic and organic International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

3 R025, Page 2 contamants the air and the ability of usg a lower regeneration temperature(oberg and Goswami,1998a). The absorber is the most important component to fluence on the efficiency of liquid desiccant system. In order to maximize the heat and mass transfer and to mimize the pressure losses the absorber, various designs of the absorber component have been vestigated(johannsen.1984,chung et al.1998,khan et al.1996). Out of above these basic configurations, Howell(1987)vestigated a cooled counter-flow absorber built of a fallg film f-tube exchanger,usg TEG as desiccant.he found that the experimental data could be correlated with predictions of a simulation model only if a very small fraction of the exchanger surface was supposed to be wetted by the desiccant solution. Queiroz (1988)used an approach similar to a coolg tower design to experimentally and theoretically determe the heat and mass transfer parameters for known let and exit conditions of air and solution an ternally-cooled absorber. Beckmann and Albers(1996) developed a liquid desiccant coolg system usg LiBr-H 2 O solution as desiccant an absorber which is periodically built up two different types of chambers which are separated by th metal heat transfer walls.in one chamber the air is dehumidified whereas the other chamber the heat of absorption is removed by the evaporative coolg of air flow.khan(1998) his paper presented a heat and mass transfer performance analysis of an ternally-cooled liquid desiccant absorber. Numerous models have been studied by different scholars to express the heat and mass transfer processes the dehumidifier, which can be divided to simplified models and complicated models. In complicated models, a step-wise heat and mass balance across the dehumidifier is used to determe the performance of the dehumidification process. Sce the outlet conditions of the desiccant are unknown, an iterative solution is necessary until the results converge to the known let conditions. In general, the amount of computation is huge and complex. Gandhidasan(2004) developed a relatively simple model for the prelimary design of an air dehumidification process occurrg a packed bed usg liquid desiccant through dimensionless vapor pressure and temperature difference ratios. Empirical correlation of moisture effectiveness based on the experimental counter-flow dehumidifier has been given by Ullah(1988) et al. and Chung(1994), which is correlated by flow rate ratio of air to desiccant, let temperature ratio of air to desiccant, desiccant let concentration etc. The purpose of the present study is to vestigate experimentally a liquid desiccant dehumidifier with coolg capacity usg compressible heat pump system, and develop a correlation of moisture effectiveness to predict the dehumidifier performance. 2. EXPERIMENTAL SETUP A schematic diagram for the proposed liquid desiccant system is shown Fig.1.The system uses lithium-chloride-water solution as desiccant. As the figure depicts, the system consists maly of a evaporator as dehumidifier, a condenser as regenerator, a compressor, a expansion valve, a solution tank, two heat exchangers, fans etc. This system is referred to as heat pump type dehumidifier, The blower power is 37W, compressor rated power is 450W, COP of heat pump is 3.0. The desiccant at required temperature and flow rate is sprayed from the top by nozzles to effect a fallg film at the surface of the fned-tube evaporator where the process air is crosswise cooled and dehumidified (referrg to Fig.2). International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

4 R025, Page 3 The diluted solution that leaves the dehumidifier passes through a heat exchanger where it is first heated by the strong solution leavg the regenerator a desiccant-to-desiccant heat exchanger. And then the diluted solution is pumped to regeneration sector(condenser) and is heated and concentrated aga. The energy resources to regenerate the desiccant is from the heat produced by the condenser of heat pump. The heated, strong solution leavg the regenerator is cooled by dilute liquid desiccant from dehumidifier and coolg water cyclg the water-to-desiccant heat exchanger respectively and send to solution tank where it is stored for dehumidification. Fally, the dry and cool air can be obtaed.. Solution tank Evaporator Diluted solution Coolg medium (water) Heat exchanger Compressor Expansion valve Heat exchanger Out Condenser Concentrated solution Fig.1 Diagram of experimental setup Nozzle Refrigerant Concentrated solution Warm humid air Cool dry air out Refrigerant out Diulted solution Diluted solution to regenerator Fig.2 Diagram of the ternally cooled liquid desiccant absorber 3. PERFORMANCE INDEXES Durg an air dehumidification process, the water vapor from the humid air transfers to the desiccant solution. Many design variable affect the performance of the dehumidification process. The drivg force for the mass transfer between the liquid desiccant solution and the air is the difference between the vapor pressure of the desiccant solution and the partial pressure of water vapor the air. The desiccant solution vapor pressure is a function of its concentration and temperature whereas the partial pressure of water vapor the air is a function of the air humidity ratio.in the present study enthalpy International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

5 R025, Page 4 effectiveness and moisture effectiveness are adopted to predict the dehumidifier performance usg liquid desiccant. Enthalpy and moisture effectiveness are defed as the ratio of the actual enthalpy or humidity ratio variance of the air passg through the dehumidifier to the variance under ideal conditions as shown below. h h out η h = 1 h h where h a, and h a, out are the air let and outlet enthalpy, respectively. h e, is the enthalpy of the air, which is at equilibrium with the desiccant solution at a particular concentration and temperature. e, η m ω ω out = (2) ω ω e, where ω and ω out are the water contents of the let and outlet air streams, respectively. ω e, is the water content of the air,which is at equilibrium with the desiccant solution at a particular concentration and temperature. The overall energy balance for the dehumidifier can be written as m & ah m& s, hs, q = m& ah out + m& s, out hs, out + (3) where m& a is the air flow rate, m & s, and m & s, out are the desiccant let and outlet flow rate, respectively, q is the quantity of coolg from the evaporator. Desiccant outlet flow rate can be written as m& = m& + m& ω ω ) (4) s, out s, a ( out The effect of desiccant flow rate on the rate of condensation of water is given terms of the desiccant concentration at the let and outlet of the dehumidifier as 1 1 m& = (1 + ) (5) ξ ξ m& o where ξ and ξ o are the desiccant let and outlet concentration, respectively. m& is the mass flow rate of water condensed from the humid air. The mass flow rate of water condensed from the humid air and absorbed by the strong desiccant solution is given by s m& = m& a ω ω ) = m& η ( ω ω ) (6) ( out a m e, Once the moisture effectiveness is determed, the mass flow rate of water condensed from the humid air to the desiccant solution can be obtaed with known let parameters based on above Eqs. Sce the complexity of combed heat and mass transfer processes the dehumidifier, the effectiveness should be given based on experimental results. The followg will give the empirical correlations of moisture effectiveness. Moisture effectiveness can be rewritten the form of Eq.(7), accordg to Eq.(2) 1 ω out / ω ηm = (7) 1 ω / ω e, International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

6 R025, Page 5 The denomator of Eq.7 can be obtaed directly by the air and desiccant let parameters, hence only the ratio of air outlet humidity ratio to the let humidity ratio( ω a, out /ω ) is required to predict moisture effectiveness. Ullah et al.(1988) have given the empirical correlation of moisture effectiveness, as shown Eq.8. η m = m& a b 1 b3 1 { b0 ( ) exp[ b2 ( )] / ξ m& s, t s, 1 ω e, / ω t } (8) where t s, is the desiccant let temperature. The constants the above equation are newly fitted by nonlear regression of the present experimental data. The exponents for b 0, b 1, b 2 and b 3 Eq.(8) are , , and , respectively. 4. RESULTS AND DISCUSSION In order to vestigate the performance of LiCl dehumidifier with coolg capacity, several parameters which affect the performance were studied. The parameters that were considered this work cluded air let humidity ratio, desiccant concentration, desiccant flow rate, desiccant let temperature and air let temperature. The air flow rate is kg/s, the air let humidity ratio was varied from to 22.85g/kg. the desiccant flow rate was varied from15 to35g/s,the desiccant let concentration was varied from 36% to 44%, the air let temperature was varied from 26 to30 at an crement of 1, the desiccant let temperature was varied from 25 to 45 at an crement of 5.The let parameters for the experimental runs are shown Table1. Table 1: Inlet parameters for the experiment runs shown Figs.3 7 m& a (kg/s) t a, ( ) ω (g/kg) m & s, (kg/s) t s, ( ) ξ (%) variable Fig t a, Fig m & s, Fig ω Fig t s, Fig ξ 0.8 Rate of water condensed(g/s) Experimental value Calculated value Air temperature( ) International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

7 R025, Page 6 Fig.3 Influence of let air temperature on dehumidification Figs.3 7 show that the experimental results and calculated values with Eq(6) and Eq(8). It is seen from the figures that the adapted present empirical correlation shows very good agreement with the experimental fdgs. The variables to have significant effect on the dehumidifier performance are: desiccant flow rate, let air humidity ratio and desiccant concentration. The water condensation rate almost unchanged with let air temperature and desiccant temperature, it can be explaed because air and desiccant to the dehumidifier are cooled by the evaporator of heat pump(fig.3 and Fig.6). 0.8 Rate of water condensed(g/s) Experimental value Calculated value Desiccant flow rate(kg/s) Fig.4 Influence of desiccant flow rate on dehumidification 0.8 Rate of water condensed(g/s) Experimental value Calculated value Air humidity ratio(g/kg) Fig.5 Influence of let air humidity ratio on dehumidification The fluence of desiccant flow rate on the air dehumidification process is given Fig.4, the other five let parameters are shown Table 1. It is obvious from the figure that The water condensation rate creases remarkably with creasg desiccant flow rate. Those effects may be attributed to that with the desiccant flow rate creasg, the variation of the desiccant concentration through the dehumidifier decreases. As the result, creasg the desiccant flow rate decreases the variation of the surface vapor pressure of the desiccant through the dehumidifier and, hence, creases the average water vapor pressure difference between the air and desiccant the dehumidifier. It is seen from Fig.5 that the water condensation rate creases with the let air humidity ratio creasg, the other five let parameters are shown Table 1. It happens because a higher humidity International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

8 R025, Page 7 ratio implies a higher air vapor pressure and consequently higher potential for mass transfer. Fig.7 shows the effect of desiccant let concentration on the dehumidifier performance, the other five let parameters are shown Table 1. As shown Fig.7, the water condensation rate creaseds significantly with creasg desiccant let concentration. The reason is as follows. Increasg the desiccant let concentration decreases the desiccant surface vapor pressure and, so, creases the average water vapor pressure difference between the air and desiccant the dehumidifier, leadg to lower air outlet humidity ratio. 1.0 Rate of water condensed(g/s) Experimental value Calculated value Desiccant let temperature( ) Fig.6 Influence of let desiccant temperature on dehumidification 0.8 Rate of water condensed(g/s) Experimental value Calculated value Desiccant concentration(% by mass) Fig.7 Influence of let desiccant concentration on dehumidification. 4. COMPARISON WITH OTHER DESICCANT SYSTEMS In order to analyze the performance of liquid desiccant dehumidifier the present study, comparison of the mass flow rate of water condensed from the humid air and the moisture effectiveness was made between the dehumidifiers with and without coolg capacity. Reliable sets of experimental data for dehumidification of air usg lithium chloride as the liquid desiccant are presented by Fumo and Goswami(2002). A comparison of typical experimental results for fourteen cases as presented by Fumo and Goswami, with the results obtaed from the current study, is given Table 2. Durg the comparison, for the present dehumidifier with coolg capacity, the mass flow rate of water condensed from the humid air and the moisture effectiveness were calculated by Eq(6) and Eq(8) with known let International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

9 R025, Page 8 parameters presented by Fumo and Goswami. The results shown that The mass flow rate of water condensed from the humid air and moisture effectiveness of the dehumidifier with coolg capacity the present study are much higher than the experimental results of the packed bed absorption tower without coolg capacity Fumo and Goswami research. This can be attributed to absorption process with coolg capacity that improves the dehumidifier performance significantly. Table 2: Comparison of calculated results with experimental values other literature Predicted values Experimental data by Eq.(6) and (8) m& a. t a, ω m & s, t s, ξ provided by Fumo for the present (kg/s) ( ) (g/kg) (kg/s) ( ) (%) and Goswami study m& (g/s) η m (%) m& (g/s) η m (%) CONCLUSION This paper experimentally studied the dehumidification performance of liquid desiccant(licl) dehumidifier with coolg capacity usg compression heat pump system. Reliable sets of data for air dehumidification were obtaed. It was found that the water condensation rate creased with creasg desiccant flow rate, air let humidity ratio and desiccant let concentration, and changed very little with air let temperature and desiccant let temperature. The adapted correlations shows very good agreement with the experimental fdgs. It is found from the comparison with other literature that absorption process with coolg capacity can improve the dehumidifier performance significantly. ACKNOWLEDGMENT The work that has been presented this paper was supported by the Chese foundation committee of nature and science, under the projects, No International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006

10 R025, Page 9 REFERENCES Oberg,V.,Goswami,D.Y.,1998a.A review of liquid desiccant coolg.in:boer,k.w.(ed), Advances Olar Energy,vol.12.American Solar Energy Society,Boulder,CO,pp Johannsen,A.,1984.Performance simulateon of a solar air-conditiong sysyem with liquid desiccant.international Journal of Ambient Energy 5, Chung,T.W.,Wu,H.,1998.Dehumidification of air by aqueous triethylene glycol solution a spray tower.separation Science and Technology 33, Khan,A.TY.,1996.Parametric analysis of heat and mass transfer performance of a packed type liquid desiccant absorber at part-load operatg conditions.ashrae Transactions 102, Howell J.R.1987.Design of liquid desiccant dehumidification and coolg systems.in: Solar Energy Utilization,NATO Advanced Science Institute Series,PP Martus Nijhoff Publishers. Queiroz.A.G.,Orlando A.F.and Saboya F.E.M.(1988) Performance analysis of an air drier for liquid dehumidifier solar air conditiong system.transactions of the ASME.Journal of Solar Energy Engeerg 110, Beckmann J.and Albers W.(1996) Air conditiong and dehumidification by a lithium bromide/lithium chloride liquid desiccant technology.in:ab-sorption96,pp, international Ab-Sorption Heat Pump Conference, Natural Resources Canad Quebec,Canada. Khan,A.Y.,1998.Coolg and dehumidification performance analysis of ternally- cooled liquid desiccant absorbers.applied Thermal Engeerg 18, P.Gandhidasan, A simplified model for air dehumidification with liquid desiccant.solar Energy 76(2004) Ullan MR,Kettleborough CF,Gandhidasan P. Effectiveness of moisture removal for an adiabatic counterflow packed tower absorber operatg with CaCl 2 -air contact system.trans ASME J Solar Energy Eng 1988;110(2): Chung TW.Predictions of moisture removal efficiencies for packed-bed dehumidification systems.gas Sep Purif 1994;8(4): Fumo N,Goswami D.Y. Study of an aqueous lithium chloride desiccant system:air dehumidification and desiccant regeneration. Solar Energy 2002;72(4): International Refrigeration and Air Conditiong Conference at Purdue, July 17-20, 2006