International Journal of Modern Trends in Engineering and Research e-issn No.: , Date: April, 2016

Transcription

1 International Journal of Modern Trends in Engineering and Research e-issn No.: , Date: April, 2016 A Review on Internally Cooled Liquid Desiccant Air Dehumidifier Jignesh R Mehta 1,Niyati M Shah 2, Krunal N Patel 3 1 Mechanical Engineering Department, Faculty of Technology and Engineering, The Maharaja Sayajirao University of Baroda, j.r.mehta-med@msubaroda.ac.in 2 Aeronautical Engineering Department, SardarVallabhbhai Patel Inst. of Technology, niyati_svit@yahoo.com 3 Mechanical Engineering Department, Faculty of Technology and Engineering, The Maharaja Sayajirao University of Baroda, krunal_2191@yahoo.com Abstract Better indoor environmental quality at lower energy consumption and less environmental impact are the goals of researchers working in the field of air conditioning. Liquid desiccant based air conditioning (LDAC) system seems to have good potential as an alternative air conditioning system for achieving above goal. Dehumidifier of an LDAC system is an air-liquid desiccant contacting device responsible for air conditioning. Most of the times, the dehumidifier used in LDAC systems is an adiabatic device. High LD flow rate necessary in such systems increase parasitic power consumption in these systems. An internally cooled dehumidifier cools LD as well as air helping to achieve sensible cooling as well as reducing flow rate of LD. Development and analysis of such a device is a bit difficult and getting more attention only recently. The current work focuses on the internally cooled dehumidifier and presents a review of the work reported in literature. This work provides direction and a good starting point for researchers working in this field. Keywords-Air Conditioning, Contacting Device,Dehumidification,Liquid Desiccant (LD). I. INTRODUCTION Creating good indoor environmental quality withleast environmental impact and energy consumption are the major challenges in the field of air conditioning. At present, the most widely used air conditioning systems are based on the vapour compression refrigeration (VCR) system, which is driven by electrical energy. However, the increasing use of air conditionerscontributes to energy scarcity and environmental degradation. Alternative air conditioning systems like evaporative cooling, vapourab/adsorption and desiccant based air conditioning systems are being investigated by researchers to overcome above problems. Ability to use low grade thermal energy, high density energy storage near ambient conditions, possibility of multi-staging, air washing capability and flexibility in laying the components are some of the favourable features of the LDACsystems. Commonly used liquid desiccants (LDs) are generallyaqueous solutions of salts like calcium chloride, lithium chloride and potassium formate. These solutions have high affinityfor watervapour at lower temperature and higher concentration. Thus, they can provide dehumidification of air or latent cooling of the spacewhen brought in contact with it in an air-ld contacting device [1]. Hybrid LDAC and VCR systems can cater to sensible as well as latent load more effectively with lower energy consumption as compared to stand-alone VCR All rights Reserved 1089

2 1.1. Principle of Liquid Desiccant Dehumidification Figure. 1 shows general hardware for desiccant based dehumidification systemand common processes for these systems[2]. While the desiccant removes moisture from air on one side in dehumidifier, it needs to reject moisture into ambient in a component called regenerator in order to continue working in a cycle. Sensible cooling accompanies, precedes or follows dehumidification of air as the latent heat of water vapour gets converted to sensible heat in the dehumidification process and is picked up by air as well as LD. Supplementary process of evaporative cooling may be used after dehumidification to reduce temperature of air further. Air and LD are brought in contact with each other for heat and mass transfer in the dehumidifier and the regenerator of LDAC system. So, they are called contacting devices, which are discussed in following sections. Figure 1. Curves of water vapour pressure atdesiccant surface as a function of temperature[2] 1.2. Liquid Desiccant Air Contacting Devices The contacting devices can have different configurations like packed bed tower, spray tower and falling film columns(figure. 2). Depending upon whether the cooling is provided inside it or not, the dehumidifier can be categorized as adiabatic or internally cooled dehumidifier Packed bed tower Packed bed tower is the most common type of contacting device for LDAC systems found in literature. The liquid desiccant is sprayed homogeneously on the packed bed with the help of nozzles.ld makes thin film over the packing media over which the air is passed with the help of a fan.structured packing is more common as compared to random packing due to lower air side pressure drop. Air velocity is kept low, in the range of 1.2 m/s to 4 m/s to avoid entrainment of droplets in the air stream. Packed bed dehumidifier needs wider air passages for air in order to prevent the desiccant from restricting the airflow. This increases the size and cost of equipment. Absolute humidity change in the range of 2 to 11 g/kg is reported in literature[3] Spray tower It is an empty cylindrical shell which is made of steel or plastic. Liquid desiccant is sprayed from the top of the vessel with the help of nozzles. This device offers lower pressure drop on air side but the chances of carryover of liquid droplets is high due to finerdroplets of LD created to increase surface area of contact. Lower dehumidifier efficiency, in the range of 30 to 60% is reported in literature All rights Reserved 1090

3 to shorter residence time. There is no deposition of scale or dirt in the tower and it is highly suitable for highly viscous solutions [4] Falling film type contacting device A downward falling thin film of liquid desiccant is created over multiple parallel surfaces of this device. The air moves in counter or cross-flow from the passages between these surfaces[5]. Low air side as well as LD side pressure drops are the advantages of these devices, but uniform distribution of the fluid over multiple surfaces as well as across the flow cross-section are the major challenges here. Figure 2. Various LD-Air contacting devices [2,5] II. INTERNALLY COOLED DEHUMIDIFIER In an adiabatic contacting device, as LD absorbs moisture from air and latent cooling converts into sensible heating, the temperature of LD and air rises. As the temperature of LD rises and concentration decreases, the ability of LD to absorb moisture rapidly decreases. A sensible cooling media may be used to cool LD and air inside the dehumidifier[3]. This not only provides total cooling of air in same device, but also widens the concentration range of LD by keeping it at a lower temperature. Such systems can have lower parasitic power consumption due to lower mass flow rate of LD [6].The cooling media in an internally cooled dehumidifier can be cooling tower water, refrigerant, chilled water or ground water. Such an internally cooled dehumidifier may be a plate heat exchanger or a finned tube heat exchanger. Due to complexity of the configuration as well as analysis, the internally cooled dehumidifier is studied less in literature.the heat and mass transfer processesin various configurations of such devices need to be completely understood and characterized Plate Heat Exchanger as Dehumidifier Khan and Martinez developed a mathematical model to predict the performance of an internally cooled parallel plate absorber with nearly isothermal operation using LiCl as LD[7]. The model was based on the control volume numerical method and the absorber was divided into a finite number of control volumes in the process air flow direction. With counter flow arrangement, return air was cooled and dehumidified to 23 C and kg/kg da from process air at 32 C and kg/kg da humidity. Chang et al analyzed the performance of a parallel platecross flow dehumidifier and found that, moistureremoval rate in internally cooled dehumidifier was 20% higherthan the adiabatic one under the same operating conditions. The effect of flow pattern, especially the flow direction of air to desiccant on the internally cooled dehumidifier performance was numerically analyzed. Adopting the heat and mass transfer model as well as experimental results from the literature, they developed the model to predict the performance of the dehumidifier within 13% accuracy. The results showed that counter-flow configuration of air and desiccant is superior for dehumidification, while parallel flow configuration performs the poorest. They also proved that the internally cooled dehumidifier All rights Reserved 1091

4 better mass transfer performance compared with the adiabatic contacting device with external heat exchanger [8-9]. A 128-plate conditioner was designed to cool and dry3200 m 3 /h (1900 cfm) of air from 27.5 C and kg/kg to 25.3 C and kg/kg when itwas cooled with 23.4 C water. At this operating conditioner, the desiccant-to-air mass flow ratio(m l /m a ) was 0.04 and the desiccant was a 39.7% lithium chloride solution. The sensible and latentcooling was 3.0 kw and 15.4 kw respectively. Complete wetting of theplate surfaces was not achieved and the delivered latent cooling was only 55% of the designvalue. [10]. Yin et al. investigated the behavior of a novel internally cooled counter-current dehumidifier based on the plate-fin heat exchanger (PFHE) by experimentationand found that there was better contact between air and liquid desiccant and the internally cooled PFHE has 13-20% higher humidity ratio (ΔW) change compared to adiabatic contacting device[11] Finned Tube Heat Exchanger as Dehumidifier Khan used a simple NTU-effectiveness method for solving a steady state two dimensional modelnumerically[12-13].air was in cross-flow to the desiccant and the cooling water (or refrigerant) was also in cross-flow to the desiccant.khan estimated the average heat transfer coefficient by using fully developed turbulent flow correlations reported in previous literature[13]. Ren et al. investigated four possible flow arrangements and three types of commonly used liquid desiccants[14]. Their onedimensional analytical model was considered the variations of heat and mass transfer coefficients, surface wettability and Lewis factor along the height of the exchanger (Figure. 4). Their study however, was confined to the cases where the changes in the solution concentration and the flow rate are relatively small. Figure 3.Low Flow Water Cooled LD-Air conditioner[10 ]Figure 4. Physical Model for ICLD Dehumidifier [15] A cross-flow fin-tube internally-cooled dehumidifier was designed and investigated by experimentation as well as simulation by Yimo et.al [15]. Influences of various inlet parameters on the dehumidification performance were analyzed to derive empirical correlation for heat transfer coefficient as a function of solution mass flow rate and concentrationx s, pipe outer diameter d o and conductivityand kinematic viscosityof air at average temperatures and air velocity at the narrowest cross section v fmax. It was predicted that the optimum air flow rate was 0.05 kg/s with mass ratio of 2.8 and the best length in the direction of air flow was 0.3 m. A, B, C and D are constants. h = A e ( ) (C (m 0.15) + D) All rights Reserved 1092

5 Chung and Wu experimentally tested a U-shape tunnel with eliminators and fin coils as spray-tower absorber operating with tri-ethylene glycol as LD[4,16]. Both air and LD were introduced at the top of the tower with co-current flow with air.they found that the absorption efficiency increases as the air flow rate decreases, the solution flow rate increases or as the solution temperature decreases (Figure 5). The overall mass transfer coefficient increased with increasing air and solution flow rates, solution concentration and inlet air humidity. Performance of the spray towers with and without fin coils was compared and it was found that the efficiency of the absorber with fin coils was 20-30% higher. Figure 5. Absorption Efficiency for variations in Liquid Flow Rate and Inlet Temperature[4] Yamaguchi et al. developed a mathematical model to predict COP of hybrid liquid desiccant airconditioning system consisting of a liquiddesiccant system and a vapour compression heat pump using R407C as refrigerant and coolant[17]. The air was cooled from 30 to 22.2ºC with system COP of 2.71 and the heat pump COP of 3.82.The system could dehumidify 5.9 g/kg da under the conditions ofsummer. The results of simulation were validated with only one set of experimental results. Mehta and Badrakia could significantly enhance the capacity of cooling water coil for total cooling (32% for water at 22 o C when inlet humidity was 20.3 g/kg da ) by creating LD film over it with marginal increase in electrical power consumption in the system. Latent cooling could be achieved at even at such a high temperature of cooling water[18]. Distribution of LD over the fins was done with the help of a perforated tube, but this was found to be insufficient for good wetting of fins. Table 1 summarizes the heat and mass transfer performance of the internally cooled LD-Air contacting devices as dehumidifier from literature. Dehumidification effectiveness and moisture removal rate were mainly adopted as indices to describe the combined heat and mass transferperformances. Table 1. Summary of Review on Internally Cooled LD Air contacting Devices Reference Scheme and Methodology Conclusion Lavemann, (2006) Experimentalperformance, 128-plate heat exchanger as conditioner, LD: 39.7% LiCl Complete wetting of the plate surfaces was not achieved Latent cooling was only 55% of the design value Chung and Wu Experiments with refrigerant coolant Mass transfer coefficient using vapour pressure of (1998) Khan and Martinez, 1998 Liu 2009 Cooling fluid air/water, LD: LiCl, coolant water and air, ε-ntu method Mathematical model for all flow directions of LD and air desiccant solution using Buckingham Pi method Moisture effectiveness and enthalpy effectiveness introduced as number of mass transfer units NTU All rights Reserved 1093 where h m : mass transfer coefficient Introduced concept of number of heat transfer units and mass transfer units based analysis

6 Yin (2008) Ren (2007) Seiichi Yamaguchiet.al Mehta and Badrakia Plate heat exchanger made of stainless steel as dehumidifier/ regenerator, co-current flow pattern for both air and desiccant Parallel or counter flow pattern, LiCl, LiBr and 35-40% CaCl 2, 1-D differential model solved with Runga-Kutta 4 th order integral approach 1-D mathematical model, hybrid system usingrefrigerant R407C Usedfinned tube cooling water coil as dehumidifier,experimental work Film thickness is not considered Internally cooled system had change in absolute humidity ratio (ΔW) 0.5 g/kg more than that of the adiabatic dehumidification Considered the effects of solution heat and mass transfer resistances, variations of solution flow rates and non-unity value of Lewis number, incomplete surface wetting Considered compressor efficiency and heat exchanger efficiency, only a single run of experiment Latent heat transfer could be achieved even at cooling water temperatures higher than dew point REFERENCES [1] ASHRAE Fundamentals. Atlanta, GA, USA2009. [2] GrossmanG. and JohannsenA, Solar Cooling and Air Conditioning, Prog. Energy Combust. Sci., vol. 7, pp , [3] Lowenstein,A., Review of Liquid Desiccant Technology for HVAC Applications. HVAC & R Research, vol. 14, pp , [4] Chung, T.W.and Wu, H., Comparison between spray towers with and without fin coils for air dehumidification using triethylene glycol solutions and development of the mass-transfer correlations, Journal of Industrial and Engineering Chemistry Research, vol.39, pp , [5] Gao, W.Z., Shi, Y.R., Cheng, Y.P. and Sun, W.Z., Experimental study on partially internally cooled dehumidification in liquid desiccant air conditioning system, Energy and Buildings, vol.61,pp , [6] Lowenstein, A., and Dean, M.H., The Effect of Regenerator Performance on a Liquid Desiccant Air Conditioner. ASHRAE Transactions, Issue 98, vol. 1, pp , [7] Khan, A.Y., and Martinez, J.L., Modeling and parametric analysis of heat and mass transfer performance of a hybrid liquid desiccant absorber, Energy Conversion and Management, Issue 10, vol.39, pp , [8] Chang, X.M., Liu, X.H., and Jiang, Y., Performance numerical analysis on an internally cooled liquid desiccant dehumidifier, Proc. IBPSA International Building Performance Simulation Association Conference, pp , [9] Liu, X.H., Chang, X.M., Xia, J.J., and Jiang, Y., Performance analysis on the internally cooled dehumidifier using liquid desiccant, Building and Environment, vol. 44, pp , [10] Laevemann, E., Peltzer,M., Hublitz,A., Kroenauer,A., Raab,U., and Hauer A., Thermo-chemical storage for airconditioning using open cycle liquid desiccant technology,ecostock 2006, Stockton College of New Jersey, Pomona, NJ [11] Yin,Y.,Zhang, X., Wang, G., and Luo, L., Experimental study on a new internally cooled/heated dehumidifier/regenerator of liquid desiccant systems, International Journal of Refrigeration, Issue 5,vol. 31, pp , [12] Khan,A.Y., and Sulsona,F.J., Modelling and parametric analysis of heat and mass transfer performance of refrigerant cooled liquid desiccant absorbers, Int. J. Energy Res.,Issue 9, vol. 22,pp , [13] Khan,A.Y., Cooling and dehumidification performance analysis of an internally-cooled liquid desiccant absorber, Applied Thermal Engineering, Issue 5, vol.18,pp , [14] Ren, C.Q., Tu, M., and Wang, H.H., An analytical model for heat and mass transfer processes in internally cooled or heated liquid desiccant air contact units, International Journal of Heat and Mass Transfer, vol.50, pp , [15] Luo,Y., Shao, S., Xu, H., Tian, C., and Yang, H., Experimental and theoretical research of a fin-tube type internally-cooled liquid desiccant dehumidifier, Applied Energy, vol.133, pp ,2014 [16] Chung, T.W., and Wu, H., Dehumidification of air by aqueous triethylene glycol solution in a spray tower, Separation Science and Technology, Issue.8, vol.33,pp , [17] Yamaguchi,S., Jeong,J., Saito,K., andmiyauchi,h., and Harada,M., Hybrid liquid desiccant air-conditioning system: Experiments and simulations, Applied Thermal Engineering,vol. 31, pp , Mehta,J. R., and Badrakia H. C., Fresh Air Dehumidification in a Novel Liquid Desiccant-Air Contacting Device, Journal of Mechanical and Civil Engineering,vol.4, Issue11, pp.79-82, All rights Reserved 1094

7