THEORITICAL STUDY FOR COMPACT LIQUID DESICCANT DEHUMIDIFIER / REGENERATOR SYSTEM
Air may be dehumidified when it is brought into contact with a suitable liquid desiccant. Different types of liquid desiccants are available in the market and the application of the proper desiccant in hot humid climates would improve the dehumidification effectiveness. The driving potential for a dehumidification process is the difference in the pressure of the water vapor in the air and the water vapor saturation pressure corresponding to the airdesiccant solution interfacial temperature and concentration of water above the desiccant. The vapor pressure of a liquid desiccant is a function of its temperature and concentration. Among the various desiccants available, lithium chloride, lithium bromide, calcium chloride, and triethylene glycol have received much attention. The present study aims to evaluate numerically the performance of the proposed liquid desiccant dehumidifier system that utilizes calcium chloride solution as a liquid desiccant. The performance parameters for the air dehumidifier were the reduction ratio of the air humidity ratio and the dehumidifier effectiveness. Several benchmarks were carried out under the following operating conditions: The cooling water temperature (10 o C-18 o C), desiccant solution temperature (26 o C-33 o C), air flow rate (3.4-6 l/s), air inlet temperature (38 o C-51 o C), air inlet humidity ratio (21-25 gw/kgda), desiccant solution mass flow rate (0.04-0.13 kg/s), desiccant solution to air mass flow rate ratio (10-26), heating water temperature (42 o C-51 o C), and desiccant solution concentration (20% -45%). The results show that the humidity ratio at the exit from the dehumidifier decreases with increasing the desiccant solution concentration and with decreasing of the desiccant solution temperature. The desiccant solution moisture content decreases with increasing of the desiccant solution temperature and mass flow rate, but it decreases with decreasing of the air inlet humidity ratio. Also, the air temperature leaving the dehumidifier decreases with increasing the desiccant solution concentration and the air inlet humidity ratio, but it decreases with decreasing of the desiccant solution temperature. The desiccant solution temperature decreases with increasing the desiccant solution concentration and with decreasing both of the desiccant mass flow rate and the cooling water temperature, but it is not affected with the air inlet humidity ratio. The desiccant solution moisture content gain increases with increasing each of the desiccant solution concentration, the air inlet humidity THEORITICAL STUDY FOR COMPACT LIQUID DESICCANT DEHUMIDIFIER/REGENERATOR SYSTEM Vol. 3, No. 11, Dec. 2008 353 ratio and the air mass flow rate. It also increases with decreasing desiccant solution mass flow rate and temperature. Also the results show that both of the dehumidifier reduction ratio of the air humidity ratio and the effectiveness increase with the increase of the heating water temperature, the desiccant solution mass flow rate, the desiccant solution to air mass flow rate ratio and the desiccant concentration. Both the reduction ratio of the air humidity ratio and the effectiveness of the dehumidifier decrease with the increase of the cooling water temperature, the desiccant solution temperature and of the air flow rate. The performance parameters were almost unaffected with the inlet air temperature. The dehumidifier reduction ratio of the air humidity ratio slightly increases with the increase of the inlet air humidity ratio, but the dehumidifier effectiveness is almost unchanged with the increase of the inlet air humidity ratio.
Simplified Heat and Mass Transfer Model for Cross-Flow and Countercurrent Flow Packed Bed Tower Dehumidifiers with a Liquid Desiccant System
A mathematical model is developed using the Matlab/Simulink platform to investigate heat and mass transfer performance of cross-flow and counterflow dehumidifiers with Lithium Chloride (LiCl) solution. In the liquid desiccant dehumidifier, the orthogonal polynomial basis is used to simulate the combined processes of heat and mass transfer. The temperature profiles on cross-flow and countercurrent flow dehumidifiers are demonstrated. The resultant counter flow air changes the temperature profile of the LiCl solution in the longitudinal direction because of the drag forces. In addition, when inlet airflow rate reaches 15 kg·s −1 , the temperature effect becomes less obvious and may be reasonably negligible. Under these conditions, the air changes the design factor and determines the interfacial temperature. It is demonstrated that the mathematical model can be of great value in the design and improvement of cross-flow and countercurrent flow dehumidifiers.
Numerical and experimental results for coupled heat and mass transfer between a desiccant film and air in cross-flow
Abstract Heat and mass transfer with a uniformly distributed falling liquid desiccant film in a cross-flow parallel plate gas-liquid heat exchanger is predicted, and the results compared with experiment. The process air is in cross-flow over the desiccant film. Refrigerant flows through the heat exchanger tubes and removes the heat of absorption for dehumidification and supplies the heat for regeneration. The governing equations, boundary conditions, and interfacial equilibrium conditions for the concentration, temperature and pressure are presented. The problem is more complex than for cocurrent or countercurrent flow, as it is fully three-dimensional.