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Desiccant dehumidification and air-conditioning systems require careful design to minimize large parasitic power requirements for pumps and blowers. Each component depends to some extent on overall system characteristics, yet each component must be carefully modeled in itself. The most challenging part of optimizing a system is the absorber design. In order to increase the heat and mass transfer rate while minimizing the pressure loss in the absorber, a direct contact falling film fin-tube heat exchanger is analyzed with water flowing on the tube side and desiccant flowing as a falling film on the fin side. Air is cooled and dehumidified by the water and desiccant as it is circulated through the absorber. A model to analyze the heat and mass transfer in the absorber has been developed. An optimum design has been selected based on not only thermal performance but also practical and economic considerations. Application of the absorber design is then made to design of a complete desiccant system for use with low-temperature heat as an energy source.

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Two efficient design configurations of a liquid desiccant cooling/dehumidification system that can be powered by low grade solar or geothermal energy and promise low parasitic power requirements for blowers and pumps are proposed and analyzed in this work. A mathematical model for each component in this system has been developed and synthesized for system simulation. The system performance is predicted and optimized through parametric studies using the computer code LIQSYS. It is shown that the exhaust recirculation mode (ERM) gives better performance at full capacity while the process recirculation mode (PRM) produces cooler and drier air. c c c min E" F H T H s Hr Le m a) QL QR S T, m Nomenclature = transfer surface area per unit length of evap-orator, m 2 /m = weight concentration of water in desiccant solution, kg H 2 O/kg solution = dry air heat capacity, k J/kg ° C = total heat capacity of the hotter stream, kJ/°Ch = total heat capacity of the colder stream, kJ/°Ch = minimum of the C h and C c , k J/ ° C h = heat capacity of water, kJ/kg°C = fin efficiency factor = wetting factor of the transfer surface = enthalpy of wet air, kJ/kg dry air = enthalpy of liquid desiccant solution, kJ/kg desiccant = enthalpy of wet air at desiccant temperature and W e , kJ/kg air = specific enthalpy of water vapor, kJ/kg, and specific enthalpy of water vapor at -17.78°C (0°F), kJ/kg = enthalpy of water, kJ/kg = enthalpy of water vapor, k J/kg = integral heat of solution, kJ/kg solution = inlet enthalpy rate of desiccant solution to the regenerator, kJ/h = outlet enthalpy rate of desiccant solution from the regenerator, kJ/h = convective heat and mass transfer coefficient, respectively; kJ/m 2°C h, kg/m 2 kg/kg/h = Lewis number, defines as h c /h m c pa w w = mass flow rate of air, desiccant solution and water, respectively, kg/h = heat required to evaporate water in the dilute desiccant solution, kJ/h = heat required in the regenerator, kJ/h = channel width of the finned-tube arrangement, mm = inlet temperature to the evaporator, °C T 0 = outlet temperature from the evaporator, °C T hiin , T hiOUi = inlet and outlet temperatures of the hotter streams, °C r cin , r cout = inlet and outlet temperatures of the colder streams, °C U a ,U T =bulk mean velocities of air and desiccant solution, m/h W = humidity ratio of air, kg water/kg air W e = equilibrium humidity ratio for the desiccant solution, kg water/kg air Z = tower axial length in the air flow direction, m e = heat exchanger effectiveness p at p T = densities of air and desiccant solution, respectively, kg/m 3