Analytical and experimental study of a liquid desiccant heat and mass exchanger operating near water freezing temperature
Abstract
With the rising costs of electricity due to increasing demand of electric power, liquid desiccant systems have received significant attention as a way to reduce latent loads on air conditioning systems. In particular, the performance of liquid desiccant systems in humid climates has shown significant reductions in energy consumption. In general, these liquid desiccant systems are composed by an absorber or dehumidifier and a regenerator that utilizes a heat source to reject the water from the diluted liquid desiccant. As the humidity of the air is absorbed at the dehumidifier, the temperature of the liquid desiccant increases due to the addition of heat from the enthalpy of condensation of the water vapor. Thus, many designs of liquid desiccant absorbers include the flow of a cooling fluid that removes heat from the liquid desiccant lowering its temperature.
A different application of liquid desiccant systems corresponds to the localized removal of moisture from the air inside low temperature rooms that contain relatively high levels of humidity such as refrigerated warehouses for the food industry. The purpose is to reduce the formation of ice at the surface of the evaporator. Due to the low temperature of the air inside these rooms, no cooling fluid is necessary for the removal of heat from the liquid desiccant. Thus, the designs of the absorbers differ from the designs used in more conventional applications.
In this thesis, mathematical models of the heat and mass transfer for an adiabatic parallel-plate absorber for which a thin film of liquid desiccant flows down its walls and dehumidifies the air in cross-flow configuration are developed. Numerical results are obtained and the performance of the absorber as a function of several parameters including inlet air temperature and relative humidity, inlet liquid desiccant temperature, mass flow rates of air and liquid desiccant, and liquid desiccant concentration is analyzed. The results are compared with experimental data available for an absorber using calcium chloride as the liquid desiccant. The geometry of the absorber is optimized to improve the dehumidifying performance of the core.