Experimental Analysis of Heat and Mass Transfer Coefficients of a Liquid Desiccant System

摘要

Heat and mass transfer system was developed and experimentally analyzed to calculate heat and mass transfer coefficients for a liquid desiccant - air contact system in a cross flow configuration for dehumidification of air and regeneration of liquid desiccants. The examined dehumidifier and regenerator are the core of a demonstration plant of a liquid desiccant system for drying hay bales. The system was set up in an agricultural domain. The dehumidifier and the regenerator are designed to overcome the present obstacles such as the carryover of the sorbent into the air stream and the flow mal-distribution of the sorbent over the exposed surfaces. The basic concept of this system is to directly reduce the moisture and warm up the air, which will be used for drying, only few kelvins above the ambient temperature. The dehumidifier consists of plate type heat and mass exchanger with a total exposed surface of about 75 m~2 made up of polycarbonate plates. The desiccant regeneration system consists of a tube type heat and mass exchanger of copper pipes, protected from corrosion with a thin powder coating layer. Textile sleeves are applied over the copper tubes. The total exposed surface area of the regenerator is about 9 m~2. Hot water supplied by solar collector field flows through the copper tubes to heat the desiccant solution in order to concentrate it again. First measurements of the demonstration plant showed promising results of the dehumidification of hay bales. The drying time for a hay bale could be reduced significantly. The air stream temperature was increased by about 10 K while relative humidity was reduced by about 40%-points during the tests. The results were analyzed and compared with results from a finite difference model. Two models, a finite difference and an effectiveness model, were developed for cross flow plate type heat exchanger. Both models can be operated with and without internal cooling or heating of the sorption process.