Waste heat and water recovery characteristics of heat exchangers for dryer exhaust

In many industrial drying processes, water is evaporated in the drying section by a large volume of fresh air supply, which contains a large amount of sensible and latent heat. Recovering heat and water from the dryer exhaust is a potential way to realize energy and water conservation in drying systems. In this paper, the experimental and theoretical investigations of the waste heat and water recovery characteristics of heat exchangers for dryer exhaust are presented. A hot air and water vapor mixture was used to simulate dryer exhaust. Convection-condensation heat transfer and water recovery characteristics were first obtained under various water vapor mass fractions, flow velocities, and humid air inlet temperatures through experiments, which revealed that the convection-condensation heat transfer coefficient increased with the increase in water vapor mass fraction and humid air velocity, and decreased with the increase in inlet temperature. The condensed water rate increased with the increase in water vapor mass fraction and humid air velocity, while inlet temperature had almost no effect on it. The condensing ratio decreased with the increase in water vapor mass fraction and humid air velocity, while improving the inlet temperature slightly reduced it. Then, a theoretical calculation model was developed to predict the heat and water recovery characteristics of heat exchangers under a wide working condition. In this model, the pipe and suction effects caused by condensation on the tube wall were considered. The properties of humid air were changed according to the temperature and vapor content instead of regarding them as constant values. The calculation results were compared with the present experimental data and those of previous studies. The good prediction accuracy and the universality of the present model were verified. This study is of great significance to the design of heat exchangers.