Medium temperature and humidity evolution of closed heat pump drying and its influence on energy consumption

Abstract

Objective： This study aimed to resolve the interactions among the material dehydration, temperature and humidity of dry medium, and the energy consumption. Methods: The dehydration characteristics, temperature and humidity evolution of dry medium, and energy consumption of system component had been determined by testing under control parameters of 60 ℃ and 10% RH. Results: Dehydration rate of the materials was relatively fast in the early stage, 96 percent of the moisture was removed from the material after 1.5 hours of drying. At this stage, the relative humidity of drying medium remained above 24 percent, the moisture in drying medium was rapidly condensed and discharged from the surface of the evaporator, and the maximum value of specific moisture extraction ratio was 1.03 kg/kW. Heat pump unit was the main energy consuming part in the drying process, and its power increased with the temperature of the drying medium. The specific moisture extraction ratio decreased rapidly in the later drying stage, and its value was only 0.004 kg/kW after 2 hours. Conclusion: The energy utilization of heat pump drying was closely related to the temperature and humidity of medium and moisture content of material. Drying had a high specific moisture extraction ratio when the moisture content of material was high, and fell off as the relative humidity decrease. The constant temperature and humidity control method was not suitable for the energy saving of the closed heat pump system in the later drying stage.

Keywords

• closed heat pump
• dehumidification drying
• temperature and humidity of medium
• specific moisture extraction ratio