Theoretical analysis and experimental study on a low-temperature heat pump sludge drying system

Abstract

A low-temperature heat pump sludge drying system was constructed and experimentally studied in the present study. The initial and final moisture contents on dry basis of sludge are 1.53 and 0.2, and the drying duration is 5 h. Theoretical models for heat pump circulation and moist air circulation are established and validated with the experimental results. Based on the validated theoretical models, sensitivity analysis of evaporating temperature, condensing temperature, and air mass flow rate on the temperature and humidity of moist air at the outlet of drying closet, drying rate and specific power consumption (SPC) are carried out. The temperature of moist air at the outlet of drying closet increases with increasing evaporating temperature and condensing temperature, while decreases with increasing air mass flow rate. The drying rate decreases with increasing evaporating temperature while increases with increasing condensing temperature, and first increases and then decreases with increasing air mass flow rate. The appreciated air mass flow rate ranges between 0.8 and 1.0 kg/s, and the drying rate around 6.0 kg/10min can be achieved under these conditions. The relative humidity of moist air at the outlet of drying closet shares a similar trend with the drying rate while SPC shows a contrary trend.