Thermal characteristics of a novel enclosed cascade-like heat pump dryer used in a tunnel type drying system

Abstract

In order to overcome the drawbacks of the traditional air source heat pump dryer (HPD), a novel enclosed cascade-like HPD was proposed. The tunnel type drying system with the HPD was designed and manufactured. The hawthorn cake was taken as one kind of the drying mater whose moisture content changed from 0.99 to 0.3 kg water/kg dry matter in the HPD or HAD system. The performance of this HPD was investigated. The drying air temperature, relative humidity, and velocity were tested with different conditions at the drying time of 120–130 min and air flow velocity of 1.5–3 m/s. The results were compared with a conventional hot air dryer (HAD). Compared with the HAD system, the energy consumption of the HPD system decreased by 32.55%. The effectiveness of the HPD was verified. The average heating coefficient of performance (COP) of the whole HPD system was about 3.45. The average specific moisture extraction rate (SMER) of the HPD system was 0.93 kg water/kW h that was much higher than HAD system. The results of the exergy analysis showed that the maximum exergy loss appeared near the location of the tunnel exit. According to the experimental data, the modified page model was proved an accurate tool to predict the hawthorn cake drying process.