Effects of compressor frequency and heat exchanger geometry on dynamic performance characteristics of heat pump dryers

A heat pump dryer (HPD) exhibits significant transient behaviors owing to the combined system of the heat pump cycle and closed-loop air cycle. However, a complete transient simulation for HPDs has not been yet developed owing to its complicated equations and lack of experimental data. In this study, a complete transient simulation of an HPD was developed using the finite volume method and was validated with experimental data, presenting validation errors less than 6.1%. Using the developed simulation model, the dynamic behavior and performance of the HPD were analyzed by varying operation modes and heat exchanger length. The operation mode with a higher compressor frequency showed a 16.5% higher moisture extraction rate (MER) due to the decreased drying time, whereas that with a lower frequency showed a 34.8% higher specific moisture extraction rate (SMER) due to the decreased energy consumption. As the heat exchanger length increased from 80% to 120%, the MER increased by 10% and the SMER increased by 14%. Additionally, as the compressor frequency increased, the optimum heat exchanger length for achieving maximum performance increased. In conclusion, the compressor control logic and heat exchanger length should be optimized according to the drying time, power consumption, and initial cost.