Performance study of a heat pump assisted dryer system for specialty crops

This thesis is concerned with the technology of heat pump assisted drying of specialty crops, the benefits and potential of the process in the agriculture sector. The purpose of using a heat pump for drying of specialty crops is to dry them at lower temperature than would otherwise be possible using conventional drying techniques. Low temperature drying of specialty crops reduces the risk of loss in nutrient content and damage to physical properties which are important aspects considering their high commercial value. Heat pump drying is potentially more energy efficient since it is possible to recover latent heat from the humid dryer exhaust air. In this research, cabinet and prototype continuous bed heat pump dryers were studied. Simulation models for the heat pump dryers were developed to predict the performance of the dryer, i.e. the drying rate of the material and psychrometric conditions of the air, and also to determine the refrigerant mass flow rate and corresponding temperatures at the condenser and evaporator coils of the heat pump system, based on the psychrometirc conditions of process air inside the drying chamber. The accuracy of the predicted results is later verified with the experimental results. Chopped alfalfa was dried in a cabinet dryer in batches and by emulating the continuous bed drying using two household dehumidifiers. The reason for using alfalfa instead of specialty crops like ginseng, herbs, echinacea, feverfew, etc. was that the material and its drying properties were readily available. Also the structure of alfalfa leaves and stems is similar to that of many herbs and specialty crops. Results showed that alfalfa was dried from an initial moisture content of 70% (wb) to a final moisture content of 10% (wb ). It was noticed that batch drying took about 4.5 h while continuous bed drying took 4 h to dry the material. The initial weight of alfalfa in each tray was 400 g. The average air velocity inside the dryer was 0.36 m/s. Low temperatures (30-45°C) for safe drying of specialty crops were achieved experimentally. Specific moisture extraction rate was maximum when relative humidity stayed above 40%. The household type dehumidifiers used in this study were about 50% more efficient in recovering the latent heat from the dryer exhaust compared to the conventional dryers. It was concluded that continuous bed drying is potentially a better option than batch drying because high process air humidity ratios at the entrance of the evaporator and constant moisture extraction rate and specific moisture extraction rate values can be maintained. Simulation results for a prototype heat pump continuous bed dryer system suggested that the change in dryer inlet temperatures of the process air has an insignificant effect on drying of the material. Therefore, based on the results, it was concluded that the dryer inlet air temperature could be kept as low as 30°C, if required, to maintain product quality. The material was dried to a safe limit of 10% moisture content. The material mass flow rate was only 3-4.5 kg/h, indicating that it might be advisable to use the heat pump dryer in combination with some other technique. ****Disk with program codes was included with the original thesis***