Study On Two-stage Controlled Freezing Point And Low Temperature Heat Pump Drying System Based On Marine Delicacy

Recently, with the rapidly development of aquaculture industry, the production of marine delicacy has grown fast. Fresh marine delicacy has high water content so that it is not convenient to store and transport. Drying is one of the most important process. Heat pump drying with low drying temperature is mainly adopted in drying precious seafood, the nutrition is well kept and higher organoleptic properties are approached. Freezing point drying is operating at-2~0℃, during this temperature, the breeding of the bacteria would be decreased and the structure of the cell would not be destroyed. Moreover, the quality of the sample was high. However, the energy utilization and the drying economical efficiency of the appropriative drying machine are low, so the cost of drying sample is added. With the adoption of the combinational drying of freezing point temperature drying and heat pump drying, it would not only satisfy the requirement of high quality, but avoid the long drying period of single freezing point temperature drying.A heat pump drying system was designed and manufactured that consist of two stages in the drying system named low temperature drying period(20~30℃) and controlled freezing point drying period(-2~0℃) respectively. To realize the simultaneous working of two stages,the outlet air of two drying chambers was mixed. The temperature and humidity would be controlled in appropriate range. Moreover, the air state of each testing point could be monitored and controlled by PC and intelligent controllers.Experiments on drying system operation properties were carried out and the effect of the recycle air volume, cooling air volume and hybrid ratio on the air state of inlet and outlet of condenser, evaporator and hybrid air were explored. Results indicated that the hybrid air state was influenced by all the factors. The temperature of the outlet of evaporator was intensely affected by the recycle air volume and the regulation scope was about 7.9℃. The effect of the cooling air volume on outlet of condenser Ⅰ was obvious and the regulation scope was about4.8℃, while the effect on the outlet of evaporator was not evident that the regulation scope was only 2.7℃. The effect of hybrid ratio on both evaporator and condenser Ⅰ was not significant.Experiments on drying characteristics of Ruditapes Variegate were carried out. The results showed that when the conversion of freezing point temperature drying into heat pump drying happened during the constant drying period of freezing point temperature drying, the color changes was small and the rehydration rate was high. However, the drying time was shorter with the longer time of freezing point temperature drying. The optimum drying method was freezing point temperature drying for 10 h and convert to heat pump drying and the drying time was 26 h, color changes was 13.30 and the rehydration rate was 74.29%.