Experimental study on the performance for heat pump hot air drying of Tibetan medicine at low-temperature

Tibetan medicine has a long history to serveas a national medicine second only to Chinese medicine. Fresh Tibetan medicine cannot be directly used as medicine. Drying process is essential as a preliminary processing step. The heat pump drying method can be suitable for this case, due to its energy saving, environmental protection, and low cost. In this study, a noveldrying system mode of heat recovery andair-jet enthalpy heat pump was proposed to improve the drying characteristics of Tibetan medicine under low-temperature environments. A systematic experiment was conducted to explore the stability and energy efficiency for the continuous operation of system, thereby to obtain the changing characteristics of continuous operation onthe heat pump in different load modes at low temperature conditions. In order to explore the performance characteristics of heat pump drying for the Tibetan medicine at low temperature environment in winter, a drying oven was built with the size of 3.9m ×2.1m ×2.3m (length ×width ×height)and a volume of about 15m3, as well asa rated load of 1.5 t. An ordinary ring scroll compressor,and anair jet enthalpy compressor in a heat pump system were selected to carry out the load experiment test at the ambient temperature of -11.4−18.3 and -13.39−16.69 ℃. The ordinary scroll compressor in a heat pump system workedin the segmented drying mode, and the continuous drying operation mode was used to dry Tibetan medicine. The air-jet enthalpy-increasing heat pump system performedday and night for the continuous drying. Results were as follows: Under the severe cold and large temperature difference (30 ℃) in the winter plateau, the averageefficiencyof heat transfer in the air-jet enthalpy-increasing heat pump system (-13.39−16.69 ℃) was 9.56%, and that in the ordinary scroll compressor heat pump system (-11.88−18.29 ℃) was 13.58%. It infers that the efficiency of heat transferin a heat exchanger was relatively low, asthe ambient temperature decreased.The compressor selection has no significant effect on the heat exchange efficiency of heat exchanger. The minimum coefficient of performance (COP) of air-jet enthalpy-increasing heat pump system(-13.39−16.69 ℃) increased by 51.5%, compared with the ordinary scroll heat pump system (-11.4−18.3 ℃). The average value of COP increased by 16.9% during the whole progress, indicating effectively improved the heating energy efficiency of a system at low temperatures. The placement and thickness of amaterial presenteda significant influence on the drying effect. When the loading capacity of drying box was about 100% and stacking thickness was 20 cm, the finalproduct behavedbright color, uniform drying, and the high specific moisture extraction rates (SMER). Compared with the traditional air-drying method, the drying time of a heat pump was only 1/9 of conventional dry method for the same amount of materials, while,the drying cost was reduced by 1 160 yuan/t. The recoveryandair-jet enthalpy heat pump system can meet the energy supply that required for the drying of Tibetan medicinal materials under the low-temperature conditions.