The Experimental Study Of Transcritical CO₂Heat Pump Dryer

Heat pump can absorb thermal energy from a low temperature surrounding to improve its energy grade. Then the thermal energy can be used in a higher temperature place. Thermal energy is transformed between refrigerant and ambient through refrigerant phase changing process because of compressing and throttling. Since heat pump can improve the energy grade by taking use of small amount energy for compressor working consumption. Heat pump dryer can use this feature to absorb energy of the drying medium out of drying chamber and make the drying medium cooling and moisture condensation. Heat pump drying has the advantages of high energy efficiency, low operating costs, environmental protected, easy to control and the drying process does not be influenced by outside ambient. CO2, a natural refrigerant, has incomparable advantages of environmentally friendly, safe, stable and good thermal properties.According to the characteristics of trans-critical CO2, the effects of different parameters on drying process are studied. Results show that CO2heat pump drying system performance is mainly influenced by the evaporating temperature, gas cooler outlet temperature and compressor discharge pressure. Evaporating temperature increasing and gas cooler outlet temperature decreasing can both improve system performance, and effects of discharge pressure associated with evaporating temperature and gas cooler outlet temperature. At a certain evaporating and gas cooler outlet temperature, compressor discharge pressure has an optimum value which makes the system performance maximum. The optimum pressure changes with various evaporation temperatures and gas cooler outlet temperatures.CO2has the properties of low viscosity, high density and small surface tension at supercritical state, which make the CO2has excellent properties of heat transfer. In supercritical state, CO2has a pseudo-critical temperature, the specific heat of CO2reach to a peak value, the heat transfer performance increases near the pseudo-critical temperature. In the process of CO2evaporating in tube, the main type of flow is annular flow. The drying up occur with evaprating process, and it would make the heat transfer performance reduced. The physical properties and characteristics of heat transfer of CO2are studied and a trans-critical CO2heat pump drying apparatus is designed according to calculate results. The3HP Dorin compressor is used, and the heating capacity is7.02kW under design conditions. Temperature and pressure sensors are used to measure the parameters at different points for experimental study. Experiments are conducted under different working conditions by the CO2heat pump drying equipment. Influence factors of the drying on carrots and sausages as materials are studied under different drying conditions. Results show that increase drying temperature and air velocity can both improve the drying rate of carrot and sausages. However, the temperature has greater influence than the air velocity on carrots drying and the air velocity has obvious influnce on sausages drying at the beginning stage and shows a trend of weakening influence with drying process. According to drying experimental results, the mathematical model on carrot and sausages drying is established. The model calculating results are compared with experimental data. The results show that the Page model is well to describe the carrot drying and the Henderson and Pabis model is well to describe the sausages drying. Establishment of the model has great significance to direct actual drying production of carrot and sausages.