Abstract: This paper proposes a single stage/cascade double mode heat pump drying system, which can adapt to the temperature variation of environment and then satisfy the drying system's heating demand. Meanwhile the maximum temperature of drying air can be elevated using cyclic heating. Moreover, a prototype of the single stage/cascade double mode heat pump is developed and an experimental study is carried out. On the basis of experimental results, the performance variation of the heat pump at different modes and operating condition is analyzed. According to the results, it can be concluded that the maximum drying air temperature of cascade mode is higher than that of single stage with the same environment temperature and its difference decreased with the environment temperature. At the same environment temperature, both the heating capacity and power consumption of cascade mode are larger than single stage and the difference of power consumption increased with the environment temperature. With the increase of environment temperature, there existed a maximum COP of cascade mode and the COP of single stage kept increasing. The pressure ratio of the high stage compressor varied greatly at different operating mode and its selection should be reasonable.

Key words: cascade, heat pump drying, performance analysis, prototype testing