Thermodynamic analysis of novel heat pump cycles for drying process with large temperature lift

Abstract

Heat pump–assisted drying has been recognized a prospective technology to meet the requirement of energy saving. However, large temperature lift will be resulted by the single-stage heat pump cycle during the high-temperature drying, especially operating with low ambient temperature for open-loop drying, which leads to insufficient heat output, high compression ratio, and low coefficient of performance (COP). Two heat pump cycles, namely, multitemperature cascade cycle and combined single-stage cycle, are proposed to address the above problems in the drying process with large temperature lift in this paper. The effects of varying operation parameters on the heat pump cycles are analyzed to optimize the cycle performance. Afterwards, the above two cycles as well as a conventional cascade cycle, a two-stage compression cycle, and a single-stage compression cycle are compared with each other in terms of cycle performance and drying performance under specified drying conditions. It is indicated by the results that the COPs of the multitemperature cascade cycle and combined single-stage cycle are about 95% and 88% higher than that of the single-stage compression cycle, respectively. As for the two cascade cycles (ie, conventional cascade cycle and multitemperature cascade cycle), 49% more water evaporation with the same power input can be resulted by the added condenser and evaporator. Among the five analyzed cycles, the multitemperature cascade cycle is the most promising to be used in the retrofitting of the drying equipment with large temperature lift.