Sensitivity analysis of structural parameters of a low-temperature heat pump sludge drying system

Optimizations of the heat exchanger of the heat pump drying system were usually carried out individually, linkage effect of the whole system in practical operating was usually not considered. In addition, for a heat pump sludge drying system, optimizations mainly concentrated on the operation parameters, few theoretical analyses focused on the effect of structural parameters to the system performance. In the present study, based on the laws of energy conservation and mass conservation, a steady-state model for the heat pump sludge drying system, which interconnects the refrigerant circulation and the moist air circulation, was established. Three variables which are coefficient of performance (COP), dehumidification rate (DR), and specific moisture extraction rate (SMER) are simultaneously introduced to evaluate the system's operating and drying performances. Effects of the number of horizontal tube rows, the number of longitudinal tube rows, and the fin spacing to the above three variables are analyzed. The results show that the COP decreases with increasing the number of horizontal tube rows while increases with increasing the number of longitudinal tube rows and fin spacing. The DR increases with increasing the number of horizontal tube rows and fin spacing and decreases with increasing the number of longitudinal tube rows. The SMER has a similar variation trend to that of the DR due to their directly proportional relationship. Finally, based on the sensitivity analysis, the heat pump sludge drying system achieves better operating and drying performances when the fin spacing is 2.85 mm, the numbers of horizontal tube rows of the evaporator and condenser are 18 and 14, and the numbers of longitudinal tube rows are respectively 12 and 9.