An experimental comparison of two heat exchangers used in wastewater source heat pump: A novel dry-expansion shell-and-tube evaporator versus a conventional immersed evaporator

A novel dry-expansion shell-and-tube evaporator (DESTE) integrated with function of defouling was developed as a component of wastewater source heat pump (WWSHP). To compare the performance of the DESTE-WWSHP with the conventional immersed evaporator (IE) based wastewater source heat pump (IE-WWSHP), an experimental platform was built by installing a DESTE and an IE unit in parallel in a same WWSHP system. The performance tests were conducted with the wastewater collected from downstream of a commercial sauna center in Shenzhen, China. Several operating parameters were investigated, including water heating capacity, coefficient of performance (COP), hot water temperature settings, wastewater discharge rate/pattern, and hot water discharge mode (continuous or intermittent). Results showed that the DESTE-WWSHP delivered comparable heat transfer performance to the IE-WWSHP, and thus, could serve as a replacement for the latter design. Specifically, the water temperature in the water storage tank was more uniformly distributed in the DESTE-WWSHP than the IE-WWSHP; the DESTE-WWSHP exhibited higher water heating capacity than the IE-WWSHP in the continuous hot water discharge mode. The DESTE has a heat transfer coefficient 3.1 times higher than that of the IE unit on the wastewater side. At the same heating capacity, the DESTE was far more compact with its compactness 7 times higher and its volume 91.71% smaller than the IE unit, leading to considerable savings in room space and materials. Additional benefits offered by this DESTE-WWSHP included reduced labor and power costs for cleaning heat exchanger, and decreased odor nuisance.