Experimental evaluation of heat transfer performance of falling film evaporator for wastewater source heat pump with backwash system

Urban wastewater contains large amounts of waste heat and the wastewater source heat pump (WWSHP) offers an effective way to use this heat. Increasing the overall heat transfer coefficient (OHTC) of the evaporator is an effective mean to improve the performance of the heat pump. In this work, falling film evaporator(FFE) is used in direct WWSHP. A WWSHP unit that can switch evaporators is first established. The effects of practical application of this WWSHP are then tested and compared with simulation results. Under the test conditions, the average OHTC of the wastewater FFE (WFFE) reaches 1960.90 W m(-2) K-1, while the average OHTC of a wastewater flooded evaporator (WFE) 1224.28 W m(-2) K-1. The average OHTC of the WFFE was thus 60.17% higher than that of the WFE and COP of the WFFE is 9.52% higher than that of the WFE. The experiment also explores the wastewater backwash, which can ensure stable operation of the unit. When using the backwash mode, the backwash time interval was set to be as short as possible to ensure that the average OHTC would not decrease. A mathematical model of the OHTC of the WFFE is presented. Using the wastewater condition as a basis, the mathematical model is used to synthetically calculate the shell side HTC of the WFFE, the convection HTC of the wastewater and the heat resistance of the tube walls and the fouling. From the experimental results, an influence factor for the wastewater down-and-up mode is proposed to revise the formula. (C) 2020 Elsevier Ltd and IIR. All rights reserved.