Effect of reversely-variable circuitry on finned-tube heat exchanger performance in air source heat pump under annual performance factor (APF) conditions

Abstract

The finned-tube heat exchanger (FTHX) in air-source heat pumps (ASHPs) shows a contradiction that the evaporator prefers more circuits while the condenser likes fewer. Hence, the reversely-variable circuity is proposed in our previous work to address this problem by incorporating the respective optimal circuits for the FTHX in both modes. In this article, the reversely-variable circuitry is adopted to maintain 7 circuits in the evaporator mode but 4-2-1 flowpath (4 circuits merging into 2 then 1) in the condenser mode of the outdoor FTHX. Compared with the original 7-circuit reversely-fixed one, the reversely-variable FTHX yields 2.1% higher annual performance factor (APF) for the ASHP due to higher capacity and COP in all 6 cooling conditions, when the outdoor FTHX is used as the condenser and the fewer circuits increases the overall heat transfer coefficient. Moreover, the performance enhancement with the reversely-variable FTHX is higher in lower-temperature ambience and part load conditions, since refrigerant flowrate is lower and the increment in overall heat transfer coefficient is more dominant. Different possible transformations of the reversely-variable circuitry is obtained with the principles of single-way valves and distributors described, so that it could be popularized to wider applications.