17 June 2005

Designing a heat pump for minimum refrigerant charge

This paper reports results from a project aiming at reducing the refrigerant charge in a 5 kW liquid-to-liquid heat pump (heating only) to 150 g of propane. The work started by determining the fluid inventory in each part of the heat pump, with the result that the main effort has been on redesigning the heat exchangers using mini-channel tubes. At present, the system operates with about 200 g of propane. It should be possible further to decrease the charge by redesigning the condenser and by reducing the amount of propane in the compressor, either by using oils in which propane is not soluble or by using compressors with a low charge of oil. The resulting designs may open the way to safe use of flammable refrigerants with high energy efficiencies, such as R152a, R32, R290 or R600a.