30 May 2005

DESIGNING A HEAT PUMP FOR MINIMUM CHARGE OF REFRIGERANT


Independently of which refrigerants will be used in the future there are strong incentives to reduce
the charge of refrigerant in each system. This paper reports on results from a project aiming at reducing
the charge of refrigerant in a 5 kW liquid to liquid heat pump (heating only) to 150g of propane. First, the
fluid inventory in each part of the heat pump was determined. As a result, the main effort has been on
redesigning the heat exchangers using mini-channel tubes. At the present time, the system operates with
about 200 g of propane. Further decrease of the charge is possible 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 low charge of oil. The designs suggested may open up for the safe use of
flammable refrigerants with high energy efficiency such as R152a, R32, R290 and R600a.