R-134a Liquid Over-Feeding Mobile Air Conditioning Systems

For most air conditioning systems, usually only 85% of the evaporator is effectively used. The other 15% is used to superheat the refrigerant so that the compressor will be protected from liquid slugging, but this practice results in excessive evaporator volume. In mobile air conditioning (MAC) systems where the space available for the evaporator is very limited, the evaporator should be fully used, yet MAC designs have not reflected this need. This study reports on the designing and testing of a novel liquid over-feeding (LOF) MAC system that can use 100% of the evaporator effectively.

A LOF system is designed so that not all liquid refrigerant is evaporated in the evaporator. The low-pressure liquid and vapor flow into an accumulator-heat exchanger where the liquid is boiled off by the warm, high-pressure liquid leaving the condenser. This concept not only allows 100% evaporator utilization, but will also reduce system power consumption and improve compressor reliability. Preliminary test results of LOF MACs, compared with thermal expansion valved MACs, indicated increases in cooling capacity of 20%, and refrigerant mass flow rate of 19%, coefficient of performance (COP) of 20%, and a reduction in power consumption of 5% at a compressor speed of 1560 revolutions per minute (rpm). This design is expected to only marginally increase the cost of MAC systems.