Compact heat exchangers for mobile CO2 systems

The natural refrigerant carbon dioxide (CO2) with all its advantages offers new possibilities of efficient heating and cooling at different climates. In reversible air conditioning systems the capacity and efficiency in cooling mode is seen to be most important. The efficiency of the reversible interior heat exchanger depends on the design of the heat exchanger. Efficiency reduction is among other factors caused by refrigerant- side pressure drop, heat conduction, refrigerant- and air maldistribution.Uniform air temperatures at the outlet of the heat exchanger are an important aspect regarding comfort and control of the mobile HVAC system.    A prototype CO2 system with a separator, refrigerant pump and a single pass heat exchanger was tested under varies conditions. The tests were performed at low compressor revolution speed i.e. idle conditions. Compared with a baseline heat exchanger of equal size, the use of an external operated pump to circulate the refrigerant through the heat exchanger, increased the cooling capacity at equal pressure levels by up to 14 %. At equal cooling capacities the COP increased by up to 23 %. Airside temperature distribution was more uniform with this way of operating the system.