Prototype liquid chiller using carbon dioxide as refrigerant

A 40 kW cooling capacity liquid chiller using carbon dioxide (CO₂) has been designed, built and tested experimentally. The prototype had the possibility to reject heat both to air and water. Both the water chiller and the gas coolers are new designs adapted for CO₂. The water-cooled gas cooler was a compact multi tube-in-tube counter flow concept, whereas the air-cooled gas cooler was a conventional round tube in fin. The liquid chiller/evaporator was based on a tube-in-shell concept. The compressor was a semi-hermetic piston type. Experiments were conducted with varying cooling water temperatures and at steady state and cool-down situations. Measured cooling capacity at steady state conditions showed a cooling capacity of 13 to 33 kW, depending of high pressure and cooling water temperature. The COP varied with high pressure and cooling water temperature. Optimum high side pressure was 70 bars for cooling water temperatures below 18, whereas 80 bars above. Calculated U-value of the evaporator was 3100 W/m2.K. Measured values were in the range of 1100 to 1400 W/m2.K. The reduction is probably due to suppressed nucleate boiling caused by oil film. The heat rejector worked well as gas cooler, whereas relatively poor as condenser. This initial study shows that the CO₂ liquid chiller technology is promising. However, there is room for performance improvements.