Experimental investigations on adiabatic capillary tube in a transcritical CO2 heat pump system for simultaneous water cooling and heating

Experimental studies for a prototype CO2 heat pump system employing capillary tube as an expansion device for simultaneous water heating and cooling have been presented. Tests were conducted with two stainless steel capillary tubes having specifications of ID = 1.71 mm, L = 2.95 m, εc = 3.92 μm and ID = 1.42 mm, L = 1.0 m, εc = 5.76 μm. System performance is significantly influenced by gas cooler water inlet temperature, whereas the effect of water flow rate on system performance is modest. An optimum charge is also recorded at which the system yields the best COP with a capillary tube. Performance deterioration is more severe at undercharged condition than at overcharged condition. Comparisons of test data with simulated results show that maximum deviation in mass flow rate of refrigerant is about ±10% and the flow characteristics exhibit a reasonably good match with the test data.