Performance evaluation of a tube-in-tube CO2 gas cooler used in a heat pump water heater

In this study, investigation of the performance of a tube-in-tube counter-flow water-cooled CO₂ gas cooler operating above and near critical pressure is presented using a heat pump water heater with CO₂ flowing in the annulus side. A tube-in-tube heat exchanger model applicable for supercritical fluid CO₂ and water was also developed and validated. The measured total heat transfer capacity ranged from 1.31 to 4.06kW at various test conditions. The calculations show good agreement with the experimental results. The results demonstrate that the variation of CO₂ temperature tends to show very slow decreasing near the pseudo-critical region when compared to the inlet region. Yet this phenomenon becomes more pronounced as the inlet pressure is close to the critical pressure (73.8bar). The calculation also reveals a peculiar phenomenon that the local heat transfer rate of the heat exchanger peaks within the heat exchanger near the pseudo critical region due to the drastic rise of specific heat (C_P value).