Pinch point analysis and design considerations of CO2 gas cooler for heat pump water heaters

•Pinch point occurring in water-cooled CO2 gas cooler was studied.•Critical mass flow ratios were proposed to control approach temperature difference.•Standard errors of UA value estimated from commonly used LMTD method were analyzed.

Due to strong nonlinear variation of supercritical CO2 specific heat capacity with temperature, pinch point would occur in water-cooled CO2 gas cooler, which has great impacts on the heat transfer characteristics of gas cooler and overall system performance. Pinch point analysis was conducted for CO2 gas cooler in the present study. The effects of refrigerant pressure, mass flow ratio (mw/mc), inlet water temperature and heat transfer area on pinch point location, approach temperature difference and heat transfer rate were analyzed in detail. Based on the analysis of pinch point location in CO2 gas cooler, the critical flow ratios were proposed to effectively control the approach temperature difference. Furthermore, the actual conductance of gas cooler was calculated and compared with that estimated by LMTD method. The results showed that CO2 gas cooler may be undersized by as much as a factor of 30–60% for different pressures if LMTD method is used. However, the UA value evaluated by LMTD method also may be overestimated under high refrigerant pressures when the approach temperature difference tends to be zero. Results of the present study are helpful to practical designs of CO2 gas cooler and heat pump water heaters.