Effects of lubricating oil on thermal performance of water-cooled carbon dioxide gas cooler

A water-cooled concentric gas cooler model was developed to examine the negative effects of the presence of polyalkylene glycol (PAG)-type lubricating oil on convective heat transfer. The model was used to analyze the gas cooler performance in detail at different oil concentrations, tube diameters, operation pressures, mass flow rates, and inlet temperatures on the water side. The results show that the entrained lubricating oil had a dominant negative effect on gas coolers with a diameter of less than 2 mm. For CO2 heat pump/refrigeration systems equipped with water-cooled micro- or mini-channel gas coolers, the lubricating oil retained in the heat exchanger should be minimized. The negative effects of the lubricating oil are dominant for a wide range of operation pressures. However, the deterioration in thermal performance becomes more apparent as the pressure approaches the critical pressure value. Proper determination of the coolant fluid mass flow rate increases the thermal effectiveness of the gas cooler for a wide range of water inlet temperatures, but the reduced heat transfer performance is more pronounced at higher mass flow rates.