Visualization study on the flow and migration characteristics of R290/PAG oil in a room air conditioner during startup and defrosting processes

PAGs are now being considered as the lubricating oil in R290 room air conditioners (RAC) owing to their low pour points and low solubility with R290. However, little research concerning the flow characteristics and migration behavior of the partly miscible PAG in the R290 RAC during dynamics processes has been reported. In this study, visual observation was conducted to analyze the migration of oil during startup and defrosting process. Results show that, during startup process, though excessive oil was discharged from saturated compressor, annular oil film and oil crystal was observed at EEV outlet, the oil can finally return to compressor in the form of linear flow when system parameters gradually reached equilibrium, even though at extremely low ambient temperature. During defrosting processes, the separated oil droplets were found at the EEV inlet when the RAC switched from heating to defrosting, or from defrosting to re-heating mode. The observed adhesion behavior of these oil droplets near EEV increased the potential risk of oil blockage. The larger EEV opening was supposed to be used during this period to increase the reliability of RAC.