Experimental and Theoretical Investigation of Oil Retention in Vapor Compression Systems
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The design of any system needs to consider a number of parameters according different needs. In heating, ventilation and air conditioning systems the overall efficiency, the reliability of the components, the cost and volume, and the refrigerant/oil charge are only some examples of variables that can be optimized. An important aspect is the selection of lubricants that provide the same or improved characteristics relative to traditional mineral oils. In HVAC systems, the oil exists only because the compressor requires it for lubrication and sealing. Proper oil management is necessary in order to improve the compressor reliability, increase the overall efficiency of the system, and minimize the system cost by avoiding redundancy. Several literature sources focused on oil/refrigerant properties (Thome, 1995), oil return characteristics (Biancardi et al., 1996) and oil transport phenomena (Mehendale, 1998). An analytical and experimental study of the oil retention has been developed for automotive air conditioning systems using carbon dioxide (Jun-Pyo Lee, 2002). However, a general comprehensive model for oil retention and oil distribution in heat pump systems using other refrigerant/oil mixtures does not exist and is of importance to future design considerations. The purpose of this thesis is to experimentally and theoretically investigate the physics of oil retention and oil transport in different components of the system. Condenser, evaporator, suction and liquid lines are studied using different pairs of refrigerant-oil mixtures. Oil retention is measured directly using an experimental apparatus, and oil film thickness is estimated. At oil mass fractions of 8 wt.%, the pressure drops increase up to 40% in the suction line, 20% in the evaporator and 30% in the condenser as compared to oil-free operating conditions. New pressure drop correlations need to include this penalty factor due to oil retention. An analytical model for vapor and two-phase refrigerant flows utilizing minimal empirical data is developed. The model is able to estimate the oil distribution in the entire system providing good design guidelines for the selection of the proper refrigerant/oil mixture, the optimization of the component geometries, and the management of the oil/refrigerant charge.