A Comparison of the Oil Return Characteristics of R-22/Mineral Oil, and Its HFC Alternatives (R-407C & R-410A) with Mineral Oil and POE in a Residential Heat Pump

Oil return from the evaporator to the compressor crank case is a critical requirement affected by the refiigerantllubricant properties, gas velocities and piping geometry. Refrigerant R-22 has been successfully used with mineral oil in residential air-conditioning and heat pump systems (AC/HP). R-407C and R-41 OA are two chlorine free HFC refrigerant candidates to replace R-22. This paper describes the test methods and the results from controlled oil return experiments in a 3 ton residential split system. The HFC refrigerants were evaluated with mineral oil (MO) and polyol ester lubricants (POE) and are compared to the R-22/MO baseline. The results support the decision to use POE's with HFC refrigerants in residential split heat pump systems. INTRODUCTION The performance and reliability of a residential heat pump system are affected by the refrigerant and lubricant properties. The solubility and miscibility characteristics affect the oil return characteristics. The air conditioning and commercial refrigeration industries have successfully applied refrigeranVIubricant pairs with varying miscibility O.e. CFC-12/MO systems with complete miscibility, R-22/MO systems with partial miscibility, and R-502/MO systems with poor miscibility). The objective of this study is to evaluate the_ oil return characteristics of some recently introduced refrigeranVIubricant pairs. R-407C and R-41 OA are two HFC substitute refrigerants for R-22 in AC/HP system applications• Based on the commercial refrigeration experience with HFC substitutes for R-12 and R-502, POE lubricants are the leading candidates for the HFC refrigerants-• However, POE lubricants have two major drawbacks: hygroscopicity and high cost. There is interest and need to know to what extent is it possible to replace POEs with a lower cost, but immiscible, oil such as mineral oil. It is the purpose of this study to experimentally investigate the oil return behavior of mineral oil and the HFC's most likely to replace R-22. Specifically, the following refrigeranVIubricant pairs are investigated in a residential heat pump; R-22/MO, R-407CIPOE, R-407C!MO, R41 OAIPOE, and R-41 OA/MO. TEST FACILITY The test facility consists of primarily two components, an indoor loop and an outdoor chamber • The indoor unit of the AC/HP system is located in a closed air loop simulating indoor conditions while the outdoor unit of the AC/HP system is placed in an environmental chamber that simulates outdoor conditions. The test facility meets or exceeds ASHRAE Standard 116-1983 requirements • The test unit is designed for R-22 and is rated at 10.6 kW (3 tons). The indoor heat exchanger of the AC/HP system is an A-coil with four parallel passes and a cross-over pattern between the two sections of the A. The outdoor heat exchanger of the test unit is a plate-fin coil, two rows deep and has four parallel passes. The indoor and outdoor heat exchangers utilize smooth tubes. Thermostatic expansion valves are used for both the heating and cooling modes. The system is not designed for an accumulator and one is not used for the data presented here. Two scroll compressors are utilized in this study, one for the medium pressure refrigerants (R-22 and R-407C) and one for the high pressure refrigerant (R-410A). Neitherofthe compressors poses a crank case heater. Each ofthe compressors is modified by installing a sight tube and sight glasses, as shown in Figure 1. The sight tube, which is graded in millimeters, allows the liquid level in the compressor to be quantified while the sight glass allows the fluid flow to be