Miscibility, Lubricant, Refrigerant, Evaporator
The phase interaction of liquid refrigerant with liquid lubricant is commonly described as miscibility. Lubricant and refrigerant combinations can be described as single liquid phase (miscible) or multiple liquid phases (immiscible). In specific system designs, good miscibility between the compressor lubricant and refrigerant play important roles in system functionality. Inadequate miscibility can lead to lubricant accumulation in the evaporator, which can cause degraded heat transfer properties as well as lubricant starvation in the compressor. Governments are implementing stricter environmental standards in the HVAC&R industry including higher efficiency targets and lower GWP (global warming potential) refrigerants. Equipment design changes are increasingly required in order to meet these regulations. Changes in refrigerant chemistry or operating conditions are causing lubrication challenges due to generally increasing refrigerant dilution and lower working viscosity. Maintaining sufficient miscibility ranges while upholding all other lubricant properties is no longer guaranteed. The miscibility boundary is generally described as a transition from one single phase to two distinct phases, however that is rarely the case. Within this paper a broader range of miscible and immiscible phase interactions will be discussed. Interesting and unique phenomena that occur during miscibility testing will be looked at with the ultimate goal of understanding how these behaviors translate into evaporator performance and lubricant return.