In any compressor with an oil-pumping system the oil is pumped under pressure to various parts and performs several tasks. This type of setup is common on larger package rooftop units as well as larger split systems.
When a compressor is at rest it is under static load. The weight of the crankshaft, motor and all related components are basically subject to gravity. Their force is all in one direction, downward. While at rest, the weight of the metal parts are able to overcome most of the oil and displace it.
When a compressor starts, it's at that point that the components are most vulnerable to damage and significant wear patterns can be found. Compressors that frequently short cycle display this type of premature wear pattern.
When a compressor is running, it's in dynamic load. The parts are moving, the oil pump is pumping and the oil is under hydraulic pressure. The combination of all of the above conditions is enough to overcome any metal-to-metal contact.
The rotating components are separated and float on a fluid bed of oil, similar to that of car tires traveling at a high rate of speed on a wet road surface. If tires are worn to the point of being bald, the water cannot be displaced fast enough. The water is capable of overcoming the load and literally separating the car from the road, putting the vehicle in a hydroplane condition. When the tires are new or have a reasonable amount of treads they are able to displace the water allowing greater contact between the tires and the road surface.
This is why bearings are finely machined and polished, and tolerances are of great importance. Any scratches or disfiguring of the mating surfaces would allow for displacement of the oil, thereby allowing the two surfaces to meet under load.
This, too, is why excess refrigerant in the oil sump of the compressor is undesirable. The foaming that takes place causes the oil to lose its hydraulic properties and again allow surfaces to meet because the refrigerant vapor can be compressed.
In addition, this film that the oil is leaving within the compressor actually helps its efficiency. When the oil is splashed on the cylinder walls and rings — as well as deposited on the suction and discharge valves — it helps to seal the gaps of those tolerances. This decreases what is known as the “blow-by” effect, which can be compared to an old hand-operated tire pump. If the rubber piston in the pump was dry or slightly distorted, put a few drops of oil in the cylinder and your pumping capacity is greatly increased.
Refrigeration oil also aids in heat removal. Because it is a liquid it has good heat transfer properties. The heat that the oil absorbs is either rejected in the condenser coil along with the refrigerant or brought down to the oil sump.
Yet another task that the oil performs is to continuously cleanse the internal surfaces of the compressor. The oil is the medium used to carry away any microscopic metal particles that may occur from wear. This is especially true on the break-in period of a compressor. The oil washes the particles down to the compressor sump. There, they are suspended in the oil until they eventually drop to the bottom and are picked up and held by magnets that are typically placed at the bottom of the compressor.
Miscibility is a small subset of the overall solubility characteristics. By far, this would be one of the greatest determining factors in oil compatibility for a particular refrigerant. Miscibility is the capability of the two products, oil and refrigerant, to mix in their liquid state.
Notice that liquid is emphasized. It is actually what is going on in the liquid line or receiver of the refrigeration system that is paramount. If the two do not dissolve in each other as a liquid, then there would be, of course, separation. In this case the specific weights would play a key factor.
Picture a liquid receiver in a large rooftop unit. The oil and refrigerant not being soluble would have the oil floating on the top with the refrigerant on the bottom, much like oil and vinegar. The refrigerant would circulate in the system because the outlet of the receiver is at the bottom, but any oil that left the compressor would not return but be trapped in the top of the receiver. Even in smaller systems it would be detrimental.
To carry this forward, picture the oil and vinegar that is shaken up in a salad-dressing bottle. That is what the mixture would look like in the liquid line. That mixture would cause hunting of the thermostatic expansion valve, resulting in at least reduced capacity, if not compressor failure, because the system never achieved a steady state operation.