Reprinted with permission from RSES Journal
The Important Role of Refrigeration Oil
Understanding the key role oil plays in refrigeration and air conditioning, and how to diagnose problems can reduce the potential for premature compressor failure
By Mark Stefura, CM
Oil is a critical part of any air-conditioning or refrigeration system. Without it, the compressor would most definitely seize up resulting in a costly repair.
We know that the three types of oil widely used in hvacr are mineral, alkybenzene, and polyolester. Each type has its own distinct characteristics involving viscosity floc point, dielectric strength, neutralization number, flash point and fire point.
Viscosity is the oil’s resistance to flow. As the temperature of the oil drops it becomes thicker, thereby, creating a situation where the oil becomes difficult to pump. The change in viscosity with temperature is measured by the viscosity index.
A high viscosity index lubricant will act like a multi-weight oil where it becomes easier to pump at low temperatures and maintains a higher viscosity at bearing temperatures. It also creates a greater challenge to get the oil that is circulating in the system to return back to the compressor.
Floc point is a measurement of the amount, if any, of wax in the oil. This pertains only to mineral oils. Dielectric strength is its measure of resistance to electrical current. All oils used with internal electric motors must possess this characteristic. The neutralization number refers to the amount of acid or caustic present in the oil. Finally, flash point and fire point both relate to the oil’s burning properties and volatility.
Other factors in determining oil selection are its lubricity, miscibility and solubility with a particular refrigerant. The lubricity has to do with preventing wear of moving components within the refrigeration system. Hydrodynamic lubrication is defined as the separation of moving parts by a film of oil.
What’s happening in there?
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.
Out in the field
Now let’s look at some of what the technician comes across in the field. First on the agenda is the crankcase heater. Check it and make sure it works. As mentioned earlier, oil and refrigerant are miscible in their liquid state and that holds true for the compressor sump as well as the liquid line.
The refrigerant that makes its way back to the compressor as a liquid or condenses in the compressor because of cooler temperatures would flash and vaporize at the point of lubrication — this is especially a problem during flooded starts.
If a given compressor shuts down and is off all night, the refrigerant could migrate and accumulate in the oil sump. At the time that the compressor is most vulnerable and needs the oil the fastest, it would be getting refrigerant-thinned oil instead.
Let’s take a look at static load. Under an initial start from static to dynamic load, the bearings would not see the proper amount of oil under the proper hydraulic pressure. The oil would be accompanied by liquid refrigerant, which would act as a degreaser and wash the oil off the moving parts. It would also see refrigerant vapor, which interrupts the continuous oil layer in the bearings. Many compressors are needlessly lost due to a failed crankcase heater.
Next, let’s briefly discuss piping. Although piping isn’t an issue in packaged rooftop units, it is a concern for large split system air-conditioning units. Many charts are available to aid the technician in proper line sizing. Suction line sizing is important for proper oil return. Charts are based on pressure drop and velocity of the refrigerant as it travels through the piping.
Oil and refrigerant are miscible when both are in liquid states but are not when the refrigerant is in a vapor state. Rather, the oil rides the walls of the piping and is aided by the refrigerant velocity. Historically, the industry has used a successful standard of 400 feet per minute (fpm) as a starting point for air-conditioning equipment.
This may vary depending on the circumstances and specific manufacturers. An oversized line would reduce velocity and hinder oil return. An undersized line would increase velocity but also increase the pressure drop back to the compressor, effecting system performance.
The use of an oil failure safety on a semi-hermetic compressor also can be used to prevent premature compressor failure. As well as ensuring that are there sufficient quantities of oil being delivered under a minimum pressure, the oil safety can at times let you know if there are other problems in the equipment. For instance, an oil safety could potentially protect a unit that was undercharged or losing refrigerant possibly before the low-pressure safety tripped.
For example, suppose there was a large rooftop unit or split system that developed a microscopic leak. Eventually, the low-pressure safety would shut the system down once it reached its cutout point. But what if the cutout point isn’t reached because the unit at this point has only lost a portion of the charge? The oil safety could trip. Why? Because of the reduced mass in the system, thereby reducing the velocity of the refrigerant in the suction line.
Systems that run under a partial loss of charge or have a partial restriction of the metering device often cannot maintain the minimum velocity required. The oil that is circulated in the system would not return. This would eventually cause the level in the compressor to drop and trip the safety.
If the technician knows the cfm (cubic feet per minute) rating of the compressor, then one can use a P-H diagram to analyze system operation. This same method of diagnosis can find hidden issues with compressor unloader staging as well as metering device problems and systems running under low ambient load conditions.
What to do
Checking the oil pressure in a semi-hermetic compressor is not a difficult procedure. The technician would use the same gages, or if desired, have a special gage just for oil. Compressors are provided with oil-pressure taps at the pump end of the compressor. If there is an oil safety switch connected, then the technician may need to install a swivel-tee fitting with a depressor if one is not already provided.
To take an oil pressure reading, the technician needs to obtain the net oil pressure. This is the difference between the oil pressure at the pump and the suction pressure in the compressor. For the reading to be accurate, take the suction reading on the compressor body. Do this because there is a small pressure drop across the service valve and barrel of the motor, which could affect the readings.
Mark Stefura, CM, is president of CFM Commercial hvac Inc. He also is the education chairman of the RSES Reading, Pa., Chapter and a member of ASHRAE.