Compressor oil is a necessary component in any air conditioning system that uses compressor in its compression system. There are three main purposes of the oil. They are used for lubrication, removal of heat and for sealing. Lubrication cooling is needed in reciprocating compressor as the piston compresses the refrigerant gas. The sealing of the piston in the cylinder needs to be cooled as well.
Similarly, rotary compressor needs lubrication for the contact between the cylinder and the rotor of the motor. Screw compressor superheat has to be removed by the lubricant during its operation.
Compressor Oil Properties
Viscosity is a measurement of the thickness of the oil which tells us its ability to resist shearing stress. One of the units of measurement of viscosity is SSU or Saybolt Seconds Universal. This is done by placing the oil sample on the standard equipment at a fixed temperature with a standardized orifice and letting it flow. The time taken for it to flow is recorded. The higher the viscosity number, the thicker is the oil and the slower it flows.
The metric unit for viscosity is poise. Most of the oil measured is less than 1 poise hence the smaller scale called centi-poise or 1/100 poise is commonly used.
In general, bigger compressor with bigger gaps and parts needs thicker oil for effective functioning of the equipment.
When using the oil, always follow the compressor manufacturers recommended oil as some oil may react with the refrigerant used in the system. If chemical reaction happens, sludge, carbon and acids may be formed which will affect the performance and lifetime of the compressor.
Dielectric Strength & Moisture
As the oil used in a hermetic compressor is usually in contact with the coil of the motor, it is critical that it has good dielectric strength. The higher the dielectric strength, the better the electrical insulating quality of the oil.
Most oils will dissolve a certain amount of moisture and water making them undesirable. When water is presence in the refrigerant, ice will form when the temperature goes below 0 °C and caused problem to the expansion valve.
Copper plating may occurred with the presence of refrigerant, oil and moisture in the system. Moisture in the oil also caused the dielectric strength of the oil to be reduced significantly as water is a reasonably good conductor. Hence, as far as possible, try to reduce the amount of moisture in contact with the oil.
Typical Compressor Oil & Refrigerants
The two categories of oils commonly used in the compressors are mineral or synthetic oils.
One type of mineral oil (MO) called Naphthenic is commonly used.
Synthetic oils such as glycols, esters and alkylbenzenes (AB) have been used in the refrigeration applications for some time without any problem.
CFC refrigerants such as R12, R13, R113, R114 and R115 are using mineral oil or alkylbenze as their lubricants. Similarly, HCFC refrigerants such as R22, R123, R401A and R409A are also using these lubricants in their design.
In recent years CFC and HCFC refrigerants usage began to dwindle due to their ozone-unfriendly properties. Usage of new HFC refrigerants such as R23, R32, R134a, R407A, R407C and R410A have been increasing in HVAC equipment. These new refrigerants use Polyol esters or POE as lubricant.
One setback of POE is that it absorbs moisture many times more compared to mineral based oils. Hence proper procedures must be used when handling this oil to reduce the contact of this oil with the atmosphere. Metal containers are used instead of plastic containers to prevent moisture from entering the containers.
When servicing the compressor, always consult the manufacturers of the compressor to find out the type and amount of oil that can be used for that particular refrigerant.