Refrigeration Lubrication

If a compressor is the heart of a refrigeration system, lubricants are its lifeblood. The right compressor oil must lubricate moving parts effectively and satisfy requirements not demanded of other general-purpose lubricants. Additionally, oil is expected to give many years of trouble-free service often without replacement or other maintenance attention. 

Lubrication, of course, is considered the primary function of compressor oil. In addition, it must have certain other properties because: 

1. It mixes with the refrigerant used in the system. 

2. It is carried over in small amounts into the refrigerant lines, condenser, receiver, evaporator and accumulator. 

3. It is in direct contact with the motor windings of hermetic compressors. 

4. Refrigeration compressor lubricants are exposed to substantial temperature extremes, very high temperatures at compressor discharge valves and very low temperatures at expansion devices and evaporators. 

Examples in which unique lubricant characteristics are required include: 

1. Lubricant in the compressor crankcase that is diluted with refrigerant must have a viscosity that will ensure satisfactory lubrication of bearings, cylinder walls and more. 

2. In hermetic compressors the lubricant must resist chemical reaction between motor windings and other metals at elevated temperatures in the presence of refrigerant. It must have good dielectric properties to prevent electrical leakage and must not cause deterioration of motor winding insulation. 

3. At the hot-discharge valves of the compressor, hard carbon or other deposits from the lubricant cannot be tolerated. 

4. The small amount of lubricant that is carried into the condenser tubes must remain fluid with the liquefied refrigerant so that it does not adhere to tube walls, where it would retard flow and heat transfer. 

5. Lubricants must not contain or produce solid materials that could plug capillary lines, expansion valves or oil return orifices. 

6. In very low temperature parts of the system, the lubricant must not precipitate wax crystals, contain moisture or congeal. Any such action would retard flow and reduce capacity and efficiency. 

Lubricant properties 

Refrigeration compressor lubricants consist of two major types: mineral oils and synthetics such as alkylbenzenes, polyol esters (POEs) and polyalkylene glycols (PAGs). 

Mineral oils have been used for decades with ammonia, CFC and HCFC refrigerants. To produce a mineral oil that will meet the above requirements, the refiner must carefully select the proper crude oil stock and choose the processing steps that will give the desired end product. 

If the refiner selects a stock that is too paraffinic, wax will separate from the oil at low temperature, clogging expansion valves or other metering devices, screens and orifices. Under-refined oil is unstable and sludge-forming; over-refined oil can be reactive and acid-forming. 

For satisfactory performance, all refrigeration lubricants – mineral oil or synthetic – must be compatible with the refrigerant in the system and have the following requirements: 

1. Good miscibility and solubility to assist in good oil return to the compressor, where it belongs. 

2. Chemical stability to resist chemical reaction with the refrigerant or other materials present in the system. 

3. Thermal stability to eliminate excess deposits at compressor hot spots. 

4. Low wax content to prevent separation of flocculent wax from the oil mixture at the low temperature points in the system. 

5. Low pour point to prevent separated lubricant from congealing and restricting flow. 

6. High dielectric strength to ensure good insulating properties. In hermetic units, the lubricant-refrigerant mixture serves as an insulator between the motor and the compressor body. 

7. Proper viscosity, even when diluted with refrigerant, to ensure high film strength at elevated operating temperatures and still provide good fluidity under coldest operating conditions. 

8. No contamination to prevent scarring of bearing surfaces, plugging of lines or oil ports and general deterioration. 

Properties of alkylbenzene oil 

Alkylbenzene is a synthetic oil derived from alkylated benzene. It is similar in many ways to mineral oil and has some superior properties that make it particularly valuable in ultra-low temperature refrigeration and air-conditioning applications. 

Some major compressor manufacturers prefer alkyl 

benzene refrigeration oil for some applications with HCFC refrigerant blends such as R-22, R-123 and R-401A. 

However, alkylbenzene refrigeration oil with the proper viscosity can be used with most CFC and HCFC refrigerants as well as hydrocarbons and ammonia in most refrigeration and air-conditioning applications. 

The benefits of high-quality alkylbenzene lubricants are high miscibility, low foaming, excellent thermal stability, very low floc points and good compatibility: 

1. High miscibility: Miscibility is the ability of the refrigerant and lubricant to stay together as one homogeneous solution. Alkylbenzene has excellent miscibility with CFC and HCFC refrigerants, resulting in the oil and refrigerant remaining as one mixture at a wide range of temperatures and pressures. 

2. Low foaming: The low foaming quality of alkylbenzene reduces carryover at compressor startup and subsequent oil loss from the crankcase. 

3. Excellent thermal stability: Alkylbenzene can enhance the life of refrigeration systems by providing better thermal stability in the presence of CFC and HCFC refrigerants. It resists change under high temperatures, reducing problems with sludge, acids and copper plating. 

4. Very low floc points: The floc point is the highest temperature at which wax-like materials precipitate from the oil in the refrigeration system. Because alkylbenzene is a synthetic lubricant, it contains little or no paraffin or wax, which can plug up parts of a system. This can be very desirable in low-temperature applications. 

5. Good compatibility: Alkylbenzene can be blended with mineral oil of the same viscosity. It will not affect motor insulation and is compatible with most elastomers and additives often used to improve lubricity. 

Preventing contamination problems is extremely critical in the refining and handling of all refrigeration oils. Great care must be used to assure that refrigeration oil is free of moisture and other contaminants. Service technicians must ensure that oil remains clean and dry. 

Polyol esters and polyalkylene glycols

As stated earlier, mineral oils were long the refrigeration lubricant of choice. However, these oil formulations do not provide satisfactory performance with HFC refrigerants; synthetic refrigeration lubricants need to be used. 

POEs and PAGs have a range of critical features, including excellent solubility, miscibility, viscosity and lubrication performance across the wide temperature spectrum encountered in all refrigeration systems. POEs have a distinct advantage in low- and ultra-low temperature applications because they contain absolutely no wax that can cause blockage of expansion valves, capillary tubes or oil return orifices. POEs of the same viscosity may be used with CFCs and HCFCs after consultation with compressor manufacturers. 

PAGs were developed early on for use with R-134a and are used extensively in automotive air conditioning. POEs are used with HFCs to serve most industrial, commercial and residential air-conditioning and refrigeration systems. 

Both POEs and PAGs are highly hygroscopic (affinity for moisture). Of the two, PAGs are substantially more hygroscopic than POEs. You must take great care to minimize exposure to the atmosphere in both cases. 

When retrofitting CFC or HCFC systems to HFC refrigerant and POE or PAG lubricants it is imperative that the mineral oil is removed prior to charging the system with HFC refrigerant. At least 95 percent of the mineral oil must be removed. 

To do this, follow the lubricant supplier's retrofit procedure. For satisfactory results, always install liberally sized filter-driers and evacuate to a low micron range before charging the system with HFC refrigerant. 

Watch for these conditions 

Here are some other conditions to watch for regarding refrigeration lubricants: 

1. Refrigerant flooding can dilute the refrigerant lubricant in the compressor crankcase and cause a loss of lubrication. This often is referred to as wash-out. 

2. Compessor short-cycling can cause excessive oil-trapping in condensers, evaporators, suction lines and system vessels, not allowing it to return to the compressor crankcase, where it belongs. 

3. Excessive foreign matter can restrict or block oil passages in the compressor, causing loss of lubrication. 

4. Excessive oil temperatures, especially at discharge valves, will cause refrigeration oil breakdown. Typical beginning breakdown temperatures are: 

• Mineral oils: About 350° F. 

• Alkylbenzene: About 390° F. 

• Polyol esters: About 490° F. 

The temperatures listed above are for a clean environment. Conditions will vary if contaminants are present. 

5. Occasionally the question is asked; Will refrigeration lubricants boil and/or evaporate if evacuated into a very low micron range? The vapor pressure of refrigeration lubricants is less than 1 micron at 80° F. No vacuum pump will get close to a 1 micron vacuum. 

Changes and improvements are constantly being made in refrigeration lubricants. All POEs, PAGs, alkylbenzenes and mineral oils are not the same in quality. Therefore, it is important to use only the brand, type and viscosity of lubricant approved by the compressor manufacturer. Make sure you keep up-to-date with these advances. 

The Rghit way to handle lubricants

Here are some rules for properly handling refrigeration lubricants: 

1. Use clean tools such as pumps, funnels and tubing while working with oil. 

2. Recap the container immediately after use. Refrigeration oil containers left open to the atmosphere will absorb moisture. 

3. Keep refrigeration oil in its original container, be sure that the container is clean and dry. 

4. Properly dispose of oil contaminated with dirt or moisture, do not risk using it. It is much less expensive to replace a bottle of refrigeration oil than to repair or replace a compressor.