Oil and ammonia mix continuously in refrigeration compressors. The primary function of the oil is to lubricate and cool the compressor, but a small amount of oil is always circulating with the refrigerant that leaves the compressor. This oil comes in contact with all components of the refrigeration system, a fact that must be considered when replacing compressor oil.

Clearly, oil plays a critical role in protecting compressors, ensuring optimum performance and life of the system. Often overlooked is what impact many important lubricant properties have on other parts of the refrigeration system and the vital importance of using the right oil.

The use of oil not proven to work with ammonia has led to issues for end users in the ammonia refrigeration industry, causing compressor failures, contaminating systems and in some cases increasing safety concerns. As an example, lubricants with high aromatic content or excessive foaming tendency increase oil carryover and increase the frequency of required oil draining from a system’s low-side components.

Also, oils with wrong aniline point can lead to gasket swelling or shrinkage with resultant ammonia or oil leaks.

“Oil moves all around the system in small amounts, so it can cause problems throughout the system if it’s not tested and proven for all the different components,” says Joe Pillis, Director of Technology for Industrial Refrigeration at Johnson Controls. “Refrigeration oils have a much bigger job than just lubricating the compressor.”

Third-party lubricants not tested for ammonia compatibility may contain unproven or harmful additives. When unproven oil additives react with ammonia, the result can be unpredictable. “Deposits sometimes form that can plug orifices and prematurely wear shaft seals and bearings,” Pillis says. “People don’t think about all this when they choose an oil. They just think, ‘Isn’t all oil the same?’”

Deposits or sludge in the filters and separators increase pressure drop and hurt efficiency. “I have seen cases where oil separators had to be cut apart to remove solid sludge from such a reaction,” Pillis says.

Refrigeration oils need to have proper viscosity and viscosity index for anticipated operating temperatures. Too low viscosity can shorten bearing life, while too high can increase frictional power consumption. Too high a pour point can foul heat transfer surfaces or impair oil recovery from re-circulators, reducing system efficiency or reducing capacity. Very low pour points generally don’t hurt performance, but can triple oil price and be a waste of money for higher evaporator temperatures.

Previously, most reciprocating compressors for ammonia systems used naphthenic oils, chosen because of their natural low pour point of around minus-30 degrees. However, their low viscosity index, high volatility and high aromatic content resulted in high carry-over. Carbon and sludge buildup impacted compressor performance and maintenance and required frequent oil changes. Today’s screw compressor systems run much lower discharge temperatures and it is rare to actually “wear out” oil.

With proper filtration and oil analysis, it is possible for refrigeration oils to run for many years without replacement. The incentive to change from the original equipment manufacturer’s oil to lower cost aftermarket alternatives is very low when oil rarely needs to be changed.

Pillis recommends an OEM-supplied oil that has been tested and proven for long-term use with ammonia. OEMsupplied oil may have a high viscosity index that provides temperature stability and reduces vaporization of oil into the discharge side of the compressor. The viscosity of the lubricant changes less as temperature increases, keeping viscosity in the proper range for various sections of the compressor.

OEM-supplied oil may have lower aromatic content, reducing volatility and resulting in less carry-over loss into the system. With lower levels of the more volatile components of the oil — to vaporize at operating temperatures — viscosity increase does not occur, and the oil stays in proper viscosity grade thousands of hours longer than other oils. This can extend compressor life and extends the drain interval.

Although OEM-supplied oil may be more expensive than a third-party aftermarket oil, end users can be confident that it will not damage key system components. “Buy an oil from somebody you can trust, who has been in the business and understands ammonia and your equipment,” Pillis says. “There are still people out there selling oil that’s untested in ammonia refrigeration systems and it causes problems.

“I also caution users about putting any aftermarket ‘efficiency improving’ additives into their compressor oil. These have often led to equipment failure and expensive clean-ups,” he says. “If someone claims they can improve your system efficiency by 20 percent by putting their ‘magic additives’ into your system, the best advice is to run the other way and lock the engine room door.”