Air Compression – Make the Right Lubricant Choices

By Steve Gahbauer 

The precision machines that produce compressed air for air tools or gas for refrigerants all require delicate care and proper maintenance to ensure optimum performance, and lubrication is key.

It serves three distinct purposes: lubricating, sealing and cooling. Compressor lubricants are exposed to high temperatures, compression of reactive gases, dilution, condensation and abrasive contaminants. Fluids must be of the proper composition and contain the correct type and amount of specially selected additives to ensure long life and smooth operation.

Using synthetic hydrocarbons, silicones or polyglycol oils prolong compressor service life. Their advantages include high oxidation resistance, high natural viscosity index, low pour point, increased film strength, lower volatility and lower carbon-forming tendencies.

Good compressor lubricants depend on the selection of appropriate lubricant base stock and additives, correct application and diligent monitoring, said Inga Kuksis, Petro-Canada Lubricants Inc.’s product and fluids specialist who discussed air compressor fluids at a Society of Tribologists and Lubrication Engineers (STLE) section meeting in Toronto.

Service life is influenced by composition, operating temperatures, film thickness, viscosity, volatility and shear ability. Mineral oils that meet these requirements are base oils such as polyalphaolefins (PAO), polyalkylene glycol (PAG) industrial gas compression esters, polyol esters, and silicone diesters. Phosphate esters are often added for fire resistance. Bear in mind oxidation stability demands on lubricants increases as plant operations expand because of hotter discharge temperatures and higher cooling requirements, and that high ambient air temperatures reduce the cooling ability of oil.

Corrosion control and water separation are also important, and oil compatibility is definitely an issue. Many compressor fluids are compatible, said Roy Hoppe, a technical advisor for fuels and lubrication at Shell Canada Products. For instance, mineral oils are compatible with PAO and diester synthetic fluids, but the addition of mineral oil has an adverse effect on the performance of synthetic fluids. Mineral oils and PAO synthetic fluids are not compatible with PAG and silicone-based fluids. Be sure to do a thorough cleaning and before changing oil or adding new lubricant.

There are various laboratory tests to ensure that compressor oils are compatible and meet high standards. These tests check pour point, shear stability, anti-wear, volatility, flash point and corrosion resistance. Hot room tests and field trials are also available. It’s a good idea to request a list of test results before buying compressor oils.

Additives – chemically active or inert – impart or reinforce a desirable property of the lubricant. Since they affect how well the oil performs, it’s also important to choose them carefully.

In a recent presentation to the Society of Tribologists and Lubrication Engineers (STLE) in Hamilton, Don Johnston, regional manager for Lubrizol Canada, explained the functions of the nine most common oil additives – detergents, dispersants, anti-wear agents, extreme pressure agents, foam inhibitors, friction modifiers, oxidation inhibitors, corrosion inhibitors and viscosity modifiers.

Detergents clean internal engine parts, neutralize combustion acids while inhibiting corrosion and oxidation. They contain metals that produce ash and they’re based on various substrates. Certain types of detergents react with water or acidic contaminants to cause the oil to thicken and ultimately gel.

Keeping parts clean

Dispersants stabilize colloidal dispersions, preventing them from agglomerating and coming out of oil as dirt. They also help to keep parts clean at lower temperatures, and they protect against contaminant particle growth. But dispersants interact with other additives and affect the viscometrics of the oil, making it difficult to meet certain low temperature criteria.

Anti-wear and extreme pressure agent additives are effective under mixed film and boundary lubrication conditions. They produce an easily sheared boundary film on the metal surface to prevent seizures under conditions of high temperature, heavy loading or extended periods of operation. But because they typically contain phosphorous, sulphur, chlorine and/or boron, they can corrode soft metals such as copper or lead.

Anti-oxidants contain inhibitors and often enhance performance by impeding synergism. Rust and corrosion inhibitors prevent or mitigate ferrous, yellow metal and lead corrosion, as well as decomposing metal conditions in engines and equipment components such as bearings and seals.

Foam inhibitors are compounds of poor oil solubility. They break foam bubbles by adsorbing on the surface of the bubbles and changing surface tension.

Friction modifiers combat heat and high power consumption, but friction will never be completely eliminated. Modifiers reduce friction and wear and align in regular pattern to reinforce fluid film. They adsorb on the metal surface rather than chemically reacting with the surface as extreme pressure agents do. Their strong anti-compressive behaviour helps to separate contacting surfaces. They also remove heat from metal-to-metal contact regions.

Viscosity modifiers provide thickening at high temperatures and minimum thickening at low temperatures, which allows the formulation of multigrade oils.

Choosing the right compressor fluid is a complex business. Smooth operation of your plant’s air compressors requires making the right decisions about lubes and additives.

Steve Gahbauer is an engineer and Toronto-based freelance writer who is the former engineering editor of PLANT and a regular contributing editor. E-mail gahbauer@rogers.com. 

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