Air Compression: Choosing the Optimal Lubricants for Longevity and Efficiency

By Steve Gahbauer 

Precision air compressors and gas compressors depend on meticulous maintenance and proper lubrication to achieve peak performance.

Lubrication serves three essential functions: reducing friction, sealing critical components, and dissipating heat. Compressor oils are constantly exposed to high temperatures, compression of reactive gases, dilution, condensation, and abrasive contaminants, so the fluid must have the correct composition and additives to ensure long life and smooth operation.

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

Good compressor lubricants depend on the selection of an appropriate base stock and additives, correct application, and diligent monitoring, explained Inga Kuksis, product and fluids specialist at Petro‑Canada Lubricants Inc. during a Society of Tribologists and Lubrication Engineers (STLE) meeting in Toronto.

Service life is influenced by composition, operating temperatures, film thickness, viscosity, volatility and shear ability. Mineral oils that meet these requirements include 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. Oxidation stability demands increase as plant operations expand, with hotter discharge temperatures and higher cooling requirements, while high ambient air temperatures reduce oil cooling capacity.

Corrosion control and water separation are also critical, and oil compatibility is a key consideration. Many compressor fluids are compatible, said Roy Hoppe, technical advisor for fuels and lubrication at Shell Canada Products. Mineral oils are compatible with PAO and diester synthetic fluids, but adding mineral oil can degrade the performance of synthetic fluids. Mineral oils and PAO synthetic fluids are not compatible with PAG and silicone‑based fluids. A thorough cleaning before changing oil or adding new lubricant is essential.

Various laboratory tests verify that compressor oils are compatible and meet high standards. These tests evaluate pour point, shear stability, anti‑wear properties, volatility, flash point and corrosion resistance. Hot‑room tests and field trials are also available. It is prudent to request a list of test results before purchasing compressor oils.

Additives – chemically active or inert – impart or reinforce desirable properties in the lubricant. Because they influence performance, careful selection is essential.

In a recent presentation to the STLE in Hamilton, Don Johnston, regional manager for Lubrizol Canada, outlined 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 and inhibit corrosion and oxidation. They contain metals that produce ash and are based on various substrates. Certain detergents react with water or acidic contaminants, causing the oil to thicken and eventually gel.

Keeping parts clean

Dispersants stabilize colloidal dispersions, preventing them from agglomerating and leaving the oil as dirt. They also help keep parts clean at lower temperatures and protect against contaminant particle growth. However, dispersants interact with other additives and can affect the oil’s viscometrics, making it difficult to meet specific low‑temperature criteria.

Anti‑wear and extreme‑pressure additives are effective under mixed‑film and boundary‑lubrication conditions. They create a sheared boundary film on metal surfaces to prevent seizures under high temperature, heavy load or prolonged operation. Because they typically contain phosphorous, sulphur, chlorine or boron, they can corrode soft metals such as copper or lead.

Oxidation inhibitors enhance performance by preventing oxidative degradation, while corrosion inhibitors mitigate ferrous, yellow‑metal and lead corrosion, protecting components such as bearings and seals.

Foam inhibitors are poorly soluble in oil. They break foam bubbles by adsorbing on the bubble surface and altering surface tension.

Friction modifiers reduce heat and high power consumption. They do not eliminate friction entirely but lower it by adsorbing on metal surfaces rather than reacting chemically, thereby maintaining a strong anti‑compressive film and removing heat from metal‑to‑metal contact regions.

Viscosity modifiers thicken the oil at high temperatures and thin it at low temperatures, enabling the formulation of multigrade oils.

Choosing the right compressor fluid is a complex decision. Smooth operation of your plant’s air compressors requires making informed choices about lubricants 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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