Oil Free Vs. Oil Lube Air Compressor

Oil Type Portable Air Compressor

The terms oil-free and oil-lube type air compressor systems have unfortunately taken on good-bad catch-word connotations that often cloud the key issues in technical decision-making. Both oil-free and oil-lube type air compressors have operating characteristics that lend themselves to specific applications both in heavy industry as well as in recreational uses as fine as air-brushing. Consider both methodologies and the criteria experts use to select systems for their needs.

Oil-Free and Oil-Type Compressors

To better understand oil-free compressors, it is helpful to explain oil-type compressors first. Oil type compressors use lubricating oil for three main purposes: lubrication of moving parts in the compression mechanism, cooling of the mechanism and the exiting air and entraining contaminants that enter the compression process through the air inlet. Oil-free compressors do not directly involve oil in the compression mechanism for any of the above three purposes. They may have oil involved in other parts of the compressed air process such as motors and gearboxes, however.

The Key Issue

The controversy centers on whether the compressed air user can tolerate the inadvertent trace amounts of residual oil in the air from oil-types--either as mist or vapor--in their process. Some tooling and motors count on it, and some processes requiring absolutely pure air such as biological, pharmaceutical and food cannot tolerate any oil whatsoever. This is not to speak of the environment inside the plant where the compressed air is consumed and vented to ambient.

Compressed Air Uses

Compressed air-driven impact wrench

Compressed air is used in every area of human endeavor, from industry, transportation, medical, services, defense, power generation and art. It is used to drive tools and machines, for inflation, for propelling materials through ducts as well as for the production of chemicals, pharmaceuticals and food. Air cools machining operations, clears contaminants from surfaces and can propel projectiles at the speed of sound. Compressed air is used as a dynamic spring as well as a cushion for delicate processes. Each of these processes requires compressed air with distinct properties, and there are a number of compressor methodologies to produce it.

Types of Air Compressors

Compressed air tanks

The basic types of air compressor are reciprocating piston type, screw and centrifugal. The first two are positive displacement type, and can either be oil-type or oil-free. The latter is a dynamic type, compressing the air by imparting kinetic energy to it from a high speed impeller. Centrifugals are intrinsically oil-free.

Efficiency

Oil-type compressors cool the compression process, resulting also in cooler exiting air and greater efficiency, since more energy goes into raising the pressure rather than the temperature. Oil-free systems tend to need more ancillary equipment after compression to cool exiting air, remove moisture and deal with corrosion resulting from condensate mixing with incoming impurities and attacking metal parts.

Selection Criteria

Professionals use several criteria to make final choices about whether to use oil-type or oil-free compressors. Once they define the exact nature of the compressed air needed, the most important is whole-life cycle costs--capital investment costs, power usage and the ancillary resources required to result in the compressed air quality they seek. Since either system can supply air to any requirement with proper after-treatment, the best overall economic model for the given application usually comes out on top.

References

• Plant Services: Oil Free vs Oil-Type Compressors and selection analysis
• Atlas-Copco: Advantages and Benefits of Oil Free Compressors
• FS Curtis Co.: Manufacturer of Both Oil and Oil Free Comrpressors

Resources

• Engineering Toolbox: Compressor types
• Frost: Example of Regulation Influencing Technology