Oil free choice

Published:  28 March, 2018

Is an oil free air compressor right air for your application? The British Compressed Air Society reports.

Compressed air is used in many applications where air quality is paramount for the end product and production process: including food and beverage processing, pharmaceutical manufacturing and packaging, chemical and petrochemical processing, semiconductor and electronics manufacturing, the medical sector, automotive paint spraying, textile manufacturing and many more. It’s considered by some that air in its pure state has no moisture or contaminants. However, in reality, this isn’t the case. In these critical applications, contamination by even the smallest quantities of contamination can result in costly production downtime and product spoilage.

Air compressor types

Over the past 25 years, both rotary screw and sliding vane air compressors have firmly established themselves for mainstream industrial use, due in part to their compact size, low noise and overall efficiency. Air compressors are typically supplied in one of two configurations, oil lubricated or oil-free. Approximately 75% or screw compressors sold are oil lubricated (higher if vane is included), however many companies are now opting for oil free variants. Before making the choice between oil lubricated or oil-free, a number of factors should always be considered.

Compressed air contamination

There are 10 contaminants found in compressed air, originating from four different sources, the ambient air, the air compressor, the air receiver and the distribution piping. Whilst oil-free compressors reduce the amount of oil carried into the compressed air system, they do not provide contaminant free (or totally oil free air) due to the oil in the ambient air being compressed, concentrated, then cooled & condensed in the compressed air system.

So what are the main differences between an oil injected and oil free compressor? In simplified terms oil lubricated compressors use oil to seal between the rotors of the air end (allowing compression), for cooling and for lubrication of vanes & bearings. Typically, an oil lubricated compressor utilises a single ‘air end’ to compress the air. Oil free variants however do not inject oil directly into the compressor stage, instead opting for faster rotation speeds, specialist air end coatings and tight machining tolerances to compress the air. As the oil in a lubricated compressor cools the air, temperatures in an oil-free machine are typically higher, therefore compression typically takes place in two stages with an inter-cooler placed between the stages for intermediate cooling. Almost all oil-free compressors still use oil for lubrication of bearings and the gearboxes connecting the two air ends.

Important note: Oil free compressors are often specified in industries requiring “technically oil free air” as there is a perceived risk with lubricated machines from bulk oil carryover should an air / oil separator fail, however it must be noted that with regular maintenance and use of genuine parts, separator failures on modern lubricated compressors are very rare indeed.

Total cost of ownership

When selecting an air compressor, it should not be done based upon purchase cost or running cost alone. It is best practice to always consider the Total Cost of Ownership (TCO) over a reasonable period (e.g. 10 -12 years). TCO should be purchase cost, full maintenance costs plus operational costs.

Calculating TCO will require the compressor manufacturer include both regular maintenance items such as oil & oil filters as well as costlier wearing parts such as air ends or vanes and integrated HOC dryer components. Including all of these items will prevent any ‘unforeseen’ costs in the future (for example, the maintenance periods on a two stage compressor may be longer than found on a single stage machine, however the cost of replacing two air ends is higher than replacing one plus more frequent oil changes).

For further information please visit: www.bcas.org.uk

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