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A Closer Look
Oil-Free Compressed Air...What is it Really?
The term "oil-free compressed air" is often used to describe compressed air that is generated by oil-free compressors. Examples of these types of machines are centrifugal, oil-free rotary screw and non-lubricated reciprocating compressors. In each of these examples, lubricants are not used within the compression chambers and therefore the compressed air generated is "oil-free." Under normal circumstances, lubricants and hydrocarbons can only enter these compressed air streams in vapor states through the inlet air filter.
In compressors with lubricated compression chambers, the compressed air unavoidably picks up some oil. The most common examples of these types of compressors are oil-flooded rotary screw machines, lubricated reciprocating, and rotary vane compressors. In most cases, manufacturers of these types of compressors report oil carryover in PPMW or parts per million by weight.
Consider for example a 200 HP oil-flooded rotary screw air compressor rated at 1000 ICFM. Most manufacturers of this size and type of compressor flood approximately 82 GPM of lubricant into the compressor air end (compression chamber) to seal and cool the compressor. The compressor manufacturer must then find a way to separate most of this oil such that downstream oil contamination is kept to a minimum. Most manufacturers claim that new oil-flooded screw compressors reduce downstream oil contamination to 2 PPMW.
Continuing with the 200 HP compressor example, oil carryover can be calculated as follows:
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2 PPMw=
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.05 Quarts
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(100 Hrs) (100 SCFM)
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Assuming that this compressor operates efficiently at 2 PPMW and that run time is 8,000 hours per year, actual oil carryover can be calculated:
(.05 Quarts) (8,000 Hrs.) (1,000 SCFM)
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= 40 Quarts/Y
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(100 Hrs.) (100 SCFM)
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In the compressed air business, it has been our experience that most people have a difficult time relating to 2 PPMW. However, 40 quarts of oil downstream is a different story. Everyone can relate to that.
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Oil Carryover
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PPMW
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Quarts / Year
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2
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40
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5
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100
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10
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200
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15
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300
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20
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400
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25
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500
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Moreover, the above example assumed an efficient running air compressor. In the real world, many lubricated compressors run at considerably higher oil carryover levels. In fact, we know of many installations that frequently run over 25 PPMW. Most often, high oil carryover on flooded screw compressors can be attributed to inefficient oil separators, frequent starting and stopping, and reservoir blowdown.
The following table shows oil carryover in both PPMW and quarts per year for the same 200 HP air compressor.
Suffice it to say, oil-flooded rotary screw air compressors are not oil-free. At 25 PPMW carryover, 500 quarts of lubricant actually pass downstream each year.
For information on oil-free air compressors or filtration products designed to remove oil, please contact Fluid Energy.
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