Oil Free vs Oil Less vs Filtration with Lubricated Air Compressor

Robert N. Littman
12/18/2003

There are distinct differences between “oil free”, “oil less” and oil lubricated compressed air with filtration. Often, the terms “oil free and “oil less” are used interchangeably because they both refer to compressors with no oil in the compression chamber. More specifically, “oil less” compressors have no oil anywhere in the running gear, crankcase or compression end. Typically “oil-less” compressors refer to reciprocating (piston style) units whereby the crank bearing and bushings are sealed and permanently lubricated, as are the wrist pin bearings. The valves consist of aluminum alloys (orãand stainless steel for enhanced life. Lubrication is achieved by Teflon similar material) composite rider and compression rings which expand as they wear. The advantage of oil less reciprocating technology is that this type of compressor is relatively efficient and produces totally oil free air. Additionally they are typically economical to purchase. Familiar manufacturers for this type of compressor include Quincy (QRD series) and Powerex (OT series). The size ranges from ½ HP to 30 HP. They are popular with laboratory, medical and pharmaceutical applications. The biggest drawbacks of the oil less reciprocating compressors are the higher maintenance requirements, vibration and noise issues.

Another type of oil less compressor is the rotary scroll. (orãThe rotary scroll uses no oil for lubrication and a series of Teflon similar material) seals are employed similarly to the reciprocating unit. The scroll utilizes a rotating orbiting scroll within the stationary scroll, thus causing a crescent shaped compression cavity. There are essentially no moving parts and the maintenance requirements are virtually nil. The big drawback with scrolls is the limited size of the individual pumps (3-7.5 hp). Powerex is the most well known manufacturer scroll type oil less compressors in the United States and Canada. Applications range from medical /pharmaceutical to laboratory and electronics. Scrolls are vibration free and can operate at very low sound pressure levels. Additionally they are relatively inexpensive compared to some other oil less technologies.

The most popular oil free compressor technology is the two-stage oil free rotary screw compressor. Other oil free compressors include rotary lobe, centrifugal and oil free piston compressors consisting of oil in the crankcase, but not in the compression end. The two stage rotary compressor has oil-lubricated gears and a series of sophisticated seals, which prevents oil from migrating into the compression end. Compression coated.ãis achieved with two sets of helical rotors, which are typically Teflon The rotors never actually touch each other and timing gears precisely control their speed. Two stage rotary screw compressors produce totally oil free air and are popular for any oil free compressed applications requiring oil free air. The two-stage oil free rotary screw compressors range in size from 20 hp to 1000 hp and pressures up to 150 psig. Two-stage oil free rotary compressors offer superior reliability, reasonable maintenance costs, low noise and little or no vibration. Popular applications include semiconductor manufacturing, high tech, pharmaceutical, medical air, solar panel manufacturing, general electronics, research, control air in power plants, nitrogen generation and fine bubble diffusion. The largest producer of two stage oil free rotary compressors in the oil free rotaryãUSA is Rogers Machinery Corp, manufacturer of Kobelco compressors. Other manufacturers include Atlas Copco and Ingersoll-Rand.

There are many applications whereby a lubricated rotary compressor equipped with filtration equipment is requested for a critically clean application. In my experience I have found that a good rule of thumb is that if no oil or oil vapor can be tolerated in the air lines or in the process then there is really no substitute for an oil free or oil less compressors. Lubricated compressors, especially rotary screw compressors, directly introduce oil into the compression chamber where it becomes atomized and mixed with the compressed air stream. The oil is removed by the demister system before it enters the distribution system. At this point the compressed air is still oil laden with up to 20 ppm of oil mist and oil vapor. To remove the remainder of the residual oil it is typical to install coalescing, particulate and carbon adsorption filters in line with the air drying system. As long as the filter elements are properly maintained and the air flow does not exceed the maximum rating of the filter housings, the distributed compressed air should remain oil free. Typically, however, trace amounts of oil vapor, oil additives and actual oil bleed into the system. Depending on the tolerance of the process for oil contamination, this may or may note pose a serious problem.

Case Study: I was asked to examine a compressed air system installed in a Cambridge, MA biotech firm. The process required that compressed air be introduced into fermentation chambers. The compressed air system was comprised of several oil lubricated rotary style compressors with five stages of filtration coupled to desiccant dryers. Considering the level of filtration and drying one would expect that the compressed air quality would meet the most stringent requirements. None the less, there were consistent batch failures inside the fermentors, which proved to be a mystery to the lab and facilities personnel. I suggested that we experiment with a small totally oil less reciprocating compressor on a newly acquired fermentor as a pilot study to determine whether or not the compressed air was causing batch failure. The results were immediate and remarkable. As it turned out the compressed air, which was heavily filtered and dried, contained enough trace oil vapor and additive volume to harm the cells in the fermentors. Shortly afterward, all of the compressors and filters were removed and replace with oil less compressors and shortly thereafter batch failures became rare.

Again, if no oil or oil vapor or associated additives can be tolerated for any compressed air usage, then an oil free rotary or oil less compressor should be selected. There is no way to guarantee oil free air with lubricated air compressors because so many variables will impact the efficacy of the air treatment system. It is important to consider the cost of lost productivity if a process is contaminated or destroyed by oil carryover. I have developed a couple of quick self tests to determine the direction of a newly engineered compressed air system in terms of deciding on filtration vs oil free air.

What are the costs of oil contamination on the air lines if it occurs?
What type of damage to product, process or equipment will occur if oil is introduced?
What will air treatment cost in terms of filter maintenance (i.e. filter elements and manpower)?
Will the capital budget allow for oil free compressed equipment?
If oil in the air line can not be tolerated, then why put it there in the first place just to remove it again?

Finally, it is important to take notice of the fact that once air lines are contaminated with oil it is virtually impossible to thoroughly clean and purge the contaminants back out of the piping network. Needless to say replacement of the air lines can be costly and time consuming.

More often than not lubricated compressor work extremely well and reliably as long as they are matched to the process and demands of the facility they are installed in. They tend to be less costly than oil free compressors and will operate trouble free for many years. Conversely, if oil free air is required it is highly recommended that an oil free or oil less compressor is selected. The equipment being offered by manufacturers today for both oil free and lubricated compressors is likely to yield many years of reliable operation and the prospect of excessive maintenance is becoming a nominal factor in choosing a compressor.