Due to the sheer amount of different options available, specifying the right replacement or new industrial compressor can become a daunting task.
The promise of reduced costs can make oil-free compressors an attractive proposition. While there are pros and cons when compared to oil-injected alternatives, a number of misconceptions about oil-free compressors have spread throughout the industry.
This post aims to break these misconceptions down and offer insight into some important fundamentals to consider when selecting your next compressor.
It is often thought that because oil-free compressors do not use oil in the compression chamber, the delivered compressed air will be totally oil free. This is an untruth as contamination within the compressed air system comes from four different sources. The atmospheric air that is drawn into the compressor is already contaminated by the oil vapour in the ambient air and the oil-free compressor is unable to remove it.
While oil-free compressors reduce the amount of oil that would need to be removed from compressed air, they do not have the ability to reduce any of the other contaminants that are typically present. Generally, compressed air is contaminated by water and oil (in different phases) as well as particulates and mircoorganisms. To ensure processes are not affected by these contaminants, it is essential to remove or reduce them to an acceptable level.
Common practice when installing an oil-free compressor is to omit some or all of the downstream filtration. Purification equipment in a compressed air system will typically incorporate a dual-filter set-up, and it is often believed that one of these filters is a particulate filter and the other is an oil removal filter. The two filters are coalescing filters, and they are the most important pieces of purification equipment in the system.
Both filters perform the same contamination removal functions; however, they do so to differing levels of filtration. The first filter is a general purpose filter which protects the high-efficiency filter from bulk contamination. This dual-filter installation ensures a continuous supply of high-quality compressed air with the additional benefits of lower operational costs and minimal maintenance compared to a single high-efficiency filter.
Compressed air dryers are designed to remove only water vapour and not water in a liquid or aerosol form. They also require the use of coalescing filters to work efficiently and achieve dew point.
By design, oil-free compressors do not use oil in the compression chamber; therefore, oil does not contact the air being compressed. However, oil is often used for the lubrication of moving parts and cooling of the machine.
Oil-free compressors typically use elaborate and complicated sealing systems to prevent the lubricating oil from entering the compressed air; however, should the sealing mechanism fail, then there is the possibility of oil entering the compressed air system. Oil vapour from the crank case breather can also be drawn back into the compressor intake.
ISO8573-1 is the International Standard for Compressed Air Purity and provides differing classifications for air quality. Class 0 is the most stringent of the classifications, and many compressor manufacturers claim oil-free compressors are compliant with Class 0.
A number of compressor manufacturers claim the delivered air from their oil-free compressors is in compliance with Class 0. Class 0 does not mean zero contamination. If the compressor was tested in clean room conditions, the contamination detected at the outlet would be minimal. Should the same compressor be installed in an urban environment, the level of contamination will be dependent upon what is drawn into the compressor intake, rendering the Class 0 claim invalid.
If Class 0 was delivered at the outlet of a compressor, there is no guarantee that the air will still be Class 0 at the point of use, as contamination already in the air receiver and distribution piping will contaminate the compressed air.
It is wrongly assumed that Class 0 refers only to oil, whereas ISO8573-1 has classifications for Particulate, Water & Oil. When claiming Class 0, specifications should state which contaminant the Class 0 refers to.
It is often stated that using an oil-free compressor reduces the user's overall cost of ownership because the need for filters can be eliminated, resulting in lower pressure losses and lower ownership costs.
No matter what type of compressor is installed, the same level of purification equipment is required. As purification equipment is based upon the contaminants entering through the compressor intake, contaminants added by the compressor, and rust and pipe scale in the compressed air system.
Typically, the purchase price of an oil-free compressor is significantly higher than that of an equivalent lubricated compressor, and maintenance of air ends is generally more expensive than lubricated equivalents. In some models, complete replacement of the air end is required after only a short number of years, which actually increases the overall cost of ownership significantly.
Even though liquid oil should not be present, oil vapour is still drawn into the compressor, meaning even oil-free compressors can produce oily condensate.
Oil vapour drawn into the intake can condense into liquid oil that can mix with condensed water in the compressor. It is the responsibility of the compressed air user to identify the legal requirements for oil in water discharges in their country and comply with those requirements, disposing of oily compressor condensate in a legal and responsible manner. Depending upon the discharge limits and the volume of oil vapour in the atmospheric air, the use of an on-site oil / water separator may also be required on oil-free compressor installations.
Hopefully, this post has helped to dispel some of the contradictions that you may have come across when researching the pros and cons of a new compressor.
Now, read on and find out why filtration is an essential part of your compressed air system.