If your processes require ultraclean compressed air, you may think that you need an oil-free air compressor. But in most cases, clean air requirements can be met without the expense and maintenance hassle of an oil-free compressor. Before you spend the money, take a look at the pros and cons of oiled vs. oil-free compressors and see which one is right for you.
What is an Oil-Free Air Compressor?
Rotary screw air compressors come in two basic varieties: oil-injected (or oil-flooded) and oil-free. As the name implies, an oil-free air compressor does not use oil for lubrication in the compression chamber. Instead, they rely on rotors with very tight mechanical tolerances to reduce friction. Air or water is used for cooling instead of oil.
- An oil-free (or oil-less) rotary screw air compressor typically uses a 2-stage compression process with an intercooler between the two compression chambers. This allows the air to cool down between compression stages and prevents overheating of the compressor.
- Piston (or reciprocating) air compressors also come in oil-free versions. An oil-free piston-style air compressor will use a coating (such as Teflon) on the cylinders to reduce friction.
It is important to remember that “oil-free” refers only to the compression chamber itself. Lubricants may still be needed for bearings and other moving parts outside the compression chamber.
Pros and Cons of Oil-Free Air Compressors
Oil-free air compressors were invented to solve the problem of oil carryover, or leftover oil residue in the compressed air stream. Their primary advantage is cleaner, drier air. However, this advantage may not be as big as you think—and oil-free air compressors can come with some pretty big drawbacks.
Since there is no oil used within the compression chamber, you may think that compressed air would be completely oil-free. Not so fast! While it is true that oil carryover from compression processes will be zero, any oil brought in through the air intake (such as vaporized metalworking fluids, lubricants used in production processes, or hydrocarbon residues from running motors in the vicinity) will still end up in the compressed air supply. Unless the intake air is pristine, filters will still be needed to remove oil residue from the air.
What are the other drawbacks of an oil-free air compressor?
- Capital costs. Cost is the biggest downside. Oil-free air compressors can be as much as double the cost upfront compared to an oil-lubricated air compressor with a similar CFM rating.
- Maintenance costs. Oil-free air compressors also have higher maintenance costs—especially rotary screw oil-free air compressors. While you may have fewer filters to change, the compression screws in the air-end are much more likely to fail than those in a lubricated air compressor. When this happens, repairs can be very costly. And remember, the oil-free rotary screw air compressor has two stages of compression instead of just one, potentially doubling your repair costs.
- Service life. An oil-free air compressor will have a shorter life overall than a lubricated rotary screw air compressor. While the air-end for an oil-flooded rotary screw air compressor can easily last 150,000 hours or more, the typical lifespan for the air-end for an oil-free air compressor is closer to 50,000 hours.
When Do You Need an Oil-Free Air Compressor?
Oil-free air compressors are typically found in industries requiring ultraclean compressed air, such as food processing and packaging, pharmaceutical and nutraceutical production, electronics, and medical or laboratory applications. Companies needing to reduce contaminants in compressed air to an absolute minimum have traditionally relied on oil-free air compressors. However, high-efficiency compressed air filtration can now produce air of comparable quality using an oil-flooded air compressor.
Oil-flooded air compressors with proper inline filtration are a reliable alternative to oil-free air compressors for nearly all applications requiring ultraclean, oil-free air. Inline air filters and oil separators can reduce oil carryover in compressed air to practically zero.
Reducing Oil Carryover in an Oil-Flooded Air Compressor
If you require ultraclean compressed air in your processes, it is important to reduce oil carryover as much as possible. Here’s what you can do.
Use Intake Filters to Reduce Contaminants Coming into the Compressor
The intake filter is your first line of defense against contaminants in your compressed air supply. The intake filter will remove large particulates and some oil mists from atmospheric air entering the air compressor.
Use Inline Filtration to Remove Oil and Particulates from Compressed Air
The inline filters do most of the work in reducing oil carryover from an oil-flooded air compressor.
- A high-efficiency coalescing inline filter removes particulates down to 0.01 microns and has a maximum oil carryover of 0.008 PPM.
- Activated carbon inline filters remove hydrocarbon mists, vapors, and odor with a maximum oil carryover of 0.002 PPM.
With these filtration efficiencies, there is very little meaningful difference in compressed air quality between an oil-free and oil-flooded air compressor. In fact, inline filtration for an oil-flooded air compressor may be more efficient than a filter used with an oil-free compressor, because the filters work best when they have some oil in them.
Change inline filters at least once a year or after 8,000 hours of operation.
Use an Oil/Water Separator
The oil/water separator is used to separate lubricants from condensate generated by the air compressor. The oil/water separator removes excess oil from the compressed air condensate and allows for proper disposal of liquid wastes according to EPA guidelines. The oil/water separator must be properly maintained, and filter cartridges should be changed at least annually.
How is Oil Removed from Inside an Oil Flooded Compressor?
Internal to the compressor is an air/oil separator, this device removes oil from the compressed air discharge so that the air has less than 3 PPM of oil in it, the inline filtration takes care of the rest. At one time years ago the separator was a common failure point for lubricated air compressors. However, with modern designs, that is rarely an issue anymore. An air receiver tank can be added after the compressor to act as a failsafe to prevent oil from reaching the compressed air filtration and air distribution system in the unlikely event of a failure.
Test Your Air Supply Monthly
If ultraclean air is a necessity, make sure you test your compressed air supply at least monthly to make sure oil carryover isn’t rising. If you start to see an uptick in oil carryover (measured in PPM), check your inline filtration. It may be time to replace the filters.
Can You Get Oil-Free Air from an Oil-Flooded Air Compressor?
Can an oil-flooded air compressor be used for applications requiring ultraclean compressed air? In most cases, the answer is yes. Fluid-Aire Dynamics can help you transition from oil-free to oil-lubricated air compressors with high-efficiency filtration. We helped one manufacturer make the switch nearly 30 years ago—and their air has passed every air quality test since.
Want to know which kind of air compressor is right for you? Talk to a compressed air expert at Fluid-Aire Dynamics. Call or contact us online.
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