Everything You Need to Know About Air/Oil Separators
Kaishan USA | February 26, 2025| Uncategorized
Most compressed air applications in industry can use oil-flooded compressors, which need air-oil separators to remove the oil mist from the air they are producing.
If you have an oil-flooded air compressor, you have an air-oil separator. And it’s a consumable. Knowing those two facts is good for your compressor, your company, your workforce and your health.
Here’s why: the original equipment manufacturer of your compressor specifies an air-oil separator according to the compressor model, the type of oil, the pressures you need and the ambient heat in your facility. Ignore those recommendations at your peril.
As we note in our blog post, “How to Maintain Your Air Compressor with Genuine Spare Parts,” inappropriate replacement of an air-oil separator can result in fires, explosions and even injuries to workers.
And while dramatic events like fires and explosions rarely happen, a problem with an air-oil separator can have many other negative consequences, including:
- Oil pass-through, resulting in poor air quality
- Pressure drop
- Increased cost
- Damage to downstream equipment
As a result, maintaining an air-oil separator is critical to the operation of an oil-flooded machine, such as Kaishan’s KRSP2 premium rotary screw air compressor.
Carefully maintaining its air-oil separator is critical to the operation of a rotary screw air compressor, such as Kaishan’s KRSP2 premium rotary screw air compressor.
Let’s look at this specialized component more closely.
What Is an Air-Oil Separator?
An air-oil separator is a specialized device featuring a mesh filter element located in a tank, also called a sump.
The combination of the swirling action in the tank with the filtration action of the element removes oil mist from the air leaving an oil-flooded screw compressor.
The swirling action in the air-oil separation tank, combined with the filtration action of the element, removes oil mist from the air leaving an oil-flooded screw compressor.
Why Does My Compressed Air Have Oil in It?
Any oil-flooded rotary screw air compressor has an oil lubrication system that performs the following functions:
- Seals the compression cycle
- Lubricates components
- Removes the heat of compression, lowering your system’s operating temperature
- Captures any contaminants in the air
Oil reduces friction and wear, controlling contamination and protecting your unit’s components from oxidation and corrosion.
Rotary compressor oil is so central to your compressor's operation that we consider it the lifeblood of your system.
But while that oil is critical to the operation of an oil-flooded rotary screw compressor, it causes problems if it gets into the downstream tools—everything from pneumatic handling systems to nail guns—that are powered by the compressed air.
As a result, the air-oil separator plays a critical role in removing that oil from the compressed air. Here’s how it works.
How An Air-Oil Separator Works
An air-oil separator has different systems to circulate the air and the oil. Let’s review each type of movement.
The Air Movement
After leaving the compression chamber, the air enters the separator tank or sump, which forces the air into a circular movement. That cyclonic effect forces out most of the oil.
The air then is drawn into the separator element, a mesh filter that removes all but a tiny fraction of the oil—3 parts per million, to be exact.
That’s clean enough for most industrial processes. But it’s not good enough if:
- Your products are ultimately consumed by humans, such as in the food, pharmaceutical or healthcare industries.
- You’re in the semiconductors or electronics industries, which require ultra-clean air.
- You apply coatings or finishes that even the tiniest oil droplets would mar.
In those situations, you may be able to get the air quality you need with additional filtration to remove oil or other particles in the air. Or you may want to go with an oil-free rotary screw air compressor, such as Kaishan’s KROF two-stage oil-free rotary screw air compressor. For more on deciding whether you need an oil-free compressor, see our blog post, “Ultimate Guide to Rotary Screw Air Compressor System Design, Part One: 10 Important Questions To Get You Started.”
The Oil Movement
The oil that falls to the bottom of the sump is drained by a scavenge line. It goes through a filter (to remove impurities) and a cooler (to remove heat), then back to the main oil tank to start the process over again.
Keeping your air-oil separator in good condition is essential for the performance of your rotary screw air compressor.
How to Maintain Your Air-Oil Separator
There are several key actions you need to take regularly to keep your air-oil separator operating correctly:
- Change the oil separator and lubricant every 4,000 hours if it’s a spin-on type, every 8,000 hours of operation (or annually) if it’s a standard drop-in type, or as indicated in your oil-sampling recommendations.
- Check the oil separator pressure differential alarm and replace it if the pressure drop through the separator is over 10 PSIG. Remember, a two-PSIG pressure drop caused by blocked filters will raise your energy consumption by an average of 1%.
And don’t forget to check the scavenge line! Clogged scavenge lines and orifices are the top cause of oil carry-over.
As mentioned, you’ll want to follow your manufacturer’s recommendations for any replacements you make.
Why Following Your Manufacturer’s Recommendation Is Good for Your Health
OEMs design their air-oil separators to fit tightly and ensure proper grounding on the inside. Unfortunately, many off-brand or pirated oil separators have a staple at the top. If that staple doesn’t make contact with the metal lid of the unit, it may not sufficiently ground the unit.
If a spark occurs in the sump, the tank could catch fire or even explode, potentially damaging equipment and causing severe injuries or death to nearby workers.
Kaishan adds a piece of copper on the top of the separator to enhance conductivity and reduce the risk of sparks, fires or explosions.
As mentioned, all manufacturers design air-oil separators to ensure a good fit, proper functioning and safe operation. That’s why we recommend following your manufacturer’s recommendations in using replacement parts and maintaining your compressor.
For more information on the benefits of using original OEM parts, check out our blog post, “How to Maintain Your Air Compressor with Genuine Spare Parts.”
Air-Oil Separator Troubleshooting
A few key findings point to specific issues with your air-oil separator:
- Oil carryover. If you’re getting oil in the air downstream, check the scavenge line and the filter element and clean if necessary. As mentioned, clogged scavenge lines and orifices are the top cause of oil carry-over.
- Contamination. Likewise, blocked filters and drains can cause dirt and other particles to linger in the air, decreasing quality and harming equipment.
- Pressure drop. A clogged separator will reduce air flow and increase energy consumption as your compressor struggles to produce the pressure and flow your system requires.
Technical Help Where and When You Need It
Diagnosing and correcting air-oil separator issues may require expert help. And replacing one of the larger separator elements can be a challenging job. That’s why we recommend you work with your local compressed air consultant.
We work with a nationwide network of independent distributors, who can provide on-site assistance and consultations as needed. These factory-trained air compression experts have experience with all aspects of compressed air systems and can service your compressor and your entire compressed air system.
Key Takeaways
- Any oil-flooded rotary screw air compressor has an oil lubrication system that seals the compression cycle, lubricates components, removes the heat of compression, lowers your system’s operating temperature and captures any contaminants in the air.
- Air-oil separators are critical in removing that oil from the compressed air.
- Maintaining an air-oil separator is critical to operating your air compressor and your compressed air system.
- We recommend that you closely follow your manufacturer’s recommendations in using replacement parts and maintaining your compressor.
Let Us Help
Maintaining the air-oil separator of your rotary screw air compressor is essential for the proper functioning of your compressed air system and all the tools and equipment that depend on it. If you need assistance with the operation and maintenance of your separator, reach out to the experts at Kaishan. Contact us today.
