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Air Compressor Oil Separator Guide – Air Oil Separators & Oil Water Separators

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An air compressor oil separator is a key component in determining the air quality within a compressed air system. Oil, if able to get down your lines and reach your pneumatic tools, may cause serious damage and reduce your tools lifespan.

Below we will take a look at oil separators: what they are, how they work and where you can find them!

Table of Contents

What is an Air Compressor Oil Separator?

An oil separator does exactly what its name tells you… it’s a filter within an air compressor system that separates oil from the compressed air to protect systems components and your air tools at the end of the line.

If oil were to reach your pneumatic devices, serious damage could be caused which will result in short lifespan and greater costs. Some air compressors may be oil free rather than oil lubricated, so you must know which yours is. For more information visit our guide on this topic!

Of course there are environmental concerns with oil, you cannot just dump it anywhere and especially in your water discharge! And even if you’re not concerned about the environment, you’ll want to keep in like with the EPA’s regulations. You don’t want to face criminal or civil lawsuits resulting in fines, so separate your oil from the air/water supply!

There are two main types of oil separators and I will discuss both in greater detail to follow:

  • Air-Oil Separator
  • Oil-Water Separator

Air-Oil Separator

During the compression of air within air compressors, oil is injected to lubricate, seal and absorb the heat of the air compression, depending on the type of air compressor of course. As not all air compressors use oil as a lubricant, but in this case, imagine a rotary screw compressor which is very dependent on oil lubricant.

So after this compression, a mixture of compressed air and oil is prevalent. The oil separator separates the oil from the air as the oil is needed to stay inside the compressor to keep lubricating the system, while the compressed air needs to exit the compressor free of oil so it can reach the pneumatic devices at the end of the line without damaging them.

How Does an Air Compressor Air-Oil Separator Work?

Most of the oil is separated from the air by centrifugal forces. The air-oil mixture enter the separator vessel at an angle so that most of the oil is pushed outside and drops into the oil vessel. This does not however remove all of the oil, as some is exists as droplets and mist.

The oily compressed air is forced to flow through the separator and as it does, small droplets of oil get stuck, and eventually combine into larger droplets which then fall and collect at the bottom of the separator filter.

The oil can then be sucked up and removed from the system by a scavenge line (recycled and sent back to the compressor for lubrication).

Where can you find an Air-Oil Separator on an Air Compressor?

Air-oil separators typically used on rotary screw compressors, are located just after the compressed air leaves the compressor, installed in the compressors discharge line. This is so the oil can be removed and recycled back into the lubrication process.

Of course not all the oil will be removed, as some oil vapors may pass through the air filter material and remain in your air supply. Therefore, it is important you have other filters in place downstream to ensure these vapors do not reach your pneumatic devices.

An oil-water separator is one that should definitely be used.

Oil-Water Separator

The oil-water separator works by separating the oils and lubricants from liquid water. The oil-water separator collects condensate from the air compressor tank, filters and dryer then removes the oil from the water prior to draining it.

How does an Air Compressor Oil-Water Separator Work?

There are several ways that you can separate oil from water vapor within compressed air systems. It’s important to separate the both as due to environmental considerations you can’t simply dump the two together out and into atmosphere, therefore oil separators are vital.

They work via a number of methods, depending on the type you buy they may use:

  • Gravity Separation
  • Chemical Absorption
  • Non-Chemical Absorption
  • Absorption

Gravity Separation

The simplest method and therefore not the most effective. It involves letting the oil and water settle naturally. Due to oils being the less dense of the two, they will typically situate themselves at the top of the separator.

However, it is still possible for some of the oil vapor to become emulsified within the water during compression so gravity alone will not be able to separate these.

Chemical Absorption

A chemical media like activated carbon is used in this method to attract bonds with oil and repel the water. Oil reacts chemically with the activated carbon media, which enables the filter to trap the oil particles within itself, so that the water can be drained.

Non-Chemical Absorption

It’s possible to have a media made out of materials that attracts oil and repels water without the chemical bonding process previously described. An example would be polypropylene, this material is oil-absorbent and water-repelling.


Similar to absorption, but instead of focus on the core of the media, adsorption attracts the oil to the surface of the material. Adsorbent materials will have multiple tiny pores within them which help maximize the amount of oil it can adsorb by attracting it electrostatically.

Oil Separator Stages

The majority of oil separators work using a fine combination of both absorption and adsorption. These systems typically work via the following stages of filtration:

  1. Oil and water condensate from the drain valves enters the depressurisation chamber, where compressed air is allowed to enter too before it is ventilated back out into atmosphere so that it does not interfere with the filtration process.
  2. Liquids drain down into tan oil filter cartridge that is made out of the polypropylene fibers allowing the oil to be collected and stabilized within these fibers. Absorbing as much oil as possible here using an absorbent material allows for the maximisation of the activated carbon.
  3. The activated carbon is the final stage of the filtration process itself. As the activated carbon is adsorbent, it contains a large number of those extremely small pores that the liquid is therefore filtered through. As the liquid is drained through the micro-pores, oil is attracted to the walls and so only the water passes through!
  4. The remaining water can be flowed out of a drain at the bottom of the separator either manually or automatically depending on the model.

Oil-Water Separators vs Water Separators

It’s important to understand that both these filters provide the same function, they’re the same product. They’re most commonly referred to as water separators, but are far more accurately referred to as an oil-water separator because they remove oil not just water from the air flow within a compressor.

We have a page with further information on the best air compressor water separators

Where can you find an Oil-Water Separator on an Air Compressor?

An oil-water separator or water separator will typically be found attached in-line between your air receiver tank and your end application. Most air tools require varying pressures so depending your system lay out, and whether it includes multiple drops or not, regulators may be placed at each drop to reduce the pressure for your desired air tools.

Due to the pressure drop across a regulator, air is able to cool down and therefore moisture increases, so a water separator can come after a regulator so remove the extra moisture from the air supply.

However, it’s imperative to not allow oil to enter your regulator! With this in mind, oil-water separators should be place just before the regulator, so that the oil vapor is not damaging the regulator, allowing it to work efficiently.

FAQs (Frequently Asked Questions)

How do you replace an oil separator?

It’s important first of all to turn the air compressor off and unplug it, and relieve all the air pressure from the tank. I’d recommend waiting approximately 5-10minutes or more if necessary for the system to cool off.

Once the system has cooled you may replace the air oil separator.

Be aware of oil temperature as it can be very hot, so take caution.

How do you know when you need a new separator?

By conducting some basic checks you should be able to determine whether your oil separator needs replacing or not…

Physically inspect the separator, should there be a lot of oil inside then the scavenge line is faulty not the separator.

Check to see if the separator feels heavy and looks dark in colour, this will mean the filter is saturated, which means the filter media is filling up with oil, so it needs replacing.

What is the effect if the oil return from oil separator to the compressor is blocked?

The obvious answer being the oil level will fall down within the compressor as it is not receiving its recycled oil. This will also cause a lot of oil build up within the separator itself and therefore you may have a faulty scavenge line or filter that is clogged and needs replacing.

Additional oil reading:

If you have any questions regarding oil separators, please leave a comment, with a photo if applicable, so that someone can help you!

By Aidan Weeks

A passionate Mechanical Engineer with endless enthusiasm for fluid power - building off the back of over 18 years of high quality contribution and discussion stimulated by Bill Wade here at About Air Compressors. With both practical and theoretical experience in pneumatics and hydraulics, I'm putting my knowledge to work - and working my grey-matter through my research, assistance and publishing work here at About Air Compressors. Feel free to reach out any time! P.S. A HUGE shout out to Doug who really offers such great value to all visitors to About Air Compressors - once again, feeling like I'm standing on the shoulders of GIANTS by getting to work alongside such a great community

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