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Coalescent Filters – Coalescing Filters For Air Compressors Explained

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Coalescent filters are sometimes used as a compressed air treatment when the presence of compressor oil or lubricating oil amongst other contaminants are in the air stream creates an issue.

Standard particulate filters may not remove most submicron contaminants which could become a real problem for your air compressor and pneumatic tools. Coalescing filters are specifically designed to trap and remove the submicron sized contaminants from compressed air.

I will provide you with answers to all your questions surrounding coalescing filters, what they are, how they work and where you can find them.

Table of Contents

What is a Coalescing Filter?

Coalescing filters are used on air compressors to mainly remove water and oil aerosols from compressed air systems. They are quite probably one of the most important pieces of equipment found in a compressed air system.

Coalescing filters can treat and remove six out of the ten main contaminants that are commonly found in compressed air. Those six are:

  • Oil aerosols
  • Water aerosols
  • Atmospheric particulate
  • Micro-organisms
  • Pipe scale
  • Rust

If coalescing filters were not prevalent on air compressors, serious damage could be done to the system. The contaminants would be allowed to hurt the performances of valves and other instruments, damage seals between components, cause erosion of tubes/pipes, and damage desiccant air driers.

The source of the oil in the compressed air stream is quite often from the air compressor itself. Oil-lubricated compressors use lubricating oil either to relieve friction between mechanical parts in the compressor pump or in the actual process of compressing the air like rotary screw compressors do.

Rotary screw air compressors have oil separation equipment at the compressor discharge, and the oil sumps of reciprocating compressors are supposed to keep the oil in the sump and out of the air. Yet, in both cases, some of that lubricating oil can still get into the compressed air stream. It is this oil vapor, which is normally not compatible with seals in downstream air components, which has to be removed by the coalescent filter as the oil vapor is negative to downstream air seals and components.

Coalescent compressed air filter from Norgren

How Does a Coalescing Filter Work?

The coalescent element inside a coalescent compressed air filter is often made of glass fibers.

The coalescent element is designed to force the compressed air stream through tortuous air paths. In so doing, the coalescent element will trap free oil onto the filter material. And, by the process of adsorption, “coalesce” oil vapor in the compressed air stream into tiny droplets of oil which gather and drip down – usually on the inside of the coalescent element – into the “quiet zone” at the bottom of the filter bowl.

When the filter bowl is drained, either manually or with an automatic drain, the contents of the filter bowl, the trapped water, and the oil mix removed from the compressed air stream, are eliminated.

The waste material from the coalescent filter bowl is considered “hazardous waste” in terms of its impact on the environment, so care must be taken in the disposal of it.

Coalescent filters contain filter elements that function similar in concept to the general purpose (30 – 40 Micron sized) filter elements found in a general purpose air filter, but their size can be as small as 0.01 micron. These extremely fine fiber meshes can intercept and capture aerosol particles as well as other contaminants by amalgamating smaller particles into larger ones.

They, too, filter out contaminants moving down the air stream from the compressor, through the filter element, and out to your air tool or air valve.

Coalescing Filter Mechanisms

Coalescing filters typically have three different mechanisms that allow them to remove contaminants from the air flows. These mechanisms depend on the sizes of the particles and are:

  • Diffusion (0.1 micron and smaller)
  • Interception (0.1 to 1 micron)
  • Direct Impact (1 micron and larger)


These particles in the range of 0.1 micron and smaller are subject to random motion which causes them to collide with the filter and therefore getting trapped. Solids stick to the membranes of the filter whilst the liquid contaminants join with each other and form larger droplets.


Particles in the range of 0.1 to 1 microns experience turbulent flow forcing them into the typical coalescing filter elements and being captured. The smaller the fiber sizes in the filter elements, the greater the efficiency.

Direct Impact

This applies to particles that are 1 micron or large. It’s as simple as it sounds, the particles simply collide through direct impact with the filter elements and become trapped.

Coalescing Filter Stages

A coalescing filter works based on the following 4 stages:

  1. Air Intake
  2. Filtration Process
  3. Coalescing of Particles
  4. Air Outlet

Now let’s better understand what each stage plays in the overall process…

Air Intake

First of all, the coalescing filtration collection bowl will be empty and ready to collect contaminants.

Compressed air is then allowed to flow through the inlet of the coalescing filter and into its filter housing where it will meet the coalescing filter element.

Filtration Process

This is the part of the coalescing filter element where it will get rid of the contaminants from within the compressed air. As the compressed air flows into the filter housing through the coalescing filter element, it allows compressed air to pass through, whilst the filter media will trap or absorb the oil and water contaminants and retain them within the filter media.

The amount of contaminants that the coalescing filter eliminates will solely depend on the micron rating of the coalescing filter elements, and how small of the liquid and particulate contaminants it is able to block.

Coalescing of Particles

As the compressed air flow continues through the coalescing filters, more and more oil and water vapor will get trapped in the filter media.

Due to this increase, eventually, they will coalesce together and form larger oil and water droplets that become large enough to condense and fall due to gravitational pull.

They then will collect at the bottom of the coalescing filter in its bowl which can be drained either manually or automatically depending on the model of the coalescing filter.

For more information on bowls Filter Bowls For Compressed Air visit our page!

Air Outlet

The clean contaminant-free air will then be able to leave the coalescing filter through the outlet of the coalescing filter element and continue its path down the line towards your application.

The youtube video below provided by Beko Technologies helps you visualize the three mechanisms and the stages of the coalescing filters operation.

Why are Coalescing Filters Installed in Pairs?

The particulates that are found in compressed air systems are of varying sizes and as the filter is normally very fine, the particles will become blocked rapidly.

As the filter becomes blocked, the differential pressure from the inlet and outlet of the coalescing filter increases. That’s not good news, right? Not only is the coalescing filter reducing the downstream pressure but it is also forcing the air compressor to work hard to generate higher pressurized air to account for the pressure loss.

This results in greater operational costs. Therefore, it is crucial to keep the pressure loss as minimal as possible!

Now how do you battle this problem? Some people think changing the filter element frequently (3-6 months) will suffice, or oversizing the filter but neither are cost-effective solutions.

The most cost-effective solution that will reduce your pressure losses is installing the coalescing filters in pairs. Combining the two filters will allow for the 6 contaminants to be reduced to different levels of purity.

A general-purpose filter typically comes first and protects the second, highly efficient particulate filtration from bulk contaminants. Removing the heavy contaminants before they reach the second and most effective filter, reduces the amount of work the second filter has to do and therefore becomes far more efficient.

This not only improves the filtration as you now have 2 filter elements instead of one, but it most importantly reduces the pressure losses and therefore costs. It can also extend the lifespan of the filter from the previous 3-6 months to a far more admirable 12 months.

Where are Coalescing Filters Located on an Air Compressor?

Typically, coalescing filtration components are installed as close to the air compressor as they can so they will come just after the receiver tank.

Experts generally recommend that you install a prefilter as such before you install coalescing filters so that you can remove liquid contaminants and solid particles before reaching the coalescing filter. This can significantly increase the efficiency of the and life of the coalescing filter.

Because coalescent filters are so very efficient in trapping airborne contamination, you want to have a general purpose 3-5 micron filter unit upstream from the coalescent unit, and then upstream from the 5 Micron unit, you’ll want a general purpose filter with a 30-40 micron element to remove the larger debris.

Though it is more expensive to plumb your air supply to your application in this way in the short term, the payback will be manyfold in longer, downstream filter element life and less maintenance time troubleshooting why your machine or system lacks sufficient air as a result of the 5 micron or coalescent filter elements becoming plugged solid.

Some manufacturers can replace a general-purpose element in their standard filter bowl housing with a coalescent filter element. It becomes difficult to tell then if the filter contains a regular element or a coalescent element. Care should be taken to ensure the right filter is installed in the correct location.

It can be advised to only install coefficient filters only in point-of-use locations that demand extremely clean air as this can save you money!

Advantages vs. Disadvantages of Using Coalescing Filters

Many companies and individual workers at home are now using coalescing filters as a vital part of their air compression system. So let’s take a look at the advantages and disadvantages.

Advantages of Using Coalescing Filters

Here are some benefits of installing a coalescing filter in your compressed air system:

  • Ability to customize the coalescing filter or upgrade existing filters
  • They’re extremely efficient and can be used in the filtration process of a variety of fluids
  • Micron ratings that are capable of producing up to 99.99% levels of efficiency
  • Eliminates maximum amount of liquid and solid particles from air systems for pure air
  • Reduces expenses of operations and maintenance of air compressors
  • Wide surface areas which can reduce pressure drop and increase efficiency
  • Ensures that downstream equipment is safe from dangerous contaminants
  • Efficient filtration system that lasts longer than usual thus reducing downtime and maintenance costs

Disadvantages of Using Coalescing Filters

There are a few limitations that may provide you with setbacks:

  • Initial costs of purchasing and installing coalescing filters can be quite high
  • Replacing the coalescing filter will of course cost you
  • Heavy hydrocarbons are prone to filling the filtration pores quickly therefore rendering the coalescing filtration element and making it useless
  • The absorbent bed deterioration will lead to a lack of eliminating the heavy hydrocarbons and allowing them to build up

FAQs (Frequently Asked Questions)

Are there alternatives to installing a pair of coalescing filters?

Yes, an alternative includes just the single filter. The only problem you will face is that they do not always provide the benefits you want from a coalescing filter.

A great number of manufacturers now offer 2 in 1 filters in an attempt to reduce the pressure losses associated with coalescing filters. They claim to provide the same level of purification but in reality, they do not always deliver to the same standard.

Do coalescing filters remove water?

Yes, coalescing filters remove oil and water contaminants from the compressed air. The coalescing filter elements allow for water to be blocked from passing through. These liquid droplets are trapped and then become larger droplets that fall into the bottom bowl of the filter housing.

The water can then be drained from the bowl, either manually or automatically depending on the type of coalescing filter you have!

How do you choose coalescing filter element?

You can choose a coalescing filter element by either looking at the micron rating of the coalescing filter element or the filtration media.

The micron rating of the coalescing filter element is the pore size that you intend to choose. It’s advised that you settle on a micron rating in accordance with the fluid you intend to filter and the environment in which you’re operating in.

Coalescing filter media vary according to the fluid you’re separating from the compressed air, and this may also affect the coalescing filter element micron size.

How do you determine the efficiency of coalescing filters?

The main factor you should be looking at to determine the efficiency of the coalescing filter is the micron rating. The higher the micron rating of the coalescing filter media, the higher efficiency of the coalescing filter elements.

Another technique would be conducting a multipass test. These tests establish the number of particles that are larger than the micron rating that can pass through the filter.

What features should you look at when purchasing a coalescing filter?

There a numerous features you should take under consideration when purchasing a coalescing filter. The material of the bowl can be an important one, and so can the fluid in question.

The main features are substance exposure, pressure and temperature, manual or automatic drain, air purity class, visual service indicator, and its weight and size.

Metal bowls are compatible with more filtration fluids than others, and can withstand high pressures and low temperatures of coalescing filtration.

Some filters may have an indicator that will alert you in case the bowl is full and you have a manual drain. Otherwise, you could choose an automatic drain and not have to worry about this.

The efficiency must be considered for how many contaminants it can eliminate from the air steam.

How are coalescing filters rated?

Coalescing filters are rated by their common selection parameters like their pressure limits and flow capacity along with their filter element micron size. But, they can also be rated by their filter efficiency, a measure of the percentage of contaminants of a specific micron size that the coalescing filter element captures.

Filters typically have ratings in the range of 90% to 99.99% depending on the product. Users should therefore decide upon products based on their application requirements, because the more efficiently rated filters typically cost more, have a higher pressure drop and certainly have short life spans or require more maintenance.

If you have any questions regarding coalescent filters, please leave a comment below, 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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