Within any air compressor system, exists a real need for filtration. Why? Well, air compressors are known to generate a certain amount of particulate contamination so to speak. This particulate contamination may include water vapour, solid particles like rust or even lubricates.

Therefore, to counter this somewhat inevitable problem, compressed air particulate filters should be applied to each application at the output of any compressor.

I will present to you in greater detail in this article, what compressed air filters does, how they and their components work and where they’re located!

Table of Contents

What does a Compressed Air Filter do?

Ultimately, compressed air filters play a pivotal role in the compressed air process. Choosing the right air filter for your air compressor will help provide you with greater air quality, increase the efficiency of your unit as well reducing energy costs, protecting your tools and even protecting your employees from harmful chemicals and particulates.

Compressed air filters stop the contaminates from reaching your air tools or equipment which would otherwise cause damage which is certainly admirable to avoid. Not only this, but without air compressor filters removing such contaminates, it’s possible that the operation of the systems critical components like the valves for instance, may be compromised.

Different types of air filters are an essential component of any compressed air system in providing air quality and purity and can also even prevent seals from swelling and undergoing premature wear.

Compressed Air Contaminants

Of course, it heavily depends upon the environment in which you use your air compressor for which contaminants may be liable for getting into your system. There may be various kinds, and most generally tend to be microscopic but the great thing is air compressor filters are capable of catching particles SO small that they are measured in microns (one-millionth of a meter).

Determining what kinds of contaminants you will need to remove from your pressurized air will depict what air compressor you will need.

Now what are these compressed are contaminants you must wonder? Here are a few:

  • Water Vapour
  • Oil and Lubricant
  • Dust and Dirt Particles
  • Micro-Organisms

However, all these particulates are removable if you have the correct filters in your system. Of course, they require specific filters and I’ll introduce the specific types of filters available later but, you can find the right one to suit your compressed air systems need. Allowing you to purify the air at intake and more importantly at the outlet heading towards your application.

Water Vapour

Water vapour is a naturally occurring chemical compound within air. With high humidity within workshops, in and around the air compressor system, these water vapour particles can potentially form into water droplets and cause problems within the compressed air.

This results in potential maintenance expenses due to damages and thereafter a shorter lifespan of the air compressor unit itself. Therefore, it is vital to air an air filter that is capable of removing water vapour from the system.

Oil and Lubricant

Oil particles can certainly be prevalent n compressed air systems, as you will know, because they’re very commonly used as lubricants for a lot of varying types of machines. The amount that passes through will be very dependant on the design type of machine, its age and condition.

In some compressor designs, lubricant is used within the air compression chamber and therefore, a filter MUST be used afterwards in order to remove the oil from the air that is heading towards your application.

Dust and Dirt Particles

No matter where you’re using your air compressor, whether that be in a workshop, a industrial factory or even at home in your garage, you certainly run the risk of dust and other small dirt particulates entering your compressor. They’re so naturally prevalent in air it’s almost impossible to avoid.

So what should you do? Ensure you have an air filtration system in place so that you’re not putting your pneumatic tools at risk!

Micro-Organisms

It’s extremely common for micro-organisms being a result of the contaminating particles mixing inside a compressed air system. Bacteria and viruses can certainly multiply under the conditions of water and oil residues mixing, further contaminating your pneumatic tools or devices at the outlet.

How does a Compressed Air Filter work?

The principle is simple, they trap and collect particles to purify the air flow. Though, it may vary slightly depending on the type of air filter you have, whether that be coalescing filters, particulate filters or water vapor filters.

Compressed air filters stops water, oil and dirt from reaching your pneumatic devices, making them an essential component for the overall health of your air compressor system. You don’t want these contaminants reaching your equipment if it has the potential to damage or even destroy them!

To actually describe how the air filter works, it’s best I introduce to you the different components and the part they play in the contaminant removing process.

Air Filter Components

Standard compressed air filters will typically contain the following components shown in the image below. The numbers on the picture of the compressed air filter picture correspond to the descriptions to follow .

Compressed Air Filter
Compressed Air Filter

1) Air Inlet

Air flows through the inlet. The cap is plumbed internally to force the air to flow downwards and spiral into the filter bowl. This creates a cyclonic action in the compressed air stream to throw free water and debris that may be in the air stream against the walls of the bowl, where it will flow down into the bottom.

2) The Filter Cap

The filter cap may be composite plastic or of metal construction depending on the manufacturer.

Satisfactory compressed air filters operation depends on the air flowing through the filter cap in the correct direction from the supply line out the filter discharge.

The correct air flow direction will usually be identified on the cap of the compressed air filter, by an arrow. The arrow points in the desired direction of air flow from the supply line to the filter discharge.

It’s important to note: The air filter will not work properly if you inadvertently reverse the air flow by installing the filter backwards.

3) The Air Path

The dotted line in the diagram depicts the flight path of the compressed air as it flows toward the filter discharge. In order to exit the filter through this path, the compressed air must have undergone the cyclonic action phase, and then passed through the filter element, further purifying the airstream.

4) Filter Discharge

The filter discharge allows the flow of the filtered compressed air from the air filter and down the air line to your desired applications.

The fitting size of the discharge port will usually be the same as the inlet port. Some compressed air filter manufacturers offer adapters to allow you to change the port and fitting size to one that best suits your application.

NOTE: While it is in some cases feasible to use a larger filter unit on a small air line, attempting the reverse may impede compressed air flow. Do not do this unless you have checked to ensure that the flow of the smaller air filter has sufficient capacity for your application. This will result in a choke point if you under size the air filter.

5) Filter Element

This is the filter element. Insufficient air supply problems encountered downstream from your compressed air filter are often caused by the element becoming plugged and choking your air supply. If your air tool or applications isn’t getting enough air, check the filter unit. Clean it or replace it, depending on the type.

Some manufacturers offer a Delta P gauge as part of their filter assembly. The Delta P will indicate air pressure drop across the filter, and by extension – the filter element .

Increasing air pressure drop on the gauge indicates that the filter unit is doing its job, it is trapping debris and removing it from the compressed air stream, and getting plugged up in the process. The more contaminants in your air, the sooner your filter element will plug.

Filter elements have a specific flow capacity measured in CFM (cubic feet per minute), and a particulate size rating measured in Microns.

6) Filter Bowl

The bowl is the biggest part of the filter, and is connected to the cap by either threading or twisted it to lock into place.

Sometimes the bowls of the air filter are plastic with a metal shroud, or are completely metal construction. That being the case, why use metal or metal shrouded bowls filter bowls? There’s a page about metal filter bowls for compressed air.

7) Filter Quiet Zone

Inside of almost every air filter bowl there will be a device that separates the bowl, horizontally, into two sections. This barrier, often made of a plastic or plastic composite, is usually installed hanging from the bottom of the filter element inside the bowl.

It has the purpose of blocking the cyclonic incoming air and preventing that fast moving compressed air from reaching the puddle of debris, water and oil that the filter is collecting in the bottom of the filter bowl.

The barrier creates a quiet zone in the filter bowl, allowing the contamination that collects onto the sides of the bowl to flow down, out of the cyclonic air stream, and to remain – without getting entrained or re-entrained back into the same air stream, until the waste liquid can be expelled from the drain at the bottom of the bowl.

8) Filter Drain

All industrial compressed air filters will have a drain of some sort in the bottom of the filter bowl.

These drains may be manual, a float type, or can be electric auto drains. They need to be opened regularly to allow collected water and debris to escape from the filter bowl.

Failure to drain the filter bowls often enough will mean that the water and debris in the quiet zone will fill past the barrier, and once there, be reintroduced into the cyclonic air stream, onto, and sometimes even through the filter unit.

In some cases debris and water from a full filter bowl will flood the element so badly that it seems to become almost no filter at all, and is actually contributing a steady stream of crud downstream, to damage the air components, and choking your air supply to absolute system failure.

Where Do You Put A Compressed Air Filter?

Compressed air filters typically come in-line after the pressurized air has left the tank and is on its way to your desired application. Here, you can have a series of filters to remove all possible contamination ensuring you have the highest air quality possible.

It is also possible to have compressed intake filters that act as the first line of defence before the air enters the compression chamber from atmosphere.

Types of Compressed Air Filters

There typically tends to be 3 main types of compressed air filters that are used on air compressors to ensure the air exiting the system to your desired tools at its purity requirement. These types ultimately produce the same result however operate on slightly different principles.

The three types of compressed air filters are:

  • Coalescing filters
  • Vapor filters
  • Dry particulate filters

Knowing what the needs of your compressor are before choosing your air filter can help ensure you choose the right one.

Coalescing Filters

Coalescing filters are capable of handling very fine particles that can in some cases be smaller than a single micron. They remove particles such as aerosols, water, oil, and other liquids however, they cannot catch vapors.

Vapor Filters

Vapor removal filters, or otherwise known as, activated carbon filters are typically used to remove the gaseous lubricants (oil vapors) that go through coalescing filters.

Vapor filters must be placed after coalescing filters as they cannot capture aerosols due to their absorption process which would allow the aerosols to saturate the filter.

The activated carbon inside the filter provides a broad surface area that naturally attracts oil vapor onto it and away from the air stream.

Dry Particulate Filters

Dry particulate filters operate in a similar manner to coalescing filters by capturing and retaining particles within their filter units. They particularly target dirt, dust and other similar fine particulates like rust from corrosion inside the compressor unit.

Filter Selection

The proper selection and use of compressed air filters will prevent many short and long term problems with your compressed air equipment and systems whilst saving you substantially in energy costs, down-time and component replacement costs over the life of your compressed air system.

Micron Sizing in Filters

I’ve mentioned microns a few times in this article, so I thought it would only be right to give you a better idea of them in relation to filters. The chart below tells you the actual size particle that a specific Micron rating represents in inches.

Micron Size Chart Filters
Micron Size Chart Filters

General purpose filter elements are 30 or 40 Micron sized. For some applications, you might need a finer filtration, and opt for a 5 Micron element. Depending on your air quality, a 5 Micron particulate size element might clog too quickly. Therefore, you should use a general purpose filter upstream from the filter with the 5 Micron element, to increase the finer element filtering capacity of your compressor.

So too, when your air filtration requirements dictate a move into even finer air filter element such as a coalescing filter type. You will definitely want to have a general purpose filter first, then a 5 Micron filter unit to further rid the air of particulates before the air stream reaches the extremely fine coalescing filter element.

Filter Size

How can you determine the right filter size for your compressed air system? One rule of thumb is that the filter inlet and outlet ports should be the same size as the O.D. of the air line on which you are installing a compressed air filter.

If you are plumbing an application using 1/4″ polyethylene tube for example, the appropriate filter port size would be 1/4″ NPT.

That rule is not always valid, but is a reasonable guideline in the absence of any other information though it is possible in some cases, with a 1/4″ NPT ported compressed air filter, you will have oversized the compressed air filter for that application, and it is possible that with the same 1/4″ NPT ported filter you will have undersized the air filter for another. More on this farther along.

Portable Compressors Too Compressor Components
Portable Compressors Too Compressor Components

Filter Flow Characteristics

Each filter manufacturer will provide the flow characteristics of their filters.

The flow characteristic of a given compressed air filter may change, depending on the Micron rating of the element inside it even though the housing and the port size of that air filter remains the same.

For example, a 1/2″ NPT filter housing can have a variety of flow capacities depending on what size of filter element is installed inside.

Please note that you cannot tell what the mesh of the filter element is from looking at the outside of the air filter.

Recognize too that the flow through the filter can be determined by the compressed air demand of the application, not necessarily the compressor size.

A 25 HP compressor can theoretically produce 100 CFM of flow at about 90 PSI, so with a 25 HP compressor it would be logical to think that you would need a filter with a capacity of 100 CFM to handle all of the air from that compressor.

But, what if you have a 200 gallon reservoir, and the downstream demand is for 150 CFM when a specific piece of equipment starts? The 25 HP compressor can produce 100 CFM at 90 PSI, the filter is sized for 100 CFM, yet if the receiver is big enough and the downstream air demand high enough, the flow of air to that application from the receiver might exceed the capacity of the filter for a short time.

It is important then to have a pretty good idea what the maximum plant CFM needs could possibly be if all demand was on stream through one compressed air filter at one time to have enough filter capacity at the receiver for that total demand if that should occur.

Point Of Use Filters

As the compressed air plumbing reaches the compressed-air-consuming locations downstream from the compressor, you will want to know what each application will consume in compressed air when it is running at capacity and install the appropriately sized compressed air filter for each application just before it.

The reason is that even though the system will have an appropriately sized air filter at the receiver or compressor tank outlet, the nature of compressed air is that there will likely be condensation of water in the air lines after the initial filter. You will need to install a filter at each air consumption point to remove the water that has condensed in the air lines between the tank filter and the air using component.

There is more information about determining the consumption of air in your plant to help size the appropriate air filters, or to on air cylinder consumption on the pages of this site.

Filter Flow Rating

One of the frustrating things about compressed air filters is that it is virtually impossible to tell from the outside what the flow rating of that filter is. You cannot be expected to know just by looking at the air filter installed in an air line, or off the store shelf.

If it is a filter that contains a general purpose element, and the NPT port size is the same inch size of the air line, usually you will be OK with using that filter in a typical air line / air tool application.

If in doubt, locate the vendor / manufacturer / model number information on the filter head or housing, and contact the vendor for flow ratings for that unit.

After you have dumped the compressed air from the line, and removed the filter bowl, by describing the element to the vendor or distributor, they should be able to identify the type of element and the flow rating of that compressed air filter with that element installed.

Water Filter Symbol for Compressed Air System Schematic

Here are the generally accepted symbols for drawing compressed air filters in your circuit schematic.

Reader Comments and Answers

Filters and Plumbing Air Tools

by Herb Frye
(Hopewell Township, PA)

I bought a used compressor that I want to use for some painting and also run air tools.

It does not have any kind of filters.

I looked at a filter that was dual for removing water dirt etc. but also added oil for air tools. I don’t want oil in my paint.

Do I need separate filters and a separate lubricator? Do I need to use separate air hoses etc.?

Herb

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Compressed air lubricator
Compressed air lubricator

Hello Herb. Thanks for visiting.

I take it that some of your tools want lubrication, but you don’t want that lubrication to go to the paint sprayer. Is that correct?

If so, plumb the discharge line from your compressor into a manifold with at least two outlets.

Take one feed from the manifold through a general purpose air filter to your air tools. This will remove free water and any particulate matter. If you wish lubrication, add a lubricator after the filter.

Take another feed to your spray paint equipment. Upstream from the spray gun, install a coalescent air filter to remove any transient oil that’s coming from the air compressor and then a general purpose air filter to remove free water and help stop fish eyes.

If water is a real problem you might consider an in-line compressed air drier next, and then plug that into your air tool lubricator, which should be installed right before your air tool.

Hope this helps.

Further Reading

Metal Filter Bowls for compressed air

Filter for air tools

Deliquescent air dryers

FRL Air Compressor Filter Regulator Lubricator Guide

Coalescent Filters – Coalescing Filters For Air Compressors Explained

Breathing Air Filters for Compressors Guide

Best Air Compressor Water Separators & Moisture Filters 2021

Refrigerated Air Dryer Guide – Best Refrigerated Air Dryers

Deliquescent Air Dryer

Desiccant Compressed Air Dryer Guide

Compressed Air dryers Guide – Which Type of Air Dryer Do I Need?


If you have any questions regarding compressed air filtration, please leave a comment below, with a photo if applicable so that someone can help you!