How to Choose an Inline Filter for Your Compressed Air System?

Picking out an inline filter is not an art, but it could be confusing if we do not ask the right questions or understand the process that requires filtration. Before we move any further, let’s take a step back and get a general idea of filtration and why it plays an important role within the compressed air system.


Why do you need a filter?

choosing a filter to dry the compressed air
When you have a taste for coffee, it is simple to go to your nearest coffee bar or use a popular β€˜cup’ machine, but we want more. The example I’d like to use consists of using the drip coffeemaker and roasted beans. One of the most important items in making a good cup of coffee in such machine is to use a proper filter in order to avoid a cup full of grainy particles and any other remaining pieces of the coffee bean. As insignificant as it might seem, the paper filter plays a vital role in making coffee that you can enjoy.


Although, a different entity than a coffeemaker, your compressed air system needs a filter as well. As we have learned in past articles, compressed air is full of particles, aerosols and oil vapors (in oil injected pistons and screws) that contaminate the compressed air and can cause potential harm to its end users (equipment that uses compressed air). Incorporating the correct inline filter can help you eliminate the unwanted particulates as well as aerosols and vapors. The amount of filters and types needed will be dependent on the quality of air your application and or process requires. Let’s take a look at the different types of filters and the questions that will help you choose the correct filter.

How clean does my air need to be?

In order to understand how clean your air needs to be, you have to identify and assess the application and process that utilizes the compressed air. Not all applications and processes that use compressed air require the same level of filtration, which is why having this information is the first step in choosing the correct filter. Compressed air utilized for pneumatic purposes can oftentimes be supported by a standard dry particulate filter that provides filtration down to 1 or 0.01 micron, however, if your process requires an OSHA-approval and elimination of oil vapors, then a charcoal activated filter will have to be utilized. Let us get a better understanding of what contaminants are and how they affect compressed air system. Contaminants within a compressed air system can originate from ambient air that is utilized, as well as the system (compressor) itself. There are three main contaminants that are found in compressed air: particulates, aerosols and vapors.


Particulates: Particulates in compressed air system are small pieces of solid material such as dust, dirt and or pollen from the ambient air, as well as loose metal particles that can be caused by pipe corrosion. Depending on the sensitivity of your application and or process, contact with particles can be damaging to the end product, therefore causing delays in production and quality control issues, not to mention potentially unsatisfied customers.


Aerosols: Aerosols consist of small droplets of liquid that can be found within compressed air system, especially in those using oil-injected compressors. Aerosols are created from the lubricant, in this case, the oil used in the compressor and can be harmful to both products and people if not treated properly.
Vapors: In the compressed air system, vapors consist of lubricants as well as any other liquid that has converted to a gas. Such vapors require a special carbon activated filter in order to be removed from the system.
Now that we have a better understanding of the contaminants above, let us take a look at what types of filtration methods are used in eliminating each type of contaminant.
Removing dry particulates
There are three main mechanisms that are utilized in dry particulate filters to remove solid particles of all sizes out of compressed air.
Inertial Impaction: Inertial impaction is a process where particles that are too heavy to flow with the compressed air stream get trapped in the fiber media. The larger the particles are, the easier it will be to separate them.


Interception: Smaller particles can follow the airstream, but if the diameter of the particle is larger than the gap of the filtermedia, it will get caught by the filter media, making it easier to eliminate larger particles than smaller ones.


Diffusion: Diffusion happens when small particles move erratically throughout the surface, instead of following the compressed air stream. This irregular movement path is caused by the particles colliding with other gas particles, an occurrence called Brownian movement. Since the particles have a free range of motion and can move around freely, it is much easier to and more likely that they get intercepted by the filter media and removed from the compressed air stream. Through diffusion, separation of smaller particles is easier than separating larger ones.
These three forces contribute to the overall efficiency of the filter.


Removing aerosols and vapors

There are two types of filters used to remove aerosols and vapor. The coalescing filters are utilized to remove liquids as well as some particulates, while vapor filters use adsorption to remove vapors from compressed air.
Coalescing: Coalescing filters are used to remove aerosols and particulates, but are not effective in the removal of vapors. The coalescing process consists of bringing small droplets of liquid together in order to form large droplets. As the droplets increase in size, they fall from the filter into a moisture trap, resulting in a cleaner and dryer compressed air stream.
Adsorption: Adsorption is a chemical process is used to remove gaseous lubricants or vapors. The process consists of vapors bonding with the surface of the media (adsorbent), which commonly uses activated charcoal in the filters due to its high surface area and attraction to the oil vapor. As the oil vapor covers the surface of the activated charcoal over time, it is essential to change it before it is fully saturated. If not, this would lead to a breakthrough of the oil into the air system. It is also necessary to use a dust filter after the activated charcoal filter, since small charcoal particals could break out and enter the airstream.
Can my process be damaged by lubricants like oil?


In order to assess the potential damage that oil can cause within your compressed air system, you have to understand the basic requirements for your industry or equipment that uses the compressed air. If your industry has strict health codes and or your equipment is sensitive to oil or vapor exposure, it is crucial to use proper filtration. Let’s take a closer look at lubricants and understand the effects they can have on your end product.


Similar to particulates, lubricants can enter your compressed air system from ambient air as well as from the compressor itself. Facility operations, like a motor exhaust releases hydrocarbons like oil aerosols into the ambient air, which can compromise air quality and cause equipment failure. Oil injected air compressors will also release lubricants into the compressed air system, resulting in increased operational and maintenance costs. Industries such as electronics and semiconductor are especially exposed to lubricant contamination, which can result in product loss, missed deadlines and unsatisfied customers.


Poor filtration is oftentimes leads to pipe corrosion, increased pressure drops and can cause equipment damage, resulting in costly downtime and unexpected repair costs. Corrosion can also cause excess debris in your piping system, which in turn results in your compressor working harder, leading to higher energy consumption and excess ware on compressor parts. Proper filtration is key in achieving desired results when strict codes or purity classes are enforced.
The only way to protect your product fully from unwanted oil in your compressed air system is to utilize oil-free compressors, as this type of technology will eliminate the risk of contamination, resulting i clean, high quality compressed air.


Be sure to check out our online shop, where we have a range of Inline Filters to suit your needs:


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Ash Air has been around in New Zealand since 1979, and we’ve grown into a nationwide company with international support and a reputation for quality and reliability.We look after all things compressed air for your business!

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