Aug. 11, 2025
Particulates: Particulates in compressed air are small pieces of material like dust, dirt, and/or pollen, as well as loose metal pieces. Depending on the sensitivity of your application and or process, contact with particles can be damaging to the end product. They can also cause delays in production and quality control issues, as well as unsatisfied customers.
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Aerosols: Aerosols consist of small droplets of liquid found within a compressed air system, especially in oil-injected machines. Aerosols are created from lubricant. Therefore, oil used in the compressor can be harmful to both products and people if not treated properly.
Vapors: In a 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.
There are three main mechanisms utilized in dry particulate filters to remove solid particles from compressed air. These three forces contribute to the overall efficiency of the filter.
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 of compressed air. The larger the particles are, the easier it will be to separate them.
Interception: Smaller particles can follow the air stream. However, if the diameter of a particle is larger than the gap of the filter media, it will get caught by the filter media. This makes 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, it is more likely that they become intercepted and removed by the filter media. Through diffusion, separation of smaller particles is easier than separating larger ones.
Two types of filters are used to remove aerosols and vapor. 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 used to remove gaseous lubricants or vapors. This process involves vapors bonding with the surface of the media (adsorbent). Activated charcoal filters are commonly used since they attract oil vapor.
As the oil vapor covers the surface of the activated charcoal over time, it is essential to change the filter before it becomes 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. This is because small charcoal particles could break out and enter the air stream.
To assess the potential damage oil can cause to your compressed air system, it's important to understand your equipment and basic industry requirements. If your industry has strict health codes and or your equipment is sensitive to oil / 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, release 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.
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A look at how to match filter specs to your inline filter needs.
We all realize that it is important to select the right filter so that something doesn’t go wrong later. For example, perhaps the efficiency of flow is too low. How do you know you made the right choice? Well, we already know about chemical compatibility requirements (see my previous post, How to Choose an Inline Filter – Chemical Compatibility). Now we need to think about operating conditions. I will go over flow rate, volume of solids, operating pressure and operating temperature in this post.
Flow Rate
Filter flow rates show how much fluid, air or gas passes through the filter. These rates are based on testing under controlled conditions. Size of connections and housing size are important. But flow rate depends on two considerations, the size of the most restrictive flow path and how much flow the filter material or media allows.
More about filter media flow ratings
It is important for all of us to understand more about how filter media controls flow rate. The percent of open area (POA) of filter media and its total surface area may limit flow. The POA needs to be large enough to handle the expected flow through the filter. Low POA can still work if you use a filter with a larger surface area.
A note of caution
Relying simply on flow ratings could be risky. This is because these ratings are based on ideal test conditions. The best way to measure flow is to test samples in the actual process. Other variables that could influence flow when comparing flow ratings to actual flow include temperature and pressure.
Volume of Filterable Solids and Clogging
Please be sure the filter pores are the right size to catch the particles being filtered out. Manufacturers usually give filter pore sizes in microns. See our Mesh and Microns page for information about microns. More surface area means more solids are captured without clogging. Clogging reduces flow rate. Clogged filters increases pressure both across the filter and upstream of it. This reduces flow efficiency. It may cause filter failure too. Filter failure can damage downstream machinery. Choose filters with the right surface area and micron rating for your application. Then monitor those filters for clogging making sure that they are easy to service and/or replace.
Operating Pressure Ratings
Maximum operating pressure is the highest pressure that does not damage the filter media. Differential pressure is the difference in pressure between the filter inlet and the filter outlet. Burst pressure is the maximum pressure filter housings can handle before failing.
Why differential pressure matters
In general, differential pressure must exist for flow to take place. It makes sense then that more fluid could pass through a filter when pushed harder. Also, certain inline components function properly for flow in one direction only. This means you probably will need to add check valves to protect filters from back flow. In addition, thick fluid can dramatically increase differential pressure across a filter. This pressure could become so high that flow rate is significantly reduced or even stopped. The pump might even be damaged.
Operating Temperature
Higher temperatures may lower filter pressure ratings. This is because higher temperatures can change chemical compatibility. It may also cause plastics to soften.
Now you can see why it is helpful to know flow rate, the amount of filterable material, operating pressure and operating temperature when choosing a filter.
Think maximum operating pressure and burst pressure. And don’t disregard the effects of temperature both inside and outside the process.
Look for our upcoming blog article about filter connection types:
How to Choose an Inline Filter – Physical Connections
Have you had a particular difficulty in matching a filter to a process? Was pressure or flow rate your nemesis? Do you have any tips to pass on? Help us by telling others about what you learned.
Previous posts in our How to Choose an Inline Filter series:
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