What is a Coalescing Filter?

Coalescing filters clear compressed air of fine dust, tiny water drops, and oil mist. 

By keeping the air clean, they stop damage to dryers, tools, and production equipment.

Dirt and other things get in during compression, especially in compressor systems that use oil as a lubricant. When oil and water mix inside the system, they start to damage pneumatic tools and spoil end products. Fixing that kind of problem takes time – and it can get expensive fast.

How Does a Coalescing Filter Work?

A coalescing filter traps fine aerosol droplets – it then makes them merge into bigger, heavier drops. Those larger drops can then be taken out of the air stream with ease. The process happens in three stages – capture, combine, and drain.

1. Capture: As compressed air flows through a dense borosilicate glass microfibre element, contaminants are trapped in three ways. Larger particles are caught by inertial impaction. Medium ones by direct interception. Finally, the smallest aerosols are caught by diffusion.
2. Coalescence: As more aerosols are captured, they collide on the fibres and combine to form much larger liquid droplets.
3. Drainage: These now-heavy droplets are drawn into a collection bowl by gravity as they travel down the filter element’s exterior.  The collected liquid is subsequently released from the system by an automated drain valve located at the bowl’s bottom.

Why are Coalescing Filters Necessary?

The process of compressing makes contaminants more concentrated. The air around us naturally contains dust and water vapour. Oil-lubricated compressors also introduce small oil droplets into that air during operation.

Standard particulate filters cannot remove these sub-micron liquid contaminants. This is what makes coalescing filters essential.

This aerosol mixture makes an acidic condensate. It eats away at pipes, gets into pneumatic tools, and makes food, drugs, or paint products dirty.

This pollution also makes energy use and maintenance costs go up. For operational efficiency, good filtration is very important.

Where is a Coalescing Filter Installed in a System?

Install coalescing filters after the particulate stage and before the carbon filter. This sequence keeps the system efficient and extends filter life.

1. Particulate Filter (Pre-filter): This filter goes first to remove bulk dry contaminants like dust and rust. It protects the fine coalescing filter from clogging.
2. Coalescing Filter: This second stage removes the fine liquid aerosols and sub-micron particles. Learn more in our guide to Types of Air Compressor Filters and How they Work.
3. Carbon Filter: This final stage removes oil vapour (a gas) to eliminate odours and tastes. A carbon filter must follow a coalescer, as liquid oil will foul the media and destroy it.

Proper filter staging – particulate, coalescing, then carbon – is key to maintaining efficiency. It also helps you to achieve ISO-compliant air quality over time.

How Do Coalescing Filters Relate to Air Quality Standards?

To reach the ISO 8573-1:2010 standard for compressed air quality, your system needs coalescing filters in place. Fitting them into your system helps ensure the air stays clean and compliant. 

This standard uses a three-digit code to show how pure the air is for [Solid Particulates: Water: Oil].

A high-efficiency coalescing filter can remove oil to Class 1 purity – just 0.01 mg/m³.

This performance supports both BS EN 12021 for breathing air and Good Manufacturing Practice (GMP) standards. To stay compliant, a professional should test the air quality and breathing air regularly – this helps to confirm the filters meet the required levels.

What is a Coalescing Filter

What Coalescing Filters Does Atlas Copco Offer?

As a Premier Distributor for Atlas Copco, Search Air provides a range of high-performance filters designed to work as an integrated system.

• DD+ Series: General-purpose pre-filter removing particles $\geq$ 1 $\mu$m and oil aerosols $\leq$ 0.1 mg/m³.
• PD+ Series: High-efficiency coalescer removing particles $\geq$ 0.01 $\mu$m and oil aerosols $\leq$ 0.01 mg/m³ (ISO Class 1).
• QD+ Series: Activated carbon stage removing oil vapour down to 0.003 mg/m³.

The correct sequence for highest purity is DD+ $\rightarrow$ PD+ $\rightarrow$ QD+. These air drying and filtration solutions form a complete Atlas Copco system supplied and maintained by Search Air. You can buy air compressor filters for all major brands directly.

When Does a Coalescing Filter Need Maintenance?

Replace coalescing filters regularly. The typical interval is every 4,000–8,000 hours or when the pressure drop exceeds 8–10 psi.

Filters that coalesce are things that can be used up. You can’t clean them and use them again.

Using a clogged filter past its useful life causes high-pressure drops and wastes energy. It also puts your filter at risk of failing, which lets contaminants that have been caught back into your system.

Routine replacement reduces energy loss and prevents unplanned downtime. Search Air includes these critical filters in all planned maintenance Service Kits.

What Happens to the Drained Contaminants?

Condensate is the oily water that collects after the filtration process. It’s classed as hazardous waste – making it illegal to discharge it straight into a drain or sewer.

You need to use an oil-water separator on this liquid. These systems strip the oil out of the water so it can be disposed of safely and according to the law.

Learn more about oil-water separators and how to handle condensate to stay in compliance. Handling it the right way protects the environment and keeps you compliant with UK wastewater regulations.

A coalescing filter is an important part of keeping compressed air systems safe. It needs to be the right size, installed correctly, and kept up with to make sure the product is of good quality, the system works well, and the law is followed.

Keep your equipment safe and the air clean by following ISO standards. Call Search Air today to set up an air quality audit or talk about your needs for compressed air filtration.