3 Types of compressed air filters

July 21, 2016

6 minutes

Filters play a vital role in the compressed air process. Depending on the end use, strict purity standards require a variety of contaminants to be removed, including oil aerosols, vapors and particulates. Contaminants can enter the compressed air from a variety of sources. Intake air can introduce dust or pollen particulates, while corroded pipes can add harmful particulates from within the compressor system. Oil aerosols and vapors are often a byproduct of using oil-injected compressors and must be filtered out before end use. There are distinct purity requirements for different compressed air applications, but the presence of contaminants can surpass acceptable levels, leading to damaged products or unsafe air. Filters fall into three categories: coalescing filters, vapor removal filters and the dry particulate filters. While each type ultimately produces the same result, they each operate on different principles.

Why compressed air filters are essential to your system

Even in the cleanest industrial environments, compressed air is never truly pure at the point of compression. The air drawn into a compressor contains naturally occurring contaminants including water vapor, dust, pollen, and atmospheric microorganisms. The compression process itself concentrates these contaminants, and in oil-injected compressors, introduces oil aerosols and vapors into the air stream as well. Without the right compressed air filters in place, these contaminants travel through your piping system and into your tools, processes, and products.

The consequences of unfiltered or inadequately filtered compressed air range from accelerated wear on pneumatic equipment and increased maintenance costs, to product contamination, failed quality inspections, and in regulated industries such as food and beverage or pharmaceuticals, serious compliance risks. This is why selecting and maintaining the right compressed air filters for your specific application is not optional, it is a fundamental requirement of a well-designed compressed air system.

International standards such as ISO 8573 define the acceptable levels of contaminants in compressed air for different quality classes, covering solid particles, water, and oil content. Understanding which ISO 8573 purity class your application requires is the most reliable starting point for determining which compressed air filters you need and where they should be installed in your system.

Coalescing filters: Coalescing filters are used for removing water and aerosols. Small droplets are caught in a filter media and merged into larger droplets that are then taken out of the filter. A re-entrainment barrier prevents these droplets from reentering the air. Most of the liquid coalescing filters remove is water and oil. These filters also remove particulates from compressed air, trapping them within the filter media, which can lead to pressure drops if not changed regularly. Coalescing filters remove most contaminants very well, reducing particulate levels down to 0.1 micron in size and liquids down to 0.01 ppm.

A mist eliminator is a low-cost alternative to a coalescing filter. While it does not produce the same level of filtration as coalescing filters, a mist eliminator offers a smaller pressure drop (about 1 psi), allowing systems to operate at lower pressure, thus saving on energy costs. These are typically best used with liquid condensate and aerosols in lubricated compressor systems.

Vapor removal filters: Vapor removal filters are typically used to remove gaseous lubricants that will go through the coalescing filter. Because they use an adsorption process, vapor removal filters should not be used to capture lubricant aerosols. Aerosols will quickly saturate the filter, rendering it useless in a matter of hours. Sending air through a coalescing filter prior to the vapor removal filter will prevent this damage. The adsorption process uses activated carbon granules, carbon cloth or paper to capture and remove contaminants. Activated charcoal is the most common filter media because it has a large open pore structure; a handful of activated charcoal has the surface area of a football field.

Dry particulate filters: Dry particulate filters are usually employed to remove desiccant particles after an adsorption dryer. They can also be implemented at point of use to remove any corrosion particles from the compressed air. Dry particulate filters operate in a similar manner as a coalescing filter, capturing and retaining particles within the filter media.

How to choose the right compressed air filters for your application

With three distinct types of compressed air filters available, each targeting different categories of contamination, selecting the right combination for your system requires an understanding of both your contamination profile and your air quality requirements. Here is a practical guide to help you choose:

If your primary concern is liquid water and oil aerosols, a coalescing filter is your first line of defense. Coalescing filters should be installed upstream of other filtration and treatment equipment, including dryers and vapor removal filters, to protect them from liquid ingestion and premature saturation. For systems using oil-injected compressors, a coalescing filter is always recommended.

If your application requires the removal of oil vapors and odors from the compressed air, a vapor removal filter using activated carbon media should be installed downstream of the coalescing filter. This sequence is critical: the coalescing filter removes liquid aerosols first, protecting the vapor removal filter from being rapidly saturated and rendered ineffective.

If your system includes a desiccant dryer, a dry particulate filter installed at the dryer outlet will capture any desiccant dust or particles that may have been carried over into the air stream. Dry particulate filters are also useful at the point of use in systems with older or corroded pipework, where rust particles and pipe scale can contaminate the air close to the end application.

For most industrial compressed air systems, a combination of coalescing and dry particulate filtration provides an effective baseline level of air treatment. Applications with more stringent air quality requirements, such as food contact, pharmaceutical production, or breathing air, will typically require the full sequence of coalescing, vapor removal, and dry particulate filtration, along with appropriate drying equipment, to meet the relevant purity standards.

Knowing the needs of your compressed air system can help you chose the right filter. Whether your air need a high level of filtration or basic contaminants removed, cleaning your air is an important step in the compressed air process. Check out Atlas Copco’s line of filters today or call a representative and learn how Atlas Copco can help you achieve cleaner, safer air.

Frequently asked questions

What are compressed air filters used for?

Compressed air filters are used to remove contaminants from compressed air before it reaches downstream equipment, tools, or processes. Contaminants in compressed air can include water, oil aerosols, oil vapors, dust, pollen, pipe scale, desiccant particles, and atmospheric microorganisms. Depending on the application and the required air purity level, one or more types of compressed air filters may be needed to achieve the necessary air quality. Without proper filtration, contaminants can damage equipment, reduce product quality, and create compliance risks in regulated industries.

Where should compressed air filters be installed in a compressed air system?

The placement of compressed air filters within the system depends on the type of filter and what it is designed to remove. Coalescing filters should be installed as close to the compressor outlet as practical, and always upstream of dryers and vapor removal filters, to remove bulk liquids before they can damage downstream equipment. Vapor removal filters should be installed downstream of the coalescing filter. Dry particulate filters are typically installed at the outlet of desiccant dryers to capture desiccant carryover, and at point-of-use locations to remove any particulates that may have entered the distribution pipework. For guidance on the optimal filter placement for your specific system, consulting a compressed air expert is always recommended.

How often should compressed air filters be replaced?

Compressed air filter elements should be replaced on a regular schedule, typically every 12 months or sooner if the pressure drop across the filter increases significantly. A clogged or saturated filter element creates excessive pressure drop, which forces the compressor to work harder and increases energy consumption. For coalescing filters, a differential pressure indicator is a useful tool for monitoring filter condition and identifying when replacement is needed before pressure drop becomes problematic. Vapor removal filters should also be replaced on schedule, as saturated activated carbon loses its adsorption capacity and can begin releasing captured contaminants back into the air stream.

What is the difference between a coalescing filter and a dry particulate filter?

Both coalescing filters and dry particulate filters capture and retain particles within their filter media, but they are designed for different types of contamination. A coalescing filter is specifically designed to capture liquid contaminants, including water droplets and oil aerosols, by merging small droplets into larger ones that drain away from the filter. A dry particulate filter, by contrast, is designed to capture solid particles such as desiccant dust, rust, and pipe scale, and is not intended for liquid removal. Using the right filter type for the right contaminant is essential to achieving effective filtration and protecting downstream equipment.

Do I need compressed air filters if I have an oil-free compressor?

Yes. While an oil-free compressor eliminates the risk of oil contamination from the compression process, it does not eliminate all contaminants from the compressed air. Atmospheric air drawn into the compressor still contains water vapor, dust, pollen, and other particulates that must be removed before the air reaches sensitive processes or equipment. Coalescing filters and dry particulate filters are still recommended for oil-free compressor installations, and depending on the air quality requirements of the application, additional filtration and drying equipment may also be necessary.

How do compressed air filters affect energy efficiency?

Compressed air filters have a direct impact on system energy efficiency through the pressure drop they create across the filter media. A clean, properly sized filter creates a minimal pressure drop, allowing the system to operate efficiently. However, as filter elements become clogged with captured contaminants over time, the pressure drop increases, forcing the compressor to generate higher pressure to compensate, which increases energy consumption. Regularly replacing filter elements and selecting correctly sized filters for your system flow rate are two of the most effective ways to minimize the energy impact of compressed air filtration and keep operating costs under control.