Prevent Defects with Proper Compressed Air Filter Maintenance

More than 90% of production facilities worldwide use compressed air as part of their processes. However, this powerful medium is not without problems — compressed air contamination is a serious threat to daily operations. That is why an air purification system is an essential part of any compressed air installation, preventing contamination and its harmful effects. Compressed air filters are a crucial component of this system.

The basis of any purification system is filtration, which is responsible for removing (or reducing to acceptable levels) 9 of the 10 most common contaminants found in compressed air systems.

Coalescing Filters

Coalescing filters take care of 6 of these contaminants. If you look into a typical compressor room, you will find several of them in use. In many cases, these coalescing filters are supported by dry particulate and oil vapour filters, which to the untrained eye often look very similar.

All of these purification technologies use filter housings that contain replaceable filter elements or cartridges.

To ensure consistently high-quality compressed air — and to prevent unnecessary energy consumption — these filter elements must be replaced periodically. Their service life is not unlimited. There are many myths regarding replacement intervals; this article aims to bring clarity.

How Coalescing and Particulate Filters Work

The components of coalescing and particulate filters are almost identical. They generally consist of:

• A housing (filter head and bowl)
• A filter element
• A drain (automatic float drain for coalescing filters, manual drain for particulate filters)

In many cases, differential pressure gauges or indicators are supplied as standard or optional accessories.

Compressed air enters the filter housing and flows from the inside to the outside of the filter element. This traps dirt particles within the inner filter layers, while water and oil aerosols coalesce on the outer water-repellent layer and are discharged via the drain.

Because the filter material is located inside the element, saturation is not visible from the outside — which often causes confusion about when the element should be replaced.

Why Replace a Filter Element That Still Looks Clean?

Compressed air filters operate under extreme conditions:

• Pressure fluctuations
• Pulsating air demand
• Temperature swings (day/night & summer/winter)
• Humidity variations (from 100% saturated air to extremely low dew points)
• Chemical attack by acidic condensate and lubricant additives

Coalescing filters are constantly soaked in oily condensate, while particulate filters operate under very dry conditions. Over time, these stresses weaken and degrade the filter material.

Every filter element has a finite service life. A tiny pinhole can have catastrophic consequences: compressed air will always take the path of least resistance. Once damaged, air will break through and tear the material further, allowing contamination into the compressed air system.

Sudden pressure surges can also rupture filter material. This is why filters should always be pressurised gradually by opening valves slowly.

Recommended Replacement Interval

It is recommended to replace filter elements every 12 months to prevent damage to your compressed air system — even if the system does not run 24/7.

Because filter elements are continuously exposed to acidic condensate, they begin degrading from the moment they are installed.

Only if a differential pressure gauge indicates clogging should the element be replaced earlier than the recommended 12 months.

The Limitations of Differential Pressure Gauges

Many filter housings are fitted with a differential pressure gauge or indicator. These devices show pressure drop via a moving needle, pop-up indicator, or digital display.

Although common, their accuracy and purpose are often misunderstood:

• Accuracy is typically only ±25%
• Calibration certificates are not available
• Indicators may show coloured zones (green/orange/red), but can give a false sense of security

A ruptured filter element produces almost no pressure drop — meaning the indicator will still show “green” while contamination passes through into the system.

Why Use a Differential Pressure Gauge at All?

The true purpose of a differential pressure gauge is to detect clogging at an early stage. However, the 12-month replacement interval remains essential.

Frequently Asked Questions About Filter Maintenance

Can filter elements be washed?

No. Contamination cannot be rinsed out of the filter material. Exposure to soap, solvents, or hot water will cause damage.

Why replace my filter element if it doesn’t look dirty?

What you see is only the water-repellent outer layer. The main filter medium is inside, trapping aerosols and particles down to submicron sizes (1 micron = 1 millionth of a metre). The human eye can only see particles around 40–50 microns — so blockages or damage are invisible.

Do I need to replace the float drain when changing the element?

No, but it is necessary to clean the float with hot water and a degreaser when replacing the element. This prevents clogging and ensures proper drainage. The float system is delicate and can be easily disturbed if handled incorrectly. Replacing the float during maintenance minimises the risk of malfunction.