Coalescing Filters: Protecting Downstream Equipment

A coalescing filter, also known as a coalescer filter, plays a critical role in compressed air systems by removing contaminants before they can damage downstream equipment. Its primary function is to separate different petroleum-based liquids, liquid aerosols, but it is also effective at eliminating water droplets and solid particulates.

By purifying the compressed air, these filters safeguard essential components such as dryers, valves, and pneumatic tools. Companies, particularly factories in South Wales and the West, rely on these filters to maintain their production standards.

What Is a Coalescing Filter?

A coalescing filter separates liquid and solid contaminants from a gas stream. It forces small droplets to merge into larger drops. Gravity drains these larger drops to the bottom of the filter housing. This stops oil and water going downstream. Coalescing filters do not remove oil vapour. They remove liquid droplets only.

What Is the Difference Between Aerosols and Vapours?

Service providers distinguish between aerosols and vapours. Aerosols are liquid droplets suspended in the air stream. These range in size from 0.01 to 10 microns (μm). Vapours are contaminants in a gaseous state. Oil vapour molecules are smaller than 0.001 microns.

It’s important to understand this distinction, as it affects the type of filtration required. Coalescing filters remove aerosols. Vapours pass through the filter media. An “oil smell” indicates the presence of vapour. Oil vapour requires removal via adsorption. Activated carbon filters perform this task. Oil vapour can recondense on cold pipework. This creates contamination downstream. Vapour also bypasses desiccant dryers. This leads to false dewpoint readings.

How Does Filtration Physics Work?

Coalescing filtration use borosilicate glass microfibres. These fibres catch particles through three specific mechanisms.

Diffusion (Brownian Motion)

Diffusion captures particles below 0.1 microns. These particles move erratically due to thermal energy. This Brownian motion causes them to collide with filter fibres.

Interception

Interception captures particles between 0.1 and 1 micron. These particles follow the airflow lines. They graze fibres when passing within one radius of the surface.

Inertial Impaction

Impaction captures particles larger than 1 micron. These particles possess high momentum. They travel in straight lines. They strike fibres directly.

Why Does MPPS Matter in Media Design?

The Most Penetrating Particle Size (MPPS) is between 0.3 and 0.6 microns. Capture efficiency is lowest in this range. Filter media is designed specifically to target this size. Atlas Copco elements achieve >99.97% efficiency at the MPPS. The media is treated to be oleophobic and hydrophobic. This repels oil and water. It prevents fibres from swelling. The drainage layer on the exterior stops liquid re-entrainment.

What Filter Technologies Does Atlas Copco Offer?

Control Gear supplies the Atlas Copco filtration range. Each series targets specific contaminant levels.

DD+ Series (General Purpose)

The DD+ removes liquid aerosols to 0.1 mg/m³. It traps particles down to 1 micron. This filter typically follows the water separator.

PD+ Series (High Efficiency)

The PD+ reduces oil aerosol content to 0.01 mg/m³. It captures particles down to 0.01 microns. The PD+ requires upstream protection from a DD+ unit. Bulk liquid will saturate the PD+ rapidly.

UD+ Series (Nautilus Technology)

The UD+ series uses a spiral-wound geometry. This combines the coarse grade of a DD+ and the fine grade of a PD+ into one element. The spiral path increases residence time inside the element. This improves coalescence at lower differential pressure. The UD+ achieves a 40% lower pressure drop than a DD+/PD+ combination. It reduces the installation space by 50%. It matches the filtration performance of the PD+.

QD/QDT Series (Activated Carbon)

The QD series removes oil vapours and odours. It reduces total oil content to 0.003 mg/m³. This filter must follow a coalescing filter. Liquid oil destroys activated carbon media.

How Should the System Be Designed?

Correct filter hierarchy is necessary for system function.

1. Water Separator: Removes bulk condensate.
2. Coalescing Filter (UD+ or DD+): Removes aerosols.
3. Air Dryer: Removes water vapour.
4. Particulate Filter (PDp): Removes desiccant dust.
5. Carbon Filter (QD): Removes oil vapour.

Why Do Desiccant Dryers Fail?

Oil aerosols must be removed before the dryer. If aerosols enter the desiccant tower, the beads become coated. This reduces adsorption capacity. The dewpoint rises. The desiccant must then be replaced.

What Are Common Failure Modes?

Rapid filter saturation often indicates a compressor fault. A blocked scavenge line fills the separator tank with oil. This sends oil slugs downstream. Slugs overwhelm the coalescing element. Older housings may suffer from a “wet band”. Turbulence re-entrains liquid from the sump in these designs. Modern Atlas Copco housings eliminate this issue.

What Are the Compliance Standards?

UK industry uses ISO purity standards. See our guide to compressed air quality filters (/all-you-need-to-know-about-compressed-air-quality-filters/).

ISO 8573-1:2010 Requirements

• Class 1 Oil: ≤0.01 mg/m³. Requires UD+ or PD+.
• Class 2 Oil: ≤0.1 mg/m³. Achievable with DD+.
• Class 4 Water: +3°C dewpoint. Requires Refrigerant Dryer.
• Class 2 Water: -40°C dewpoint. Requires a Desiccant Dryer.

BCAS Best Practice Guideline 102

The British Compressed Air Society defines grades for food and beverage safety.

• Direct Contact: Requires ISO Class [2:2:1].
• Indirect Contact: Requires ISO Class [2:4:2].

What Is the Financial Impact of Filtration?

Filtration strategy affects energy costs. See our expert guide to industrial filtration (/expert-guide-to-industrial-filtration-choosing-the-right-system/).

The Cost of Pressure Drop

Every 1 bar of pressure drop increases compressor energy consumption by 7%. Consider a 75 kW compressor running 8,000 hours per year. Assume an electricity cost of £0.17/kWh. An excess pressure drop of 1 bar wastes approximately £7,140 annually. Clogged filters cost more in electricity than the price of a replacement. Filters should be changed based on running hours. This is typically 4,000 to 8,000 hours. Do not rely solely on pressure gauges. A ruptured element reads zero pressure drop. It provides zero filtration.

How Can Control Gear Support Your System?

Control Gear serves the South Wales and West region. Locations include Cardiff, Newport, Swansea, Bristol, Swindon, and Stroud. See our guide on oil versus oil-free compressors (/oil-versus-oil-free-air-compressors-guide/).

Technical Services

• Supply & Installation: Engineers install Atlas Copco filters and pipework.
• Retrofit Solutions: Control Gear offers HYDAC Betterfit elements. These fit into existing competitor housings. This lowers running costs without replacing the assembly.
• Air Audits: Engineers perform breathing air quality tests.
• Leak Detection: A single 3mm leak costs £1,000–£2,650 per year. Leak surveys identify these losses.

Industries such as pharmaceuticals and automotive rely on these services. Food processing facilities use them to meet compliance.

Coalescing Filters: Applications in Various Industries

Coalescing filters are essential in a wide range of industries where clean, dry compressed air is critical. These filters ensure the removal of oil, water, and other contaminants, safeguarding equipment and processes. Here’s a look at some key applications:

•   Pharmaceuticals: Maintaining sterile air for production and packaging.
•   Food and Beverage: Preventing contamination in food processing and packaging.
•   Automotive: Ensuring clean air for painting and other critical processes.
•   Electronics: Protecting sensitive electronic components from damage.
•   Petrochemical Industries: Removing contaminants from process gases.

Troubleshooting Common Coalescing Filter Problems

Like any piece of equipment, coalescing filters can experience problems. Here are some common issues and how to address them:

• Excessive Pressure Drop: Indicates a clogged filter element. Replace the element.
• Oil Carryover: Suggests a damaged or saturated filter element. Replace the element and investigate the source of the oil.
• Water Carryover: Indicates a problem with the drainage system. Check the drain valve and ensure it’s functioning properly.
• Reduced Filter Life: May be caused by excessive contamination in the compressed air. Consider installing a pre-filter.

What are the Different Types of Coalescing Filter Media?

Coalescing filters utilise various media types, each with unique performance characteristics. The most common include:

• Borosilicate Glass Microfibres: Offer high efficiency and are suitable for a wide range of applications.
• Synthetic Fibres: Provide excellent resistance to chemicals and high temperatures.
• Pleated Media: Offer a large surface area and high dirt-holding capacity.

How Do Coalescing Filters Work?

Coalescing filters work by using a multi-stage process to remove liquid aerosols and solid particulates from a gas stream. Here’s a breakdown of the key steps:

1.  Impaction: Larger droplets and particles collide with the filter media.
2.  Interception: Smaller droplets and particles are captured as they pass close to the filter fibres.
3.  Diffusion: Very fine particles are captured by Brownian motion.
4.  Coalescence: Captured droplets merge to form larger droplets.
5.  Drainage: Larger droplets drain to the bottom of the filter housing and are removed.

What is the Difference Between a Coalescing Filter and a Particulate Filter?

While both types of filters remove contaminants from compressed air, they target different types of contaminants. Coalescing filters are designed to remove liquid aerosols (oil and water), while particulate filters remove solid particles (dust, dirt, and rust). In many compressed air systems, both types of filters are used in combination to achieve optimal air quality.

Frequently Asked Questions (FAQs) About Coalescing Filters

Here are some frequently asked questions about coalescing filters:

What does a coalescing filter do?

A coalescing filter removes liquid aerosols (oil and water) from compressed air.

How does a coalescing filter work?

It forces small droplets to merge into larger drops that can be drained away.

What are coalescing air filters used for?

They are used to protect downstream equipment from damage caused by liquid contaminants.

How often should I change my coalescing filter?

Typically every 4,000 to 8,000 hours, but it depends on the application and the level of contamination.

What are the signs of a failing coalescing filter?

Increased pressure drop, oil carryover, and water carryover are common signs.

Can I clean a coalescing filter?

No, coalescing filters are typically not cleanable and should be replaced when they become saturated.

How does temperature affect the performance of coalescing filters?

High temperatures can reduce the efficiency of coalescing filters.

Conclusion

Coalescing filters protect equipment. They ensure compliance with ISO 8573-1. Technologies like the Atlas Copco UD+ reduce pressure drop. This reduces energy waste. Maintenance must follow running hours.

Call to Action

Contact Control Gear to arrange a filtration assessment. Ask about HYDAC Betterfit upgrades.