Clean compressed air is essential for meeting UK quality standards.
It is also vital for preventing expensive damage to your downstream equipment.
Facilities across Scotland rely on consistent air quality. When that stability slips, the consequences follow quickly – contaminated product, failing equipment, and audit issues that are hard to explain away.
Coalescing filters are the critical barrier in your system. They get rid of aerosols before they have a chance to hurt your dryers and tools.
This guide tells you how these filters work, what the Atlas Copco UD+ technology can do for you, and how to stay in line with ISO 8573-1.
What Is a Coalescing Air Filter?
A coalescing air filter is a purification device. Its job is to remove liquid oil, liquid water, and fine particulate matter from compressed air.
Its primary purpose is protection. It protects your dryers, instruments, and pneumatic tools from contamination that comes from below.
It’s important to be clear about what each filter can – and can’t – deal with.
- Coalescing Filter: Removes solid particles and liquid aerosols, including water droplets and oil mist.
- Activated Carbon Filter: Targets odours and oil vapour – contaminants that remain in a gaseous state.
A common mistake is assuming a coalescing filter will deal with the smell of oil. It won’t. Coalescing media can stop liquid aerosols, but oil vapour passes straight through, unchanged.
If you’re aiming for breathable air or food-grade purity, the setup has to go further – a coalescing filter paired with an adsorption (carbon) unit, working together.
How Does a Coalescing Filter Work?
The filter forces compressed air through a deep bed of specialised glass fibres.
The filtration process relies on three distinct physical mechanisms. These simply describe how different particle sizes behave inside the media.
- Diffusion: At this size – below 0.1 micron – particles don’t behave predictably. They jitter and wander rather than flowing straight through. Eventually, one of those random movements brings them into contact with a fibre, and that’s where they’re captured.
- Interception: Medium-sized particles flow near fibres and get caught, like burrs sticking to fabric.
- Inertial Impaction: Heavier particles (>1 micron) cannot change direction fast enough. They crash directly into the media and get trapped.
Why the MPPS Matters
You might hear engineers talk about the MPPS – the Most Penetrating Particle Size. It usually sits between about 0.3 and 0.6 microns.
Particles in this range are the most difficult to capture. They fall into a narrow gap where the usual trapping mechanisms are least effective. Higher-grade filter media is designed with this in mind, tightening that gap so those particles don’t slip through.
The Coalescing Process
Once captured, small liquid droplets merge with others. This merging process is called coalescence.
The droplets grow in size until gravity pulls them down to the bottom of the filter bowl. An automatic drain then ejects this liquid condensate from the system.
What Are the Benefits of Atlas Copco UD+ Technology?
Design Air (Scotland) Ltd supplies Atlas Copco compressed air filters because of their engineered efficiency.
Standard setups often use two filters: a DD+ (general purpose) followed by a PD+ (high efficiency). The Atlas Copco UD+ Series replaces this two-stage train with a single unit. It uses “Nautilus” technology.
How Does Nautilus Technology Work?
The UD+ filter uses a unique spiral-wound design. This geometry reduces the density of the media without losing efficiency.
Crucially, the spiral shape reduces the “wet band” at the bottom of the element. This prevents liquid droplets from being picked up again by the airflow.
What Does This Mean for Your Operation?
- Lower Energy Bills: The UD+ achieves a 40% lower pressure drop compared to a traditional two-filter setup.
- Space Savings: It combines two filtration grades into one housing. This reduces your installation footprint by 50%.
- High Efficiency: It maintains oil aerosol removal down to 0.01 mg/m³.
Why Is Pressure Drop Critical for UK Businesses?
Pressure drop is a direct financial cost.
Every restriction in your air line forces the air compressor to work harder. It needs more power to maintain the same system pressure.
The 7% Rule
Add unnecessary pressure drop and the compressor pays for it. An extra 1 bar – around 14.5 psi – typically pushes energy consumption up by about 7%.
The Cost of Inefficiency
Run a 75 kW compressor more or less continuously – say 8,000 hours across the year – and the energy bill soon dominates the conversation. At today’s electricity prices, hovering around £0.17 per kWh, you’re looking at something in the region of £102,000 just to keep it running.
Now introduce clogged or inefficient filters. A 1 bar pressure drop doesn’t look dramatic on paper, but it forces the compressor to work harder – wasting around 7% of that energy for no useful gain.
That equals £7,140 per year in wasted electricity. The cost of a new filter element is a fraction of this wasted energy.
What Are the Compliance Standards (ISO 8573-1 & BCAS)?
UK facilities are required to meet defined air quality standards.
The main reference point is ISO 8573-1:2010 – a classification system that grades compressed air based on solid particles, moisture content, and total oil.
Oil Class Limits
- Class 1: Total oil concentration ≤ 0.01 mg/m³. This is required for critical applications like food contact and pharmaceuticals.
- Class 2: Total oil concentration ≤ 0.1 mg/m³. This is the standard for general pneumatic power.
BCAS Best Practice Guideline 102 (Food & Beverage)
The British Compressed Air Society (BCAS) dictates specific purity levels for food production.
- Direct Contact: Air touching food must meet Class [2:2:1]. This requires a desiccant dryer to achieve Class 2 Water (-40°C dewpoint). A refrigerant dryer (Class 4 Water) is not compliant here.
- Indirect Contact: Air exhausted into the production room must meet Class [2:4:2].
Design Air (Scotland) Ltd provides ISO 8573-1 air quality testing to verify your system meets these legal obligations.
Where Should Filters Be Installed in the System?
Correct placement is essential for protecting your investment. A typical filter train follows this sequence:
- Water Separator (WSD): Removes bulk liquid water.
- General Coalescing Filter (UD+ or DD+): Removes aerosols and protects the dryer.
- Air Dryer: Removes water vapour.
- High-Efficiency Particulate Filter (PDp): Catches dust released by desiccant dryers.
- Activated Carbon Filter (QD): Removes oil vapours and odours.
What Is the “Death of Desiccant”?
You must never install a desiccant dryer without a high-efficiency coalescing pre-filter.
If oil aerosols reach the dryer, the oil coats the desiccant beads. This blocks the microscopic pores required for adsorption.
The result? The dryer stops removing moisture, the dewpoint crashes, and the media requires expensive replacement.
When Should You Replace the Filter Element?
Filter elements don’t last indefinitely. They’re consumables, and once they load up, performance drops away.
For that reason, manufacturers usually define a service window of roughly 4,000 to 8,000 operating hours, with an annual change acting as a sensible fallback.
Why Can’t I Rely on the Gauge?
Many operators wait for the differential pressure gauge to turn red before changing filters. This is dangerous.
Pressure surges can rupture a saturated filter element. If the media tears, air flows through the hole with zero resistance.
The gauge drops to zero (green), falsely indicating a clean filter. In reality, contaminants are bypassing the filter entirely.
How Do I Troubleshoot Common Issues?
High Oil Carryover
If a new filter saturates immediately, check the compressor’s scavenge line.
A blocked orifice or failed scavenge check valve prevents oil from returning to the airend. This forces large “slugs” of liquid oil downstream, overwhelming the coalescing filter.
Condensate Buildup
Any liquid that’s been coalesced still has to go somewhere. That’s why the auto-drain on the filter bowl matters.
If it isn’t working, liquid builds up in the bowl. Once the level gets high enough, the airflow picks it up again. From there, it’s carried out of the filter and sent straight downstream.
Final Thoughts
Coalescing filters are not optional accessories. They are the primary defence for your pneumatic machinery and product quality.
Upgrading to Atlas Copco UD+ technology reduces energy bills by lowering pressure drop. It also simplifies your maintenance schedule.
Design Air (Scotland) Ltd stocks replacement filter elements at our Airdrie headquarters.
We support facilities across Glasgow, Edinburgh, Dundee, Fife, Perth, and Stirling, providing rapid maintenance support alongside detailed air quality audits.
Ensure your compliance and protect your equipment.
Contact Design Air (Scotland) Ltd today for a filter inspection or site audit.
