Step one is not choosing a filter — it is identifying the problem
Most engineers approach gas filtration by looking at products first: housings, elements, specifications. But the most important question comes before all of that: what exactly is contaminating your gas?
The answer determines everything — the filter type, the media, the element grade, and whether one stage is enough or you need multiple. Get the diagnosis wrong, and no amount of filtration will solve the problem.
The three types of gas contamination
All gas contamination falls into three fundamental categories. Each requires a different filtration mechanism:
Solid Particulate
Dust, rust, scale, pipe debris, weld spatter, catalyst fines. Discrete solid particles that can be trapped by a physical barrier.
Liquid Aerosol
Oil mist, water fog, hydrocarbon condensate, electrolyte droplets. Sub-micron liquid droplets suspended in the gas stream like a fog.
Vapour / Gas Phase
Oil vapour, solvent vapour, odour compounds, chemical traces. Gaseous contaminants dissolved in the carrier gas — invisible, pass through any mechanical filter.
Why this distinction matters
Particulate elements trap solids by mechanical filtration. Coalescing elements capture and drain liquid aerosol by coalescence. Adsorbers (activated carbon) remove vapour-phase contaminants by adsorption. Using the wrong type for your contaminant is like using a sieve to catch fog.
How to identify what you are dealing with
In many cases, you can identify the primary contamination type from observable symptoms — without laboratory analysis:
| Symptom | Contamination Type | Filter Required |
|---|---|---|
| Black, brown, or grey deposits in pipework or on surfaces | Solid particulate (rust, soot, scale) | Particulate element |
| Visible haze or mist at exhaust or vent points | Liquid aerosol (oil or water mist) | Coalescing element |
| Liquid pooling in downstream equipment, drains, or low points | Heavy liquid loading (condensate or oil) | Coalescing element + bulk separator |
| Oily smell in compressed air or exhaust stream | Oil vapour (gaseous phase) | Activated carbon adsorber |
| Instrument drift, sensor failure, false analyser readings | Mixed (particulate + aerosol) | Multi-stage: particulate + coalescing |
| Rapid filter blockage (weeks instead of months) | Heavy particulate or wrong element type | Pre-filter + correct element grade |
| Sticky residue on equipment surfaces | Aerosol + vapour combination | Coalescing + activated carbon |
Particulate contamination in detail
Solid particles are the simplest contamination to understand and the most common in pipeline systems. Typical sources include:
- Pipe corrosion: Rust and scale from carbon steel piping — the most common source
- Construction debris: Weld spatter, thread sealant, pipe shavings from installation or maintenance
- Compressor wear: Metal particles from bearings, valves, and piston rings
- Process contaminants: Catalyst fines, powder, product particles carried by the gas
Particulate elements are available in grades from PF (coarse, >5 µm) through UX (ultra-fine, 99.9999% at 0.01 µm). The grade you need depends on what you are protecting downstream.
Liquid aerosol contamination in detail
Liquid aerosol is the most frequently misunderstood contamination type. These are sub-micron droplets — typically 0.1 to 1.0 µm — that behave like a gas rather than visible liquid. They pass straight through particulate elements.
Common sources include:
- Compressor oil carry-over: Oil-lubricated compressors inject oil aerosol into the discharge
- Condensation: Water vapour condensing as temperature drops below dewpoint
- Process liquids: Electrolyte droplets (electrolysers), cooling water (scrubbers), hydrocarbon condensate
- Vacuum pump exhaust: Oil mist from the sealing oil
The particulate trap
A particulate element will capture some large droplets — giving the impression it is working. But the sub-micron aerosol passes straight through. Within weeks, downstream equipment shows oil contamination despite the filter looking clean. This is the most common misdiagnosis in gas filtration.
Vapour-phase contamination in detail
Oil vapour and chemical vapours exist in the gas phase — they are invisible, odourless (usually), and pass through any mechanical filter. Neither particulate nor coalescing elements can remove them.
Activated carbon adsorption is the standard solution. Carbon adsorbers use the enormous internal surface area of activated carbon granules to capture vapour molecules by physical adsorption.
Adsorbers need protection
Activated carbon adsorbers must always follow a coalescing filter. If liquid aerosol reaches the carbon bed, it floods the pore structure and destroys the adsorption capacity almost immediately. The sequence must be: particulate → coalescer → adsorber.
When you have multiple contamination types
In practice, most gas streams contain a combination of contamination types. Compressed air from an oil-lubricated compressor carries particulate, oil aerosol, water aerosol, and oil vapour simultaneously. The solution is multi-stage filtration in the correct sequence:
Stage 1: Pre-filter
Removes bulk particulate and large liquid slugs. Protects downstream elements from premature loading.
Stage 2: High-efficiency coalescer
Removes sub-micron liquid aerosol (oil and water) to below 0.01 mg/m³. The workhorse of any treatment system.
Stage 3: Activated carbon adsorber
Removes oil vapour and chemical traces. Only needed when vapour-phase contamination must be controlled.
Key Takeaway
Before selecting a filter, identify the contamination: solids need particulate elements, aerosols need coalescers, vapours need adsorbers. Most real-world applications need at least two stages. The diagnostic table above covers the most common symptoms — if yours matches, the filter type follows directly.
Know your contamination? Find the right filter.
Select your application and contamination type in the Engineering Tool — it recommends the correct housing, element, and filtration stage.
