Compressed Air Filter Selection – Air Compressor Manufacturers

Select wrong compressed air filter, two things can happen. One is equipment damage, cylinder scoring, valve core wear, seals fail prematurely. Costs money and time to fix. Other is product rejection, paint with fish eyes, residue on circuit boards, food fails inspection. Losses even bigger. Before selecting filter element, first figure out which consequence you're afraid of. Decisions after that are easier.

ISO 8573-1 divides compressed air quality into classes. Particles, moisture, oil each have a set of numbers. Equipment manual says "supply air requirement Class 1.4.1", that's referencing this standard. Filter manufacturers also use same system when marking performance. Easy to compare.

Where Filter is Installed, Easier to Get Wrong Than Which Model to Select

Many people obsess over which brand, which class of filter element. Actually installing in wrong location is more common problem.

Industrial compressed air filtration system

Filtration Systems

Compressed Air Treatment Installation

Before dryer needs coarse filtration. Refrigerated dryer relies on cooling to condense water vapor. If inlet carries lots of liquid water and oil droplets, internal separator can't handle it. Dew point control fails, heat exchange surfaces get coated with oil film. Coarse filter's job is block this stuff for the dryer. Filtration precision 3 micron, residual oil 5 mg/m³ or so. Doesn't sound impressive. Not meant to produce clean air anyway.

After dryer install T-grade element. Dryer itself produces contamination. Desiccant powder, internal component wear particles, these things need to be caught before entering pipe network. T-grade output has oil content around 0.5 mg/m³, particles controlled at 1 micron. Ordinary cylinders and valves, this level is enough.

At use point, situation changes. From compressor station to paint booth might be dozens of meters of pipe. In between are elbows, fittings, reducers, dead legs. Pipe wall oxide scale will flake off. Thread sealant will fragment. Dead legs will condense and grow stuff. Central filtration no matter how good, can't control what happens along this stretch. So high-requirement use points need their own precision filter. 0.01 micron type, right up against equipment inlet.

Activated carbon element location matters even more. It handles gaseous oil molecules, not droplets. Liquid oil goes in, adsorption sites get saturated. Carbon bed ruined in days. Must install after precision filter. Let upstream handle liquid oil first.

Coalescence and Adsorption Are Two Different Things

Don't understand difference between these two mechanisms, easy to get confused on selection.

Coalescence

Microfiber Separation

Adsorption

Carbon Bed Filtration

Coalescing element inside is glass microfiber. Small oil droplets attach to fibers, merge with other droplets, keep getting bigger. Big enough for gravity to pull it down, it drips. Eventually collects at filter housing bottom and drains out. T-grade and A-grade elements both this principle. Difference is fiber coarseness and layer count. Finer fibers catch smaller droplets, pressure differential also higher.

Adsorption completely different. Activated carbon surface full of micropores. Oil vapor molecules drill in, stick to pore walls by intermolecular forces. This process produces no liquid. Pressure differential barely changes. When carbon bed adsorption capacity exhausted, you can't tell from pressure gauge. Can only judge by time or cumulative air volume when to change.

Understanding this difference, you know why gaseous oil must use activated carbon. Coalescing element can catch oil mist down to 0.01 mg/m³, but gaseous portion passes straight through, no resistance at all. What food and pharmaceutical plants care about is precisely this part. Gaseous oil brings odor, no matter how low the content, affects product.

Combination Configuration Has No Standard Answer

Configuration tables you can find online are all "pneumatic tools use C+T, painting use C+T+A, food use C+T+A+H." These are starting points, not endpoints.

Pneumatic tools situation is simplest. C-grade protects dryer, T-grade protects tools, two stages enough. But if tools are marked "oil-free operation," T-grade output of 0.5 mg/m³ might still be too high. Need to add precision filter after. Compressor oil and lubricating grease specified when tool was designed might not be compatible. Sometimes air carrying a bit of oil actually flushes out the original grease.

Paint spraying very sensitive to oil contamination. Silicone compounds especially troublesome. Cause crater defects in paint film. Volcano-crater shaped depressions. Touch-up paint can't cover it. Whole part needs rework. 0.01 mg/m³ A-grade filtration is basic requirement. Each paint booth gets its own set. Don't try to save money relying on central filtration.

Automotive

Paint Booth

Pharmaceutical

Clean Production

Electronics

Circuit Assembly

Food and electronics industries go four-stage full set. Logic similar but focus points different. Food cares about odor transfer. Electronics cares about organic residue interfering with soldering and conformal coating adhesion. Activated carbon stage can't be skipped.

Pharmaceutical thinking is a bit different. Rather than fighting hard to filter downstream, better to use oil-free compressor from the source. Scroll type, dry screw, centrifugal. No oil going in, downstream pressure much less. Downstream C, T, A grades mainly handle atmospheric particles and trace contamination that compressor bearing seals might bring. Activated carbon actually might not be needed.

Pressure Differential Matters

Filter element blocked, pressure differential rises. That's common knowledge. But when to change, what happens if changed too early or too late, many people haven't calculated this bill.

<1.5 psi

New Element ΔP

7 psi

Attention Threshold

10 psi

Must Change

New element installed, at rated flow, pressure differential usually under 1.5 psi. If higher right from installation, either sized too small, or installed backwards, or element was defective from factory. First time installed, note the number. Have baseline for future comparison.

When differential hits 7 psi start paying attention. At 10 psi must change. Running past 10 psi, filter media has risk of rupture. Once element breaks, accumulated dirt releases concentrated into downstream. Paint booth might have whole day's parts needing rework.

Changing too late costs more than just that. Every 1.5 psi increase in differential, compressor uses about 1% more power to maintain system pressure. One filter dragging at 9 psi differential for three months, extra electricity cost might buy several new filter elements.

Activated carbon element is exception. When carbon bed saturated, pressure differential change very small. Can't use differential gauge to judge. Some manufacturers install oil vapor detector downstream of carbon bed. Breakthrough triggers immediate alarm. Without detector, can only change by time. Manufacturer says how long, follow it. Be conservative.

Few Easily Overlooked Details

Automatic drain at filter housing bottom often gets forgotten. Float type jams and doesn't drain, liquid fills up then gets re-entrained by airflow. Previous filtration wasted. Timer-controlled drain, if timing set wrong, either doesn't drain clean, or leaks too much air. Some plants use electronic level-sensing drains. More expensive but less hassle.

Filter element installation direction can't be wrong. Coalescing element airflow goes from inside out through fiber layer. Installed backwards, coalescing effect greatly reduced. Some elements can't tell direction by appearance. Can only rely on housing arrows or manual.

Precision filter and activated carbon filter have requirements on upstream air quality. Manual says "inlet oil content must not exceed so-and-so," not written casually. Exceeded, element life greatly shortened. Activated carbon especially sensitive to this. Liquid oil goes in, breakthrough in days.

When changing filter element, record differential and hours used. After accumulating a few times, can figure out the pattern. Some stations have heavy contamination, change frequently. Some stations are clean, can extend interval. Adjusting based on actual data is more economical than one-size-fits-all following manufacturer recommended interval.

Starting Point for Selection

Back to the question at beginning: afraid of equipment damage or afraid of product rejection?

Afraid of equipment damage, focus on protecting pneumatic components and dryers. C+T two-stage configuration, differential monitoring in place, change elements on schedule. Basically enough.

Afraid of product rejection, work backwards from use point. Paint, electronics, food, pharmaceutical each has its own contamination sensitivity points. Figure out specifically whether afraid of particles, afraid of oil mist, or afraid of oil vapor. Then decide what level to configure to. Install filters individually at use points. Don't try to save trouble relying entirely on central filtration.

Filtration stage in compressed air system looks insignificant. But select wrong or install wrong or don't change on time, cost shows up on equipment repair bills and product rejection rates.