Compressed air leaves a compressor hot, wet, dirty and oily. Everything downstream, cylinders, valves, tools and the product itself, is damaged by one or more of those four. Air preparation is not an accessory bolted on at the end of the design; it is the difference between seals that last years and seals that last months.
This guide covers what is actually in the air, what each stage of preparation removes, where the units belong in the line, and the two decisions that cause most of the trouble: filtration grade and whether to lubricate at all.
What is in compressed air
Ambient air drawn into a compressor contains water vapour, dust and, in an industrial setting, oil aerosols and hydrocarbons. Compression concentrates all of it.
Water. Air holds less water as pressure rises, so compression forces vapour to condense. A 10 kW compressor in a temperate climate can produce tens of litres of condensate a day. Water carries rust into cylinders, washes grease out of seals, freezes in outdoor lines and ruins anything it reaches in a spray application.
Particulate. Compressor wear debris, pipe scale, rust from the receiver and whatever the intake filter missed. Particulate scores cylinder bores and jams valve spools.
Oil. From a lubricated compressor, carried over as aerosol and vapour. Harmless in some applications, unacceptable in food, pharmaceutical, paint spraying and instrument air.
Heat. Air leaves the compressor hot; as it cools in the distribution main it deposits its water there, which is why the main itself has to be arranged to shed condensate.
The three units and what each one does
Filter. Removes particulate and bulk liquid water. A general-purpose filter element is typically rated 5 to 40 µm and is enough for cylinders and valves. Finer grades exist and are the right answer only where the application needs them, because each finer grade costs pressure drop and element life.
| Grade | Typical rating | Where it belongs |
|---|---|---|
| General purpose | 5 to 40 µm | Cylinders, valves, air tools |
| Fine | around 1 µm | Instrumentation, sensitive valves |
| Coalescing | 0.01 to 0.1 µm, removes oil aerosol | Paint spraying, food contact, electronics |
| Activated carbon | Removes oil vapour and odour | Breathing air, food, pharmaceutical |
Coalescing and carbon filters are staged, not alternatives. Putting a 0.01 µm coalescing element straight after the compressor will clog it in days; it belongs downstream of a general-purpose filter that has already taken out the bulk.
Regulator. Holds a stable downstream pressure regardless of upstream fluctuation and flow demand. Two things are worth knowing.
Regulators are sized on flow, not on thread. An undersized regulator droops badly under demand, so the machine that worked in commissioning slows when the whole line runs.
A regulator set higher than the application needs is the most common source of wasted energy in a plant. Every extra bar costs compressor power and increases leak losses through every joint in the system.
Lubricator. Adds a controlled oil mist to the air for components that need it. This is where most plants get it wrong in one direction or the other.
To lubricate or not
Most modern pneumatic components do not need added lubrication. They are supplied pre-greased for life, and adding oil mist can wash that grease out. Fitting a lubricator to a machine built from modern cylinders and valves is usually a mistake.
Some components do need it, particularly air motors, older cylinders, percussion tools and anything the manufacturer explicitly specifies as requiring lubricated air.
The decision is not a matter of taste, and the rule is one-way: once a component has run on lubricated air, it must continue on lubricated air. The oil mist washes out the original grease, so removing the lubricator later leaves the component with nothing.
Two practical consequences:
- Decide per branch, not per plant. Put lubricated tools on their own drop, downstream of their own lubricator, and leave the rest of the system dry.
- Never fit a lubricator upstream of a branch that feeds food-contact, paint or instrument applications.
Where the units belong in the line
Order is filter, regulator, lubricator, and it is not arbitrary. The filter protects the regulator from particulate that would damage the seat. The regulator gives the lubricator a stable pressure so the oil delivery rate stays consistent. A lubricator upstream of a filter simply has its oil removed again.
Position matters as much as order. Air preparation belongs close to the machine it serves, not only at the compressor house. Condensate forms in the distribution main all the way along, so a filter at the compressor does not protect a machine forty metres away.
Take drops from the top of the main. A branch taken from the bottom of a horizontal main collects everything that has settled there. Taken from the top, with a drop leg and a drain at the bottom of the main, the water stays in the main and is removed deliberately.
Fall and drainage. Distribution mains are laid with a slight fall in the direction of flow, with drain points at the low ends. Without that, the main becomes a reservoir and the machine at the far end gets a slug of water at the worst moment.
Sizing and pressure drop
Every element in the preparation set costs pressure. A filter, a regulator and a lubricator in series can easily consume a bar between them once the elements are part-way through their life.
Size the set on the peak flow the machine draws, not on the average, and not on the port thread of the machine. A set that is one size larger than necessary costs a little more once; one that is undersized costs pressure and therefore energy for as long as it stays installed.
Fit a pressure gauge downstream of the regulator and, on critical machines, a differential indicator across the filter. Elements are usually changed on a calendar schedule, which means they are changed either too early or too late; a differential indicator changes them when they need it.
A specification checklist
- What the air touches. Product contact, breathing air and paint all change the filtration requirement completely.
- Filtration grade, staged rather than jumping straight to the finest.
- Regulator sized on peak flow, with the setting as low as the application allows.
- Lubrication decided per branch, and documented, because the decision cannot be reversed cheaply.
- Preparation set close to the machine, not only at the compressor.
- Drops taken from the top of the main, with drain points at the low ends.
- Drainage: automatic drains on filters where a manual drain will not reliably be operated.
- Element replacement triggered by differential pressure where the machine matters.