Water running from the drain, drops coming out of the blow gun, rust in the fittings: water in compressed air is not a sign of a fault. Every compressor produces it, whatever its technology. What matters is getting rid of it before it reaches the tools.
Where does the water come from?
Ambient air always contains invisible water vapour. The compressor draws in this air and compresses it: the same amount of vapour ends up concentrated in a much smaller volume. As long as the compressed air is hot, the water stays as vapour. As soon as it cools down, in the air receiver, the pipes or the hoses, it condenses into droplets.
Two factors change the amount of water:
- The temperature and humidity of the intake air: warm, humid air holds far more vapour. Drains discharge the most in summer and in poorly ventilated rooms.
- The compressor's air flow: the more air it draws in, the more water it draws in. Depending on the size of the machine and the season, this ranges from a few litres to several tens of litres a day.
What water damages
- Pneumatic tools: it washes out their lubrication, so they rust and wear faster.
- The network: rust forms inside steel pipes, breaks off and clogs filters and couplers.
- The air receiver: water standing at the bottom eats it away from the inside.
- Finishes: in painting, varnishing or soft blasting, a few drops are enough to spoil the job.
- Networks exposed to cold: water can freeze in pipes that run outdoors or through an unheated room.
The points to drain
Liquid water settles wherever the air slows down or cools. Each of these points needs a drain:
| Where | Why water collects there | Recommended drain |
|---|---|---|
| Bottom of the air receiver | The air cools and slows down there: this is where most of the water drops out | Automatic drain, or valve opened every day |
| Dryer outlet | The dryer condenses the remaining vapour and has to discharge it | Drain built into the dryer, to be checked |
| Under the filters | The pre-filter and oil removal filter catch droplets and oil | Filter bowl drain, manual or automatic |
| Low points of the network | Water that escaped treatment runs down to the bottom of the pipes | Tapping with a drain valve at the low point |
In a well-designed network, the pipes have a slight slope towards a drain point, and drops to the workstations leave from the top of the main line, so that water does not fall straight into the tools. Our article on building a leak-free compressed air network covers these rules.
Manual drain or automatic drain?
On a small piston compressor, the air receiver is often drained by hand: at the end of the day, valve open at low pressure until the air comes out dry. It works, as long as nobody forgets.
As soon as the compressor runs a lot, or the workshop is closed part of the time, an automatic condensate drain removes the risk of forgetting. There are two main types:
- The timed drain opens at adjustable intervals. Simple and inexpensive, but it releases a little air every time it opens, even when there is no water.
- The level-sensing drain only opens when water is present. It does not waste compressed air.
A drain needs watching: stuck closed, it lets water back up into the network; stuck open, it blows air continuously and becomes a leak. You should hear it discharge regularly.
Draining is not enough: the role of the dryer
Draining removes liquid water. But the air leaving the air receiver is still saturated with vapour: as it cools further along the network, it condenses water again, right into the tools. To prevent this you need a dryer, which removes the vapour before the network.
- A refrigerated dryer suits most heated workshops.
- A desiccant dryer gives much drier air, for networks exposed to frost or demanding uses.
To choose between the two, read which air dryer to choose. Our air dryers are offered by screw compressor power. The K series 4-in-1 compressors have the air receiver, refrigerated dryer and filtration built in on the same frame.
What to do with the condensate?
The water coming out of the drains is not clean water. From a lubricated compressor it contains oil, and it picks up dust and rust. It must not go straight into the sewer: run it through an oil/water separator before discharge, in line with local regulations. The recovered oil is handled as waste.
Water at the tools: the checklist
If water still reaches the workstations, check in this order:
- Draining the air receiver: is it done every day, or is the automatic drain working?
- The dryer: is it running, and is its flow rating at least equal to the compressor's?
- The filter and dryer drains: a bowl full of water no longer holds anything back.
- The room: a room that is too hot makes the dryer less effective and increases the amount of water to treat.
- The network: are there low points without a drain, or drops that leave from the bottom of the line?
If everything is in order and the water persists, contact our after-sales service, or book maintenance for your installation.
For your trade: Compressor for spray booths, Compressor for body shops, Compressor for garages.