The air your compressor draws in always contains water vapour. Compressing it concentrates that water; as it cools in the air receiver and the piping, it condenses. Without treatment, it ends up in your tools, cylinders and spray guns. The dryer is the component that solves this problem at the source.
What water does to your installation
- In tools: it washes away lubrication and speeds up wear and corrosion.
- In piping: it promotes rust, which breaks off and clogs filters and tools.
- In painting: a few drops are enough to cause finish defects.
- In winter: in networks that run through cold areas, the water can freeze.
Draining the air receiver and the low points of the network removes liquid water, but not the vapour that remains in the air. That is the dryer's job.
The key concept: pressure dew point
The pressure dew point is the temperature at which the water contained in compressed air starts to condense. The lower it is, the drier the air. Choosing a dryer means choosing the dew point your application needs.
A simple rule: the dew point must be lower than the coldest temperature the air will encounter in your network. Otherwise, water will condense somewhere.
The three main technologies
The refrigerated dryer
This is the most common solution in workshops. It cools the compressed air in a heat exchanger, like a refrigerator: the water vapour condenses and is removed through a drain, then the air is reheated before going back into the network.
- Dew point: a few degrees above zero, sufficient for the vast majority of heated workshops.
- For whom: garages, body shops, joinery shops, production workshops.
- Strengths: simple, robust, low maintenance.
- Limitation: it cannot go below 0 °C, otherwise the water would freeze in the heat exchanger.
The desiccant dryer
The air passes through a desiccant material that captures moisture. Two towers work alternately: while one dries the air, the other regenerates.
- Dew point: very low, well below zero.
- For whom: networks exposed to frost, demanding industrial processes, applications where the air must be very dry.
- Strengths: very dry air.
- Limitations: more expensive, and regeneration consumes part of the compressed air or energy.
The membrane dryer
The air flows through hollow fibres that let water vapour pass through to the outside.
- For whom: low air flows, isolated points of use.
- Strengths: compact, no electricity required.
- Limitation: only suitable for low air flows.
How to size a dryer
A dryer is chosen first and foremost on the air flow it has to treat: it must be at least equal to the compressor's air flow. But this rated flow is given for reference conditions. Three factors affect it:
- The air temperature at the dryer inlet: the hotter the incoming air, the less efficient the dryer.
- The ambient temperature: a poorly ventilated room reduces the dryer's capacity.
- The working pressure: it changes the amount of water to be removed.
Dryer manufacturers provide correction factors for these three parameters. In a hot room, choose a dryer larger than the compressor's air flow alone would suggest.
Good installation practice
- Place the dryer after the air receiver: the air has already cooled there and part of the water has been drained.
- Install a pre-filter upstream of the dryer to protect it from particles and oil.
- Check that the automatic condensate drain is working.
- Ventilate the room: a refrigerated dryer gives off heat.
And with an AIRSTAHL compressor?
The screw compressor 7.5 kW 4-in-1 has a built-in 420 L air receiver, refrigerated dryer and filtration: the air comes out dry and filtered, with nothing to add. The 5.5, 11, 15 and 22 kW models are supplied without an air receiver or dryer: we will offer you, on quotation, the configuration suited to your air flow and your application.
For the order of components between the compressor and the workshop, also read our guide Connecting your compressor to the air network.