Compression squeezes water out of air, and a 100 L/min compressor running 8 hours condenses about 0.4 L a day in 20 °C air at 60% relative humidity and about 1.4 L in saturated 30 °C air. Cool the air before it enters the receiver, separate the water in a trap with a drain at the bottom, slope every pipe toward a drain point, and empty every drain daily, because the water rusts the tank and pipes from inside and freezes in tool exhausts.
Part of Pneumatic Systems
Compressed air carries water vapour that condenses as the air is squeezed and cooled. Removing it protects the vessel, the pipes, the tools and any finish being sprayed.
Compression Squeezes Water Out, and the Arithmetic Shows How Much
Air at a given temperature can hold only so much water vapour. Compress it, cool it, and the vapour that no longer fits becomes liquid. The saturation vapour pressure of water is about 2.34 kPa at 20 °C, 4.25 kPa at 30 °C and 7.38 kPa at 40 °C (recalled steam-table values). From these:
- Saturated air at 20 °C holds 2,339 x 0.018015 / (8.314 x 293.15) = 0.0173 kg/m³, or 17.3 g/m³
- Saturated air at 30 °C holds 30.3 g/m³
- At 60% relative humidity and 20 °C the intake holds 0.6 x 17.3 = 10.4 g/m³
After compression to 8 bar absolute (7 bar gauge) and cooling back to 20 °C, the most vapour each cubic metre of free air can still carry is 0.622 x 2.339 / (911.9 - 2.339) x 1.204 kg/m³ = 1.9 g/m³ (the mixing-ratio formula and air density are recalled). The rest condenses.
Worked example: 100 L/min for 8 hours.
- Air handled = 0.1 m³/min x 480 min = 48 m³ of free air
- At 20 °C and 60% humidity: condensate = 10.4 - 1.9 = 8.5 g/m³, so 48 x 8.5 = about 405 g, 0.4 L a day
- In saturated 30 °C air: 30.3 - 1.9 = 28.4 g/m³, so 48 x 28.4 = about 1,364 g, 1.4 L a day
The carrier gives 1-3 L a day for this duty. Those figures need very warm, humid air; 3 L would need intake air saturated at above 40 °C (about 2.4 L at exactly 40 °C). Plan for the range in the worked example and, more to the point, for a volume that varies wildly with the weather.
Where the Water Goes Depends on the Temperature
The water condenses wherever the air cools to a new low. If air leaves the compressor at 40 °C, it can still carry 6.1 g/m³ of free air (the same formula with 7.38 kPa), more than three times the 1.9 g/m³ it carries at 20 °C. The extra 4.2 g/m³ condenses later, in the pipes, at the tool or in a cold corner, where nobody has fitted a drain. So cool the air at the compressor, where the water can be caught.
Removal Works in Stages
- Aftercooler: a coil of copper tube between the compressor and the receiver. Air cools, water condenses and drains from the bottom.
- Receiver: the tank is itself a settling vessel; its drain removes what falls out.
- Water trap: a larger-diameter pipe section, 60-80 mm per the carrier, where the air slows and water separates, with a drain at the bottom.
- Slope and drain legs: slope the pipes 1-2° toward drain points, and take branch connections from the top of the main so water cannot run into tools.
Drying the air beyond this (for spraying finishes or cold exhausts) takes more cooling or a drying stage; no figure is sourced here, so test the air by holding a cold mirror or a piece of glass in the jet from a tool outlet and looking for fog.
Common Mistakes
- An undrained trap. It fills, and the water goes straight on to the tools.
- Branches taken off the bottom of the main. Water rides along the bottom of a pipe and enters a branch that leaves from beneath it.
- Draining only when the receiver feels heavy. Drain on a schedule, daily at least, and note how much comes out.
- Mixing oil and water drains into a stream. The compressor oil comes out with the water; dispose of it properly.
Safety
Condensate corrodes the vessel where you cannot see
Water sitting on the bottom of a receiver or pipe rusts the wall from the inside, thinning it until it fails by bursting. A drain valve that sticks or is never opened is a slow hazard. Open the drain slowly with your face and body to one side, since it releases air at tank pressure, and stop at once if the valve will not close. A drain that is blocked with rust is a warning that the wall may be rusting too. Frozen condensate can block a drain and a pressure line.
What’s Next
- Learn next: Safety Devices — drain valves, relief valves and gauges
- Combine with: Spray Equipment — where dry air matters most