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Drying Compressed Air

All atmospheric air contains water vapour, more at high temperatures and less at lower temperatures. When the air is compressed the water concentration increases.

Example, a compressor with a working pressure of 7 bar and a capacity of 200 l/s that draws in air at 20°C with a relative humidity of 80% will give off 80 litres of water in the compressed air line during an eight hour working day.

The term pressure dew point (PDP) is used to describe the water content in the compressed air. It is the temperature at which water vapour transforms into water at the current working pressure.

Low PDP values indicate small amounts of water vapour in the compressed air. It is important to remember that atmospheric dew point can not be compared with PDP when comparing different dryers.

Example, a PDP of +2°C at 7 bar is equivalent to -23°C at atmospheric pressure.

To use a filter to remove moisture (lower the dew point) does not work. The reason is because further cooling means continued precipitation of condensation water.

You can select the main type of drying equipment based on the pressure dew point. Seen from a cost point of view, the lower the dew point required the higher the acquisition and operating costs for air drying.