How to Efficiently Dry Compressed Air

Liquid water in compressed-air systems can lead to rusted equipment and unresponsive controls. Anyone who uses compressed air will, at some point, find liquid water in the air distribution system. This can be anything from a nuisance to a serious problem, depending on the application. If water might corrode equipment or introduce bacteria, removing it is critical.

Compressed air typically contains water as both liquid and vapor. Because removing it carries a cost, it’s important to define how dry the air must be for a given application and specify the right equipment to produce this result. First, though, it’s helpful to understand the source of water in a system.

Why and how do we dry compressed air?

Atmospheric air contains more water vapor at high temperatures and less at lower temperatures. This has an effect on the water concentration when the air is compressed. For example, a compressor with a working pressure of 7 bar and a capacity of 200 l/s that compresses air at 20˚C with a relative humidity of 80% will release 10 liters/hour of water in the compressed air line. Problems and disturbances can occur due to water precipitation in the pipes and connected equipment. To avoid this, the compressed air must be dried.

What is pressure dew point?

The term "pressure dew point" (PDP) is used to describe the water content in the compressed air. It is the temperature at which water vapor condenses into water at the current working pressure. Low PDP values indicate small amounts of water vapor in the compressed air.

It is important to remember that atmospheric dew point can not be compared with PDP when comparing different dryers. For 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. This is because further cooling leads to continued precipitation of condensation water.

You can select the main type of drying equipment based on the pressure dew point. When taking cost into account, the lower the dew point required, the higher the investment and operating costs for air drying. Five techniques exist for removing the moisture from compressed air: cooling plus separation, over-compression, membranes, absorption and adsorption drying.

How to dispose of the condensate after drying the air?

Anyone who runs a compressed air system (using oil-injected technology) needs to be aware of how to properly dispose of the condensate in a responsible manner, so as not to infringe any environmental laws. The condensate released by compressed air equipment will have tiny particles of oil in that are not visible to the naked eye, which is why it needs proper disposal. Not only is incorrect disposal detrimental to the environment, but you could also incur a fine and damage to your reputation as a responsible company.

There are many rules concerning the disposal of waste generally, as you will no doubt be aware if you have paid a recent visit to your local recycling centre and compared that to a visit only a few years ago. Don’t let your compressed air supply catch you out without doing a fairly easy first check.

Have a walk around your compressed air equipment: you should be able to see the condensate drains appearing from the back of the compressors and dryers on your site. Take a look at where these are piped to. Ideally you should see them all going to an oil/water separator of some description and then off to a foul drain. If they are going straight from a drain onto the floor or just into a standard plastic container then this should raise a red flag. Oil/water separators are very easy to install. Note that there are many rules around the disposal of condensate, even with the correct equipment in place, and the rules can vary from region to region.

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