A common problem in compressed air systems is the condensation of humidity which causes, corrosion, considerable damage to the equipment and therefore malfunction of the system. The condensation is due to the water vapor in the compressed air. When this vapor cools down it transforms to water droplets.
To avoid water condensation in a pneumatic system, an air dryer should be installed. Dryers remove water vapor from the air, which lowers its dew point (the temperature to which air can be cooled before water vapor begins to condense).
The most important basic types of industrial compressed air dryers are discussed here.
The refrigerant dryer consists of an air-to-air heat exchanger and a refrigerant-to-air heat exchanger. First, the moist compressed air is cooled down to approx. 30 °C by the air-to-air heat exchanger. A large part of the water separation is already taking place here. In the subsequent refrigerant-air heat exchanger, the air is cooled just above freezing point. Here the second water separation takes place.
Before the compressed air enters the circuit, it is reheated to approx. 20 °C via the air-to-air heat exchanger. This prevents the compressed air in the pneumatic working circuit from heating up and absorbing humidity.
This dryer consists of two containers which are filled with a drying agent (e.g. silica gel). This desiccant adsorbs the water, i.e. it is stored down to the finest pores of the desiccant, without any connection with the water. Two containers with this desiccant are necessary to alternately free one container from water storage. This is done by means of a heated hot air flow. The air flows are controlled by appropriate directional valves.
By using a desiccant, pressure dew points below the freezing point (down to - 70 °C) can be achieved.
Absorption dryers have a desiccant based on common salt or urea salts in their container. Since the water forms a chemical compound with the desiccant, it must be disposed of regularly. Due to this disadvantage, absorption dryers are hardly used in new plants any more.
The pressure dew point must of course be above the freezing point. These dryers are not suitable for inlet temperatures above 35 °C and pressures below 2 bar.