The air dryers are used to prevent moisture condensation in compressors, to prevent corrosion and to reduce the moisture content in the air.
There are 3 basic types of these machines:
1. Condensing dryers: The dryer cools the stream of compressed air below the dew point temperature of the incoming air, this way the excessive humidity is eliminated and the „new“ dew point temperature is set, according to the temperature of cooled air. These dryers achieve dew point temperatures above zero (risk of freezing). For air of better quality with very low moisture content, or lower dew point temperature, it is necessary to use adsorption or membrane dryers.
2. Adsorption dryers: The adsorption device works with a substance capable of binding atmospheric humidity. This substance is characterized by a large internal surface size, against its volume. Binding of air humidity is a physical proces (not chemical) and as such it can be reversed (reactivation). Adsorption devices consist always of two vessels. In one of them sorption occurs and reactivation in the other one. Reactivation is done either by heat or by pressure shock. Working interval of one cartridge differs, depending on the machine. At the end of the working interval, the compressed air is switched to the reactivated vessel. These machines reach dew point temperatures typically of -20 to -70 °C.
3. Membrane dryers: They work on the diffusion principle. Filtered compressed air flows inside of them through hollow fibres and through their walls, the moisture diffuses into the drier air (flush air). Flushing air is obtained by expansion of air, which already came through the hollow fibres. This flushing air is flowing around the fibres against their direction (similar to the counterstream exchanger). The advantage of this device is work without the need for electricity and without mechanical wear. Unlike with the adsorption devices, there are no particles released from the membrane dryers. Disadvantages are, on the contrary, a small capacity (it is necessary to use dryer batteries for higher flow rates) and instability of the moisture content behind the device.