The maximum water vapor content of a compressed air volume unit is caused by the temperature of the compressed air and is almost completely independent of its pressure. The water vapor content is therefore theore-tically represented by the dew point, which indicates the temperature at which the actual water vapor quantity corresponds to a relative humidity of 100% and below which condensation begins.
Drying in this context means a reduction of the dew point below the actual operating temperature. To obtain ultra-dry air (ISO 8573.1 class 1, 2 or 3), essentially only the process of adsorption, in which water is bound to a solid phase, is suitable. Adsorption is thereby defined as the attraction of a substance (the adsorbate) to the surface of a solid body (the adsorbent) via physical binding forces. Desorption, on the other hand, refers to the release of the adsorbate from the adsorbent. Since the adsorption capacity of adsorbents decreases with increasing temperature and decreasing pressure, moisture can be desorbed again by heat supply or pressure reduction.
The adsorbents used in such a drying process are high-tech desiccants and have an inner surface area of up to 1000 m2/g, due to their pore structure of macro-, meso- and micropores, where condensed water vapor can accumulate. For the drying of compressed air, silica gels, aluminum oxides (activated alumina) and zeolitic molecular sieves are most commonly used. These desiccants reach dynamic adsorption capacities above 20% by weight and dew points down to -100 °C.