In order to dry the moisture laden compressed air, it is fed through the adsorber. The partial pressure gradient from the drying medium to the moist compressed air causes the deposition of moisture from the compressed air onto the receptive drying medium. With adsorption based on the principle of heatless regeneration, moisture from the compressed air is deposited onto the external surface of the drying medium. With this drying system, the capacity of the drying medium is used up to 0.5% because only the external surface of the drying medium can be used for the deposition of moisture. In order not to overload drying materials with moisture during adsorption, relatively short cycles between adsorption and desorption have to be chosen.
Economical adsorption is achieved in a period of a few minutes only. Longer periods call for a larger adsorber with the correspondingly larger quantity of drying medium, shorter periods lead to an unfavourable relationship of the desorption and pressure build up time which runs in parallel. The loading of the drying medium with moisture from the volume of air takes place in the mass transfer zone (see paragraph 6.4.3) of the drying medium. The moisture introduced into the adsorber passes through the distance between inlet and outlet in increasing concentration. Before the break-through point of moisture reaches the outlet, the system switches over, time controlled, from adsorption to desorption.
For heatless regenerated adsorption dryers, activated alumina or a molecular sieve are utilised as adsorbents. Activated alumina is suitable for entry temperatures up to 35°C and pressure dewpoints as low as -40°C. Molecular sieves as drying medium find application for higher inlet temperatures of up to 55°C and lower pressure dew points of down to -90°C. Whereas the pressure dewpoint of -25°C is achieved in operation relatively quickly, theoretically lower pressure dewpoints, as low as -90°C are achieved only after days of continuous operation. A rise in temperature through adsorption in the bed of drying medium is relatively small with heatless regenerated adsorption dryers because the loading up with moisture is quantitatively low. The compressed air temperature at the outlet of the dryer is thus about 2-6°C higher than at the inlet, given normal operating conditions. The service life of the adsorbents in heatless regenerated adsorption dryers amounts to about 4-5 years, if correct operating conditions are adhered to and assuming one shift operation.
