These type of compressed air dryers go by various names: regen dryers, twin tower dryers, desiccant dryers, pressure swing adsorption dryers, temperature swing dryers. In any case, the two towers operate alternately. Wet compressed air flows through the desiccant bed of one tower, while the desiccant in the other tower is being regenerated or stripped of accumulated moisture. Regeneration of the desiccant bed is accomplished in various ways: by a purge of dry air drawn from the outlet of the on-line tower; by externally heated purge air; by internal heaters; by atmospheric air blown through an external heater and forced through the off-line tower; or with steam. The desiccant bed consists of a solid adsorbent material, such as activated alumina, silica gel, or molecular sieve.