A desiccant air dryer alumina gel usually has two identical towers, filled with desiccant. One tower is used to dry the compressed air, while the other tower is regenerated.

Every once in a while (controlled by a central control unit) the desiccant air dryer alumina gel switches towers. The compressed air is now dried by the second tower, while the first tower, filled with wet, saturated desiccant is dried.

On most desiccant air dryer alumina gel, there is a small sight-glass. Behind the glass you can see small beads. This is not the drying-desiccant, but an indicator. It will change color when the air the is too wet. Check it every once in a while to make sure that your desiccant air dryer alumina gel is working properly.