A desiccant dryer usually has two identical towers, filled with 4a air dryer 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 dryer switches towers. The compressed air is now dried by the second tower, while the first tower, filled with wet, saturated 4a air dryer desiccant is dried.

On most dryers, there is a small sight-glass. Behind the glass you can see small beads. This is not the 4a air dryer 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 dryer is working properly.

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