A desiccant dryer 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 portable dental unit) the dryer 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 dryers, 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 dryer is
Small desiccant dryer Photo: Atlas Copco
Regeneration or drying of the desiccant
When the desiccant has done its job for some time, it has absorbed all the water from the compressed air and is saturated with water. It cannot hold any more water. In order to re-use the desiccant, we need to get rid of all that water. This is called regeneration.
The regeneration process is done in one of the two towers of the air dryer, the one that is not operational. There are two ways the desiccant can be regenerated: by hot air or by dried compressed air.
With the hot air method, a separate system consisting of a fan with an electric heater is used to blow hot air through the desiccant to dry it. This is just normal air drawn from the compressor room.
Dryer which uses heat from the oil-free dental air compressor for regeneration. Photo: Atlas Copco
There are also special dryers available for use with oil-free screw compressors that used the heat of the compressor to regenerate the desiccant (and thus saving energy and money). This type of dryer consists of 1 rotating drum. 1 quarter is being regenerated,
while the rest is drying the compressed air.
A third method of drying the desiccant, is with dry compressed air. This is often used in the smaller dryers and is also not suitable for big dryers.
With this method, part of the compressed air that has just been dried is used to regenerate the desiccant. This means that less compressed air is available for your tools and machinery.