Desiccant Compressed Air Dryers
Desiccants are substances that attract and hold water very well. When you purchase a product that could be damaged by the presence of water you often find a little envelope full of what looks like small beads packed inside the box. This is desiccant. Two types are commonly used: activated alumina and silica gel. Unlike the salt tablets used in the deliquescent dryer, these desiccants don’t dissolve when they absorb water. They fill up with as much water as they can hold and maintain their shape. Once they’re full they need to be replaced or be regenerated (have the water removed). Some desiccants are treated with chemical indicators to change color when regeneration or replacement is needed.
Another name commonly used for desiccant dryers is “Twin Tower” dryers. This is because each dryer consists of two towers filled with desiccant, with each tower being capable of handling the rated flow capacity of the dryer. While one tower is drying compressed air, the other is being regenerated.
Desiccant dryers are used primarily where very low dew points are required. The most common ratings are at pressure dew points of –40°F and -100°F. Depending on size, their initial purchase cost can be less than or more than a refrigerated dryer. Operating cost is a more serious consideration with desiccant dryers. You don’t want to use them unless you really need the low dew point.
Operating cost of these dryers varies with the method used to remove water from (regenerate) the desiccant. This process is referred to as “purging” and the three most common methods used are (1) Heatless purge, (2) Heated purge and (3) Blower purge. What all three methods have in common is that air is used to dry out the desiccant and carry the moisture to atmosphere.