Compressed air contains moisture that is undesirable for most applications. Dryer desiccators for air treatment use granular materials (desiccants) that adsorb water molecules to its surface. When the desiccants are saturated with water molecules, hot air is flushed over the beads to remove the moisture and thus regenerate the material.

Activated alumina and silica gel are the most common desiccants. Activated alumina is aluminum oxide that comes in three bead sizes, 1/8, 3/16 and 1/4 inch. The larger beads provide less surface area in a column, but have a lower pressure drop at a given air flow rate. The 1/8 in bead is the most common in use.

The beads have low abrasion and high crush strength and do not degrade when exposed to high temperatures. They resist liquid water and will provide air dryness to minus 40 F dew point and below, depending on the dryer design.

Silica gel is 97% silica and 3% alumina. It is available in its natural brown color or with a blue die which turns pink as the gel becomes saturated with water molecules. The beads are uniform in size which provides a lower pressure drop. Silica gel fractures when exposed to liquid water. It should have a prefilter at the air intake if the presence of water is possible. The dew point of silica gel systems can be from -40 F down to -100 F depending on the system design and air flow.

The dryers come in the following configurations: External Heater, Internal Heater, and Heatless. All systems have two columns with the drying air flowing upward through one column, while the other column is being regenerated.

The heatless system taps dry air from the main line to flow through the column being regenerated. Heat created by the beads in the adsorption process helps in the regeneration process.

As a general rule, 17% of dry air is purged for -40 F dew point and 35% for -100 F dew point. Coalescing oil filters are a must when drying air from an oil injected compressor. A particle filter is recommended for oil free compressors.

The capacity is measured in scfm at the input to the unit. The rating is to standard CAGI ADF100. This is 100 psig input pressure, 100 F input temp and 5 psi max pressure drop. The standard dryness, or dew point is -40 F. A lower dew point can be achieved with lower air flow and more purge air.