Desiccant Dryer

When a compressed air application requires a pressure dew point under 0°C, a desiccant dryer is required to meet the specification. Desiccant dryers usually contain two pressure vessels, both filled with desiccant (activated alumina beads).

Adsorption Dryer

The moisture from the compressed air clings, like Velcro to the desiccant when the air passes through the first tower. After a period of time, when the beads in that tower are saturated, the compressed air is switched to pass through the second vessel while the first vessel is then flooded with air to dry out the saturated beads. This process is then repeated after a set period when the beads in the second tower are saturated, moving the flowing air back to the first vessel while the second vessel is regenerated and so on. To regenerate desiccant beads, there are two techniques that are commonly used.

Cold Regeneration

A small amount of the compressed air that has dried, expands to atmospheric pressure in cold regeneration dryers and the saturated desiccant vessel is sent over. The air is much dryer due to the compressed air expansion. The moisture moves from the desiccant to the dried air and into the atmosphere. This type of regeneration results in very short cycle times and a small amount of electricity – only for the controls. The design is simple and reliable, but up to 18% of the compressor power is used for regeneration, making it a costly solution especially for low pressures.

Heat Regeneration

In this method, moisture is released from the desiccant beads when they are heated and water retaining forces break. Air flow moves the moisture out of the saturated vessel. These are the types of heat regenerated adsorption dryers:

Heated Purge

In heated purge desiccant dryers, part of the dried compressed air expands to atmospheric pressure and heat is sent to raise the temperature to 150-180°C. Saturated desiccant sends hot air through the vessel, moisture is released and moves to the atmosphere with the air. In heated blower purge desiccant dryers, a blower is installed that moves ambient air to a heater. The air is warmed up to 150-180°C, and travels through the vessel with the saturated desiccant. Moisture is released and travels to the atmosphere with the air.

Heat of Compression

Heat created during compression is used in heat of compression desiccant dryers to warm the desiccant and remove moisture. Hot compressed air moves from the compressor through the saturated vessel, moisture is released, and travels with the compressed air to a cooler. The compressed air is then cooled, and moisture is removed. The compressed air moves from the cooler to the vessel containing dry desiccant. Here, the remaining moisture is eliminated and dry air enters the compressor system. Before the desiccant beads can begin adsorbing again, they must cool down with cold ambient air, or with dry or cold compressed air.