The term desiccant dryer describes a broad range of compressed air treatment equipment.  The term can encompass anything from a small point-of-use cartridge style dryer costing a couple of hundred dollars to a massive piece of equipment costing hundreds of thousands of dollars.

Let’s talk about just one subset of this category, the heatless desiccant dryer.

Heatless desiccant dryers are used to remove water vapor from compressed air lines and consist of just a few basic components, including:

• Desiccant Towers
• Switching Controls
• Control Valves

The towers are filled with an adsorbent media called desiccant.  The most prevalent form of desiccant used air dryers is activated alumina.  This media is composed of small white beads, usually 2-5 millimeter (1/8”) in diameter.  The surfaces of these beads are highly porous and water molecules attach to their lattice-like internal structure.

Wet compressed air flows upward through one tower. When air reaches the top of the tower the air is extremely dry. A portion (15%) of this dry air is diverted through an orifice to the second tower. This dry purge, or regenerating air, strips off water vapor from the desiccant in the second tower, which is at atmospheric pressure.

A series of valves continuously switches incoming and regenerating air between the towers.  The complete cycle usually lasts around 10 minutes. 

These are called heatless dryers because the purge air is not heated, as it would be with more sophisticated and high volume designs.  The sketch below shows how air moves through an MHL Series heatless dryer from Van Air Systems.

Heatless desiccant dryers achieve a -40⁰ compressed air dew point.At this dryness, outlet compressed air only retains about 100 PPM of water.It’s really, really dry.Heatless dryers are typically specified for instrumentation, process air, laboratories, and branch lines exposed to sub-freezing temperatures.

Heatless dryers do not require a lot of technical sophistication to operate or maintain.

While heatless dryer provide high quality air and are easy to maintain, keep in mind that 15% of the air introduced to a heatless dryer will be lost to the regeneration purge.

If you need help sizing a dryer or working through an application, I encourage you to email me.

Ready to buy a heatless dryer? Use the coupon code $35OFF to save on a heatless dryer purchased here.