The Case For Desiccant Compressed Air Dryers

Most people are familiar with desiccant from the small packets we find enclosed with a new pair of shoes, in a bag of beef jerky, or in some medication bottles.  These packets almost always say “Do Not Eat,” and I get that for the ones in the beef jerky or the pill bottles, but I just don’t understand why they put it on the desiccant packets bound for a shoe box…

Anyway, desiccant (in MUCH larger volumes than the household examples above) are also used to get water vapor out of compressed air.  Desiccant dryers are popular because they’re effective and reliable.  The most common design consists of two vertical tanks, or towers, filled with desiccant media – usually activated alumina or silica gel.

These materials are prone to adsorption (similar to absorption, only it’s a physical process instead of a chemical one) which means they’re good at trapping, and holding, water.  In operation, one of these towers has air coming in it straight from the compressor (after it’s become pressurized, remember, it still has just as much water vapor in it as it did when it was drawn in…up to 5% of the total gas volume.)

When that tower’s desiccant has adsorbed water vapor for long enough (it’s usually controlled by a timer,) the dryer controls will port the air through the other tower, and commence a restoration cycle on the first tower.  So, one is always working, and the other is always getting ready for work.

There are three methods by which the desiccant media can be restored:

  • Regenerative Desiccant Dryers send a purge flow of dry air (fresh from the operating tower’s discharge) through the off-line tower’s desiccant bed.  This dry air flow reverses the adsorption process, and carries the water away as it’s exhausted from the dryer.  This is simple and effective, but it DOES use a certain amount of your compressed air.
  • Heat Of Compression Desiccant Dryers use the heat from pressurized air straight from the compressor(s).  This hot air is directed through one tower, where it removes moisture from the desiccant.  It then flows through a heat exchanger where it’s cooled, condensing the moisture, before it flows through the other tower to remove any remaining moisture.  This method doesn’t add to your compressed air usage, but it only works with oil-free compressors.
  • The third method uses a hot air blower to flow heated air through the off-line desiccant bed.  It’s similar to the Regenerative type, but it doesn’t use compressed air.  However, they DO require a certain amount of wattage for the heater…remember, electricity isn’t cheap either.

As an EXAIR Application Engineer, it’s my job to help you get the most out of our products, and your compressed air system.  If you have questions about compressed air, call me.

Russ Bowman
Application Engineer
EXAIR Corporation
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Heat of Compression Dryers

A Heat of Compression regenerative desiccant dryer for compressed air

Before compressed air can be realistically utilized, it needs to be delivered to the point of use with proper volume and pressure, and it should also be clean and have some moisture removed.  We have information available regarding cleaning compressed air, but how do you dry the compressed air?  And why do you dry the compressed air?

Drying compressed air is akin to removing the humidity in the air when using an air conditioning system.  If the moisture is not removed, the effectiveness of the system is reduced and the ability to use the output of the system is reduced as well.

But, from a functional standpoint, what does this really mean?  What will take place in the compressed air system if the air is not dried and the moisture is allowed to remain?

The answer is in the simple fact that moisture is damaging.  Rust, increased wear of moving parts, discoloration, process failure due to clogging, frozen control lines in cold weather, false readings from instruments and controls – ALL of these can happen due to moisture in the compressed air.  It stands to reason, then, that if we want long-term operation of our compressed air products, having dry air is a must.

So, how can we remove the moisture in the compressed air?  One of the most common methods to remove moisture is a regenerative dryer, specifically, heat-of-compression type dryers.  A heat of compression type dryer is a regenerative desiccant dryer which uses the heat generated by the compression of the ambient air to regenerate the moisture removing capability of the desiccant used to dry the compressed air.

When using one of these dryers, the air is pulled directly from the outlet of the compressor with no cooling or treatment to the air and is fed through a desiccant bed in “Tank 1” where it regenerates the moisture removing capabilities of the desiccant inside the tank.  The compressed air is then fed through a regeneration cooler, a separator, and finally another desiccant bed, this time in “Tank 2”, where the moisture is removed.  The output of “Tank 2” is supplied to the facilities as clean, dry compressed air.  After enough time, “tank 1” and “tank 2” switch, allowing the hot output of the compressor to regenerate the desiccant in “tank 2” while utilizing the moisture removing capabilities of the desiccant in “tank 1”.

Heat of compression dryers offer a lower power cost when compared to other dryers, but they are only applicable for use with oil free compressor and to compressors with high discharge temperatures.  If output air temperatures from the compressor are too low, a temperature booster/heater is needed.

If you have questions about your compressed air system and how the end use devices are operating, contact an EXAIR Application Engineer.  We’ll be happy to discuss your system and ways to optimize your current setup.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

 

Heated Desiccant Dryer by Compressor1.  Creative Commons License