Desiccant Dryers: Heat of Compression Type

Desiccant Dryers

Desiccant dryers come in different forms.  They are designed for water sensitive areas as they can reach a dew point to -40oF (-40oC) and below.  That means that water will not condense in the compressed air lines until the temperature is below the dew point.  The desiccant inside these units will adsorb the water vapor as compressed air passes through a bed.  Once the desiccant bed is full of water vapor, it will have to be regenerated.

A typical system will use two towers that will switch back and forth.  One tower is used to remove the water from the compressed air system, and the other is used to regenerate the desiccant.  In this blog, I will cover how the desiccant can be regenerated with a Heat of Compression (HOC) type of desiccant dryer.

An air compressor is not an efficient device.  For every eight horsepower of energy to make compressed air, only one horsepower is used as work.  And for compressed air drying, the type of desiccant dryer is important.  Regeneration of desiccant beads can be done either with non-heated or heated means. The non-heated, or heatless version will use 15% of your compressed air to purge through the regeneration tank.  The air escapes into the atmosphere with the water vapor and is wasted.

With the heated type desiccant dryers, they come in three different categories.  One type uses a heater to increase the temperature of the compressed air. At the elevated temperature, the purge requirement can be reduced to 7% for the regeneration of desiccant.  But, still compressed air is wasted.  To cut the purge to zero, a blower-type heated desiccant dryer can be used.  Instead of heating the compressed air, the blower will push ambient air through a heater to regenerate the desiccant bed.  But can you get more efficient than that?

Well, what if you can remove the heater and the blower?  The heat of compression type of desiccant dryers can do that.  Remember above when I mentioned that “for every eight horsepower of energy to make compressed air, only one horsepower is used as work”.  The seven horsepower of energy that is lost is given off as heat.  The HOC dryer uses that heat to regenerate the desiccant bed.  So, the overall energy is reduced even further.  There is a restriction when using this type of dryer.  The air compressor will have to be oil-free because oil will coat the desiccant beads and stop the adsorption rate.

When the air is compressed, heat is generated.  This heated air can reach around 200oF (93oC).  With the higher temperature, air can hold more water vapor.  As the heated air passes through the desiccant bed that needs to be regenerated, the water vapor is picked up from the desiccant beads.  The saturated air would then pass through an aftercooler.  The aftercooler reduces the air temperature below 100oF (38oC) which will cause the water to drop out.  From the aftercooler, the air will then pass through the desiccant bed in the drying tower.   When the cycle time is reached, the towers will switch to regenerate the second tower.

Line Vacs can convey many things.

With these types of dryers, the desiccant beads will start to degrade from regeneration.  To help replace them, EXAIR offers a Line Vac.  Instead of climbing a ladder with many bags of desiccant, the Line Vac can do this safely and ergonomically.   EXAIR Line Vacs use a small amount of compressed air to generate a powerful vacuum by a Venturi effect.  The unique design of the generators creates a high velocity of air to create a low pressure on one side and a powerful thrust on the other.  The Line Vac can pick up and move solid material vertically up to 20 feet (6 meters).  You can watch a video on the operation of a Line Vac HERE.  The EXAIR Line Vacs are very quiet, compact, rugged, and powerful.  To replace the desiccant, it can do it quickly and safely.

If you need to convey solid materials in a quick and easy way, an EXAIR Line Vac could be a solution for you.  We have them in a variety of materials and designs to match your application.  Ergonomically, they can save the back-wrenching labor of picking up bags, climbing stairs, and dumping material into towers.  If you want to know if the EXAIR Line Vac could work for you, an Application Engineer at EXAIR can help to recommend the best unit for you.

John Ball
Application Engineer

Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Photo: Heated Desiccant Dryer by Compressor1Creative Commons Attribution-No Derivs 2.0 Generic

About Heat of Compression Dryers

Drying compressed air is similar to removing the humidity in the air when using an air conditioning system.

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.

desiccant-dryer
A Heat of Compression regenerative desiccant dryer for compressed air

 

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.

heat-of-compression-regenerative-desiccant-dryer-diagram.png

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”.

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.

Jordan Shouse
Application Engineer
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Heated Desiccant Dryer by Compressor1.  Creative Commons License

Supply Side Review: Heat of Compression-Type Dryers

The supply side of a compressed air system has many critical parts that factor in to how well the system operates and how easily it can be maintained.   Dryers for the compressed air play a key role within the supply side are available in many form factors and fitments.  Today we will discuss heat of compression-type dryers.

Heat of compression-type dryer- Twin Tower Version

Heat of compression-type dryers are a regenerative desiccant dryer that take the heat from the act of compression to regenerate the desiccant.  By using this cycle they are grouped as a heat reactivated dryer rather than membrane technology, deliquescent type, or refrigerant type dryers.   They are also manufactured into two separate types.

The single vessel-type heat of compression-type dryer offers a no cycling action in order to provide continuous drying of throughput air.  The drying process is performed within a single pressure vessel with a rotating desiccant drum.  The vessel is divided into two air streams, one is a portion of air taken straight off the hot air exhaust from the air compressor which is used to provide the heat to dry the desiccant. The second air stream is the remainder of the air compressor output after it has been processed through the after-cooler. This same air stream passes through the drying section within the rotating desiccant drum where the air is then dried.  The hot air stream that was used for regeneration passes through a cooler just before it gets reintroduced to the main air stream all before entering the desiccant bed.  The air exits from the desiccant bed and is passed on to the next point in the supply side before distribution to the demand side of the system.

The  twin tower heat of compression-type dryer operates on the same theory and has a slightly different process.  This system divides the air process into two separate towers.  There is a saturated tower (vessel) that holds all of the desiccant.  This desiccant is regenerated by all of the hot air leaving the compressor discharge.  The total flow of compressed air then flows through an after-cooler before entering the second tower (vessel) which dries the air and then passes the air flow to the next stage within the supply side to then be distributed to the demand side of the system.

The heat of compression-type dryers do require a large amount of heat and escalated temperatures in order to successfully perform the regeneration of the desiccant.  Due to this they are mainly observed being used on systems which are based on a lubricant-free rotary screw compressor or a centrifugal compressor.

No matter the type of dryer your system has in place, EXAIR still recommends to place a redundant point of use filter on the demand side of the system.  This helps to reduce contamination from piping, collection during dryer down time, and acts as a fail safe to protect your process.  If you would like to discuss supply side or demand side factors of your compressed air system please contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

Heat of compression image: Compressed Air Challenge: Drive down your energy costs with heat of compression recovery: https://www.plantservices.com/articles/2013/03-heat-of-compression-recovery/

 

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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