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: Deliquescent Type Dryers

As mentioned in my post last week.  The supply side of compressed air systems within a facility is critical to production.  The quality of air produced by your compressor and sent to the demand side of the system needs to be filtered for both moisture and particulate.  One method to dry the air, that is the topic for this blog, is deliquescent type dryers.

These dryers operate like an adsorbent dryer such as a desiccant medium dryer.  The main variance is that the drying medium (desiccant) actually undergoes a phase change from solids to liquids.  Because of this the material is used up and cannot be returned to its original state for reuse.   The liquids formed by the desiccant dissolving in the removed water vapor are then filtered out of the air stream before it is passed on to the demand side of the air system.

There are many compounds that are used to absorb the moisture in the wet compressed air.  A few options are potassium, calcium, or sodium salts and many that contain a urea base.  The desiccant compound must be maintained at a minimum level for the dryer to contain enough media to successfully dry the air.

These dryers are generally a single tank system that is fed with compressed air from a side port near the bottom of the tank.  The air then travels up past drip trays where the desiccant and water mixture fall and ultimately ends up in the bottom of the tank.  The air then goes through a material bed that must be kept at a given level in order to correctly absorb the moisture in the air.  The dry air is then pushed out the top of the tank.

As the desiccant material absorbs the liquid from the compressed air flowing through the tank it falls onto the drip trays and then into the bottom of the tank where it is drained out of the system.  This process can be seen in the image below.

 

Deliquescent type compressed air drying system
How a deliquescent air dryer works – 1(VMAC Air Innovated, 2017)

 

The dew point that this style dryer is able to achieve is dependent on several variables:

  • Compressed air temperature
  • Compressed air pressure / velocity
  • Size and configuration of the tank
  • Compression of the absorption media
  • Type of absorption media and age of media

These dryers are simplistic in their design because there are no moving parts as well as easy to install and carry a low startup cost.

Some disadvantages include:

  • Dewpoint range 20°F – 30°F (Again this is according to the media used.)
  • Dissolved absorption material can pose a disposal issue as it may not be able to be simply put down a drain
  • Replacement of the absorption material

Even with disadvantages the ability to supply the demand side of a compressed air system for a production facility is key to maintaining successful operations.  If you would like to discuss any type of compressed air dryer, please contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

1 – Deliquescent Dryer Image: VMAC Air Innovated: The Deliquescent Dryer – https://www.vmacair.com/blog/the-deliquescent-dryer/

 

Intelligent Compressed Air: Deliquescent Dryers – What are They and How do They Work?

EXAIR has written blogs about the different types of dryers that are used to remove liquid from compressed air systems. In this blog, I will be discussing the deliquescent dryer. This dryer falls under the desiccant dryer category, and unlike the regenerative cousins, it is the least commonly used type of dryer. The regenerative desiccant dryers use a medium that will adsorb the water vapor, and the deliquescent dryers use a hygroscopic material that will absorb the water vapor. This salt-like medium has a strong affinity for water, and it comes in a tablet or briquette form. Placed inside a single unit pressure vessel, the “wet” compressed air passes through the bed to become dry. The size of the pressure vessel is determined by the compressed air usage which allows for the proper amount of contact time with the hygroscopic bed. Generally, the dew point will be between 20 to 50 deg. F (11 – 28 deg. C) less than the compressed air inlet temperature. Unlike most dryers, the dew point after deliquescent dryers will vary with the inlet air temperatures.

Vessel Design

The design of vessel is very important for the function of a deliquescent dryer. A grate is required to hold the medium off the bottom. The compressed air will flow from the bottom, up through the bed, and out from the top. The predetermined space between the bed and the bottom of the vessel is used for the liquid that is generated. When “wet” compressed air passes through the bed, the hygroscopic material will absorb the water and change the tablets from a solid into a liquid. Deliquescent dryers got the name from the definition of the verb, “deliquesce” which is “becomes liquid by absorbing moisture from the air”. Once the material is turned into a liquid, it cannot be regenerated. The liquid must be discarded periodically from the vessel and new solid material must be added. With the single tower design, the deliquescent dryers are relatively inexpensive.

Some advantages in using the deliquescent dryers are that they do not require any electricity or have any moving parts. So, they can be used in remote locations, rugged areas, or hazardous locations. They are commonly used to reduce the dew point in compressed air, natural gas, landfill gas and biogas systems. Without the ability for regeneration, no additional compressed air will be lost or used. In comparing the power requirement to other compressed air dryers, the deliquescent dryers have the lowest power requirement at 0.2Kw/100 cfm of air. (This energy rating is only due to the additional power required for the air compressor to overcome the pressure drop in the dryer).

Some disadvantages in using the deliquescent dryers is that the hygroscopic material degrades. The deliquesced liquid does have to be drained and disposed, and new material does have to be added. Even though they do not have any moving parts, they still require periodic maintenance. The deliquescent material can be corrosive. So, after-filters are required to capture any liquid or dust material that may carry over and damage downstream piping and pneumatic components. Also, the variation in the dew point suppression can limit locations and areas where it can be used.

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Products, you can contact an Application Engineer at EXAIR. We would be happy to hear from you.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Photos:  used from Compressed Air Challenge Handbook