About Compressed Air Dryers – What Are They and Why Use Them

All atmospheric air contains some amount of water vapor.  When air is then cooled to saturation point, the vapor will begin to condense into liquid water. The saturation point is the condition where the the air can hold no more water vapor. The temperature at which this occurs is knows as the dew point.

When ambient air is compressed, heat is generated and the air becomes warmer. In industrial compressed air systems, the air is then routed to an aftercooler, and condensation  begins to take place. To remove the condensation, the air then goes into separator which traps the liquid water. The air leaving the aftercooler is typically saturated at the temperature of the discharge, and any additional cooling that occurs as the air is piped further downstream will cause more liquid to condense out of the air. To address this condensation, compressed air dryers are used.

It is important to dry the air and prevent condensation in the air. Many usages of the compressed air are impacted by liquid water being present. Rust and corrosion can occur in the compressed air piping, leading to scale and contamination at point -of -use processes. Processes such as drying operations and painting would see lower quality if water was deposited onto the parts.

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There are many types of dryers – (see recent blogs for more information)

  • Refrigerant Dryer – most commonly used type, air is cooled in an air-to-refrigerant heat exchanger.
  • Regenerative-Desiccant Type – use a porous desiccant that adsorbs (adsorb means the moisture adheres to the desiccant, the desiccant does not change, and the moisture can then be driven off during a regeneration process).
  • Deliquescent Type – use a hygroscopic desiccant medium that absorbs (as opposed to adsorbs) moisture. The desiccant is dissolved into the liquid that is drawn out. Desiccant is used up, and needs to be replaced periodically.
  • Heat of Compression Type – are regenerative desiccant dryers that use the heat generated during compression to accomplish the desiccant regeneration.
  • Membrane Type– use special membranes that allow the water vapor to pass through faster than the dry air, reducing the amount water vapor in air stream.

The air should not be dried any more than is needed for the most stringent application, to reduce the costs associated with the drying process. A pressure dew point of 35°F to 38°F (1.7°C to 3.3°C) often is adequate for many industrial applications.  Lower dew points result in higher operating costs.

If you have questions about compressed air systems and dryers or any of the 15 different EXAIR Intelligent Compressed Air® Product lines, feel free to contact EXAIR and myself or any of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer
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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

Intelligent Compressed Air: Desiccant Dryers – What are they and How Do they Work?

Desiccant dryer
Heat of Compression Desiccant Dryer

No matter where you are in the world, the atmospheric air will contain water vapor. As this air cools to the saturation point, also known as dew point, the vapor will condense into liquid water. The amount of this moisture will vary depending on both the ambient temperature and the relative humidity. According to the Compressed Air Challenge, a general rule of thumb is that the amount of moisture air can hold at a saturated condition will double for every 20°F increase in temperature. In regions or periods of warmer temperatures, this can create a significant problem. Some problems that can be associated with moisture-laden compressed air include:

  • Increased wear of moving parts due to removal of lubrication
  • Formation of rust in piping and equipment
  • Color variation, adherence, and finish of paint that is applied using moisture-laden compressed air
  • Create unstable conditions for processes that are dependent upon pneumatic controls. Malfunctions due to rust, scale, or clogged orifices can damage product or cause costly shutdowns
  • In colder temperatures, moisture in the compressed air flow can freeze in the control lines

To remove moisture from the lines, a dryer must be installed. One of the most commonly found dryers in a facility are referred to as desiccant dryers. There are three variations of desiccant dryers: Regenerative-Desiccant Dryers, Heat Reactivated Desiccant Dryers, and Heat of Compression Desiccant Dryers.

A Regenerative-Desiccant Dryer uses a porous desiccant that collects and adsorbs the moisture. This allows for large amounts of water to be retained with a minute amount of desiccant. Most regenerative-desiccant dryers consist of two towers. One where wet, moisture-laden compressed air flows through a desiccant bed. A second tower contains desiccant that is being regenerated. A controlled amount of dry air flows through the tower being regenerated, which causes the moisture to release from the desiccant and flow out with the purge flow. This saturated air exhausts to atmosphere. After a set time, the towers will switch and continue this cycle of drying/regenerating.

Another, and more efficient, regenerative-desiccant dryer uses heat to assist in removing water from the desiccant. As the tower is heated, the moisture is no longer adsorbed by the desiccant and is purged through the flow. This style is more efficient than the other styles of dryers because less compressed air is wasted removing the moisture in the tower.

The third type of desiccant dryer is the Heat of Compression dryer. This style utilizes the heat that is generated during compression to accomplish the regeneration of the desiccant. Typically, the heat that is generated from the air compressor is exhausted to atmosphere and wasted. With a heat of compression dryer, this heat is captured and used to regenerate the desiccant. The compressed air passes through the drying section of the desiccant bed, is dried, and exits through the discharge. A portion of the captured heat flows through the opposite side of the dryer to regenerate the desiccant. Afterwards, this hot air passes through a regeneration cooler and is combined with the main air stream. This results in ZERO loss of purge air, making this style of dryer the most efficient available.

If you have questions about how to optimize your compressed air system, contact EXAIR. An Application Engineer is standing by ready to assist you!

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@exair.com
Twitter: @EXAIR_TD

 

Heated Desiccant Dryer by Compressor1.  Creative Commons Attribution-NoDerivs 2.0 Generic.