Compressed Air Dryers : What are they Good For?

Absolutely Nothing….. err ALOT! They are really good for a lot! Specifically removing moisture/condensate from compressed air.

In almost every operation, clean, dry compressed air will result in lower operating costs. The purpose of compressed air dryers is to overcome the dew point of your compressed air by removing water from it. Compressed air can contain humidity, and in the right environments it can reach the dew point temperature and condense into a damaging liquid. This liquid can be problematic, as it can contaminate your products or equipment, causing frozen pipes, and possibly leading to corrosion and other issues.

Now that we know how important they are how do you know which one is right for you?

Types of compressed air Dryers

Refrigerant Dryer – the most commonly used type, the air is cooled in an air-to-refrigerant heat exchanger. (Here is a great blog deep diving on Refrigerant Dryers)
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). (Here is a great blog deep diving on Desiccant Dryers)
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. (Here is a great blog deep diving on Deliquescent Dryers)
Membrane Type– use special membranes that allow the water vapor to pass through faster than the dry air, reducing the amount of water vapor in the air stream. (Here is a great blog deep diving on Membrane Dryers)

The selection of an air dryer is done best by the professional who knows or learns the particular end uses, the amount of moisture which each use can tolerate and the amount of moisture which needs to be removed to achieve this level. Air, which may be considered dry for one application, may not be dry enough for another. Dryness is relative. Even the desert has moisture. There is always some moisture present in a compressed air system regardless of the degree of drying.

For compressed air, the best way to specify dryness is to cite a desired pressure dew point. Different types of dryers, therefore, are available with varying degrees of pressure dew point performance. To specify dew point lower than required for an application is not good engineering practice. (Naming a pressure dew point is how to state the degree of dryness wanted.) It may result in more costly equipment and greater operating expense.

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

Jordan Shouse
Application Engineer

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Drying Supply Side Air With Heat of Compression 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

Heat of compression image: Compressed Air Challenge: Drive down your energy costs with heat of compression recovery:

Intelligent Compressed Air: What You Need To Know About Membrane Dryers

After nitrogen and oxygen, water vapor is the third most abundant (by percentage) component of air. Because of the numerous problems that moisture causes in compressed air systems, it’s critical to have measures in place to remove it. That’s why any industrial air compressor will be equipped with a dryer.

There are a number of dryer types to choose from, all with their own “pros & cons” based on variables such as compressor size, installation environment, and specifics of what the compressed air is going to be used for.

Membrane dryers are among the newest technologies used in compressed air treatment. They work by osmosis…that’s the principle by which a selectively permeable membrane will allow some stuff (but not all stuff) to pass through. In biology, it’s how:

  • The cells in a plant’s roots draw moisture from soil.
  • Your blood picks up oxygen from your lungs & nutrients from your digestive system and delivers it to your organs.
  • Placing the textbook under your pillow tonight transfers information you’ll need for the big exam tomorrow into your brain.

OK; that last one isn’t true, and you’d know that if you’d read the textbook. In addition to the natural world, the principle is also exploited in industry for commercial gain:

  • Desalination and purification of water.
  • Nitrogen generation.
  • Removing water and water vapor from compressed air.

For the purposes of today’s blog, that last one is the one we’re interested in. The construction of a membrane dryer consists of a cylinder filled with tiny polymer tubes with a special coating on their inner walls…this is the membrane itself, which lets the water pass through, via the abovementioned principle of selective permeation.

As compressed air enters the cylinder, it’s directed through the polymer tubes, which allow water (but not air) to pass through their walls due to the difference in partial pressure between the gases (e.g., compressed air & water vapor) on the inside, & outside, of the tubes. Air flow, traveling in the opposite direction outside the tubes, sweeps the water out. The higher the sweep air flow rate, the lower the dew point of the compressed air out.

Membrane dryers have no moving parts, and use no electricity…their only utility load is the compressed air consumption of the sweep (also called “purge”) air flow. This is noteworthy, as it can be as high as 15-20% of the compressed air flow, if maximum dew point suppression is desired.

Due to their simple & compact design, they’re among the easiest dryers to install, and they’re unaffected by environmental contamination & ambient temperature. They are, however, quite sensitive to internal contamination. Membrane dryer systems typically incorporate a proprietary filtration system to remove oil/oil vapor and fine particulates from the compressed air flow.

The biggest limitation of a membrane dryer is their flow capacity – they’re going to max out at about 200 SCFM, so they’re best suited to small-to-mid size systems. If a particular area of the facility requires a lower dew point than the rest of the plant, a membrane dryer is definitely worth a look. That’s an ideal fit for its “pros”:

  • Low capital, and operating costs.
  • Ease of installation, even in a compact space
  • Maintenance free

And the “cons” are minimalized:

  • Purge air flow has to be a higher percentage of total flow for lower dew point, but it’s a percentage of a specific area’s air flow, not the whole system.
  • Lower total air flow means smaller/less expensive filtration.

EXAIR Corporation wants to help you get the most out of your compressed air system. If you’d like to find out more, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
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Membrane Dryer Schematic – From Compressed Air Challenge, Best Practices for Compressed Air Systems, Second Edition

Air Compressor System photo courtesy of thomasjackson1345 Creative Commons Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)

Intelligent Compressed Air: Refrigerant Dryers

When we talk with customers about their EXAIR Products, we also discuss the quality of their compressed air. Many of our products have no moving parts and are considered maintenance-free when supplied with clean, moisture free compressed air. One of the most critical aspects of a compressed air distribution system is the dryer.

No matter where you are in the world, the atmospheric air will contain water vapor. Even in the driest place in the world, McMurdo Dry Valley in Antarctica, there is some moisture in the air. 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 increase of 20°F. In regions or periods of warmer temperatures, this poses an even greater 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
  • Can affect the color, adherence, and finish of paint that is applied using compressed air
  • Jeopardizes processes that are dependent upon pneumatic controls. A malfunction due to rust, scale, or clogged orifices can damage product or cause costly shutdowns
  • In colder temperatures, the moisture can freeze in the control lines

In order to remove moisture from the air after compression, a dryer must be installed at the outlet of the compressor. It is recommended to dry the compressed air to a dew point at least 18°F below the lowest ambient temperature to which the distribution system or end use is exposed. A dew point of 35-38°F is often sufficient and can be achieved by a refrigerated dryer (Best Practices for Compressed Air Systems). This makes the refrigerant dryer the most commonly used type in the industry.

A refrigerant dryer works by cooling the warm air that comes out of the compressor to 35-40°F. As the temperature decreases, moisture condenses and is removed from the compressed air supply. It’s then reheated to around ambient air temperatures (this helps to prevent condensation on the outside of distribution piping) and sent out to the distribution system.

With your air clean and dry at the point of use, you’re making sure you get the most out of EXAIR’s Intelligent Compressed Air Products without adhering to pesky maintenance procedures.

Tyler Daniel
Application Engineer
Twitter: @EXAIR_TD

Compressor image courtesy of Tampere Hacklab via Flickr Creative Commons License