Tag: dryer

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

Published on by Russ BowmanLeave a comment

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

Published on by Tyler DanielLeave a comment

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
E-mail: TylerDaniel@EXAIR.com
Twitter: @EXAIR_TD

Compressor image courtesy of Tampere Hacklab via Flickr Creative Commons License

Why Dryers Are Needed in Compressed Air Systems

Published on by Brian FarnoLeave a comment

Air compressors are extremely proficient at compressing anything in the air they are intaking. With that air that is taken in, moisture is going to be present. The amount of moisture will all depend on where you are located geographically and the ambient conditions in the area. Here in Ohio, we experience all 4 seasons so the moisture content is higher in the air during the summer months, rather than the winter months. When this air is saturated with water vapor and the conditions are right, the air reaches a point it cannot hold any additional water vapor. This point is known as the dew point of the air and water vapor will begin to condense to form droplets.

When ambient air is compressed, heat is generated and the air increases in temperature. In most industrial compressed air systems, the air is then processed to an aftercooler, and that is where condensation begins to form. To remove the condensation, the air then goes into a 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 transferred will cause more liquid to condense out of the air. To address this moisture, compressed air dryers are used.

It is critical to the quality of the system and components downstream that actions are taken to prevent this condensation in the air. Condensation is generally detrimental to any point of use application and or the piping that conveys the air. Rust and/or corrosion can occur anywhere in the piping, leading to scale and contamination of the compressed air and processes. When trying to dry products off using compressed air or using the air to atomize a liquid such as paint, adding in these contaminants and moisture will cost production losses.

There are several options when it comes to the type of dryer that one may consider installing on their compressed air supply side.

• Refrigerant Dryer – the most commonly used type, the 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 of water vapor in the 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 I or any of our Application Engineers can help you determine the best solution.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

Compressed Air Supply Side: What Is A Deliquescent Dryer, And When Would You Use One?

Published on by Russ BowmanLeave a comment

As we head in to the colder months here in Ohio, I will soon be getting my humidifier out of the basement and set up in my bedroom. The dry air that accompanies the onset of winter chaps my lips, cracks the skin on my knuckles, affects my nasal passages, and oftentimes makes me wake up with a sore throat…something I definitely don’t want to happen in the middle of a pandemic! So I put some water vapor in my home’s air, on purpose, to take care of all of that.

Moisture in an industrial compressed air system, however, isn’t good for anything.  It’ll corrode your pipes, get rust in your pneumatic tools, motors, and cylinders, and spit out of your blow off devices, all over whatever you’re using your air to blow off.  Depending on the type of compressor, where, and how, it’s used, there are different types of dryers.  Today, dear reader, we’re taking a look at one of the most basic moisture removal systems: the deliquescent dryer.  The principle of operation is as follows:

  • Deliquescent dryer: how it works (1)
    Incoming compressed air enters near the base, where a form of mechanical separation occurs…the air flows back & forth, around trays of desiccant.  The simple act of changing direction causes a certain amount of free liquid to just fall out and collect in the bottom.
  • The air then flows upwards through the desiccant bed. The desiccant in a deliquescent dryer absorbs moisture (as opposed to the adsorption that occurs in a regenerative desiccant dryer) until they get so wet, they dissolve.
  • The desiccant level has to be monitored (commonly via a sight glass) so it can be replaced as it’s consumed.
  • After the desiccant does its job, moisture free air flows out the top, and gets on with it’s work.

Deliquescent dryers, owing to their simplicity, are the least expensive air dryers.  They have no moving parts and no electricity, so the only maintenance involved is replacing the desiccant media as it’s consumed.  This makes them especially popular in mobile/on-site applications involving portable or tow-behind, engine driven compressors, since they don’t need power to run.

There are several disadvantages, also owing to their simplicity:

  • The deliquescent media has to be periodically replenished.  If you don’t stay on top of it, you can find yourself shut down while you go back to the shop to get a big bag of salt.  That’s time your boss can’t charge your customer for.  Also, the cost of the new media is a continual operating cost of the dryer…something you don’t have to account for with the regenerative desiccant models.
  • Disposal of the waste media can be a concern…you definitely want to check your local environmental regulations before dumping it in the garbage.  Your boss won’t like talking to the EPA about THAT either.
  • They have to be equipped with a particulate filter on the discharge to keep the deliquescent media (which, being a salt, is corrosive in nature) from entering your system.  That would be even worse than water moisture…which this is there to prevent in the first place.
  • They don’t get near as low of a dewpoint as other dryers – the best you can hope for is 20°F to 30°F.  Which is fine, given the above mentioned nature of applications where these are commonly used.  You just wouldn’t want to use them to supply a product like an EXAIR Vortex Tube…which can turn that in to -40°F cold air, causing the water vapor to turn to liquid, and then to ice.  In a hurry.

EXAIR Corporation is in the business of helping you get the most out of your compressed air.  If you want to learn more, please follow our blog.  If you have specific questions, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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(1) – Deliquescent Dryer Image: VMAC Air Innovated: The Deliquescent Dryer – https://www.vmacair.com/blog/the-deliquescent-dryer/