What Are Compressed Air Dryers and Why are They Necessary?

desiccant-dryer

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, dry 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 Antartica, 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. There are three primary types of dryers used in the compressor industry: refrigerant, desiccant, and membrane. Each style has it’s own inherent applications and benefits.

Refrigerant type dryers cool the air, removing the condensed moisture before allowing it to continue through the distribution system. These will generally lower the dew point of the air to 35-40°F which is sufficient for most applications. So long as the temperature in the facility never dips below the dew point, condensation will not occur. Typical advantages of a refrigerant dryer include: low initial capital cost, relatively low operating cost, and low maintenance costs. This makes them a common solution used in an industrial compressed air system.

Another type of dryer is the desiccant dryer. I’m sure you’ve seen the small “Do Not Eat” packages that are included in a variety of food products, shoes, medications, etc. These are filled with a small amount of desiccant (typically silica gel) that is there to absorb any moisture that could contaminate the product. In a desiccant dryer, the same principle applies. The compressed air is forced through a “tower” that is filled with desiccant. The moisture is removed from the air supply and then passed into the distribution system. One minor drawback with a desiccant type dryer is that the desiccant material does have to periodically be replaced. Desiccant dryers can also be used in addition to a refrigerant dryer for critical applications that require all water vapor to be removed.

The third type of dryer is the membrane dryer. In this style, extremely low dew points are able to be achieved. This makes them the optimal choice for outdoor applications where the air could be susceptible to frost in colder climates. They are also ideal for medical and dental applications where consistent reliability and air quality is an absolute must. A membrane dryer does not require a source of electricity to operate and its compact size allows it to be easily installed with minimal downtime and floor space. Maintenance is minimal and consists of periodic replacement of the membrane. While they are good for some applications, they do come with limitations. They do limit the capacity of the system with variations ranging from as little as 1 SCFM to 200 SCFM. Because of this, they’re often used as a point-of-use dryer for specific applications rather than an entire compressed air system. Some of the compressed air must be purged with along with the moisture which consumes excess compressed air.

Regardless of what products you’re using at the point-of-use, a dryer is undoubtedly a critical component of that system. Delivering clean, dry air to your EXAIR Products or other pneumatic devices will help to ensure a long life out of your equipment.

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

Refrigerant Compressed Air Dryer Systems

No matter what your use of compressed air entails, moisture is very likely an issue.  Air compressors pressurize air that they pull in straight from the environment and most of the time, there’s at least a little humidity involved.  Now, if you have an industrial air compressor, it’s also very likely that it was supplied with a dryer, for this very reason.

There are different types of dryer systems, depending on your requirements.

For practical purposes, “dryness” of compressed air is really its dew point.  That’s the temperature at which water vapor in the air will condense into liquid water…which is when it becomes the aforementioned issue in your compressed air applications.  This can cause rust in air cylinders, motors, tools, etc.  It can be detrimental to blow offs – anything in your compressed air flow is going to get on the surface of whatever you’re blowing onto.  It can lead to freezing in Vortex Tube applications when a low enough cold air temperature is produced.

Some very stringent applications (food & pharma folks, I’m looking at you) call for VERY low dew points…ISO 8673.1 (food and pharma folks, you know what I’m talking about) calls for a dew point of -40°F (-40°C) as well as very fine particulate filtration specs.  As a consumer who likes high levels of sanitary practice for the foods and medicines I put in my body, I’m EXTREMELY appreciative of this.  The dryer systems that are capable of low dew points like this operate as physical filtration (membrane types) or effect a chemical reaction to absorb or adsorb water (desiccant or deliquescent types.)  These are all on the higher ends of purchase price, operating costs, and maintenance levels.

For many industrial and commercial applications, though, you really just need a dew point that’s below the lowest expected ambient temperature in which you’ll be operating your compressed air products & devices.  Refrigerant type air dryers are ideal for this.  They tend to be on the less expensive side for purchase, operating, and maintenance costs.  They typically produce air with a dew point of 35-40°F (~2-5°C) but if that’s all you need, they let you avoid the expense of the ones that produce those much lower dew points.  Here’s how they work:

  • Red-to-orange arrows: hot air straight from the compressor gets cooled by some really cold air (more on that in a moment.)
  • Orange-to-blue arrows: the air is now cooled further by refrigerant…this causes a good amount of the water vapor in it to condense, where it leaves the system through the trap & drain (black arrow.)
  • Blue-to-purple arrows: Remember when the hot air straight from the compressor got cooled by really cold air? This is it. Now it flows into the compressed air header, with a sufficiently low dew point, for use in the plant.

Non-cycling refrigerant dryers are good for systems that operate with a continuous air demand.  They have minimal dew point swings, but, because they run all the time, they’re not always ideal when your compressed air is not in continuous use.  For those situations, cycling refrigerant dryers will conserve energy…also called mass thermal dryers, they use the refrigerant to cool a solution (usually glycol) to cool the incoming air.  Once the glycol reaches a certain temperature, the system turns on and runs until the solution (thermal mass) is cooled, then it turns off.  Because of this, a cycling system’s operating time (and cost) closely follows the compressor’s load – so if your compressor runs 70% of the time, a cycling dryer will cost 30% less to operate than a non-cycling one.

EXAIR Corporation wants you to get the most out of your compressed air system.  If you have questions, I’d love to hear from you.

Russ Bowman
Application Engineer
EXAIR Corporation
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Intelligent Compressed Air: Refrigerant Dryers and How They Work

We’ve seen in recent blogs that Compressed Air Dryers are an important part of a compressed air system, to remove water and moisture to prevent condensation further downstream in the system.  Moisture laden compressed air can cause issues such as increased wear of moving parts due to lubrication removal, formation of rust in piping and equipment, quality defects in painting processes, and frozen pipes in colder climates.  The three main types of dryers are – Refrigerant, Desiccant, and Membrane. For this blog, we will review the basics of the Refrigerant type of dryer.

All atmospheric air that a compressed air system takes in contains water vapor, which is naturally present in the air.  At 75°F and 75% relative humidity, 20 gallons of water will enter a typical 25 hp compressor in a 24 hour period of operation.  When the the air is compressed, the water becomes concentrated and because the air is heated due to the compression, the water remains in vapor form.  Warmer air is able to hold more water vapor, and generally an increase in temperature of 20°F results in a doubling of amount of moisture the air can hold. The problem is that further downstream in the system, the air cools, and the vapor begins to condense into water droplets. To avoid this issue, a dryer is used.

Refrigerated Dryer
Fundamental Schematic of Refrigerant-Type Dryer

Refrigerant Type dryers cool the air to remove the condensed moisture and then the air is reheated and discharged.  When the air leaves the compressor aftercooler and moisture separator (which removes the initial condensed moisture) the air is typically saturated, meaning it cannot hold anymore water vapor.  Any further cooling of the air will cause the moisture to condense and drop out.  The Refrigerant drying process is to cool the air to 35-40°F and then remove the condensed moisture.  The air is then reheated via an air to air heat exchanger (which utilizes the heat of the incoming compressed air) and then discharged.  The dewpoint of the air is 35-40°F which is sufficient for most general industrial plant air applications.  As long as the compressed air stays above the 35-40°F temperature, no further condensation will occur.

The typical advantages of Refrigerated Dryers are-

  1.  – Low initial capital cost
  2.  – Relatively low operating cost
  3.  – Low maintenance costs

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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