Tag: refrigerated air dryers

Dew Point and Water in Compressed Air: Understanding the Effects

Published on by John BallLeave a comment

In systems, it is important to understand the type of medium that is being used.  For most EXAIR products, this will be compressed air.  As the air compressor draws in ambient air, it also brings in dust, contamination, and moisture into the system.  If untreated, the pneumatic system will have to contend with these foreign “invaders” that will affect the performance of your pneumatic devices.  One of the most common problems is water.

Water enters the compressed air system from the water vapor already present in the ambient air, which is referred to as the dew point or relative humidity.  When you take ambient air and compress it, the amount of “elbow room” for the water vapor decreases.  This causes the water vapor to condense and create liquid water.  It would be similar to a water-soaked sponge.   As you compress it with your hands, the sponge will not be able to hold on to the water.  Similarly, as the air is compressed, water will start to form and fall out into the compressed air system.  Water is a by-product of a compressed air system.

Visual depiction of the impact of water vapor contained amongst air particles and how this reduces available volume during compression.

The definition for determining if liquid water is present in your system is called the pressure dew point.  Dew point is the temperature at which water vapor will condense and form water droplets.  If the dew point temperature and the air temperature are equal, then the air is considered 100% saturated (water vapor will start to condense to form water droplets).  In compressed air systems, air dryers are used to reduce the dew point temperature.  This means that unless the ambient temperature falls below the dew point temperature, water vapor will not condense into a liquid state.

There are two major types of compressed air dryers; refrigerated and desiccant.  The refrigerated air dryers are the most common, and the dew point is measured at about 39oF (4oC).  So, unless the air temperature gets close to freezing, i.e., the piping system that goes outside in cold weather, water should not be present.  Desiccant air dryers can achieve dew points as low as -40oF (-40oC).  This compressed air is very dry and can be used for medical systems, food and beverage processing, and instrument air.  The reason is that bacteria cannot survive in compressed air that is that dry.  The other types are dewpoint reducing systems, which include membrane and deliquescent dryers. 

Good engineering practice calls for point of use filtration and moisture removal, such as that provided by EXAIR Filter Separators.

For most pneumatic devices, a Filter Separator with an auto-drain should be used as a minimum amount of protection.  Even with systems that have compressed air dryers as described above, they are mechanical devices.  So, failures can occur.  You should review your compressed air system to ensure that your pneumatic system, including EXAIR products, is operating at peak efficiency.  This will include your supply system, compressed air leaks, and blow-off devices. 

Moisture-laden compressed air can cause issues such as increased wear on the pneumatic tools, the formation of rust in piping and equipment, quality defects in painting processes, and frozen pipes in colder climates.  Regardless of what products you’re using at the point-of-use, a compressed air dryer is undoubtedly a critical component of the compressed air system.  Providing clean, dry air to EXAIR Products or other pneumatic devices will help to extend the life of your equipment.  If you wish to discuss more about your compressed air system or how EXAIR can provide a more efficient way to use that compressed air, an Application Engineer will be happy to assist you.

John Ball
International Application Engineer


Email: johnball@exair.com
Twitter: @EXAIR_jb

Refrigerated Air Dryers

Published on by John BallLeave a comment

Whenever air gets compressed, it reduces the space for the water molecules to remain as a vapor; which causes condensation.  For this, compressed air dryers are an important part of a compressed air system.  They are designed to remove moisture to prevent condensation further downstream in the system.  The three main types of dryers are refrigerated, desiccant, and membrane. For this blog, I will cover the refrigerant-type compressed air dryers.

Compressed air dryers are rated with a dew point rating.  A dew point is the temperature at which the air has a relative humidity of 100%.  Since the air cannot become more saturated with water than 100%RH, water will condense and fall out like “rain”.  You can see this effect during the cool mornings when dew forms on the grass.  Compressed air dryers are designed to reduce the dew point temperature of your compressed air.  For a refrigerant type, they are near the dew point temperature of 38oF (3oC).  Like a refrigerator, they use refrigerant to cool the compressed air.  We cannot go below this temperature as it could form ice inside the dryer.  But, as long as the ambient temperature does not go below 38oF (3oC), liquid water will not be present in the pneumatic system. 

There are two main types of refrigerated air dryers; cycling and non-cycling.  Cycling type refrigerant air dryers will cool a liquid mass, generally a glycol-water mixture, to a set-point and turn off.  The liquid will go through an air-to-liquid heat exchanger to remove the heat from the compressed air.  Referring to the cycling action, when the liquid mass goes above the set point, the refrigeration system will restart and cool the liquid mass again.  The cycling refrigerant air dryers are more expensive, but they are more efficient. 

Non-cycling refrigerant air dryers are more common.  The refrigeration system continues to run through an air-to-air heat exchanger to cool the compressed air.  It is similar to your AC system in your car.  With this type of system, they are more susceptible to the environment, i.e., temperature, elevation, and humidity.  So, adjustments are required for proper installation. 

With both types of refrigerant dryers, the internal compressed air section is very similar.  They will have a filter separator to remove the liquid that is created from the condensation from the cold temperatures.  They also have an additional air-to-air heat exchanger.  This will provide two important features for the refrigerated air dryers.  As the cold air leaves the refrigerant section, it helps to cool the incoming compressed air.  This will make the system more efficient.  And as the hot incoming compressed air helps to warm the cold air leaving the dryer, it will stop the condensation of liquid water on the outside of the pipes.  Like the dew forming on the grass during cool mornings, the same will occur with the compressed air piping system. 

Moisture-laden compressed air can cause issues such as increased wear on the pneumatic tools, the formation of rust in piping and equipment, quality defects in painting processes, and frozen pipes in colder climates.  Regardless of what products you’re using at the point-of-use, a compressed air dryer is undoubtedly a critical component of the compressed air system.  Delivering clean, dry air to your EXAIR Products or other pneumatic devices will help to ensure a long life out of your equipment.  If you wish to discuss more about your compressed air system or how EXAIR can provide a more efficient way to use that compressed air, an Application Engineer will be happy to assist you. 

John Ball, CCASS


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

Photo: Grass morning dew by RuslanSikunov.  Pixabay License

What Are Compressed Air Dryers and Why are They Necessary?

Published on by Tyler DanielLeave a comment

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

Published on by Russ BowmanLeave a comment

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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