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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Cabinet Coolers and Water?

I enjoy the days in Fall when you have the cool mornings, and the sunny afternoons. Have you awakened in the morning, poured yourself a hot cup of java, and looked outside your window? You notice that the grass, the leaves on the trees, and the seat of your lawn tractor are wet. The reason for this is attributed to dew point. Dew point is the temperature at which water vapor will condense and form water droplets. That same term applies in compressed air. If the dew point temperature and the air temperature are equal, then the air is 100% saturated (water vapor can start condensing to form water droplets).

Another way to get water in your compressed air system is by pressurizing it. When you take ambient air and compress it, the amount of “elbow” room for 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, like your compressor, the sponge will not be able to hold onto the water, and it will release the excess. Under that same hand pressure, the sponge is still fully saturated (i.e. if you continue to squeeze the sponge and dip it back into the water, it will not be able to absorb any more water). The compressed air system is the same. As soon as the air is compressed, water will start to form and fall out of the compressed air. Now you have water in your compressed air lines.

     A customer asked me about our Cabinet Cooler® system. He said that if we reduce the temperature by 54 ⁰F (30 ⁰C) in an electrical panel, will water condense onto the circuitry? Electricity and water can be a disaster but in this case we can be confident of no condensation on the circuitry. I researched this phenomenon a little further, see the details and analogy below.

NEMA 12 EXAIR Cabinet Cooler

NEMA 12 EXAIR Cabinet Cooler

Most facilities have some type of compressed air dryer in their system. This will reduce the dew point of the compressed air system. As an example, a refrigerated dryer will reduce the pressure dew point to 40 ⁰F (4.5 ⁰C). This means that liquid water will not be present in your compressed air line until the temperature is below 40 ⁰F (4.5 ⁰C). I also know that when you expand the air from 100 psig (6.9 barg) to atmospheric pressure, the air will become dryer (or the dew point will become less). Just like the example of the sponge, if you loosen your grip (going from a pressurized system to a non-pressurized system), the sponge will become “dryer” and can now absorb more water. As we combine these two concepts, we can determine if water will condense from the compressed air and become “dew” on the electrical components. If we take a typical 70 ⁰F (21 ⁰C) plant, the Cabinet Cooler® will cool the air to 16 ⁰F (-9 ⁰C). (The specification of our Cabinet Cooler® at 100 psig (6.9 barg) and 54 ⁰F (30 ⁰C) temperature drop). Let’s calculate the dew point temperature of the air exiting the Cabinet Cooler®. In looking at an elevated pressure/atmospheric pressure dew point chart, the 40 ⁰F (4.5 ⁰C) dew point of your compressed air line will drop to -6 ⁰F (-21 ⁰C) when it expands to atmospheric pressure. Thus, the temperature of the air coming out of the Cabinet Cooler® is 20 ⁰F (-7 ⁰C), and the dew point is -6 ⁰F (-21 ⁰C). So, no water will condense from the compressed air. With proper filtration, the efficiency and effectiveness of your Cabinet Cooler® will last you a long time and keep your electrical components cool and dry.

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

 

Image courtesy of Windell Oskay. Creative Comment License

You Have Too Much Water in Your Compressed Air

I have been working with a couple of overseas distributors lately who have projects involving vortex tubes that are tasked with cooling a chamber down to temperatures in the -10 to -20°C range. One of the projects was having some difficulty though. It seemed that in the beginning of the day, the vortex tube would function perfectly, but as the day wore on, the vortex tube would “stop working” as the customer would say.

Vortex tube

We went back and forth a few times and I finally determined that the customer’s compressed air supply had a dew point that was higher than the -20°C output flow they were achieving in the beginning of the day. The mechanics of what was happening is that the vortex tube would cool an initial volume of compressed air and the moisture was condensing out of the airflow and lying inside the vortex tube body. When the subsequent “on” cycles would occur, that left over water would freeze up inside the vortex tube generator, eventually plugging it up.

And so the cycle after that point was reached would be one of freeze, plug up, thaw and freeze again. Not a very reliable way to go about the application. And so, in order for the customer to have a more reliable process, they had to dry their compressed air with a refrigerant type air dryer to drop that dew point before the compressed air entered the vortex tube. Once they did that, no more troubles.

Neal Raker, Application Engineer
nealraker@exair.com

Compressed Air and Dew Point

Today’s discussion is on dew point of air as it has a significant impact on a compressed air system. The dew point is the temperature at which the water vapor in the air  can no longer stay in a vapor form, and condenses from a vapor into a liquid. The amount of water vapor contained in air is directly proportional to its temperature. The warmer the air the more space there is between molecules thus it is able to hold more water vapor.Capture

It is when air temperature drops below the dew point that issues develop in a compressed air system. Let’s take the example of a warm summer day at 90 F and 50% relative humidity. From the chart we see the dew point temperature to be 70 F. So at night, when all the equipment is shut down and the temperatures drop into the 60’s, water will condensate throughout the entire system. In the morning when the equipment is turned on, water blows through sensitive valving.

Compressing air will increase the dew point. Hot compressed air exiting the compressor and cooling while it makes its way through distribution systems is one reason for condensate in compressed air lines. Drying the compressed air is recommended to reduce or eliminate water condensate problems in a compressed air system.

There are several methods to dry out your compressed air. Each have their advantages and disadvantages. The following short review of the various options will help you decide which is best for your application.

After-coolers

The compressor’s after-cooler  which looks similar to a car’s radiator or the condenser in an air conditioner, is the first step to dryer air. It is placed at the compressor’s air outlet and uses either ambient air or water to cool the compressed air and condense some of the water vapor into a liquid that can be removed with a water separator.

The simplicity of design is a positive. The negative is that it can never cool below ambient but something above ambient depending on its capacity. After-cooler performance is rated by approach temperature, which is how closely the compressed air leaving the after-cooler will approach the temperature of the cooling medium used.

For example, if an air-cooled after-cooler is rated for a 10°F approach temperature, and the temperature of the ambient air is 90°F, the temperature of the air leaving the after-cooler will be 100°F. Assuming 50% relative humidity day the dew point will be 80 F.

Mechanical Water Separators

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Wet compressed air enters the separator and passes through a set of vanes that spins it in a vortex. Centrifugal force causes liquid to fly out of the compressed air stream and run down the inside of the filter bowl, where it can be drained off. These are installed at the point of use as a final defense before entering sensitive compressed air equipment. They are an inexpensive assurance of quality air. The ones EXAIR has also include a sintered bronze filter element to remove dirt and scale as well as water.

Deliquescent Dryer

A deliquescent dryer is basically a tank full of salt tablets. As the compressed air passes through the salt, the salt attracts water and dissolves into a brine that can be drained off. These are the least expensive dryers to purchase and maintain because they have no moving parts and require no power to run. The operating cost consists of the cost of more salt tablets.

Desiccant Air Dryers

These are similar to the deliquescent driers except they use a desiccant that attracts water but holds it. When they have reached their saturation limit they are either replaced or regenerated in one of three methods.

Operating cost of these dryers varies with the method used to remove water from or regenerate the desiccant.

Heatless regenerative dryers take a portion (about 15%) of the dry compressed air leaving the dryer and passes it through the desiccant to absorb the moisture out of it. Purchase cost economical but operational costs are high because if all the compressed air used to dry out the desiccant.

Heated purge regenerative dryers take advantage of the fact that hot air can hold more water than cold air. These dryers take about 5% of the dry compressed air leaving the dryer and pass it through an electric heater and then sends it through the wet desiccant bed. This dryer cost more than the heat less dryer but is offset by using half the compressed of that used by the heat less dryer.

Blower Purge Dryers

These are similar in concept to the had dryers found in restrooms but on a larger scale. Heated air is sent trough the desiccant with a blower. These are not quite as efficient because they are heating up ambient air which would not be as dry as compressed air.

Membrane Air Dryers

These dryers use pass the compressed air through a membrane with pores large enough to allow air molecules through but not large enough to allow water molecules through. The lower a dew point is needed, the more purge air is required. These

Refrigerated Air Dryers

Is an A/C system that refrigerate  the compressed air as close to freezing as possible in order to condense out as much water as possible then use a mechanical water separators to remove the condensed water. They require electricity to operate along with the associated cost of operation and maintenance.

Hopefully this gives you a better understanding on how to qualify your compressed air.

Feel free to contact me at any time with questions or concerns, or if I can be of any further assistance. I genuinely appreciate the opportunity! 1-800-903-9247 or click on the live chat icon in the upper left hand corner.

Joe Panfalone

Application Engineer
Phone (513) 671-3322
Fax (513) 671-3363
Web: http://www.exair.com
Twitter: http://www.twitter.com/exair_jp
Facebook: http://www.facebook.com/exair

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