Tag: dew point

How Do Membrane Dryers Work?

Published on by Russ BowmanLeave a comment

Water and water vapor can cause big problems in a compressed air system. That’s why compressed air dryers are a critical component of those systems. There are a few different types, and they all have their pros and cons. My colleague Jordan Shouse summed up the selection process pretty succinctly in a past blog: Compressed Air Dryers : What are they Good For? I particularly enjoyed his reference to Edwin Starr’s famous protest song, War (What Is It Good For?)

While they are all indeed “good for (as Jordan said) absolutely A LOT”, I wanted to take (another) deeper dive into one particular type. Membrane dryers are among the newer types of compressed air dryers. As the name suggests, they use a semi-permeable membrane whose structure allows molecules of certain fluids to pass through faster than others, thereby separating those certain fluids from the primary volume. Membrane separation technology is commonly used in:

  • Water purification and desalination (removal of particulates and salts)
  • Nitrogen separation (removal of oxygen and other trace gases)
  • Removing moisture from compressed air (removal of water vapor)

A membrane dryer is made of a cylinder containing very small polymer tubes that have a special coating inside. This coating allows the above-mentioned water & water vapor to pass through more readily than the nitrogen, oxygen, and other trace gases found in the atmospheric air that the compressor has drawn in.

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.

The advantages of membrane dryers are:

  • No moving parts to wear or break.
  • No electricity required.
  • Easy to install.
  • Unaffected by environmental contamination.
  • Compact design.

While environmental contamination isn’t a concern, they are very sensitive to internal contamination, like oil & oil vapor. Membrane dryers are commonly supplied with coalescing filters to minimize any issues there.

One disadvantage (kind of) is their limited flow capacity: the highest capacity membrane dryers are capable of passing around 200 SCFM, meaning they can only be used in small-to-mid-sized systems. I said “kind of” above because, if a certain part of the compressed air system requires a different level of dry air than the rest of the plant, membrane dryers are an inexpensive and easy way to provide that. That’s “kind of” an advantage, in that situation, actually.

Another disadvantage (kind of) is the amount of purge air they lose. Remember, membrane technology means certain fluids will pass through faster & more readily – but not exclusively – so they’re going to lose some of that brand new compressed air along with the water vapor. And the lower you want the dew point to be, the more purge air will be lost. However, the purge air loss is minimal when they’re discharging air dried to a dew point in the 40°F range, and that’s perfectly acceptable in many industrial compressed air systems.

I want to help you get the most out of your compressed air system, so when you think of compressed air, think EXAIR! If you’ve got questions, we’re here to help. Give me a call.

Russ Bowman, CCASS

Application Engineer
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Air Quality Classes: ISO 8573-1

Published on by John BallLeave a comment

Airborne particles surround us everywhere.   In a general work environment, nearly four million particles per cubic foot are floating around us at any given time.  When an air compressor brings in this air, the concentration increases substantially.  So, compressed air is not only expensive to make, but very dirty.  As the air exits your air compressor and travels into your pneumatic system, there is so much contamination, that the International Standard Organization, ISO, created an Air Quality chart with Purity Classes.

This chart is easy to follow and can be found on the International Organization for Standardization; ISO 8573-1 for Air Quality.  It is used to select a cleanliness level for your compressed air system. Contamination is categorized into three areas; Particles, Water, and Oil (reference above).  Each class is associated with a number for each category ranging from 0 (most stringent) to 9 (most relaxed).  As an example, the Air Quality value of ISO 8573-1:2010 [1.2.4] has Class 1 for Particles, Class 2 for Water, and Class 4 for Oil.  These class values will show the maximum value in each category.

To define the categories in more detail, I will separate the three to discuss the origins and solutions.

Per the descriptions above, here are the criteria by which compressed air purity is classified.

Particles: For solid particles, this part comes from many different areas.  The surrounding ambient air that is being drawn into the air compressor is filtered, but the intake filter will only remove large diameter particles.  The smaller diameter particles will go through the filter and into the compressed air system.  Another part is rust particles that come from steel air pipes and receiver tanks.  Over time, rust will flake off and create particles that can affect pneumatic equipment.  Other particles can come from components inside the air compressor, valves, etc., that wear and breakdown.  In the ISO column for Particles, it is separated into three different micron ranges and concentrations.  The removal of particles from the compressed air is done by traps and compressed air filters.  EXAIR offers two types; Filter Separators with 5-micron filtration and Oil Removal Filters with 0.03-micron filtration.  There are other types of filtration systems depending on your ISO requirement.

Water:  Humidity is a natural occurrence.  It can be measured as a dew point temperature.  This is the temperature at which water will condense and make rain.  Inside an air compressor, the air is ‘squeezed”, and the amount of space for water vapor is reduced.  So, it will condense into liquid form as “rain” inside the pipes.  Air that comes out from an air compressor will always be saturated with water.  To remove liquid water, a mechanical device can be used.  Inside a Filter Separator, a centrifugal separator will spin the air and remove the liquid water.  To remove water vapor, a compressed air dryer is required, like a refrigerant, desiccant, deliquescent, or membrane type.  Each type will have a maximum dew point range that they can reach.  As an example, a refrigerant type will reduce the dew point to 37oF (3oC).  That means that water will not condense until the temperature reaches below 37oF (3oC).

Oil: This category can be found as a liquid, aerosol or vapor, and it includes more than just oil. It contains small hydrocarbons like CO, CO2, SO2, and NOX.  Oil mainly comes from inside an oil-flooded air compressor.  As the air passes through the compressor, it will pick up remnants of oil aerosols and carry them downstream.  With high temperatures inside the air compressor, some of the oil will vaporize.  Even with oil-less type air compressors, carbon vapor can still be an issue.  Small hydrocarbons can come through the air intake and condense inside the system like water vapor above.  To remove the liquid and aerosol type of oil, Oil Removal Filters can be used.  They are designed to “coalesce” the small particles into larger particles for gravity to remove.  Oil vapor requires activated carbon to remove it.  These types of filter units will adsorb the vapor.  This helps to remove odors as well as dangerous chemical vapors that may be in the compressed air line.

There are a variety of pneumatic systems that use the ISO 8573-1 standard.  This will include breathing air operations, food and beverage, pharmaceutical, and the electronics industry.  If you need stringent requirements for your compressed air system, the Air Quality standard should be used by referring to the class numbers above.  This helps to dictate the types of filtration and air dryers that should be used within your pneumatic system.  If you have any questions about your compressed air system, an Application Engineer at EXAIR will be happy to help you.

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

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

Refrigerant Dryers for Compressed Air

Published on by EricKuhnash@EXAIR.comLeave a comment

A refrigerant air dryer is is used with compressed air for removing moisture from the compressed air system . Compressed air always contains water because your are taking in outside air which contains moisture. It is vital to have a compressed air dryer system in place to protect your equipment and tooling from damage. How does a refrigerant dryer work and why select this type of dryer system?

Refrigerant air dryers are commonly used as they are easy to operate, economical and low maintenance. Once installed it is likely that you may never have to think about it again. The system works by cooling the air to around 37 degrees Fahrenheit. At this point all the water vapor condenses into water. The water can then be removed by a simple water trap. Once the liquid water has been removed the air gets reheated to room temperature. Since most of the water has been condensed and removed the reheated air will be significantly dryer than from before.

Fundamental Schematic of a Refrigerant-Type Dryer

The cooling process of refrigerated dryers is the same process used in refrigerators and freezers. The liquid refrigerant is evaporated in a separate circuit and used to cool down the compressed air. As the air cools the refrigerant gets warmer. The refrigerant moves into a compressor and gets re-cooled in a condenser, this process is a continuous cycle as more air is introduced into the compressor.

Here a few items to consider when making your purchase:

Maximum pressure: The dryers max pressure should be the same or higher than your compressor.

Inlet Temperature: If you exceed maximum inlet temperature you are at risk of damaging parts of your equipment. Some refrigerated dryers have an after cooler making sure your compressed air stays within acceptable temperature ranges.

Maximum Flow: Make sure your dryer has the capacity needed to there are no drops in air pressure.

Maximum Room Temperature: If you are placing your refrigerated dryer into a room with a hot environment there is a change that it could overheat, Make sure that your max operating temperature for the dryer is able to accommodate the max temperatures in the room where it will be operating.

EXAIR wants you to be successful in every aspect of your compressed air system. If you have a need for any of EXAIRS Intelligent Compressed Air Systems please give any of our Application Engineers a call or contact us through our TecHelp.

Eric Kuhnash
Application Engineer
E-mail: EricKuhnash@exair.com
Twitter: Twitter: @EXAIR_EK