Tag: dewpoint

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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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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