Tag: dry air

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
Visit us on the Web
Follow me on Twitter
Like us on Facebook

Dryers On A New Level – Deliquescent Level Even.

Published on by Brian FarnoLeave a comment

Today I want to discuss dryers and not the type that I have repaired many times due to having three kids in my house. Speaking of which, the amount of hair that gets trapped within a dryer is one of the things my nightmares are made from. You’d think we have a Yeti living in our home. While the picture below is of our first dryer and washer, that platform has been repurposed into a workbench and the metals have all been recycled back into something else now. That’s not what we are here to talk about. Instead, we are going to discuss deliquescent dryers.

Deliquescent dryers can sometimes be confusing. Some compressed air dryer vendors use the terms deliquescent and desiccant synonymously, as if they were interchangeable in describing their equipment. Deliquescent dryers are not complex drying systems and are most commonly found in the petrochemical industry.

Deliquescent Dryer

Unlike any other dryer, a deliquescent dryer is also used to reduce or remove moisture before it turns to liquid water. These dryers can be installed indoors, outdoors, offshore, or in any remote location. They do not require electricity for operational purposes or have any moving parts, making them easy to maintain and economically more efficient. In a deliquescent dryer, moist air (gas) passes over a layer of deliquescent tablets that absorb moisture. The pressure dew point lowers as the tablets slowly dissolve, the condensation falls into the drain area, and the drier air flows through the outlet into the piping system.

The best deliquescent materials are salts due to their strong attraction to moisture. Deliquescent desiccants (drying tablets) are formulated from calcium chloride, magnesium chloride, potassium chloride, and lithium chloride. Not all deliquescent desiccants are equal. The final formulation and properties of the desiccant can significantly impact the design of a dryer tank. That is, the surface of the desiccant chemical, often beads or pellets, will liquefy, and the resulting liquid will flow to the bottom of the vessel. There is either a drain (manual or auto) at the base of the deliquescent dryer which is used to expel the collected fluid.

Some factors that will affect the consumption of the desiccant are the type of adsorbent, type of adsorbate, the size of the adsorbent bead or pellet, the concentration of the adsorbate in the compressed air stream, and the temperature of that air stream.

You will want to have a water trap, also known as a general-purpose compressed air filter, plumbed in line just upstream from the deliquescent dryer. Otherwise, any liquid water flowing with the compressed air into the air dryer will make short work of the desiccant chemical, requiring a more frequent—and expensive—recharge.

Compressing air generates heat. That hot, moist compressed air will consume the desiccant chemical in the deliquescent dryer much more quickly. The best practice is to ensure the airflow to the dryer is as cool as possible, with a long airline and a dwell tank before the deliquescent dryer, to allow the air to cool and have water saturate out naturally.

A deliquescent dryer can be expected to reduce the compressed air dew point by 20 – 30 deg. F, or so. The degree of drying depends how saturated the airflow is going in and on the type of deliquescent chemical used.

Unlike other forms of compressed air dryers, a deliquescent unit doesn’t guarantee the air will reach a certain dew point. The amount of water vapor in the air that exits the dryer is completely predicated on how much water vapor is in the air going into the dryer.

Brian Farno, MBA – CCASS Application Engineer

BrianFarno@EXAIR.com
@EXAIR_BF

Compressed Air Dryers : What are they Good For?

Published on by Jordan T ShouseLeave a comment

Absolutely Nothing….. err ALOT! They are really good for a lot! Specifically removing moisture/condensate from compressed air.

In almost every operation, clean, dry compressed air will result in lower operating costs. The purpose of compressed air dryers is to overcome the dew point of your compressed air by removing water from it. Compressed air can contain humidity, and in the right environments it can reach the dew point temperature and condense into a damaging liquid. This liquid can be problematic, as it can contaminate your products or equipment, causing frozen pipes, and possibly leading to corrosion and other issues.

Now that we know how important they are how do you know which one is right for you?

Types of compressed air Dryers

• Refrigerant Dryer – the most commonly used type, the air is cooled in an air-to-refrigerant heat exchanger. (Here is a great blog deep diving on Refrigerant Dryers)
• Regenerative-Desiccant Type – use a porous desiccant that adsorbs (adsorb means the moisture adheres to the desiccant, the desiccant does not change, and the moisture can then be driven off during a regeneration process). (Here is a great blog deep diving on Desiccant Dryers)
• Deliquescent Type – use a hygroscopic desiccant medium that absorbs (as opposed to adsorbs) moisture. The desiccant is dissolved into the liquid that is drawn out. Desiccant is used up and needs to be replaced periodically. (Here is a great blog deep diving on Deliquescent Dryers)
• Membrane Type– use special membranes that allow the water vapor to pass through faster than the dry air, reducing the amount of water vapor in the air stream. (Here is a great blog deep diving on Membrane Dryers)

The selection of an air dryer is done best by the professional who knows or learns the particular end uses, the amount of moisture which each use can tolerate and the amount of moisture which needs to be removed to achieve this level. Air, which may be considered dry for one application, may not be dry enough for another. Dryness is relative. Even the desert has moisture. There is always some moisture present in a compressed air system regardless of the degree of drying.

For compressed air, the best way to specify dryness is to cite a desired pressure dew point. Different types of dryers, therefore, are available with varying degrees of pressure dew point performance. To specify dew point lower than required for an application is not good engineering practice. (Naming a pressure dew point is how to state the degree of dryness wanted.) It may result in more costly equipment and greater operating expense.

If you have questions about compressed air systems and dryers or any of our 15 different Intelligent Compressed Air® Product lines, feel free to contact EXAIR, and I or any of our Application Engineers can help you determine the best solution.

Jordan Shouse
Application Engineer

Send me an Email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_JS

Drying Supply Side Air With Heat of Compression Dryers

Published on by Brian FarnoLeave a comment

The supply side of a compressed air system has many critical parts that factor in to how well the system operates and how easily it can be maintained.   Dryers for the compressed air play a key role within the supply side are available in many form factors and fitments.  Today we will discuss heat of compression-type dryers.

Heat of compression-type dryer- Twin Tower Version

Heat of compression-type dryers are a regenerative desiccant dryer that take the heat from the act of compression to regenerate the desiccant.  By using this cycle they are grouped as a heat reactivated dryer rather than membrane technology, deliquescent type, or refrigerant type dryers.   They are also manufactured into two separate types.

The single vessel-type heat of compression-type dryer offers a no cycling action in order to provide continuous drying of throughput air.  The drying process is performed within a single pressure vessel with a rotating desiccant drum.  The vessel is divided into two air streams, one is a portion of air taken straight off the hot air exhaust from the air compressor which is used to provide the heat to dry the desiccant. The second air stream is the remainder of the air compressor output after it has been processed through the after-cooler. This same air stream passes through the drying section within the rotating desiccant drum where the air is then dried.  The hot air stream that was used for regeneration passes through a cooler just before it gets reintroduced to the main air stream all before entering the desiccant bed.  The air exits from the desiccant bed and is passed on to the next point in the supply side before distribution to the demand side of the system.

The  twin tower heat of compression-type dryer operates on the same theory and has a slightly different process.  This system divides the air process into two separate towers.  There is a saturated tower (vessel) that holds all of the desiccant.  This desiccant is regenerated by all of the hot air leaving the compressor discharge.  The total flow of compressed air then flows through an after-cooler before entering the second tower (vessel) which dries the air and then passes the air flow to the next stage within the supply side to then be distributed to the demand side of the system.

The heat of compression-type dryers do require a large amount of heat and escalated temperatures in order to successfully perform the regeneration of the desiccant.  Due to this they are mainly observed being used on systems which are based on a lubricant-free rotary screw compressor or a centrifugal compressor.

No matter the type of dryer your system has in place, EXAIR still recommends to place a redundant point of use filter on the demand side of the system.  This helps to reduce contamination from piping, collection during dryer down time, and acts as a fail safe to protect your process.  If you would like to discuss supply side or demand side factors of your compressed air system please contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

Heat of compression image: Compressed Air Challenge: Drive down your energy costs with heat of compression recovery: https://www.plantservices.com/articles/2013/03-heat-of-compression-recovery/