What Makes A Compressed Air System “Complete”?

It’s a good question.  When do you know that your compressed air system is complete?  And, really, when do you know, with confidence, that it is ready for use?

A typical compressed air system. Image courtesy of Compressed Air Challenge.

Any compressed air system has the basic components shown above.  A compressed air source, a receiver, dryer, filter, and end points of use.   But, what do all these terms mean?

A compressor or compressed air source, is just as it sounds.  It is the device which supplies air (or another gas) at an increased pressure.  This increase in pressure is accomplished through a reduction in volume, and this conversion is achieved through compressing the air.  So, the compressor, well, compresses (the air).

A control receiver (wet receiver) is the storage vessel or tank placed immediately after the compressor.  This tank is referred to as a “wet” receiver because the air has not yet been dried, thus it is “wet”.  This tank helps to cool the compressed air by having a large surface area, and reduces pulsations in the compressed air flow which occur naturally.

The dryer, like the compressor, is just as the name implies.  This device dries the compressed air, removing liquid from the compressed air system.  Prior to this device the air is full of moisture which can damage downstream components and devices.  After drying, the air is almost ready for use.

To be truly ready for use, the compressed air must also be clean.  Dirt and particulates must be removed from the compressed air so that they do not cause damage to the system and the devices which connect to the system.  This task is accomplished through the filter, after which the system is almost ready for use.

To really be ready for use, the system must have a continuous system pressure and flow.  End-use devices are specified to perform with a required compressed air supply, and when this supply is compromised, performance is as well.  This is where the dry receiver comes into play.  The dry receiver is provides pneumatic capacitance for the system, alleviating pressure changes with varying demand loads.  The dry receiver helps to maintain constant pressure and flow.

In addition to this, the diagram above shows an optional device – a pressure/flow control valve.  A flow control valve will regulate the volume (flow) of compressed air in a system in response to changes in flow (or pressure).  These devices further stabilize the compressed air system, providing increased reliability in the supply of compressed air for end user devices.

Now, at long last, the system is ready for use.  But, what will it do?  What are the points of use?

Points of use in a compressed air system are referred to by their end use.  These are the components around which the entire system is built.  This can be a pneumatic drill, an impact wrench, a blow off nozzle, a pneumatic pump, or any other device which requires compressed air to operate.

If your end use devices are for coating, cleaning, cooling, conveying or static elimination, EXAIR Application Engineers can help with engineered solutions to maximize the efficiency and use of your compressed air.  After placing so much effort into creating a proper system, having engineered solutions is a must.

Lee Evans
Application Engineer

Intelligent Compressed Air: Deliquescent Dryers – What are They and How do They Work?

EXAIR has written blogs about the different types of dryers that are used to remove liquid from compressed air systems. In this blog, I will be discussing the deliquescent dryer. This dryer falls under the desiccant dryer category, and unlike the regenerative cousins, it is the least commonly used type of dryer. The regenerative desiccant dryers use a medium that will adsorb the water vapor, and the deliquescent dryers use a hygroscopic material that will absorb the water vapor. This salt-like medium has a strong affinity for water, and it comes in a tablet or briquette form. Placed inside a single unit pressure vessel, the “wet” compressed air passes through the bed to become dry. The size of the pressure vessel is determined by the compressed air usage which allows for the proper amount of contact time with the hygroscopic bed. Generally, the dew point will be between 20 to 50 deg. F (11 – 28 deg. C) less than the compressed air inlet temperature. Unlike most dryers, the dew point after deliquescent dryers will vary with the inlet air temperatures.

Vessel Design

The design of vessel is very important for the function of a deliquescent dryer. A grate is required to hold the medium off the bottom. The compressed air will flow from the bottom, up through the bed, and out from the top. The predetermined space between the bed and the bottom of the vessel is used for the liquid that is generated. When “wet” compressed air passes through the bed, the hygroscopic material will absorb the water and change the tablets from a solid into a liquid. Deliquescent dryers got the name from the definition of the verb, “deliquesce” which is “becomes liquid by absorbing moisture from the air”. Once the material is turned into a liquid, it cannot be regenerated. The liquid must be discarded periodically from the vessel and new solid material must be added. With the single tower design, the deliquescent dryers are relatively inexpensive.

Some advantages in using the deliquescent dryers are that they do not require any electricity or have any moving parts. So, they can be used in remote locations, rugged areas, or hazardous locations. They are commonly used to reduce the dew point in compressed air, natural gas, landfill gas and biogas systems. Without the ability for regeneration, no additional compressed air will be lost or used. In comparing the power requirement to other compressed air dryers, the deliquescent dryers have the lowest power requirement at 0.2Kw/100 cfm of air. (This energy rating is only due to the additional power required for the air compressor to overcome the pressure drop in the dryer).

Some disadvantages in using the deliquescent dryers is that the hygroscopic material degrades. The deliquesced liquid does have to be drained and disposed, and new material does have to be added. Even though they do not have any moving parts, they still require periodic maintenance. The deliquescent material can be corrosive. So, after-filters are required to capture any liquid or dust material that may carry over and damage downstream piping and pneumatic components. Also, the variation in the dew point suppression can limit locations and areas where it can be used.

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Products, you can contact an Application Engineer at EXAIR. We would be happy to hear from you.

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


Photos:  used from Compressed Air Challenge Handbook

Intelligent Compressed Air: Membrane Dryers – What are they and How Do they Work?

Recently we have blogged about Compressed Air Dryers and the different types of systems.  We have reviewed the Desiccant and Refrigerant types of dryers, and today I will discuss the basics of  the Membrane type of dryers.

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.

Membrane Dryers are the newest type of compressed air dryer. Membranes are commonly used to separate gases, such as removing nitrogen from air. The membrane consists of a group of hollow fiber tubes.  The tubes are designed so that water vapor will permeate and pass through the membrane walls faster than the air.  The dry air continues on through the tubes and discharges into the downstream air system. A small amount of ‘sweep’ air is taken from the dry air to purge and remove the water vapor from inside the dryer that has passed through the membrane tubes.

Membrane Dryer
Typical Membrane Dryer Arrangement

Resultant dew points of 40°F are typical, and dew points down to -40°F are possible but require the use of more purge air, resulting in less final dry compressed air discharging to the system.

The typical advantages of Membrane Dryers are-

  1.  Low installation and operating costs
  2.  Can be installed outdoors
  3.  Can be used in hazardous locations
  4.  No moving parts

There are a few disadvantages to consider-

  1. Limited to low capacity systems
  2. High purge air losses (as high as 15-20% to achieve lowest pressure dew points
  3. Membrane can be fouled by lubricants and other contaminants, a coalescing type filter is required before the membrane dryer.

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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Membrane Dryer Schematic – From Compressed Air Challenge, Best Practices for Compressed Air Systems, Second Edition




Heat of Compression Dryers

A Heat of Compression regenerative desiccant dryer for compressed air

Before compressed air can be realistically utilized, it needs to be delivered to the point of use with proper volume and pressure, and it should also be clean and have some moisture removed.  We have information available regarding cleaning compressed air, but how do you dry the compressed air?  And why do you dry the compressed air?

Drying compressed air is akin to removing the humidity in the air when using an air conditioning system.  If the moisture is not removed, the effectiveness of the system is reduced and the ability to use the output of the system is reduced as well.

But, from a functional standpoint, what does this really mean?  What will take place in the compressed air system if the air is not dried and the moisture is allowed to remain?

The answer is in the simple fact that moisture is damaging.  Rust, increased wear of moving parts, discoloration, process failure due to clogging, frozen control lines in cold weather, false readings from instruments and controls – ALL of these can happen due to moisture in the compressed air.  It stands to reason, then, that if we want long-term operation of our compressed air products, having dry air is a must.

So, how can we remove the moisture in the compressed air?  One of the most common methods to remove moisture is a regenerative dryer, specifically, heat-of-compression type dryers.  A heat of compression type dryer is a regenerative desiccant dryer which uses the heat generated by the compression of the ambient air to regenerate the moisture removing capability of the desiccant used to dry the compressed air.

When using one of these dryers, the air is pulled directly from the outlet of the compressor with no cooling or treatment to the air and is fed through a desiccant bed in “Tank 1” where it regenerates the moisture removing capabilities of the desiccant inside the tank.  The compressed air is then fed through a regeneration cooler, a separator, and finally another desiccant bed, this time in “Tank 2”, where the moisture is removed.  The output of “Tank 2” is supplied to the facilities as clean, dry compressed air.  After enough time, “tank 1” and “tank 2” switch, allowing the hot output of the compressor to regenerate the desiccant in “tank 2” while utilizing the moisture removing capabilities of the desiccant in “tank 1”.

Heat of compression dryers offer a lower power cost when compared to other dryers, but they are only applicable for use with oil free compressor and to compressors with high discharge temperatures.  If output air temperatures from the compressor are too low, a temperature booster/heater is needed.

If you have questions about your compressed air system and how the end use devices are operating, contact an EXAIR Application Engineer.  We’ll be happy to discuss your system and ways to optimize your current setup.

Lee Evans
Application Engineer


Heated Desiccant Dryer by Compressor1.  Creative Commons License