Rotary Scroll Compressors

Over the years, my EXAIR colleagues and I have blogged about different types of air compressor types including single and double acting reciprocating, rotary screw and sliding vane air compressors. You can click on the links above to check those out. Today, I will review the basics of the rotary scroll-type compressor.

The rotary scroll type compressor falls under the positive displacement-type, the same as the other types previously discussed.  A positive displacement type operates under the premise that a given quantity of air is taken in, trapped in a compression chamber and the physical space of the chamber is mechanically reduced.  When a given amount of air occupies a smaller volume, the pressure of the air increases.

Positive displacement type compressors

Each of the previous positive displacement type compressors use a different mechanism for the reduction in size of the compression chamber. The rotary scroll uses two inter-meshing scrolls, that are spiral in shape. One of the scrolls is fixed, and does not move (red).  The other scroll (black) has an “orbit” type of motion, relative to the fixed scroll. Air would be drawn in from the left, and as it flows clockwise through the scroll, the area is reduced until the air is discharged at a high pressure at the center.

How it Works

There is no metal to metal sliding contact, so lubrication is not needed.  A drawback to an oil free operation is that oil lubrication tends to reduce the heat of compression and without it, the efficiency of scroll compressors is less than that of lubricated types.

The advantages of the rotary scroll type compressor include:

  • Comes as a complete package
  • Comparatively efficient operation
  • Can be lubricant-free
  • Quiet operation
  • Air cooled

The main disadvantage:

  • A limited range of capacities is available, with low output flows

EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Jordan Shouse
Application Engineer

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Rotary Scroll GIF:  used from  Public Domain
Images Courtesy of  Compressor1 Creative Commons.


Discovery of The Vortex Tube

There are many theories regarding the dynamics of a vortex tube and how it works. Many students have studied them in hopes of advancing the physics or as part of their undergrad studies. The man that started it all was not intentionally researching it, however.

The Vortex Tube was invented by accident in 1928, by George Ranque, a French physics student. He was performing experiments on a vortex-type pump that he had developed and noticed that warm air exhausted from one end and cold air from the other. Ranque quickly changed his focus from the pump to start a company taking advantage of the commercial possibilities for this odd little device that produced both hot and cold air, using only compressed air, with no moving parts. The company was not successful, and the vortex tube was forgotten until 1945 when Rudolph Hilsch, a German physicist, published a widely read paper on the device.

A vortex tube uses compressed air as a power source, has no moving parts, and produces hot air from one end and cold air from the other. The volume and temperature of the two air streams is adjustable with a valve built into the hot air exhaust.  Temperatures as low as -50°F (-46°C) and as high as 260°F (127°C) are possible.

Compressed air is supplied to a vortex tube and passes through nozzles that are tangent to an internal counterbore. As the air passes through it is set into a spiraling vortex motion at up to 1,000,000 rpm. The spinning stream of air flows down the hot tube in the form of a spinning shell, like a tornado (in red). The control valve at the end allows some of the warmed air to escape and what does not escape reverses direction and heads back down the tube as a second vortex (in blue) inside of the low-pressure area of the larger warm air vortex. The inner vortex loses heat and exits through the other end of as cold air.

It is thought that both the hot and cold air streams rotate in the same direction at the same angular velocity, even though they are traveling in opposite directions. A particle of air in the inner stream completes one rotation in the same time of an air particle in the outer stream. The principle of conservation of angular momentum would say that the rotational speed of the inner vortex should increase because the angular momentum of a rotating particle (L) is equal to the radius of rotation (r) times it’s mass (m) times its velocity (v).  L = r•m•v.  When an air particle moves from the outer stream to the inner stream, both its radius (r) and velocity (v) decrease, resulting in a lower angular momentum. To maintain an energy balance for the system, the energy that is lost from the inner stream is taken in by the outer stream as heat. Therefore, the outer vortex becomes warm and the inner vortex is cooled.

At EXAIR, we have harnessed the cooling power of the vortex tube, and it can be found and utilized in such products as Spot CoolersCabinet Coolers, and Vortex Tubes themselves. If you have questions about Vortex Tubes, or would like to talk about any of the EXAIR Intelligent Compressed Air® Products, feel free to contact us.

Brian Farno
Application Engineer


A Brief History of Compressed Air

So where exactly did compressed air come from? How did it become so widely used and where will it go? Both of these are great questions and the answers lie below.

Compressed air can be traced all the way back to the classic bellows that were used to fuel blacksmith fires and forges.  These started as hand pumped bellows, they then scaled up to foot pumped, multiple person pumped, oxen or horse driven and then eventually waterwheel driven.  All of these methods came about due to the demand for more and more compressed air. These bellows did not generate near the amount of air pressure or volume needed for modern day practices yet they worked in the times.  These early bellows pumps would even supply miners with air.

With the evolution of metallurgy and industry these bellows were replaced by wheel driven fans, then steam came about and began generating more industrial sources of power.  The main issue with steam was that it would lose its power over longer runs of pipe due to condensing in the pipes.  Thus the birth of the air compressor was born. One of the largest projects that is noted to first use compressed air was in 1861 during the build of the Mont Cenis Tunnel in Switzerland in which they used compressed air machinery.  From here the constant need and evolution for on-demand compressed air expanded.  The picture below showcases two air compressors from 1896.

Air Compressors from the old days.

The compressors evolved over time from single stage, to two-stage reciprocating, on to compound, rotary-screw compressors, rotary vane, scroll, turbo, and centrifugal compressors with variable frequency drives.  The efficiency of each evolution has continued to increase.  More output for the same amount of input.  Now we see a two-stage compressor, considered old technology, and wonder how the company can get any work done.

All of the technological advances in compressor technology were driven by the demand sides of the compressed air systems.  Companies needed to power more, go further, get more from less, ultimately increase production.  With this constant increase in demand, the supply of compressed air increased and more efficient products for using compressed air began to evolve so the air was used more efficiently.

Enter EXAIR, we evolved the blowoff to meet the increasing demands of industrial companies to get the same amount of work done with less compressed air. We have continually evolved our product offering since 1983.  It all started with just a few typed pages of part numbers and has evolved to a 208 page catalog offering of Intelligent Compressed Air Products® for industry.  We will also continue to evolve our product designs for continued improvement of compressed air usage.  This is all to better help companies retain their resources.

EXAIR Catalog 32

If your company uses compressed air and you aren’t sure if it is efficiently being utilized, contact an Application Engineer.  Thanks for joining us for the brief history lesson, we look forward to hearing from you and seeing what the future brings.

Brian Farno
Application Engineer


Compressed air (1896) (14594022618).jpg – Wikimedia Commons – Internet Archive Book Images – Link


Common Compressed Air Symbols

When any product / system is designed drawings are made to assist in the production of the designed product. For example if a mechanical part is being machined you may see symbols like these to verify the part is made correctly:

GD&T Symbol Examples

Same with an electrical panel, they use symbols like the ones below to note the type of equipment used in a location.

Electrical Symbol Examples


Then there’s the Piping & Instrumentation Diagram (P&ID)…it depicts an overall view of a system, showing the flow (usually fluid or electricity) through that system’s components, giving the viewer an understanding of the operation, and expected results from said operation.

Some examples of symbols you might find in a compressed air system are:


The one on the left can be used for any air compressor. The others denote specific types of air compressor (from left:) Centrifugal, Diaphragm, Piston, Rotary, and Screw.


Air preparation & handling:

The symbols on the left denote the EXAIR products on the right: Automatic Drain Filter Separator, Oil Removal Filter, and Pressure Regulator

Instrumentation and control:

The symbols on top denote the EXAIR products below (left to right): Flowmeter, Pressure Gauge, and Solenoid Valve

Occasionally, we’re asked if there are standard ANSI or ISO symbols for any of our  engineered Intelligent Compressed Air Products…and there aren’t.  Perhaps one day they might make the cut, but for now, their standard convention is to choose a shape and call it out by name.  It might look something like this:

From top left, and then down: Automatic Drain Filter Separator, Oil Removal Filter, Pressure Regulator, and Super Air Knife

If you have questions about any of the quiet EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Jordan Shouse
Application Engineer
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Air – What Is It?

Air… We all breathe it, we live in it, we even compress it to use it as a utility.  What is it though?  Well, read through the next to learn some valuable points that aren’t easy to see with your eyes, just like air molecules.

Air – It surrounds us – (Yosuke,1)

  1. Air is mostly a gas.
    • Comprised of roughly 78% Nitrogen and 21% Oxygen.  Air also contains a lot of other gases in minute amounts.  Those gases include carbon dioxide, neon, and hydrogen.
  2. Air is more than just gas.
    • While the vast majority is gas, air also holds lots of microscopic particulate.
    • These range from pollen, soot, dust, salt, and debris.
    • All of these items that are not Nitrogen or Oxygen contribute to pollution.
  3. Not all the Carbon Dioxide in the air is bad.
    • Carbon Dioxide as mentioned above is what humans and most animals exhale when they breathe.  This gas is taken in by plants and vegetation to convert their off gas which is oxygen.
    • Think back to elementary school now.   Remember photosynthesis?
      • If you don’t remember that, maybe you remember Billy Madison, “Chlorophyll, more like Bore-a-fil.”
    • Carbon dioxide is however one of the leading causes of global warming.

      Moisture In The Air – (Grant)2
  4. Air holds water.
    • That’s right, high quality H2O gets suspended within the air molecules causing humidity.  This humidity ultimately reaches a point where the air can simply not hold anymore and it starts to rain.  The lack of humidity in the air leads to static, while lots of moisture in the air when it gets compressed causes moisture in compressed air systems.
  5. Air changes relative to altitude.
    • Air all pushes down on the Earth’s surface.  This is known as atmospheric pressure.
    • The closer you are to sea level the higher the level of pressure because the air molecules are more densely placed.
    • The higher you are from sea level the lower the density of air molecules.  This causes the pressure to be less.  This is also why people say the air is getting a little thin.

Hopefully this helps to better explain what air is and give some insight into the gas that is being compressed by an air compressor and then turned into a working utility within a production environment.  If you would like to discuss how any of these items effects the compressed air quality within a facility please reach out to any Application Engineer at EXAIR.

Brian Farno
Application Engineer

1 – Air – Creative Commons – Tsurutea Yosuke –

2 – DSC_0750 – Creative Commons – David Grant –

Supply Side Review: Heat of Compression-Type Dryers

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


Heat of compression image: Compressed Air Challenge: Drive down your energy costs with heat of compression recovery:


About Compressed Air Dryers – What Are They and Why Use Them

All atmospheric air contains some amount of water vapor.  When air is then cooled to saturation point, the vapor will begin to condense into liquid water. The saturation point is the condition where the the air can hold no more water vapor. The temperature at which this occurs is knows as the dew point.

When ambient air is compressed, heat is generated and the air becomes warmer. In industrial compressed air systems, the air is then routed to an aftercooler, and condensation  begins to take place. To remove the condensation, the air then goes into separator which traps the liquid water. The air leaving the aftercooler is typically saturated at the temperature of the discharge, and any additional cooling that occurs as the air is piped further downstream will cause more liquid to condense out of the air. To address this condensation, compressed air dryers are used.

It is important to dry the air and prevent condensation in the air. Many usages of the compressed air are impacted by liquid water being present. Rust and corrosion can occur in the compressed air piping, leading to scale and contamination at point -of -use processes. Processes such as drying operations and painting would see lower quality if water was deposited onto the parts.


There are many types of dryers – (see recent blogs for more information)

  • Refrigerant Dryer – most commonly used type, air is cooled in an air-to-refrigerant heat exchanger.
  • 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).
  • 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.
  • Heat of Compression Type – are regenerative desiccant dryers that use the heat generated during compression to accomplish the desiccant regeneration.
  • Membrane Type– use special membranes that allow the water vapor to pass through faster than the dry air, reducing the amount water vapor in air stream.

The air should not be dried any more than is needed for the most stringent application, to reduce the costs associated with the drying process. A pressure dew point of 35°F to 38°F (1.7°C to 3.3°C) often is adequate for many industrial applications.  Lower dew points result in higher operating costs.

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

Brian Bergmann
Application Engineer
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