Intelligent Compressed Air: How do Vortex Tubes Work

A vortex tube is an interesting device that has been looked upon with great fascination over the last 89 years since its discovery by George Ranque in 1928. What I’d like to do in this article is to give some insight into some of the physics of what is happening on the inside.

With a Vortex Tube, we apply a high pressure, compressed air stream to a plenum chamber that contains a turbine-looking part that we call a generator to regulate flow and spin the air to create two separate streams. One hot and one cold.

Below is an animation of how a Vortex Tube works:

Function of a Vortex Tube

 

The generator is a critical feature within a vortex tube that not only regulates flow and creates the vortex spinning action, it also aligns the inner vortex to allow its escape from the hot end of the vortex tube. Note the center hole on the photo below. This is where the cooled “inner vortex” passes through the generator to escape on the cold air outlet.

Vortex generator

Once the compressed air has processed through the generator, we have two spinning streams, the outer vortex and the inner vortex as mentioned above.  As the spinning air reaches the end of the hot tube a portion of the air escapes past the control valve; and the remaining air is forced back through the center of the outer vortex.  This is what we call a “forced” vortex.

If we look at the inner vortex, this is where it gets interesting.  As the air turns back into the center, two things occur.  The two vortices are spinning at the same angular velocity and in the same rotational direction.  So, they are locked together.  But we have energy change as the air processes from the outer vortex to the inner vortex.

If we look at a particle that is spinning in the outer vortex and another particle spinning in the inner vortex, they will be rotating at the same speed.  But, because we lost some mass of air through the control valve on the hot end exhaust and the radius is decreased, the inner vortex loses angular momentum.

Angular momentum is expressed in Equation 1 as:

L = I * ω

L – angular momentum
I – inertia
ω – angular velocity

Where the inertia is calculated by Equation 2:

I = m * r2

m – mass
r – radius

So, if we estimate the inner vortex to have a radius that is 1/3 the size of the outer vortex,  the calculated change in inertia will be 1/9 of its original value.  With less mass and  a smaller radius, the Inertia is much smaller.  The energy that is lost for this change in momentum is given off as heat to the outside vortex.

Adjustments in output temperatures for a Vortex Tube are made by changing the cold fraction and the input pressure.  The cold fraction is a term that we use to show the percentage of air that will come out the cold end.  The remaining amount will be exhausted through the hot end. You can call this the “hot fraction”, but since it is usually the smaller of the two flows and is rarely used, we tend to focus on the cold end flow with the “cold fraction”.  The “Cold Fraction”  is determined by the control valve on the hot end of the Vortex Tube. The “Cold Fraction” chart below can be used to predict the difference in temperature drop in the cold air flow as well as the temperature rise in the hot air flow.

Vortex Tube Cold Fraction

By combining the temperature drops expressed above with the various flow rates in which Vortex Tubes are available, we can vary the amount of cooling power produced for an application. The above cold fraction chart was developed through much testing of the above described theory of operation. The cold fraction chart is a very useful tool that allows us to perform calculations to predict vortex tube performance under various conditions of input pressure and cold fraction settings.

The most interesting and useful part about vortex tube theory is that we have been able to harness this physical energy exchange inside a tube that can fit in the palm of your hand and which has a multitude of industrial uses from spot cooling sewing needles to freezing large pipes in marine applications to enable maintenance operations on valves to be performed.

We would love to entertain any questions you might have about vortex tubes, their uses and how EXAIR can help you.

John Ball
Application Engineer

Email: johnball@exair.com
Twitter: @EXAIR_jb

Vortex Tube Cold Fractions – An Explanation

Vortex Tube Family

At EXAIR we’ve been a pioneer in the compressed air market for the past 34 years.  We’ve brought engineered nozzles to the market which reduce compressed air consumption while maintaining performance, laminar flow Air Knives, pneumatic conveyors, atomizing nozzles, air-assisted static eliminators, and a slew of other products.  One of these “other” products is our Vortex Tube, which we manufacture in various sizes while also using as a basis for our Cold Guns, Adjustable Spot Coolers, Mini Coolers, and Cabinet Coolers – all of which are built on the same Vortex Tube technology.

Theory of operation for an EXAIR Vortex Tube

The principle behind a Vortex Tube is rooted in the Ranque-Hilsch effect which takes place inside of the tube.  As a compressed air source is fed into the Vortex Tube, the air flows through a generator and begins to spin down the length of the tube, “hugging” the ID of the tube.  When this spinning air contacts a deliberate obstruction at the end of the tube, it is forced to reverse directions, which requires a change in diameter to the vortex.  The original vortex must decrease in diameter, and in order to do so, it must give off energy.  This energy is shed in the form of heat, and a portion of the incoming air is directed out of the tube with a drastically reduced temperature via what is called the “cold end”.  Another portion of the air escapes through the “hot end” of the tube, resulting in a cold airflow at one end, and a hot airflow at the other end of the tube.

Small, but powerful, Vortex Tubes really are a marvel of engineering.  And, like most useful developments in engineering, Vortex Tube technology begs the question “How can we control and use this phenomena?”  And, “What are the effects of changing the amount of air which escapes via the cold end and the hot end of the tube?”

EXAIR Vortex Tube Performance Chart

These answers are found in the understanding of what is called a cold fraction.  A cold fraction is the percentage of incoming air which will exhaust through the cold end of the Vortex Tube.  If the cold fraction is 80%, we will see 80% of the incoming airflow exhaust via the cold end of the tube.  The remaining airflow (20%) will exhaust via the hot end of the tube.

For example, setting a model 3210 Vortex Tube (which has a compressed air flow of 10 SCFM @ 100 PSIG) to an 80% cold fraction will result in 8 SCFM of air exhausting via the cold end, and 2 SCFM of air exhausting through the hot end of the Vortex Tube.  If we change this cold fraction to 60%, 6 SCFM will exhaust through the cold end and 4 SCFM will exhaust through the hot end.

But what does this mean?

Essentially, this means that we can vary the flow, and temperature, of the air from the cold end of the Vortex Tube.  The chart above shows temperature drop and rise, relative to the incoming compressed air temperature.  As we decrease the cold fraction, we decrease the volume of air which exhausts via the cold end of the Vortex Tube.  But, we also further decrease the outlet temperature.

This translates to an ability to provide extremely low temperature air.  And the lower the temperature, the lower the flow.

Red box shows the temperature drop in degrees F when an EXAIR Vortex Tube is operated at 100 PSIG with an 80% cold fraction.

With this in mind, the best use of a Vortex Tube is with a setup that produces a low outlet temperature with good cold air volume.  Our calculations, testing, and years of experience have found that a cold fraction of ~80% can easily provide the best of both worlds.  Operating at 100 PSIG, we will see a temperature drop of 54°F, with 80% of the incoming air exiting the tube on the cold end (see red circle in chart above).  For a compressed air supply with a temperature of 74°F-84°F (common compressed air temperatures), we will produce an output flow with a temperature between 20°F and 30°F – freezing cold air!

With a high volume and low temperature air available at an 80% cold fraction, most applications are well suited for this type of setup.  When you order a Vortex Tube from EXAIR we will ship it preset to ~80% cold fraction, allowing you to immediately install it right of the box.

The cold air from an EXAIR Vortex Tube is effective to easily spot cool a variety of components from PCB soldering joints to CNC mills, and even complete electrical control panels.  Contact an Application Engineer with application specific questions or to further discuss cold fractions.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

The Adjustable Spot Cooler Is Ideal For Small Parts Cooling

I recently worked with a customer who was trying to cool some small part housings after they leave a wash system. The parts are currently placed in a wash tray where they travel down a conveyor belt and into the washer where they are heated to around 160°F. As the parts exit the washer, the belt is stopped so the parts can be left to cool before an operator places the parts into their dryer system. This cooling process was taking about 15 minutes before the operator was able to safely handle the parts. This cooling delay was negatively affecting their production cycle. They were looking to eliminate the 15 minute cooling cycle by incorporating some type of air cooling system so the parts could be quickly processed to the dryer. They wanted to standardize on a single device as they manufacturer a variety of part shapes with the largest being a valve housing that measures close to 2″ x 2″.

The customer was able to send a few photos of their parts and after reviewing the information sent, I recommended they use our Model # 3925 Adjustable Spot Cooler System with dual point hose kit. Incorporating a Vortex Tube, the Adjustable Spot Cooler is able to produce cold air temperature as low as -30°F, based on ambient supply temperature. The unit features a temperature control valve that allows for simple adjustments of the temperature of the exhausting cold air as well as the volume of air being discharged.  The system includes 2 additional generators which provide more or less airflow volume through the device as well as cooling capacity (Btu/hr.) for even  more control. The dual hose kit separates the cold air into two separate airstreams to provide for a wider coverage area or in this particular case, the customer would be able to treat both sides of the housing for even cooling. The magnetic base makes for easy installation without having to make expensive modifications to their existing setup.

3925 Adjustable Spot Cooler

3925 Adjustable Spot Cooler System – for wider coverage areas with no moving parts to wear out or replace.

EXAIR offers a wide variety of spot cooling products for use in many industrial settings. For help selecting the best product to fit your needs, give me a call, I’d be glad to help.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

 

 

 

The Adjustable Spot Cooler Provides High Flexibility and Effectiveness

A customer emailed me with some questions about the using the EXAIR spot cooling technology for use on PEEK material being machined in a Swiss Turning machine. Typically, apart from drilling and parting, coolants are not necessary for thermoplastic machining operations.  In order to obtain the best surface finish and tightest tolerances, keeping the cutting area cool is required.  The ideal goal was to provide sub-zero air to the cutting area, while being quiet and easy to operate.  After reviewing the various EXAIR spot cooling products, it was determined that the Adjustable Spot Cooler System would satisfy all of the requirements.

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Model 3825 Adjustable Spot Cooler System

The Adjustable Spot Cooler System shown above is capable of producing temperatures from -30°F to room temperature, with just the turn of a knob.  Included in the package are (2) additional generators, which allow for more or less cold air flow rate, depending on the application cooling needs.  With the magnetic base, the system can be easily positioned, and the flexible hose allows for precise aim of the cold air flow. And, sound levels are kept below 75 dBA.

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Model 3825 Used in a Turning Operation

To recap, the Adjustable Spot Cooler System provides adjustable cold air temperature with the simple turn of a knob, includes additional generators to provide wide ranging flow rates, has a magnetic base to allow for positioning anywhere, on any machine, and has a flexible hose for directing the cold air wherever it is needed.

I would say that it is a Very Adjustable Spot Cooler.

To discuss spot cooling and your application, we ask you to contact EXAIR and one our  Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

Send me an email
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Twitter: @EXAIR_BB

Cooling a Thermal Manikin After a Fire

Not to be persnickety, but there is a difference between mannequins, life size model for displaying or tailoring clothes, and manikins, an anatomical model used for testing and teaching, usually with movable joints. (The enunciation is exactly the same though).  A lab designed a test for thermal protective clothing.  They had a manikin that was 6 feet in height and had 120 copper slug sensors located all over its body. The sensors would record the temperature gradients on the surface of the manikin, representing skin exposure to heat.  They would dress their manikin with thermal protective clothing from head to toe and expose it to intense fires at various temperatures and exposure times.  After each test was completed, they would record the results and cool the manikin to 26 deg. C before they started the next fire test.  These results were used for safety limits to protect wearers from second and third degree burns, very important in keeping firefighters safe.

Fire Suit under test

Fire Suit under test

In their application, they were looking to cool the sensors on the manikin as quickly as they can to increase test cycle rates. Initially they used a “cool down” area fitted with fans to blow air across the manikin.  The problem was that it took too long to cool to the 26 deg. C mark required in their testing protocol.  They decided to manually use an air gun to blow compressed air across the sensors to increase cooling.  This did reduce the cycle time, but because of the force created by the air gun, some sensors would shift and be out of calibration.  This was a huge concern for the test lab.

The design of the copper slug sensor has a small piece of copper set inside a silicone holder. To isolate the copper metal, there are small ruby spheres between the holder and copper slug.  This creates an air gap around the copper slug to help increase sensitivity to temperature changes.  A thermocouple is attached to the back side of the copper slug for analytical measurements.

Adjustable Spot Cooler

Adjustable Spot Cooler

After they discussed their application with me, I suggested the model 3725 Adjustable Spot Cooler. This base unit comes without a magnetic base and hose kit, which makes it lighter in weight. The customer could easily attach it directly to their compressed air line, replacing the air gun that was damaging the sensors.  The Adjustable Spot Cooler incorporates the Vortex Tube which makes standard compressed air into cold air.  With a turn of a knob, they could control the temperature and the velocity of the cold air.  This feature was key in determining just the right amount of force to not affect the calibration of the sensors.  An added benefit of the Adjustable Spot Cooler is if you reduce the amount of outlet cold air, the temperature will decrease even more.  This feature allowed the customer to reach their target much more quickly and without damaging the sensors.

If you need to cool things down in your application, you can contact an Application Engineer at EXAIR. We have many different styles and combinations of Vortex Tubes and Spot Coolers to give you the right form of cooling, whether it is a mannequin or a manikin.

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

 

DDI-2007-Burning Man by Interpretive Arson.  Common License.

Adjustability=Flexibility=Wide Ranging Performance

Recently, I moved into a new house, and it didn’t have bathroom vent fans.  In order to prevent mold and mildew growth from the humid shower air; we decided to install them ourselves.  Going to the local home goods store, we were faced with a wall of options, with factors such as noise level, CFM rating and of course price.  Going over the myriad of possibilities was headache inducing and of course, the bathroom size was borderline between a small and mid-size unit. The decision came down to – pay more and oversize it or risk installing an undersized unit.  If only they made a unit to cover a wide variety of conditions.

Here at EXAIR, we make many types of Intelligent Compressed Air Products.  Many times, when we are talking to customers, the right size or performance needed isn’t quite clear.  Fortunately, many of the products we make are flexible, and allow for adjustability and increased range of performance to best match the specific need of the application.

For example, The Super Air Knives and Super Air Wipes have available Shim Sets to increase or decrease the air gap and change the force and flow.  The shims are easily installed, and come in a wide choice of sizes. Super Air Knives are available, from stock, in aluminum, 303SS, 316SS, and PVDF. Super Air Wipes are made from aluminum or 303 SS.

The Super Air Amplifiers also have available patented Shim Sets to fix the air flow at a predetermined setting. Adjustable Air Amplifiers have an infinitely adjustable air gap which can easily be set to match the application needs if a predetermined flow is not preferred. Adjustable Air Amplifiers also have a smaller footprint than Super Air Amplifiers and can also use the aforementioned shims to preset the air flow in applications where space is limited. These are available in 303 stainless steel and aluminum. We also have a high temperature option.

adjustable Air Amplifier

Adjustable Air Amplifier

Within our already enormous and flexible Air Nozzles & Jets product line, the Flat Super Air Nozzles have available Shim Sets to provide the ability to adjust the force and flow for blowoff or cleaning operation.  The model 1009 Adjustable Air Nozzle has an adjustable gap setting, allowing you to “tune in” the force and flow to meet the application requirement.  Also, the Adjustable Air Jet provides an adjustable gap setting to control the air flow performance. Flexibility also comes from the available materials of zinc/aluminum, aluminum, 303SS, 316SS and PEEK thermoplastic.

For vacuum generation, the Adjustable E-Vac allows for the vacuum and flow rate to be easily adjusted.  With a simple turn of the unit, the vacuum and flow levels can be changed to overcome porosity and increase or decrease the lifting power.

Adjustable EVac

Adjustable E-Vac

There are several other Intelligent Compressed Air Products that EXAIR manufactures that offer the adjustability and flexibility, such as Vortex Tubes and Adjustable Spot Coolers.

If only EXAIR manufactured bathroom fans, I could have ensured making the right choice and be able to match the performance to exactly meet my application needs.

Brian Bergmann
Application Engineer
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Vortex Tube Cold Fractions

Vortex Tubes are the perfect solution when dealing with a variety of spot cooling applications. They use compressed air to produce a cold air stream and a hot air stream, with temperatures ranging from as low as -50°F  up to +260°F (based on ambient supply temperature) and providing as much as 10,200 Btu/hr. of cooling capacity. By simply adjusting the valve in the hot end of the Vortex Tube, you are able to control the “cold fraction” which is the percentage of air consumed by the vortex tube that is exhausted as cold air versus the amount of air exhausted as hot air. Our small, medium and large Vortex Tubes provide the same temperature drop and rise, it’s the volume of air that changes with the various sizes.

Vortex Tubes

Vortex Tubes are available in small, medium and large sizes with various flows and cooling capacities.

When looking at the below performance chart, you will see that “Pressure Supply” and “Cold Fraction %” setting all play a part in changing the performance of the Vortex Tubes. Take for example, an operating pressure of 100 PSIG and cold fraction setting of 20%, you will see a 123°F drop on the cold side versus a 26°F temperature rise on the hot side. By the using the same Vortex Tube and keeping the operating pressure at 100 PSIG but changing the cold fraction to 80%, you will now see a 54°F temperature drop on the cold side and a 191° rise at the hot end.

Vortex Tube Performance Data

Vortex Tube Performance Chart

We’ve looked at how the cold fraction changes the temperature, but how does it change the flow for the various Models?

Say you are using a Model # 3240 Medium Vortex Tube which consumes 40 SCFM @ 100 PSIG. Again with the cold fraction set at 80% (80% of the consumed compressed air out of the cold end), you would flow 32 SCFM at the cold air exhaust.

40 SCFM x 0.8 (80% CF) = 32 SCFM

Using the same Model # 3240 Medium Vortex Tube but now with a 20% cold fraction (20% of consumed compressed air out of the cold end), you would flow 8 SCFM at the cold exhaust.

40 SCFM x 0.20 (20% CF) = 8 SCFM

As you can see, to achieve the colder air temperatures, the volume of cold air being exhausted is reduced as well. This is important to consider when making a Model selection. Some other considerations would be the operating pressure which you can see also has a significant effect on performance. Also the compressed air supply temperature because the above temperatures are temperature differentials, so in the example of the 80% cold fraction there is a 115F temperature drop from your inlet compressed air temperature.

If you need additional assistance, you can always contact myself or another application engineer and we would be happy to make the best selection to fit your specific need.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

 

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