## 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
BrianFarno@EXAIR.com
@EXAIR_BF

## Max Refrigeration vs. Max Cold Temp Vortex Tubes

Here at EXAIR, our vortex tubes are offered in two separate series. The reason for this is to optimize the performance of the cold air temperature drop when operating with opposite ends of the cold fraction chart. The maximum refrigeration vortex tubes, 32xx series, perform optimally when they are set to a greater than 50% cold fraction.  The maximum cold temp vortex tubes, 34xx series, perform optimally when they are set to a less than or equal to 50% cold fraction. The cold fraction is discussed more in-depth within this link from Russ Bowman, Vortex Tube Cold Fractions Explained. This blog is going to explain a little further why one series of vortex tubes would be chosen for an application over another.

Maximum refrigeration (32xx) vortex tubes are the most commonly discussed of the two types when discussing the optimal selection of the vortex tube for an application. The 32xx series vortex tubes achieve a maximum refrigeration output when operated at 100 psig inlet pressure with around  80% cold fraction. This would give a temperature drop from incoming compressed air temperature of 54°F (30°C). The volumetric flow rate of cold air will be 80% of the input flow which means only 20% is being exhausted as warm exhaust air. By keeping the flow rate higher the air is able to cool a higher heat load and is the reason the vortex tube is given a BTU/hr cooling capacity.

Maximum cold temperature (34xx) tubes are less common as their applications are a little more niche and require a very pinpoint application. Rather than changing the temperature inside of a cooling tunnel or cooling an ultrasonic welding horn, the max cold temp vortex tube is going to have a minimum cold flow rate, less than 50% of input volumetric flow.  This minimal flow will be at temperature drops up to 129°F (71.1°C) from the incoming compressed air temperature.  This air is very cold and at a low flow. A 20% cold fraction exhausts 80% of the input volume as hot air. This type of volume would be ideal for sensor cooling, pinpoint cooling of a slow-moving operation, or thermal testing of small parts.

In the end, EXAIR vortex tubes perform their task of providing cold or hot air without using any refrigerants or moving parts. To learn more about how they work, check out this blog from Russ Bowman. If you want to see how to change the cold fraction, check out the video below. If you would like to discuss anything compressed air related, contact an application engineer, we are always here to help.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

## Adjustable Spot Cooler Removes Tackiness from Plastic Tubes

A manufacturing plant contacted EXAIR to help them with a “sticky” situation.  This company extruded PVC tubes that would be used as fuel lines on small engines.  Plasticizers are typically used to add flexibility to plastic materials.  For the PVC material above, a plasticizer was added to make it softer and more elastic.  The issue that they saw was the outer surface of the tubes were tacky from the plasticizer and heat which made it difficult to handle in packaging the tubes.

This company extruded many different diameters, but they wanted to target their most difficult size, the smallest tube.  The dimensions were given as 0.187″ (4.7mm) O.D. by .0934″ (2.4mm) I.D., and the feed rate was close to 4 feet/min (1.2 meter/min).  The problem area that they explained was at the end of the production line where the extruded tubes were cut by a blade cutter into 12” (305mm) lengths.  The tubes would then fall into a collection bin for batch processing.  Since the collection bin was setup at a slight upward angle, they wanted the tubes to gather toward one end of the bin.  Since the tubes were still hot and sticking to each other, the operators had to individually handle each tube which was counterproductive and time-consuming.  After our discussions, I suggested that cold air could harden the PVC tube enough by removing heat and help to “set”  the platicizer.  Since they manufactured different sizes and feed rates, we needed to have adjustability as well in our cold air device.

One of our most versatile spot cooling instruments is the EXAIR Adjustable Spot Cooler.  This system uses the Vortex Tube technology to convert compressed air into a cold air stream without any moving parts, refrigerants, or motors.  The Adjustable Spot Cooler is a low-cost, reliable, maintenance-free way to give spot cooling for a myriad of industrial applications.  For this customer, this product gave them the versatility that they were needing.

EXAIR stocks these units with either a single or dual point hose kit, a magnetic base, a filter separator, and two additional generators.  The control valve at the end of the unit adjusts the output temperature down to -30 oF (-34 oC) with a turn of a knob.  The generators are specifically engineered to control the amount of compressed air that is used.  Both types of controls will allow this customer to “dial in” the correct cooling capacity for the operation.  The filter separator included with the system will clean the compressed air to keep the unit and the product free of dirt and debris.  The magnetic base which this customer really liked makes the Adjustable Spot Cooler portable for use in different areas.

I recommended the model 3925 Adjustable Spot Cooler because it had the dual point hose kit to blow cold air on both sides of the tubes.  Since this company had different tube diameters and thicknesses, adjustability was very critical.  If the tubes got too cold, cracks could occur from the blade cutting machine; and, if the tubes were too warm, the tackiness on the surface of the tube would remain.   Once they installed the Adjustable Spot Cooler, this company was able to increase their packaging line for the different size PVC tubes.  Now the operators could reach into the collection bin and grab many aligned tubes instead of individually separating and sorting.

If you have a “sticky” situation, the EXAIR Adjustable Spot Cooler may be a product for you.  The company above was able to have their tubes slide together in the collection bin.  Many applications could be improved by adding cold air.  And, if you have a similar situation, an Application Engineer at EXAIR will be happy to discuss a solution.

John Ball
Application Engineer
Email: johnball@exair.com

## How it Works: Theory Behind the Vortex Tube

What is a vortex tube and how does it work? A vortex tube is a device used to separate compressed air into a cold and hot stream of air; but the main question that many people have theorized is how does this device work.

In 1928 George Ranque, a French physics student stumbled upon this phenomenon on accident while he was performing experiments on a vortex type pump. During the experiment George noticed that hot air was being exhausted from one side and the other side was producing cold air. Eventually the device was forgotten about until 1945 when the German physicist, Rudolph Hilsch published a paper describing the device, eventually causing it to gain popularity and find applications in the industrial world.

The diagram bellow is one of the widely accepted explanations for the vortex tube phenomenon.

When the vortex tube is supplied with compressed air the air flow is directed into the generator that causes spin into a spiraling vortex at around 1,000,000 rpm. This spinning vortex flows down the neck of the hot tube denoted in the diagram as red. The control valve located on the end of the hot tube allows a fraction of the hot air to escape and what does not escape reverses direction and travels back down the tube in a second vortex denoted in blue. Inside of the low-pressure area of the larger outer warm air vortex, the inner vortex loses heat as it flows back to the front of the vortex and as it exits the vortex expels cold air.

The phenomenon is theorized to occur because both the hot and cold streams rotate at the same velocity and direction. This means that a particle of air in the inner vortex makes a complete revolution in the same time that a particle in the outer vortex takes to make a complete revolution. This effect is known as the principle of conservation of momentum and is the main driving force behind the vortex tube. In order for the system to stay in equilibrium air particles lose energy, in the form of heat, as they move from the outer stream to the inner stream, creating the cold air vortex that gets expelled.

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 EXAIR and myself or any of our Application Engineers can help you determine the best solution.

Cody Biehle
Application Engineer
EXAIR Corporation
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