About Vortex Tubes

Vortex Tubes are a phenomenon of physics generally used for spot cooling and we get many questions about them. Typically we have customers ask “how do they work?” or “when did you come up with them?”. The latter question is the easy answer…

We did not come up with Vortex Tubes, George Ranque did in 1928. George was developing and testing a vortex pump he designed and observed a warm air exhaust and cold air exhaust from opposite ends of his pump. He got so excited about this development, he shut down the research and development of the pump he was working on and jumped headlong into the commercial potential of this hot and cold air product. He started a small firm which soon failed and the visibility of the Vortex Tube along with it.

Recognition of a Vortex Tube increased again in 1945 with a scientific paper published by  Rudolph Hilsch. The paper became popular enough to raise awareness and continued interest in the potential of Vortex Tubes.

The first question, “how do they work?”, is the tough one. First, here is what they do – 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 these two airstreams are adjustable with a built-in valve on the hot air exhaust. Temperatures as low as -50F (-46C) and as high as +260F (+127C) are possible.

Again, how do they work? Nobody knows for certain. If I could choose who to try to explain it I would choose Julius Sumner Miller to explain it, there is nothing this guy couldn’t explain, but alas, he is no longer with us. We can still enjoy his passion for physics (and his unique delivery) through YouTube. Here he discusses Bernoulli, the video is a bit long but take the opportunity to learn about Professor Miller if you have the time.

Nobody has been able to “do the math” to prove exactly how it functions but there is a widely accepted theory. Compressed air is supplied to the Vortex Tube and passes through nozzles tangent to an internal counterbore. These nozzles set the air in a vortex motion. This spinning stream of air turns 90 degrees and passes down the hot tube (thin tube part of a Vortex Tube) in the form of a spinning shell, like a tornado. A valve at one end of the tube allows some of the warmed air to escape. What does not escape heads back down the tube as a second vortex inside the low pressure area of the larger vortex. This inner vortex loses heat and exhausts through the other end as cold air.

While one airstream moves up the tube and the other down it, both rotate in the same direction at the same angular velocity. That is, a particle in the inner stream completes one rotation in the same amount of time as a particle in the outer stream. However, because of the principle of conservation of angular momentum, the rotational speed of the smaller vortex might be expected to increase. (The conservation principle is demonstrated by spinning skaters who can slow or speed up their spin by extending or drawing in their arms.) But in the Vortex Tube, the speed of the inner vortex remains the same. Angular momentum has been lost from the inner vortex. The energy that is lost shows up as heat in the outer vortex. Thus the outer vortex is warm and the inner vortex is cooled.

Yes, I know – you too are longing for Professor Julius Sumner Miller to explain. Thanks for reading.

Kirk Edwards
Application Engineer
kirkedwards@exair.com

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