Vortex Tube theory is one of the more interesting accidental inventions of the 20th Century…splitting a flow of air into two streams: one hot, and one cold. Georges Ranque happened upon the phenomenon in the 1930s. He patented it in 1933, but it wasn’t commercially viable at the time. In the 1940s, it caught the interest of German physicist Rudolph Hilsch, who “tweaked” Ranque’s design and published a widely read paper on it in 1947. Over the next few decades, the use of compressed air became more prevalent in a wide range of industries, eventually becoming the “4th utility” that it’s known as today. With that increase in use came improvements in air compressor design & function – improvements that finally bestowed long-awaited commercial viability on the Vortex Tube.
So, how does it work? Ranque’s patent and Hilsch’s paper both detail what it is and what it does, but to this day, nobody’s been able to offer any 100% scientific proof as to HOW it does what it does. The commonly accepted explanation involves a proven scientific principle called conservation of angular momentum. That’s a mouthful, so let’s break it down:
Momentum is a physical property of matter, defined by its mass and velocity…and it depends on both. Something with more mass will have more momentum than something with less mass, if their velocities are the same. And something moving at a higher velocity will have more momentum than something that’s moving slower, as long as their masses are the same. Unless otherwise specified, “momentum” is usually considered to be linear – the matter is moving in one direction.
Angular momentum is also defined by mass and velocity, but its value is also affected by rotational inertia, which is determined by the distribution of its mass around the center point of its rotation. If an object moving at a certain velocity is forced closer to its rotational center point, it has to speed up to maintain (or conserve) angular momentum. Physics really, really, (really) wants to make that happen, according to the laws of conservation of matter & energy. And physics ALWAYS obeys the law…which forces us to as well.
Consider figure skaters doing those dizzying moves where they spin on the ice on one skate. If the skater spins with their arms straight out and then brings their arms in, close to their body, they begin to spin faster. The skater’s mass doesn’t change, but their mass distribution around the rotational center point does…so physics gets its way by increasing the velocity. Therefore, energy (angular momentum, in this case) is conserved. It’s impressive how easy some of them make it look:
In a Vortex Tube, the airflow is discharged tangentially into the tube, making it spin inside the inner wall of the tube at a specific velocity. When it reaches the end of the tube, it’s forced to change directions and continue spinning inside that outer spinning flow, but in the opposite direction. Unlike our figure skater in the example above, though, its velocity doesn’t change. Something has to, though, because physics ALWAYS gets its way. Since the energy of its angular momentum HAS to be conserved, that energy gets converted into heat, which transfers from the outer spinning flow and exits the vortex tube’s “hot” end. When it does so, the temperature of the remaining, inner spinning air flow goes down.
That’s our story, anyway, and we’re sticking to it. In any case, it works, and it works quite well. If you’d like to find out more, give me a call.
Russ Bowman, CCASS
Application Engineer
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