VORTEX TUBES

The following resources were used to write this blog: 1.) “Vortex Tubes—Theory and Application”, by iProcessSmart; copyright 1999 and 2.) “The Ranque-Hilsch Vortex Tube”, by Giorgio De Vera, March 2010.

If you follow my postings and read any of my work, you know I mainly stay within subjects involving education and technology.  Sometimes I tackle subject matter off the “beaten path” but STEM (Science, Technology, Engineering and Mathematics) get most of my ink.

Recently, I was asked to get involved with specifying a vortex tube.  The application was very specific and frankly quite fascinating.  Well, with that said, I had to go back to school on this one.  Let’s take a look.

VORTEX TUBES:

 HISTORY:

The vortex tube was invented quite by accident in 1928. George Ranque, a French physics student, was experimenting with a vortex-type pump he had developed when he noticed warm air exhausting from one end and cold air from the other. Ranque soon forgot about his pump and started a small firm to exploit the commercial potential for this strange device that produced hot and cold air with no moving parts. However, it soon failed and the vortex tube slipped into obscurity until 1945 when Rudolph Hilsch, a German physicist, published a widely read scientific paper on the device.

Much earlier, the great nineteenth century physicist, James Clerk Maxwell postulated that since heat involves the movement of molecules, we might someday be able to get hot and cold air from the same device with the help of a “friendly little demon” who would sort out and separate the hot and cold molecules of air.

Thus, the vortex tube has been variously known as the “Ranque Vortex Tube”, the “Hilsch Tube”, the “Ranque-Hilsch Tube”, and “Maxwell’s Demon“. By any name, it has in recent years gained acceptance as a simple, reliable and low cost answer to a wide variety of industrial spot-cooling problems.

CONFIGURATION:

The tube itself is a mechanical device that separates compressed air into an outward radial high temperature region and an inner lower region. It operates as a refrigerating machine with a simplistic geometry and no moving parts.   It is used commercially in CNC machines, cooling suits, refrigerators, airplanes, etc. Other practical applications include cooling of laboratory equipment, quick startup of steam power generators, natural gas liquefaction, and particle separation in the waste gas industry.  Two JPEGs show the configurations are as follows:

 

 

Representation of Counter-Flow Type

Representation of Uni-Flow Type

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 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.

THEORIES:

Theories abound regarding the dynamics of a vortex tube. Here is one widely accepted explanation of the phenomenon as follows:

Compressed air is supplied to the vortex tube and passes through nozzles that are tangent to an internal counterbore. These nozzles set the air in a vortex motion. This spinning stream of air turns 90° and passes down the hot tube in the form of a spinning shell, similar to 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 thru the other end as cold air.  One airstream moves up the tube and the other moves down the tube while 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 becomes warm, and the inner vortex is cooled.

CLASSIFICATIONS:

There are two classifications of the vortex tube. Both of these are currently in use in the industry. The more popular is the counter-flow vortex tube.    The hot air that exits from the far side of the tube is controlled by the cone valve. The cold air exits through an orifice next to the inlet.

 

Counter-Flow

On the other hand, the uni-flow vortex tube does not have its cold air orifice next to the inlet.

Uni-Flow

Instead, the cold air comes out through a concentrically located annular exit in the cold valve. This type of vortex tube is used in applications where space and equipment cost are of high importance. The mechanism for the uni-flow tube is similar to the counter-flow tube. A radial temperature separation is still induced inside, but the efficiency of the uni-flow tube is generally less than that of the counter-flow tube.

This is a very very brief explanation of vortex tubes but hopefully, one which will pique interest for further study.  I welcome your comments.