Vortex Tubes: What is a Cold Fraction?

Have you ever needed a source of cold air but don’t want to invest in a costly chiller? INTRODUCING Vortex Tubes! Vortex Tubes use compressed air and contain no moving parts to create a cold and hot stream of air from either end of the device. Using the valve located on the hot stream a vortex tube can achieve temperatures as low as -50°F (-46°C) and temperatures as high as 260°F (127°C).

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 and wall of the hot tube. 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 center of the tube and exhausts out of the cold end. Inside of the low-pressure area of the larger outer warm air vortex, the inner vortex loses heat as it flows back to the cold end of the vortex and as it exits the vortex expels cold air. The absolute temperature drop that occurs during this process is going to be controlled by the cold fraction of the Vortex Tube and the supply pressure.

The brass screw used to control the cold fraction of a vortex tube

The cold fraction is defined as the amount of the inlet supply air that will exit out of the cold end of the vortex tube. An example would be if I had 10 SCFM supplied to a vortex tube with 60% cold fraction, then 6 SCFM would be exiting the cold discharge. Cold based on the amount of air you allow out of the hot end of the vortex tube you can control the temperature drop of the cold air. A smaller cold fraction which only allows a small amount of air to exit the cold discharge will result in a larger temperature drop; and vise versa a larger cold fraction will result in a much smaller temperature drop.

Table the shows the temperature drop and rise in correlation with the cold fraction and pressure

Here a EXAIR we have designed our vortex tubes to operate optimally at both a high cold fraction and a low cold fraction. The 32XX series designed to give you the best refrigeration, which means it will work well for cold fractions ~60% – 80%. This will give you a smaller temperature drop with more air flow which allows you to keep things cool much easier. This contrasts with the 34XX series which is designed more optimal performance at lower temperatures; this means the optimal cold fraction would be ~20% to 40%. Cold fractions this low will produce very little air flow but the temperature will be very cold (as low as -50°F). This is useful if you need to get an item down to a very low temperature.

If you have any questions about compressed air systems or want more information on any of EXAIR’s products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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