More on Vortex Tubes: Understanding Cold Fractions

vortex tube
An EXAIR Vortex Tube

I had a conversation today through our online chat feature with a customer in the Middle East who needed a bit more understanding about Vortex Tubes.  The cooling power and instantaneous ability of a Vortex Tube offers ways to remove heat from applications, but the way the Vortex Tube works was a little misunderstood.  So, we went over the basics.

A Vortex Tube transforms a compressed air supply into a stream of hot and cold air.  As the compressed air enters into the Vortex Tube, it passes through a generator which causes the air to spin.  The airstream spins down the length of the Vortex Tube until it reaches a “brake”, whereupon it changes directions and begins spinning inside of itself, giving off energy in the form of heat.  The result is a stream of cold air at one end of the Vortex Tube, and a stream of hot air at the other.

But how can we adjust the flows and temperatures?

Adjusting the flow and cold air temperature from a Vortex Tube is as simple as turning the adjustment valve at the hot end of the unit.  This valve controls the “cold fraction” of the Vortex Tube, or, to put it more simply, the amount of air which will exit the unit at the cold end.

EXAIR Vortex Tube Performance Chart
EXAIR Vortex Tube Performance Chart

For example, if we were to set a Vortex Tube to an 80% cold fraction, 80% of the air consumed by the Vortex Tube would exhaust through the cold end of the unit.  If we take the same Vortex Tube and establish a 60% cold fraction, 60% of the consumed air will exhaust through the cold end of the unit.

Why is this important?

The cold fraction is important because at various cold fractions we will product varying temperature drops, even at the same operating pressure.  So, in the example above, if we have a Vortex Tube operating at 7 BARG, set to an 80% cold fraction, we can expect a temperature drop of 30°C (54°F), relative to the temperature of the incoming compressed air.

This means that if our compressed air temperature is 25°C (77°F), we will have an outlet temperature of -5°C (23°F).  If we take the same air supply and reduce the cold fraction to 60%, we will have a temperature drop of 48°C (86°F).

The caveat here is that when we reduce the air temperature, we also reduce the flow.  So, the colder the air temperature from the Vortex Tube cold end, the lower the volume of cold air.

When determining if a Vortex Tube is right for an application, it is important to consider all the variables (operating pressure, compressed air temperature, cold fraction, required cooling) when making a model number selection.

If you have any questions or concerns when considering a Vortex Tube, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer

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