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The vortex tube has a number of features that make it attractive for industrial applications. Firstly, having no moving parts, it is a highly reliable device. Secondly, it requires no external power such as electricity or flames in order to operate, making it a comparatively safe source of heating or cooling.
The vortex tube is therefore ideal for use in environments where maintenance is difficult or where safety is critical. Nuclear reactors represent a situation fulfilling both these criteria, and my sponsors have been investigating the potential for such uses of the vortex tube.
Given such desirable features, why isn't the vortex tube more widely used? There are basically two problems with the device.
(1) The thermal power of the vortex tube is limited. The 18mm diameter apparatus at Cambridge University Engineering Department for example produces only approximately 300 W of power. Commercial tubes (from Vortec Corp. of Cincinnati) produce maximum powers ranging from 2.9W for the very smallest to 1794W for the largest.
To make matters worse, my analysis suggests that the 'normalised performance' of vortex tubes declines as their radius increases. Thus while increasing the size of a tube increases the maximum throughput of air, it also tends to reduce the maximum temperature difference acheivable and hence there seems to be an overall physical limit on the thermal power that can be obtained from tubes of 'conventional' design. There may however be some scope for a significantly redesigned vortex tube to exceed these limits...but I haven't established the details yet.
(2) The efficiency of the vortex tube, measured as (power out)/(power needed to compress inlet air) is diabolical. For example a typical vortex tube and compressor arrangement achieves a Coefficient of Performance of around 0.08 when operated as a domestic refrigerator.
Chaining vortex tubes together, such that the cold outlet of one leads to the inlet of another could in principle at least reduce the temperature of the working gas to the point where it would liquefy. Part of my work has analysed the likely behaviour of several possible such 'cascades' for the liquefaction of air.
Modelling shows that cascades of conventional tubes could only satisfy very low demands for liquid air. An improvement in vortex tube performance by a factor of 3 is needed to make liquefaction of air by vortex cascades a realistic proposition.
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