Have a look at this post related to some FAQ's about vortex tubes:
http://ottobelden.blogspot.com/2011/06/homemade-ranque-hilsch-vortex-cooling.html
You do need some back pressure on the hot end of the tube, with a 'plug valve' of some kind. The shape of the valve does have an influence on the temperature difference because of how it will redirect the flow back to toward the cold end. For the tube that I built in the link below I used a bolt and I angled the inlet holes for the air so that the vortex was spinning in the direction of the bolt threads - the air could "unscrew" on the thread and exit. I believe that this facilitated the removal of the hot air around the inside of the tube while deflecting the cold air back because the bold is ground down to a dull point.
http://ottobelden.blogspot.com/2010/12/another-home-made-ranque-hilsch-vortex.html
The high speed air that is spinning around against the ID of the tube is heated by friction and as it expands it is compressing the air near the middle of the tube. The air near the middle can escape out the cold air plate hole and expend causing further cooling. That is how I think it is working...
- Otto Belden
