Tech Bite

Question:

As you and a friend are working on cleaning out your garage, you come across your old air compressor and see that it is still charged with high-pressure air from the last time you were using it. You tell your friend that you can extract either hot or cold air (relative to ambient) from the tank at high flow rates without using any external power at all. Your friend thinks you're crazy and bets you $50 that you can't do it. You proceed to prove him wrong, and you win the bet. How did you do it?

You attach a vortex tube (also known as a Ranque-Hilsch Vortex Tube) to your compressor (you just happened to have one hanging around...) and proceed to show him that hot air is expelled from one end and cold air from the other. Vortex tubes have been around for many years and are commercially available. First discovered experimentally by Georges Ranque in 1933, and further refined and studied by Rudolf Hilsch in 1946, the vortex tube is a simple-to-construct and difficult-to-explain passive device — essentially a simple tube into which high-pressure air is introduced circumferentially. This air spins at upwards of a million rpm and through a process that to this day is still not well understood, the air separates into two flows: one in the center, which is relatively cool, and one towards the tube's wall, which is relatively hot.

In a counterflow vortex tube the cool air is allowed to flow from the center of the tube at one end and the hot air from an annulus at the other end. Usually a valve is used to vary the flow rate and, hence, the temperature separation. In a uniflow tube the two air streams are emitted out the same end, but again, the cold air comes from the center and the hot air from an outer annulus. The temperature difference for a given length/diameter counterflow tube is a function of the incoming air pressure, the center "throttling" opening diameter on the cold side, and the size of the annular "valve". For a tube set up for maximum temperature difference with ambient temperature inlet air, it is said the tube can generate a hot air flow of about 100 degrees F and cold flow at about -70 degrees F. When adjusted for maximum hot air temperature, it is said the hot side can generate a flow at about 350 degrees F. So it appears that Maxwell's demon is alive and well.