What it shows:
James Clerk Maxwell postulated that since heat involves the movement of molecules, it might be possible to separate hot and cold air in a device with the help of a "friendly demon" who would sort out and separate the fast and slow moving molecules of air. The vortex tube is such a device and does exactly that — using compressed air as a power source, it has no mechanical moving parts and produces hot air at one end and cold air at the other.
How it works:
Room temperature compressed air is supplied to the vortex tube. Upon entering the tube, the air passes through a nozzle designed to move the air tangentially inside the tube, setting the air into a vortex motion. The spinning air stream is subsequently forced by the nozzle to change its direction and travel down the length of the tube in the form of a spinning shell. A special conical valve at the end of the tube allows some of the air to escape while diverting the rest back down the tube as a second vortex inside the larger outer vortex. This second vortex then exits out the other end of the tube. This scenario is illustrated as follows:
While the outer vortex moves down to one end of the tube and the inner votex moves in the opposite direction to the other end, both rotate in the same direction and with the same angular velocity. Since the inner vortex has a smaller radius, conservation of angular momentum would have it rotate at a larger angular velocity. Thus, work was done to change its angular momentum and have it rotate at the smaller angular velocity of the outer vortex. The inner vortex loses angular momentum by interacting with the outer vortex and the kinetic energy that is lost shows up as heat in the outer vortex. The outer vortex becomes warm and the inner vortex is cooled. See reference (1) for a rigorous analysis of how it works.
The actual tube and an exploded view:
A muffler can be added to the assembly to reduce noise (desirable). Simply unscrew the cold cap and replace it with the muffler.
Setting it up:
Compressed air supply: The manufacturer recommends a line pressure of 80 to 100 PSIG for best performance. The compressed air in the Science Center is only 60 to 75 PSIG and, although it is good enough to operate it, it does affect the performance. For example, a temperature of -14˚C (6.8˚F) can be achieved at 75 PSIG (using the 15C generator nozzle) but only -8˚C (18˚F) at 60 PSIG. The use of clean air is essential — use the 5 micron element filter and connect the 1/4 NPT inlet of the vortex tube directly to it as shown in the photograph. The noise muffler is attached to the cold air exhaust on the left; also shown is a close-up picture of the vortex generator.
Adjusting the vortex tube: Hot and cold air temperatures produced by the Vortex Tube are adjusted with the slotted valve at the hot air exhaust. Opening the valve reduces the cold airflow and lowers its temperature. Closing the valve increases the cold airflow as well as its temperature. To measure the temperature accurately, the thermometer should be inserted into the cold muffler.
A nylon 3/8 NPT hose barb adaptor can be screwed into the outlet of the muffler to accomodate a piece of tygon tubing if it is desired to direct the cold air onto something.
The vortex-tube chiller was invented by George Ranque in 1928 when he observed the temperature splitting effect in a vortex-type pump: J. Phys. Radium 4, 112 (1933). Years later, a systematic study of the device was brought to the attention of the scientific community by Hilsch: Z. Naturf. 1, 208 (1946); Rev. Sci. Inst. 18, 108 (1947). Hence the various names for the vortex tube.
The vortex tube is an EXAIR model 3215. Sold as a kit (kit model 3930), it includes ten different plastic nozzles (called generators), a noise muffler for the cold end, and an Arrow Series F35 air filter. Each of the plastic nozzles is marked with a number and a letter. The number indicates the CFM air consumption and the letter indicates the type of operation — "R" for maximum refrigerated air flow and "C" for maximum cold temperature (at the expense of large air flow). According to the manufacturer, the tube can provide a maximum of 2,800 Btu/hr cooling and cold air temperature of 50˚F (28˚C) below the compressed room temperature air with the 40 CFM nozzle. With our reduced air pressure, the 15 CFM nozzle works best.
1. B.K. Ahlborn and J.M. Gordon, "The vortex tube as a classic thermodynamic refrigeration cycle," J. Applied Phys. 88(6), 3645-3653 (Sept. 2000).
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