Exorcising Maxwell’s Demon – Not Really

We all have our demons, and James Clerk Maxwell was no different. While he and I weren’t close, I have gotten to know one of his demons over the past 13 years. Okay, so in all seriousness, James Clerk Maxwell is a well-known Scottish Mathematician who throughout his life discovered a way to break the second law of thermodynamics and that became known as Maxwell’s Demon. So what exactly was his demon?

It was in 1867 when Maxwell wrote a letter and described his first encounter with the theory. In this letter, he wrote he spoke of a “finite being” that would control a massless door that separated two chambers of gas. This door would be opened and closed to permit a faster-moving molecule into the fast chamber, which also carried more heat, and then the slower-moving particles from the hot chamber into the slow chamber which was also cooling constantly. Because the velocity of a gas is dependent on the kinetic temperature of the molecule and its surrounding molecules. Since the demon was separating the hot and cold molecules it would permit one chamber to warm up while the other chamber cools down below ambient air conditions. This in turn decreases the total entropy of the system yet doesn’t do any work. Thus, violating the great second law of thermodynamics.

It wasn’t Maxwell that related this “finite being” to a demon, instead it was William Thomson, 1st Baron Kelvin who we associate absolute temperatures with in order to pay our respects of all his work. Lord Kelvin originally, published the thoughts in response to letters that Maxwell had written to other scholars. Kelvin did not mean for this to be a correlation with the malevolent being instead to be related to a daemon from Greek mythology which is a supernatural being who performs work behind the scenes. So why do we connect Maxwell’s demon to anything to do with an Intelligent Compressed Air Product?

In the spirit of Maxwell’s Demon, the separation of air molecules into a cold and hot air stream, without work being done can be directly related to a Vortex Tube. The Vortex Tube does exactly this, as an air stream enters the compressed air inlet, the air enters into the generator chamber where it is spun at very high speeds and sent down what is called the hot tube. As the air is spinning and traveling, it separates into a hot and cold air stream. The cold air is then sent down and out the cold end of the Vortex Tube while the hot air stream is exhausted out the hot end.

How a Vortex Tube Works

The percentage of cold air exit vs hot air exit is deemed the cold fraction and effects the temperature drop and rise for the respective stream of air. This gives us the ability to slightly control the demon, and thus we learn how to make this supernatural being work for us, and thus we never break the second law of thermodynamics, it’s actually this demon.

EXAIR Vortex Tube Performance Chart

In all seriousness, this thought experiment still continues to be a talk of physicists, mathematicians and scholars. They utilize Vortex Tubes in experiments to better understand how Maxwell came up with this thought and just how can they control this supernatural being.

If you would like to discuss how to utilize a Vortex Tube in your application or any of the EXAIR product lines, contact any Application Engineer today.

Brian Farno
Application Engineer

Vortex Tube Cold Fraction and how it Affects Flow and Temperature Control

Vortex Tubes are the perfect solution when dealing with a variety of spot cooling applications. They use compressed air to produce a cold air stream and a hot air stream, with temperatures ranging from as low as -50°F  up to +260°F (based on ambient supply temperature) and providing as much as 10,200 Btu/hr. of cooling capacity. By simply adjusting the valve in the hot end of the Vortex Tube, you are able to control the “cold fraction” which is the percentage of air consumed by the vortex tube that is exhausted as cold air versus the amount of air exhausted as hot air. Our small, medium and large Vortex Tubes provide the same temperature drop and rise, it’s the volume of air that changes with the various sizes.

The unique physical phenomenon of the Vortex Tube principle generates cold air instantly, and for as long – or short – a time as needed.

When looking at the below performance chart, you will see that “Pressure Supply” and “Cold Fraction %” setting all play a part in changing the performance of the Vortex Tubes. Take for example, an operating pressure of 100 PSIG and cold fraction setting of 20%, you will see a 123°F drop on the cold side versus a 26°F temperature rise on the hot side. By the using the same Vortex Tube and keeping the operating pressure at 100 PSIG but changing the cold fraction to 80%, you will now see a 54°F temperature drop on the cold side and a 191° rise at the hot end.

Vortex Tube Performance Data
Vortex Tube Performance Chart

We’ve looked at how the cold fraction changes the temperature, but how does it change the flow for the various Models?

Say you are using a Model # 3240 Medium Vortex Tube which consumes 40 SCFM @ 100 PSIG. Again with the cold fraction set at 80% (80% of the consumed compressed air out of the cold end), you would flow 32 SCFM at the cold air exhaust.

40 SCFM x 0.8 (80% CF) = 32 SCFM

Using the same Model # 3240 Medium Vortex Tube but now with a 20% cold fraction (20% of consumed compressed air out of the cold end), you would flow 8 SCFM at the cold exhaust.

40 SCFM x 0.20 (20% CF) = 8 SCFM

As you can see, to achieve the colder air temperatures, the volume of cold air being exhausted is reduced as well. This is important to consider when making a Model selection. Some other considerations include the operating pressure which also has a significant effect on performance. The compressed air supply temperature is important because the above temperatures are temperature differentials, so in the example of the 80% cold fraction there is a 115F temperature drop from your inlet compressed air temperature.

If you need additional assistance, you can always contact myself or another application engineer and we would be happy to make the best selection to fit your specific need.

Jordan Shouse
Application Engineer

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