Tag: cold fraction

Exorcising Maxwell’s Demon – Not Really

Published on by Brian FarnoLeave a comment

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
BrianFarno@EXAIR.com
@EXAIR_BF

What You Can Do With A Vortex Tube…And What You Can’t

Published on by Russ BowmanLeave a comment

Vortex Tubes are near the top of the list of the most interesting uses of compressed air: Cold (and hot) air, generated instantly, from a device with no moving parts. Why don’t we use them for EVERYTHING? It’s not that it CAN’T be done, but it can be impractical to do so. Consider:

While researching our Cabinet Cooler Systems, some callers will ask about using this technology to cool a space larger than an electrical panel, like a server room. I spoke with just such a caller once, who had 7.5kW worth of heat estimated in a server room that was under construction, and had been asked to research cooling solutions…so we did:

  • Since 1 watt equals 3.41 Btu/hr, 7.5 kilowatts equals 25,575 Btu/hr worth of cooling required.
  • Our highest capacity single Cabinet Cooler generates a cooling capacity of 2,800 Btu/hr, so we talked about ten of them, for ~10% safety factor, which was reasonable for the purposes of our discussion.
  • Each 2,800 Btu/hr Cabinet Cooler uses 40 SCFM @100psig, for a total of 28,000 SCFM. Using a common thumbrule that says a typical industrial air compressor generates 4 SCFM per horsepower, that means they’d need a 100HP compressor (or that much capacity from their whole system) just to run these Cabinet Coolers. Adding that cooling capacity to their HVAC requirements made more sense.

Of course, with every rule, there’s an exception: an independent crane operator carries a Model 3250 Large Vortex Tube with him for cab cooling in the tower cranes he’s contracted to operate. While the US Department of Energy considers “personnel cooling” to be an inappropriate use of compressed air, the small fans typically found in these cranes’ cabs offer little comfort to an operator spending all day, 50 feet off the ground, in the summer heat of the Deep South!

EXAIR offers 24 distinct Vortex Tube models with cooling capacities from 135 Btu/hr to 10,200 Btu/hr.

Another common question regards the use of a Vortex Tube with another EXAIR product…the most common being an Air Knife. These callers want to blow cold air onto something, but instead of the conical and relatively small flow pattern the Vortex Tube discharges, they want to blow a curtain of cold air. The design & function of both the Vortex Tube, and the Air Knife, work against this idea:

  • The cold air has to exit the Vortex Tube at, or very near, atmospheric pressure. If it encounters much back pressure at all, performance (as measured by the temperature and flow rate of the cold air) will deteriorate.
  • An Air Knife, by design, is pressurized all the way to the point where the compressed air flow exits the 0.002″ thick gap. That’s far too much back pressure for a Vortex Tube to operate under.
  • Even if the Vortex Tube DID supply cold air, under pressure, to the Air Knife, the tremendous amount of environmental air entrained by the Air Knife would still result in a total developed flow temperature that was much closer to ambient temperature for the area.
Since the Super Air Knife entrains air from the surrounding environment at a rate of 40:1, the resultant air temperature, regardless of the temperature of the air supply, is always going to be pretty close to ambient.

One “workaround” for this is what we informally call a “cold air knife” – that’s when you plumb the cold air from a Vortex Tube into a length of pipe with a series of holes drilled along its length. Let’s say a building products manufacturer wanted to blow cold air across a 10ft wide continuous sheet of roofing material…because they did:

  • I recommended that they take a PVC (because it’s non-conductive and wouldn’t transfer heat from ambient as fast) pipe a little longer than 10ft, cap the ends, drill 1/8″ holes every inch (total of 120 holes).
  • From the table below, we see that a 1/8″ diameter hole can flow as much as 1.1 cubic feet per minute @1psig*, so 120 of those holes will pass ~132 cubic feet per minute worth of air flow.
  • Four Model 3240 Vortex Tubes were specified: when set to an 80% Cold Fraction, 80% of the 40 SCFM that each will consume, or 32 SCFM, is directed to the cold end. 32 SCFM X 4 3240’s = 128 SCFM. Close enough. They plumbed those 4 Vortex Tubes at approximate equal distances along the length.
*I picked 2psig because that’s the maximum back pressure before it starts to change performance. I also assumed we’re not going to round the entrance of the holes, so I applied the 0.61 multiplier from the table notes.

A Model 3215 Medium Vortex Tube supplied @100psig will flow 10 SCFM worth of cold air when set to a 67% Cold Fraction**, which will give us a curtain of cold air that’s a little more than 71°F colder than the compressed air supply:

**When set to a 70% Cold Fraction (that means 70% of the compressed air supply flow is directed to the cold end), the cold flow from a Vortex Tube supplied @100psig will be 71°F colder than the compressed air supply. At a 67% Cold Fraction, it’ll be a little colder than that.

If you’ve got an application involving the need for cold air, on demand, EXAIR has a variety of products that’ll do just that. Give me a call to find out more.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
Visit us on the Web
Follow me on Twitter
Like us on Facebook

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

Published on by Jordan T ShouseLeave a comment

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

Send me an Email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_JS

How to Apply Vortex Tubes and Understand Cold Fractions

Published on by Russ BowmanLeave a comment

It’s been almost 100 years since Georges Ranque discovered the vortex tube phenomenon. Since then, they’ve become one of the best worst kept secrets in industry…I talk to callers all the time who have a piece of equipment that came with one of our Cabinet Cooler Systems installed, and they want to know how to get cold air like that for a machine tool cutting or spot cooling application. Other callers have discovered Vortex Tubes for the first time via a web search, or they saw one at a customer’s (or vendor’s) facility. They often sound like someone asking a magician to reveal the secret behind a trick. Of course, it’s not magic (not really) – but it is certainly a neat trick:

Then, the discussion turns to product selection. EXAIR Vortex Tubes come in three sizes, with multiple Models in each size range. Those different Models are all the same Small, Medium, or Large Vortex Tube, with a different Generator installed, which determines the amount of compressed air the Vortex Tube will consume…and the Cold Fraction range. These two variables go hand in hand when determining which Vortex Tube is right for the application.

‘Cold Fraction’ is the term for the percentage of the supply air that’s directed to the cold end. The higher the Cold Fraction, the higher the flow, and the temperature, of the cold air flow. Conversely, the lower the Cold Fraction, the lower the cold air flow…and temperature.

For jobs that call for rapid cooling to ambient temperature (or a little below), a “Max Refrigeration” Generator is installed in a 3200 Series Vortex Tube. They are designed to direct most of the compressed air flow to the cold end, exhausting a smaller amount out of the hot end. A Vortex Tube set at an 80% Cold Fraction is generally very close to being optimized for these applications: they’re putting out a decent amount of air flow, with a 54F temperature drop. Assuming the compressed air supply is roughly room temperature, that means you’re blowing 20 to 30F (-6.6 to -1.1C) air onto your part. Most of the time, it’ll cool it down in a real hurry. The final piece of the puzzle, then, is determining the cold air flow rate. Our lowest capacity Small Vortex Tube with a Max Refrigeration Generator will use 2 SCFM @100psig, and generates a flow of 1.6 SCFM of cold air. On the other end of the spectrum, our highest capacity Large Vortex Tube uses 150 SCFM @100psig, and gives you a cold flow of 120 SCFM. There are ten Models in between, so we can come quite close to an optimal selection for just about any size/shape of part that needs cooled.

Keep in mind that there are two variables in a convection/conduction air cooling application: the flow rate of the air, and the difference in temperature in the cooling air and the hot part. We’ll always recommend starting at the highest cold fraction, but you may find that a little bit lower flow…and the lower temperature that comes with it…might suit your needs better. Good news is, that doesn’t change the compressed air consumption, so you can optimize performance at no additional cost of operation.

Other applications call for air that’s just as cold as possible. For those, we offer our 3400 Series “Max Cold Temperature” Vortex Tubes. Where the 3200 Series’ Cold Fractions are adjustable from 50-80%, the 3400 Series can be adjusted from 20-50%. Assuming, again, that the compressed air supply is roughly room temperature, at a 20% Cold Fraction and 100psig supply pressure, your cold flow can be as low as -50F (-45.6C). If you’re trying to get something to a particularly low temperature – lab samples or circuits that need to be tested at a certain temperature, or freeze seals in piping systems, for instance – then a 3400 Series Vortex Tube is just what you’re looking for. These come in the same sizes & Models as the 3200 Series, from 2 to 150 SCFM.

Another nice thing about using a Vortex Tube for cold air is that you can turn them on and off as frequently (or as seldom) as needed. They’re generating cold air flow, at their published rated temperature, instantly. There are no moving parts to wear, so you can cycle them on and off rapidly, or let them run continuously. In fact, if you supply them with clean, moisture free air, they’ll run darn near indefinitely, maintenance free.

Here’s a short video, showing how to adjust the Cold Fraction of a Vortex Tube. If you’d like to find out more, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
Visit us on the Web
Follow me on Twitter
Like us on Facebook