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

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

What I Do

I’ve blogged before about having a fantastic wife and three smart and rapidly growing daughters. Our nightly routine is one that gets to be cumbersome and sometimes painful, at the same time, I wouldn’t change a single aspect as it gives both my wife and me one on one time with each girl. Even my pre-teen daughter still wants this one-on-one time when we just sit and calm down from the day by talking or singing in her case. I know it won’t last forever, so I always try to stay present. Here lately all three of my daughters on different days have asked me what I do at work during the day. It caught me off guard all three times.

They know that I work for EXAIR, and they know we make “stuff”, they’ve been to the company parties and even had lunch with me here in the office, they still didn’t know what I did, and at the time each one asked, even I didn’t know what I did. The answers I gave were all fairly similar. I help people figure out how to fix stuff by using the stuff we make. If they have something from EXAIR that isn’t working then I help them figure out why it isn’t working, and we try to get it fixed. Then they would ask things like, if their car is broken they call you, no that’s only when I’m at home. I tell them I also get to test products and see what they can do, even make videos of what our stuff does. Of course, they wanted to know if I made TikToks and I proudly informed them I do not and that most of this stuff is on a website or on YouTube.

The fact is that they know I love to work with my hands and see my work around the house or at other people’s homes on their cars or on their projects. They know that I value my experiences and I always try to have them recall an experience they may have already had when they are struggling with something. The best is when my oldest is learning about heat transfer. First, we did an experiment with my trusty Zippo lighter, so she experienced that holding your hand six inches over a flame you can feel the warmth but underneath you can’t. Then I showed them Vortex Tube Videos. They didn’t find it as cool as I do. (DAD PUN INTENDED!)

Lucky for me, when people are contacting me at work, they generally get excited about seeing compressed air turned into hot and cold air streams without moving parts and being able to solve heat transfer issues quickly and easily. The exact opposite reaction of young children, which helps me not feel like such a nerd.

The point of this story is that I am here to help, it’s one of the key responsibilities I hold as an Application Engineer here at EXAIR. With that, I share all of my experience that comes with over 15 years in the industry and always keep my eyes and ears open when I don’t know something. If you are at a wall with your point-of-use compressed air system or a process in your manufacturing, contact us and see how our bank of experience can help you to determine the best path moving forward.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

Vortex Tubes: What is a Cold Fraction?

How a Vortex Tube Works

EXAIR has written many different articles about how Vortex Tubes work and the applications in which they are used.  The idea of making cold air without any freon or moving parts is a phenomenon.  This phenomenon can generate cold air to a temperature as low as -50 oF (-46 oC).  In this article, I will explain the adjustment of the Vortex Tube to get different temperatures and cooling effects with reference to the Cold Fraction.

To give a basic background on the EXAIR Vortex Tubes, we manufacture them in three different body sizes; small, medium, and large.  These sizes can produce a range of cooling capacities from 135 BTU/hr to 10,200 BTU/hr (34 Kcal/hr to 2,570 Kcal/hr).  The unique design utilizes a generator inside each Vortex Tube.  The generator controls the amount of compressed air that can enter the Vortex Tube as well as initiating the spinning of the air.  As an example, a medium-sized Vortex Tube, model 3240, will only allow 40 SCFM (1,133 SLPM) of compressed air to travel into the Vortex Tube at 100 PSIG (6.9 bar).  While a small-sized Vortex Tube, model 3208, will only allow 8 SCFM (227 SLPM) of compressed air at 100 PSIG (6.9 bar).  EXAIR manufactures the most comprehensive range, from 2 SCFM (57 SLPM) to 150 SCFM (4,248 SLPM).

After the compressed air goes through the generator, the pressure will drop to slightly above atmospheric pressure.  (This is the “engine” of how the Vortex Tube works).  The air will travel toward one end of the tube, where there is an air control valve, or Hot Air Exhaust Valve.  This side of the Vortex Tube will blow hot air.  This valve can be adjusted to increase or decrease the amount of air that leaves the hot end.  The remaining portion of the air is redirected toward the opposite end of the Vortex Tube, called the cold end.  By conservation of mass, the hot air and cold air flows will have to equal the inlet flow as shown in Equation 1:

Equation 1:

Q = Qc + Qh

Q – Vortex Inlet Flow (SCFM/SLPM)

Qc – Cold Air Flow (SCFM/SLPM)

Qh – Hot Air Flow (SCFM/SLPM)

The percentage of inlet air flow that exits the cold end of a vortex tube is known as the Cold Fraction.  As an example, if the Hot Air Exhaust Valve of the Vortex Tube is adjusted to allow only 20% of the air flow to escape from the hot end, then 80% of the air flow is redirected toward the cold end.  EXAIR uses this ratio as the Cold Fraction; reference Equation 2:

Equation 2:

CF = Qc/Q * 100

CF = Cold Fraction (%)

Qc – Cold Air Flow (SCFM/SLPM)

Q – Vortex Inlet Flow (SCFM/SLPM)

EXAIR Vortex Tube Performance Chart

EXAIR created a chart to show the temperature drop and rise, relative to the incoming compressed air temperature.  Across the top of the chart, we have the Cold Fraction and along the side, we have the inlet air pressure.  As you can see, the temperature changes as the Cold Fraction and inlet air pressure change.  As the percentage of the Cold Fraction becomes smaller, the cold air flow becomes colder, but the amount of cold air flow becomes less.  You may notice that this chart is independent of the Vortex Tube size.  So, no matter the generator size of the Vortex Tube that is used, the temperature drop and rise will follow the chart above.

How do you use this chart?  As an example, we can select a model 3240 Vortex Tube.  It will use 40 SCFM (1133 SLPM) of compressed air at 100 PSIG (6.9 Bar).  We can determine the temperature and amount of air that will flow from the cold end and the hot end.  For our scenario, we will set the inlet pressure to 100 PSIG, and adjust the Hot Exhaust Valve to allow for a 60% Cold Fraction.  Let’s say the inlet compressed air temperature is 68oF.  With Equation 2, we can rearrange the values to find the Cold Air Flow, Qc:

Qc = CF * Q

Qc = 0.60 * 40 SCFM = 24 SCFM of cold air flow

The temperature drop shown in the chart above is 86oF.  If the inlet temperature is 68oF, the temperature of the cold air is (68oF – 86oF) = -18oF.  So, at the cold end, we will have 24 SCFM of air at a temperature of -18oF.  For the hot end, we can calculate the flow and temperature as well.  From Equation 1,

Q = Qc + Qh or

Qh = Q – Qc

Qh = 40 SCFM – 24 SCFM = 16 SCFM

The temperature rise shown in the chart above is 119oF.  So, with the inlet temperature at 68oF, we get (119oF + 68oF) = 187oF.  So, we have 16 SCFM of air at a temperature of 187oF coming out of the hot end.

With the Cold Fraction and inlet air pressure, you can get specific temperatures for your application.  For cooling and heating capacities, the flow and temperature can be used to calculate the correct Vortex Tube size for your application.  If you need help in determining the proper Vortex Tube to best support your application, you can contact an Application Engineer at EXAIR.  We will be glad to help.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

Video Blog: Cold Gun Maintenance Video

Is your Cold Gun still working like when it was new? If not, check out this video – you can very likely restore it to “as new” performance with a simple disassembly & cleaning.

If you have any questions about maintenance on your Cold Gun, or any of our products, give me a call.

Russ Bowman, CCASS

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