Cooling With Compressed Air: Air Knife vs. Vortex Tube Products

One of the popular applications for the EXAIR Super Air Knife is cooling. When mounted so that the air flow sweeps across the surface of a product, the laminar nature of the air flow works to maximize the contact time with the surface, which also maximizes the heat transfer…which means better product cooling than, say the turbulent air flow from a fan or blower.

Still, it’s common for us to get questions about how to provide even faster cooling.  Well, the two main variables in heat transfer are the time the air is in contact with the product, and the difference in temperature between the product surface and the air.

We’ve already touched on “time in contact”…sweeping the laminar flow across the surface at as low of an angle as you can, against the direction of travel, is ideal.  Combine that with the extraordinarily high air flow due to the entrainment level of the Super Air Knife, and you get an awful lot of air in contact with the surface, for a (relatively) long time.

Super Air Knives cool steel casting from 1,725°F (940°C) to 200°F (93°C) in under 20 minutes.

The difference in temperature, though, is a little trickier to deal with.  Because the developed flow from the Super Air Knife is mostly entrained ambient temperature air from the surrounding environment, you’re at the mercy of that ambient temperature.  One of the most common question – of the common questions about faster cooling – is, can you feed a Super Air Knife with cold air from a Vortex Tube?  The answer is no, for two big reasons:

  • The Vortex Tube’s cold flow can’t be back pressured, which would happen if you fed it through the plenum of a Super Air Knife and tried to make it come out the 0.002″ gap.
  • Even if it did work, the entrained air which, remember, makes up most of the flow, is still room temperature…meaning the total developed flow is a lot closer to room temperature than however cold the air you fed the Super Air Knife would be.

If the surface area to be blown on, to effect the desired cooling, is suitably sized, a Vortex Tube can be installed at a low angle to sweep its flow across.  The cold air flow from a Vortex Tube can also be distributed to more than one point, to cover more surface area.  That’s exactly what we do with our Dual Point Hose Kits for our Adjustable Spot Coolers, Mini Coolers, and Cold Gun Aircoolant Systems:

Dual Point Hose Kits can distribute air to both sides of a part, or onto a wider surface, than a single point discharge.

In fact, both the Single and Dual Point Hose Kits have a variety of tips they can be fitted with for tighter, or broader, flow patterns:

In some cases, multiple Vortex Tube products can be used, and, in other situations, the cold air can be directed through a manifold of some sort:

There are numerous methods to distribute the cold air flow from a lone, or a series of, Vortex Tubes.

Applications like the two on the right above (setting molten chocolate in molds, and keeping those white plastic parts during ultrasonic welding, respectively,) commonly start out as Air Knife inquiries, but the need for refrigerated air leads to creative Vortex Tube solutions.

If you’d like to discuss whether your application is best served by a Super Air Knife or a Vortex Tube Spot Cooling Product, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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Discovery of The Vortex Tube

There are many theories regarding the dynamics of a vortex tube and how it works. Many students have studied them in hopes of advancing the physics or as part of their undergrad studies. The man that started it all was not intentionally researching it, however.

The Vortex Tube was invented by accident in 1928, by George Ranque, a French physics student. He was performing experiments on a vortex-type pump that he had developed and noticed that warm air exhausted from one end and cold air from the other. Ranque quickly changed his focus from the pump to start a company taking advantage of the commercial possibilities for this odd little device that produced both hot and cold air, using only compressed air, with no moving parts. The company was not successful, and the vortex tube was forgotten until 1945 when Rudolph Hilsch, a German physicist, published a widely read paper on the device.

A vortex tube uses compressed air as a power source, has no moving parts, and produces hot air from one end and cold air from the other. The volume and temperature of the two air streams is adjustable with a valve built into the hot air exhaust.  Temperatures as low as -50°F (-46°C) and as high as 260°F (127°C) are possible.

Compressed air is supplied to a vortex tube and passes through nozzles that are tangent to an internal counterbore. As the air passes through it is set into a spiraling vortex motion at up to 1,000,000 rpm. The spinning stream of air flows down the hot tube in the form of a spinning shell, like a tornado (in red). The control valve at the end allows some of the warmed air to escape and what does not escape reverses direction and heads back down the tube as a second vortex (in blue) inside of the low-pressure area of the larger warm air vortex. The inner vortex loses heat and exits through the other end of as cold air.

It is thought that both the hot and cold air streams rotate in the same direction at the same angular velocity, even though they are traveling in opposite directions. A particle of air in the inner stream completes one rotation in the same time of an air particle in the outer stream. The principle of conservation of angular momentum would say that the rotational speed of the inner vortex should increase because the angular momentum of a rotating particle (L) is equal to the radius of rotation (r) times it’s mass (m) times its velocity (v).  L = r•m•v.  When an air particle moves from the outer stream to the inner stream, both its radius (r) and velocity (v) decrease, resulting in a lower angular momentum. To maintain an energy balance for the system, the energy that is lost from the inner stream is taken in by the outer stream as heat. Therefore, the outer vortex becomes warm and the inner vortex is cooled.

At EXAIR, we have harnessed the cooling power of the vortex tube, and it can be found and utilized in such products as Spot CoolersCabinet Coolers, and Vortex Tubes themselves. If you have questions about Vortex Tubes, or would like to talk about any of the EXAIR Intelligent Compressed Air® Products, feel free to contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

Max Refrigeration vs. Max Cold Temp Vortex Tubes

Here at EXAIR, our vortex tubes are offered in two separate series. The reason for this is to optimize the performance of the cold air temperature drop when operating with opposite ends of the cold fraction chart. The maximum refrigeration vortex tubes, 32xx series, perform optimally when they are set to a greater than 50% cold fraction.  The maximum cold temp vortex tubes, 34xx series, perform optimally when they are set to a less than or equal to 50% cold fraction. The cold fraction is discussed more in-depth within this link from Russ Bowman, Vortex Tube Cold Fractions Explained. This blog is going to explain a little further why one series of vortex tubes would be chosen for an application over another.

Cold Fraction
EXAIR Vortex Tube Performance Chart

Maximum refrigeration (32xx) vortex tubes are the most commonly discussed of the two types when discussing the optimal selection of the vortex tube for an application. The 32xx series vortex tubes achieve a maximum refrigeration output when operated at 100 psig inlet pressure with around  80% cold fraction. This would give a temperature drop from incoming compressed air temperature of 54°F (30°C). The volumetric flow rate of cold air will be 80% of the input flow which means only 20% is being exhausted as warm exhaust air. By keeping the flow rate higher the air is able to cool a higher heat load and is the reason the vortex tube is given a BTU/hr cooling capacity.

Vortex Tube Hot Valve Adjustment

Maximum cold temperature (34xx) tubes are less common as their applications are a little more niche and require a very pinpoint application. Rather than changing the temperature inside of a cooling tunnel or cooling an ultrasonic welding horn, the max cold temp vortex tube is going to have a minimum cold flow rate, less than 50% of input volumetric flow.  This minimal flow will be at temperature drops up to 129°F (71.1°C) from the incoming compressed air temperature.  This air is very cold and at a low flow. A 20% cold fraction exhausts 80% of the input volume as hot air. This type of volume would be ideal for sensor cooling, pinpoint cooling of a slow-moving operation, or thermal testing of small parts.

In the end, EXAIR vortex tubes perform their task of providing cold or hot air without using any refrigerants or moving parts. To learn more about how they work, check out this blog from Russ Bowman. If you want to see how to change the cold fraction, check out the video below. If you would like to discuss anything compressed air related, contact an application engineer, we are always here to help.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

Vortex Tube Cold Fractions Explained

Simply put, a Vortex Tube’s Cold Fraction is the percentage of its supply air that gets directed to the cold end. The rest of the supply air goes out the hot end. Here’s how it works:

The Control Valve is operated by a flat head screwdriver.

No matter what the Cold Fraction is set to, the air coming out the cold end will be lower in temperature, and the air exiting the hot end will be higher in temperature, than the compressed air supply.  The Cold Fraction is set by the position of the Control Valve.    Opening the Control Valve (turning counterclockwise, see blue arrow on photo to right) lowers the Cold Fraction, resulting in lower flow – and a large temperature drop – in the cold air discharge.  Closing the Control Valve (turning clockwise, see red arrow) increases the cold air flow, but results in a smaller temperature drop.  This adjustability is key to the Vortex Tube’s versatility.  Some applications call for higher flows; others call for very low temperatures…more on that in a minute, though.

The Cold Fraction can be set as low as 20% – meaning a small amount (20% to be exact) of the supply air is directed to the cold end, with a large temperature drop.  Conversely, you can set it as high as 80% – meaning most of the supply air goes to the cold end, but the temperature drop isn’t as high.  Our 3400 Series Vortex Tubes are for 20-50% Cold Fractions, and the 3200 Series are for 50-80% Cold Fractions.  Both extremes, and all points in between, are used, depending on the nature of the applications.  Here are some examples:

EXAIR 3400 Series Vortex Tubes, for air as low as -50°F.

A candy maker needed to cool chocolate that had been poured into small molds to make bite-sized, fun-shaped, confections.  Keeping the air flow low was critical…they wanted a nice, smooth surface, not rippled by a blast of air.  A pair of Model 3408 Small Vortex Tubes set to a 40% Cold Fraction produce a 3.2 SCFM cold flow (feels a lot like when you blow on a spoonful of hot soup to cool it down) that’s 110°F colder than the compressed air supply…or about -30°F.  It doesn’t disturb the surface, but cools & sets it in a hurry.  They could turn the Cold Fraction down all the way to 20%, for a cold flow of only 1.6 SCFM (just a whisper, really,) but with a 123°F temperature drop.

Welding and brazing are examples of applications where higher flows are advantageous.  The lower temperature drop doesn’t make all that much difference…turns out, when you’re blowing air onto metal that’s been recently melted, it doesn’t seem to matter much if the air is 20°F or -20°F, as long as there’s a LOT of it.  Our Medium Vortex Tubes are especially popular for this.  An ultrasonic weld that seals the end of a toothpaste tube, for example, is done with a Model 3215 set to an 80% Cold Fraction (12 SCFM of cold flow with a 54°F drop,) while brazing copper pipe fittings needs the higher flow of a Model 3230: the same 80% cold fraction makes 24 SCFM cold flow, with the same 54°F temperature drop.

Regardless of which model you choose, the temperature drop of the cold air flow is determined by only two factors: Cold Fraction setting, and compressed air supply pressure.  If you were wondering where I got all the figures above, they’re all from the Specification & Performance charts published in our catalog:

3200 Series are for max cooling (50-80% Cold Fractions;) 3400’s are for max cold temperature (20-50% Cold Fractions.)

Chocolate cooling in brown; welding/brazing in blue.

EXAIR Vortex Tubes & Spot Cooling Products are a quick & easy way to supply a reliable, controllable flow of cold air, on demand.  If you’d like to find out more, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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