Vortex Tubes: What is a Cold Fraction & How to use it to Your Benefit

Vortex Tubes

EXAIR has wrote 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 in reference to the Cold Fraction.

To give a basic background on the EXAIR Vortex Tubes, we manufacture three different sizes; small, medium, and large.  These sizes can produce a range of cooling capacities from 135 BTU/hr to 10,200 BTU/hr.  The unique design utilizes a generator inside each Vortex Tube.  The generator controls the amount of compressed air that can enter into the Vortex Tube.  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).

Vortex Tube Exploded View

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 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 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)

Cold Fraction is the percentage of air that flows out the cold end of a Vortex Tube.  As an example, if the control 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 has to be 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 Flow (SCFM/SLPM)

Vortex Tube Charts

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 changes.  As the percentage of the Cold Fraction becomes smaller, the cold air flow becomes colder, but also the 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.

Vortex Tube Example

How do you use this chart?  As an example, a model 3240 Vortex Tube is selected.  It will use 40 SCFM of compressed air at 100 PSIG.  We can determine the temperature and amount of air that will flow from the cold end and the hot end.  The inlet pressure is selected at 100 PSIG, and the Hot Exhaust Valve is adjusted to allow for a 60% Cold Fraction.  Let’s use an inlet compressed air temperature to be 68 oF.  With Equation 2, we can rearrange the values to find Qc:

Qc = CF * Q

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

The temperature drop from the chart above is 86 oF.  If we have 68 oF at the inlet, then the temperature is (68 oF – 86 oF) = -18 oF.  So, from the cold end, we have 24 SCFM of air at a temperature of -18 oF.  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 from the chart above is 119 oF.  So, with the inlet temperature at 68 oF, we get (119 oF + 68 oF) = 187 oF.  At the hot end, we have 16 SCFM of air at a temperature of 187 oF.

With the Cold Fraction and inlet air pressure, you can get specific temperatures for your application.  For cooling and heating capacities, these values can be used to calculate the correct Vortex Tube size.  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

EXAIR Vortex Tubes are for a Variety of Cooling Applications

Vortex tube
Cooling or Heating with the Vortex Tube

EXAIR Vortex Tubes are a low cost, reliable and maintenance free solution to a wide variety of industrial spot cooling problems.  They only requirement is a supply of compressed air as the power source.  Vortex Tubes have no moving parts and can produce temperatures that range from -50°F to +260°F (-46°C to +127°C).

Vortex Tubes produce two air streams one cold and one hot, the percentage of cold air flow from the inlet flow is referred to as the cold fraction.  The cold fraction is adjustable by the hot valve on the hot discharge side of the vortex tube.  Adjusting the hot valve results in both air temperature and air volume changes. The colder the air becomes, the volume of that cold air declines. So for very cold temperatures, a smaller volume of air is produced compared to a warmer air temperature.

For the vast majority of industrial cooling applications a larger volume of cool air will provide more efficient cooling than a lesser amount of very cold air.  Generally speaking the highest Btu/Hr values are in the 70-80% cold fraction range.

The exception to this would be in labs or special cases where the coldest temperatures are desired.  Adjusting a Vortex Tube is easy, simply insert a thermometer/thermocouple in the cold air exhaust and set the temperature by adjusting the valve on the hot end of the Vortex Tube.  You will know when you reach max refrigeration (80% cold fraction) as the cold air temperature will be 50°F (28°C) lower than the compressed air supply temperature.

EXAIR Vortex Tubes are constructed from stainless steel.  This ensures excellent wear resistance, corrosion resistance and assures years of reliable operation.  They are offered in 3 different size ranges (small, medium & large).  There are generators located inside the tube (user serviceable) that will change the volumetric flow.  The generators are available in a plastic construction or brass construction for high temperature applications.  The ranges 2 SCFM – 8 SCFM are designated as small Vortex Tubes, 10 SCFM – 40 SCFM are medium and 50 SCFM – 150 SCFM are large.  This feature allows you to customize or change your Vortex Tube for greater flexibility in a wide range of applications.

vortex_generator
Vortex generator

Large Vortex Tubes are specified when a high flow of cold air is needed. There are 16 models to choose from in total.  Capable of providing 3,400 BTU/HR up to 10,200 BTU/HR of cooling power.  These have been used to cool high heat loads that are centrally located or to help cool samples of gases for testing.

Medium Vortex Tubes are the most popular – there are twenty to choose from, depending on the cold air flow rate and temperature you’re looking for. These can produce temperatures as cold as -40°F (-40°C) when set to a 20% Cold Fraction (which is the percentage of total supply air that’s directed to the cold end) and cold air flows as high as 32 SCFM when set to an 80% Cold Fraction, which will produce a cold air temperature of about 20°F (-7°C). Some common uses are cooling ultrasonic welds and brazed joints.

The Medium Vortex Tubes are so popular, in fact, that they’re incorporated into our Adjustable Spot Cooler and Cold Gun Systems. They come ready-to-go with mufflers, cold air hose kits, and magnetic bases, so they couldn’t be easier to use.

Adjustable Spot Cooler

Cold Gun Lineup

Small Vortex Tubes are great when low flows (less cooling power) will succeed, or if compressed air supply is limited.  There are 12 models in total to choose from. These are specified for much smaller applications, like cooling the needle of a sewing machine, small drill bits, etc. You can also get one with a cold air hose & magnetic base…that’s the Mini Cooler System.

Mini Cooler

If you would like to discuss Vortex Tubes, Spot Cooling, efficiency of your compressed air usage, quieter compressed air products and/or any EXAIR product,  I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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Real Heroes Of Science: James Clerk Maxwell

People who watch way too much TV (like me) will certainly remember the “Real Men of Genius” commercials.  Here’s one of my personal favorites:

Local radio stations all over the country made parodies of these, as did sketch comics.  While trying to come up with something for my weekly blog, I saw that today was the anniversary of the passing of 19th century physicist James Clerk Maxwell.  So, if you’ll try to keep the background music from the video above playing in your head while you read this, let’s see if I can pay proper tribute:

James Clerk Maxwell…even though Albert Einstein is famous for the Special Theory of Relativity, he credited YOU for laying the groundwork.  You not only theorized the relationship between electricity, magnetism, and light, but you also proved it mathematically…so Albert didn’t have to.  He said your work was “the most profound and the most fruitful that physics has experienced since the time of Newton.”

Singer: Albert Einstein pretty much called you an “Einstein” the way we call geniuses “Einsteins.”

Professor Maxwell…you devoted your life to learning.  About EVERYTHING.  As if solving Einstein’s problem with the Theory of Relativity (40 years before he knew he had a problem with it) wasn’t enough, you decided to find out what the rings of Saturn were made of.  Over 100 years before we could send the Voyager spacecraft to find out for sure.  And you were right.

Then you discovered how to take color photographs by experimenting with light filters.

Singer: Not only did you tell us what Saturn’s rings were made of, we have color photographs of them thanks to you….

James Clerk Maxwell…your theory that a “friendly little demon” could somehow separate gases into hot and cold flows, while unproven in your lifetime, did actually come to fruition by the development of the Vortex Tube.  Which does just that.

Singer: That’s right, I just drew a straight line from Maxwell’s Demon to EXAIR’s Vortex Tube & Spot Cooling Products!

So here’s to you, James Clerk Maxwell…may we continue to recognize your brilliance, and be inspired by your drive to push forward in scientific developments.

Singer (building to final crescendo): James Clerk Maxwell, a Real Hero Of Sci-i-i-i-i-ence!

If you’d like to hear the musical parts of this actually get sung, or if you’d like to find out more about Vortex Tube products and their uses (it might be best to stick with that second part actually,) give me a call.

Russ Bowman
Application Engineer
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Intelligent Compressed Air: How do Vortex Tubes Work

A vortex tube is an interesting device that has been looked upon with great fascination over the last 89 years since its discovery by George Ranque in 1928. What I’d like to do in this article is to give some insight into some of the physics of what is happening on the inside.

With a Vortex Tube, we apply a high pressure, compressed air stream to a plenum chamber that contains a turbine-looking part that we call a generator to regulate flow and spin the air to create two separate streams. One hot and one cold.

Below is an animation of how a Vortex Tube works:

Function of a Vortex Tube

 

The generator is a critical feature within a vortex tube that not only regulates flow and creates the vortex spinning action, it also aligns the inner vortex to allow its escape from the hot end of the vortex tube. Note the center hole on the photo below. This is where the cooled “inner vortex” passes through the generator to escape on the cold air outlet.

Vortex generator

Once the compressed air has processed through the generator, we have two spinning streams, the outer vortex and the inner vortex as mentioned above.  As the spinning air reaches the end of the hot tube a portion of the air escapes past the control valve; and the remaining air is forced back through the center of the outer vortex.  This is what we call a “forced” vortex.

If we look at the inner vortex, this is where it gets interesting.  As the air turns back into the center, two things occur.  The two vortices are spinning at the same angular velocity and in the same rotational direction.  So, they are locked together.  But we have energy change as the air processes from the outer vortex to the inner vortex.

If we look at a particle that is spinning in the outer vortex and another particle spinning in the inner vortex, they will be rotating at the same speed.  But, because we lost some mass of air through the control valve on the hot end exhaust and the radius is decreased, the inner vortex loses angular momentum.

Angular momentum is expressed in Equation 1 as:

L = I * ω

L – angular momentum
I – inertia
ω – angular velocity

Where the inertia is calculated by Equation 2:

I = m * r2

m – mass
r – radius

So, if we estimate the inner vortex to have a radius that is 1/3 the size of the outer vortex,  the calculated change in inertia will be 1/9 of its original value.  With less mass and  a smaller radius, the Inertia is much smaller.  The energy that is lost for this change in momentum is given off as heat to the outside vortex.

Adjustments in output temperatures for a Vortex Tube are made by changing the cold fraction and the input pressure.  The cold fraction is a term that we use to show the percentage of air that will come out the cold end.  The remaining amount will be exhausted through the hot end. You can call this the “hot fraction”, but since it is usually the smaller of the two flows and is rarely used, we tend to focus on the cold end flow with the “cold fraction”.  The “Cold Fraction”  is determined by the control valve on the hot end of the Vortex Tube. The “Cold Fraction” chart below can be used to predict the difference in temperature drop in the cold air flow as well as the temperature rise in the hot air flow.

Vortex Tube Cold Fraction

By combining the temperature drops expressed above with the various flow rates in which Vortex Tubes are available, we can vary the amount of cooling power produced for an application. The above cold fraction chart was developed through much testing of the above described theory of operation. The cold fraction chart is a very useful tool that allows us to perform calculations to predict vortex tube performance under various conditions of input pressure and cold fraction settings.

The most interesting and useful part about vortex tube theory is that we have been able to harness this physical energy exchange inside a tube that can fit in the palm of your hand and which has a multitude of industrial uses from spot cooling sewing needles to freezing large pipes in marine applications to enable maintenance operations on valves to be performed.

We would love to entertain any questions you might have about vortex tubes, their uses and how EXAIR can help you.

John Ball
Application Engineer

Email: johnball@exair.com
Twitter: @EXAIR_jb

Vortex Tube Cold Fractions – An Explanation

Vortex Tube Family

At EXAIR we’ve been a pioneer in the compressed air market for the past 34 years.  We’ve brought engineered nozzles to the market which reduce compressed air consumption while maintaining performance, laminar flow Air Knives, pneumatic conveyors, atomizing nozzles, air-assisted static eliminators, and a slew of other products.  One of these “other” products is our Vortex Tube, which we manufacture in various sizes while also using as a basis for our Cold Guns, Adjustable Spot Coolers, Mini Coolers, and Cabinet Coolers – all of which are built on the same Vortex Tube technology.

Theory of operation for an EXAIR Vortex Tube

The principle behind a Vortex Tube is rooted in the Ranque-Hilsch effect which takes place inside of the tube.  As a compressed air source is fed into the Vortex Tube, the air flows through a generator and begins to spin down the length of the tube, “hugging” the ID of the tube.  When this spinning air contacts a deliberate obstruction at the end of the tube, it is forced to reverse directions, which requires a change in diameter to the vortex.  The original vortex must decrease in diameter, and in order to do so, it must give off energy.  This energy is shed in the form of heat, and a portion of the incoming air is directed out of the tube with a drastically reduced temperature via what is called the “cold end”.  Another portion of the air escapes through the “hot end” of the tube, resulting in a cold airflow at one end, and a hot airflow at the other end of the tube.

Small, but powerful, Vortex Tubes really are a marvel of engineering.  And, like most useful developments in engineering, Vortex Tube technology begs the question “How can we control and use this phenomena?”  And, “What are the effects of changing the amount of air which escapes via the cold end and the hot end of the tube?”

EXAIR Vortex Tube Performance Chart

These answers are found in the understanding of what is called a cold fraction.  A cold fraction is the percentage of incoming air which will exhaust through the cold end of the Vortex Tube.  If the cold fraction is 80%, we will see 80% of the incoming airflow exhaust via the cold end of the tube.  The remaining airflow (20%) will exhaust via the hot end of the tube.

For example, setting a model 3210 Vortex Tube (which has a compressed air flow of 10 SCFM @ 100 PSIG) to an 80% cold fraction will result in 8 SCFM of air exhausting via the cold end, and 2 SCFM of air exhausting through the hot end of the Vortex Tube.  If we change this cold fraction to 60%, 6 SCFM will exhaust through the cold end and 4 SCFM will exhaust through the hot end.

But what does this mean?

Essentially, this means that we can vary the flow, and temperature, of the air from the cold end of the Vortex Tube.  The chart above shows temperature drop and rise, relative to the incoming compressed air temperature.  As we decrease the cold fraction, we decrease the volume of air which exhausts via the cold end of the Vortex Tube.  But, we also further decrease the outlet temperature.

This translates to an ability to provide extremely low temperature air.  And the lower the temperature, the lower the flow.

Red box shows the temperature drop in degrees F when an EXAIR Vortex Tube is operated at 100 PSIG with an 80% cold fraction.

With this in mind, the best use of a Vortex Tube is with a setup that produces a low outlet temperature with good cold air volume.  Our calculations, testing, and years of experience have found that a cold fraction of ~80% can easily provide the best of both worlds.  Operating at 100 PSIG, we will see a temperature drop of 54°F, with 80% of the incoming air exiting the tube on the cold end (see red circle in chart above).  For a compressed air supply with a temperature of 74°F-84°F (common compressed air temperatures), we will produce an output flow with a temperature between 20°F and 30°F – freezing cold air!

With a high volume and low temperature air available at an 80% cold fraction, most applications are well suited for this type of setup.  When you order a Vortex Tube from EXAIR we will ship it preset to ~80% cold fraction, allowing you to immediately install it right of the box.

The cold air from an EXAIR Vortex Tube is effective to easily spot cool a variety of components from PCB soldering joints to CNC mills, and even complete electrical control panels.  Contact an Application Engineer with application specific questions or to further discuss cold fractions.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

EXAIR Manufactures Custom Vortex Tubes

EXAIR is based in Cincinnati, OH and it is where we design and manufacture our products. Since we are the manufacturer, we can design and build custom product when your application demands particular features. Vortex Tubes are the foundation of our cooling products and can be customized to suit your needs in many ways…

Vortex Family

The EXAIR Vortex Tube uses compressed air to generate a cold air stream at one end and a hot air stream at the other end.  This phenomenon in physics is also known as the Ranque-Hilsch tube.  It can generate very cold or very hot air without any moving parts, motors, or Freon.  Thus; making it low cost, reliable, and maintenance free.  The EXAIR Vortex Tube can generate

  • Air temperatures from -50 to +260 deg. F (-46 to +127 deg. C).
  • Flow rates from 1 to 150 SCFM (28 to 4,248 SLPM)
  • Refrigeration up to 10,200 BTU/hr (2,570 Kcal/hr)
Cooling or Heating with the Vortex Tube

With a wide range of cooling and heating applications, the EXAIR Vortex Tubes can be an ideal product for you.  They are used for cooling electronics, CCTV cameras, and soldered parts.  They are also useful for setting hot melts, gas sampling, and environmental chambers.  With its very compact and versatile design, it can be mounted in tight places to apply heated or cold air to your process.  The Vortex Tubes are used for improving process times in cooling, protecting equipment, or setting specific temperature requirements.  If you need a Vortex Tube to be more specific to your application, EXAIR can manufacture a proprietary product in the following ways:

Preset Vortex Tubes – the standard Vortex Tube has a screw on the hot end to adjust the cold and hot air temperatures.  To make the Vortex Tube tamper-resistant, EXAIR can replace the screw with a preset hot valve.  If you can supply the temperature and flow requirements for your application, EXAIR can determine the correct diameter hole to give you a consistent temperature and flow from the Vortex Tube.

Materials – The standard Vortex Tubes has a maximum temperature rating of 125 deg. F (52 deg. C).  For elevated ambient temperature, we offer a brass generator which will increase the temperature rating to 200 deg. F (93 deg. C).  If other materials are needed for food, pharmaceutical, or chemical exposure, we can also offer stainless steel for the hot plug, cold cap, and generator. I have seen Vortex Tubes made entirely from 316SS and even one made with a brass body. There are EXAIR Vortex Tubes with special material o-rings and hot valves or with customized muffler assemblies.

Fittings – Our standard units have threaded connections on the Vortex Tube to connect fittings and tubing.  In certain applications to improve mounting and assembly, special fittings may be required for ease of installation.  EXAIR can attach or modify these parts to the Vortex Tube to meet your requirements.

At EXAIR, we pride ourselves with excellent customer service and quality products.  To take this one step further, we offer specials to accommodate your applications.  As a manufacturer of the Vortex Tubes, we can work with our customers to generate a custom product with high quality, fast delivery, and a competitive price.  So, if you do need a special Vortex Tube to give you a specific temperature, ease of mounting, or a proprietary product for your OEM design, you can discuss your requirements with an Application Engineer.  We will be happy to help you.

John Ball
Application Engineer

Email: johnball@exair.com
Twitter: @EXAIR_jb

Vortex Tube Cold Fractions

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.

Vortex Tubes
Vortex Tubes are available in small, medium and large sizes with various flows and cooling capacities.

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 would be the operating pressure which you can see also has a significant effect on performance. Also the compressed air supply temperature 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.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN