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

 

More on Vortex Tubes: Understanding Cold Fractions

vortex tube

An EXAIR Vortex Tube

I had a conversation today through our online chat feature with a customer in the Middle East who needed a bit more understanding about Vortex Tubes.  The cooling power and instantaneous ability of a Vortex Tube offers ways to remove heat from applications, but the way the Vortex Tube works was a little misunderstood.  So, we went over the basics.

A Vortex Tube transforms a compressed air supply into a stream of hot and cold air.  As the compressed air enters into the Vortex Tube, it passes through a generator which causes the air to spin.  The airstream spins down the length of the Vortex Tube until it reaches a “brake”, whereupon it changes directions and begins spinning inside of itself, giving off energy in the form of heat.  The result is a stream of cold air at one end of the Vortex Tube, and a stream of hot air at the other.

But how can we adjust the flows and temperatures?

Adjusting the flow and cold air temperature from a Vortex Tube is as simple as turning the adjustment valve at the hot end of the unit.  This valve controls the “cold fraction” of the Vortex Tube, or, to put it more simply, the amount of air which will exit the unit at the cold end.

EXAIR Vortex Tube Performance Chart

EXAIR Vortex Tube Performance Chart

For example, if we were to set a Vortex Tube to an 80% cold fraction, 80% of the air consumed by the Vortex Tube would exhaust through the cold end of the unit.  If we take the same Vortex Tube and establish a 60% cold fraction, 60% of the consumed air will exhaust through the cold end of the unit.

Why is this important?

The cold fraction is important because at various cold fractions we will product varying temperature drops, even at the same operating pressure.  So, in the example above, if we have a Vortex Tube operating at 7 BARG, set to an 80% cold fraction, we can expect a temperature drop of 30°C (54°F), relative to the temperature of the incoming compressed air.

This means that if our compressed air temperature is 25°C (77°F), we will have an outlet temperature of -5°C (23°F).  If we take the same air supply and reduce the cold fraction to 60%, we will have a temperature drop of 48°C (86°F).

The caveat here is that when we reduce the air temperature, we also reduce the flow.  So, the colder the air temperature from the Vortex Tube cold end, the lower the volume of cold air.

When determining if a Vortex Tube is right for an application, it is important to consider all the variables (operating pressure, compressed air temperature, cold fraction, required cooling) when making a model number selection.

If you have any questions or concerns when considering a Vortex Tube, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

Which Vortex Tube Do I Need?

Last week, I wrote a brief introduction to vortex tubes, titled One Item Generates ¼ Ton of Refrigeration and Fits in the Palm of your Hand.” In it I introduced the Vortex Tube and the other products made from Vortex Tubes: Cabinet Coolers, Cold Guns, Adjustable Spot Cooler and Mini Coolers. I also introduced the idea of a cold fraction.  Today, I want to talk about specific Vortex Tube models.

The flow from the cold side of the Vortex Tubeis characterized in two different ways. First, we characterize the air by ΔT (temperature drop) from the starting compressed air temperature. With a supply pressure of 100 PSIG, the drop in temperature can range from 54° to 123° Fahrenheit. Second, we characterize the flow of air in Standard Cubic Feet per Minute. The different models of vortex tube are design to provide a range of flows and temperature.

Vortex Tube Specification

Vortex Tube Specification Chart

When facing this list you have numerous choices that can be daunting. My priorities for selecting a Vortex Tube for a customer are twofold. First, you need the Vortex Tube that will work in your application. Second, I want to choose the model with the least amount of compressed air in order to solve their problem with the least amount of air possible. The smallest Vortex Tube is a model 3202. It also utilizes the least amount of compressed air, 2 SCFM. At 100 PSIG and an 80 percent cold fraction, it will produce a cold flow of 1.6 SCFM at 54° F  below your compressed air temperature. If your compressed air temperature is starting at 70° F, your cold temperature will 16° F. All of the Vortex Tubes will be able produce this same temperature drop, but depending on which Vortex Tube you use will determine the volume of flow produced at that temperature.

1.6 SCFM of flow 54° F below compressed air temperature will take 135 BTU/HR away from a small 100°F box, which is enough energy to cool a needle, a small sensor, or a tiny camera, but what if you have a bigger area you need to cool. Then you need to use a Vortex Tube that will produce more flow. The 3202, 3204, and 3208 will all produce air at the same temperature, but the 3204 and 3208 will produce more volume of cold air.  With the same parameters as above (100 PSIG of inlet pressure and 80 percent cold fraction) the 3204 will produce 3.2 SCFM of cold air and cool 275 BTU/Hr. out of a 100° F environment. The 3208 will produce 6.4 SCFM of cold air and cool 550 BTU/Hr. These larger Vortex Tubes could be used to cool a closed circuit camera in a hot environment or a small drill bit where coolant is prohibited or undesired. From here our product continue to produce more volume of flow and we can go up to our largest Vortex Tube, 3299 which will use 150 SCFM of compressed and cool up to 10,200 BTU/HR.

What if you have an application where you don’t need more air but 16°F  isn’t cold enough? Then you can adjust your cold fraction. Adjusting the cold fraction will allow you to increase the temperature drop. Opening the brass hot valve, will lower the cold fraction. As more air is allowed to escape out of the hot end of the Vortex Tube, the temperature and the flow rate of the cold flow decrease.  If you need to cool below a 50% cold fraction we recommend the 3400 series Vortex Tubes. At 100 PSIG this would occur when you need more than 100° F temperature drop.

Vortex Tubes can be used in a variety of cooling application. If you have any question about the topic discussed above please contact me or another application engineer.

Dave Woerner
Application Engineer
DaveWoerner@EXAIR.com
@EXAIR_DW

Testing, Testing

One of the best criteria to know in a cooling application is the amount of cooling capacity required.  For example, if an ultrasonic weld needs to be cooled in 10 seconds and 400 BTU/hr. of cooling capacity is needed, we can recommend a suitable Vortex Tube.  (In this case, model 3208 with 550 BTU/hr. of cooling capacity.)  Or, if a specific temperature and flow of air is required, we can recommend accordingly. I recently spoke to a customer who needed a specific temperature of air at a low volume…

3408 Vortex Tube Test at 100 PSIG

EXAIR 3408 Vortex Tube tested at 100 PSIG with an inlet  compressed air temperature of 77°F (6.9 BARG, compressed air temperature of 25°C)

The photo above shows a test run for an end user that needed to achieve a specific cold outlet temperature from a Vortex Tube.  Their specific application required lower flow, lower temperature air, which led to the recommendation of our 3400 series units. The 3400 series Vortex Tubes produce the coldest air temperatures at a lower volume of air.

The project deadline for this end user had been shortened, making it imperative to find a solution which was not only suitable, but repeatable and readily available.  And, although the cold fraction percentages (which define temperature rise/drop from a Vortex Tube) are published on the EXAIR.com site and in our catalog, a quick test setup and photo can go a long way toward providing confidence within a customer that we could provide a solution.

The test proves the data from EXAIR is true to our published values, and gave the customer the confidence to order four pieces for their project.  Update:  The customer called today and ordered four more pieces.

Discussion and testing are methods we use in the Application Engineering department at EXAIR to determine if our products are suitable for an application.  If you have questions about your application and would like an EXAIR opinion, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

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