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

Vortex Tube Cools Glue On A Paper Folding Machine

I recently worked on a cooling application with an engineering company who designed a paper folding machine for their customer. As the paper enters the machine, it travels over a series of rollers or “plows” that folds the paper into the desired design. At the last step a heated glue is applied to the edge so the paper stays folded. After the paper leaves the folder it is sent to a stack machine to be processed and packaged for shipment. It was at this area they were starting to see some issues arise as the glue was retaining heat, causing it to leak onto the dividers of the stacker or other finished papers.

Example of a paper folding machine

To try and remedy the situation, the customer had installed an air nozzle to blow compressed air across the last fold and while this did work somewhat, they had to operate at really low pressure so they didn’t cause the paper to move while trying to cool the glue. This slowed the process down, which was negatively affecting their production output, so they reached out for assistance on a more reliable solution.

After further discussing the process with the design company, I recommended they use our Model # 3908 Small Vortex Tube Cooling Kit. The Vortex Tube Cooling Kits include the Vortex Tube, cold muffler, tubing, filter separator and all of the generators to change the flow rate and cooling capacity of the Vortex Tube during operation. The temperature drop from the supply air temperature and the volume of air being exhausted can be controlled by adjusting the valve in the hot end to change the cold fraction (the percentage of air being exhausted out of the cold end versus the amount of air being exhausted out of the hot end).

Items included in a Vortex Tube Cooling Kit

By incorporating the Cooling Kit into the process, the customer would be able to experiment with the airflow and temperature to achieve an acceptable balance, providing enough cold air to cure the glue, while not disrupting the process. If you have a similar process you would like to discuss, please contact an application engineer at 800-903-9247 for assistance.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

The Folding Machine 4 image courtesy of Ms. Tharpe via creative commons license

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

EXAIR Vortex Tubes: As Much Cold Air As You Need, As Cold As You Need It

If you’re looking for a reliable, consistent flow of cold air, there’s really no better way to produce it than with a Vortex Tube. There are no moving parts…the air flow and temperature from a particular model, set to a specific cold fraction, is only influenced by the compressed air supply pressure & temperature.

Pressure is easy to control…all you need is a suitable regulator.  Temperature CAN be a variable, depending on your type of compressor, if you have a dryer system (and what type it is,) and sometimes, ambient conditions…if, for example, a long pipe is run through a very hot environment like a foundry or a blast furnace operation.  In cases where supply pressure and/or temperature can be limitations, a higher capacity Vortex Tube, set to a lower Cold Fraction, may be specified.  Which brings me to the user inquiry that inspired today’s blog…

This particular customer uses our Model 3215 Vortex Tubes (15 SCFM, 1,000 Btu/hr) to provide cooling to analyzer systems that monitor certain quality parameters in their manufacturing processes.  The ability to precisely control the temperature in these systems makes for repeatable and accurate measurement of these parameters.   Their compressed air supply in this area is regulated to 80psig, they have a refrigerant-type dryer and climate-controlled facility, so their supply temperature is a consistent 70°F.  You couldn’t ask for better conditions for a successful Vortex Tube application, and they’ve worked great, for years.

Now, due to a plant expansion, they’re installing some of these analyzer systems in a location where the compressed air supply is limited to 60psig.  The required cooling capacity is going to be the same, so the Project Manager reached out to us to see if they could get the same amount of cooling with this new pressure limitation.  Here’s how they’re doing it:

We publish the rated performance of Vortex Tube products for a supply pressure of 100psig.  The Model 3215 Vortex Tube consumes 15 SCFM @100psig and, when set to an 80% Cold Fraction (meaning 80%…or 12 SCFM…of the 15 SCFM supply is directed to the cold end,) the cold air will be 54F colder than the compressed air supply temperature.  Here’s the performance table, so you can follow along:

EXAIR Vortex Tube Performance Table

Now, their supply is at 80psig.  Since air consumption is directly proportional to absolute supply pressure (gauge pressure PLUS atmospheric, which is 14.7psi at sea level,) we can calculate their units’ consumption as follows:

(80psig + 14.7psia) ÷ (100psig + 14.7psia) = 0.83 X 15 SCFM (@100psig) = 12.4 SCFM (@80psig)

So, with a 50°F temperature drop (from a supply @70°F,) they were getting 12.4 SCFM of cold air at 20°F.

As you can see from the table above, they’ll only get a 46°F drop at 60psig…and the flow won’t be as high, either.  So…we’ll need to get more air through the Vortex Tube, right?  Let’s use a little math to solve for what we need.

We still need 20°F cold air from 70°F compressed air, so, at 60psig, we’re looking at a Cold Fraction of ~70%.  And we still need 12.4 SCFM, so:

12.4 SCFM ÷ 0.7 = 17.7 SCFM @60psig (required supply)

Our Model 3230 Vortex Tube uses 30 SCFM @10opsig…at 60psig it’ll consume:

(60psig + 14.7psia) ÷ (100psig + 14.7psia) = 0.65 X 30 SCFM (@100psig) = 19.5 SCFM (@60psig)

That’s about 10% more flow than they needed, theoretically, which was close enough to start.  From there, they “dialed in” performance by regulating the supply pressure and Cold Fraction (see video, below):

If you’d like to find out more, or work through a cooling application, give me a call.

Russ Bowman
Application Engineer
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Controlling Temperature And Flow Of An EXAIR Vortex Tube

If you need a reliable, consistent flow of cold air, look no further than the EXAIR Vortex Tube:

A 1/4 ton of refrigeration in the palm of your hand!

Getting the performance you want comes down to answering two simple questions:

What temperature do I need? 

Vortex Tubes produce a DROP in temperature, so your compressed air supply temperature is our starting point to determine what the actual cold air temperature will be.  The magnitude of the temperature drop is dependent on two factors:

  • Compressed air supply pressure – the higher the pressure, the higher the temperature drop.
  • Cold Fraction setting of the Vortex Tube – this is the percentage of the air supply that’s directed to the cold end.  The same temperature drop is produced, regardless of model, for a given Cold Fraction.  The lower the Cold Fraction, the greater the temperature drop (and hence, the lower the air temperature.)

EXAIR has two distinct series, or types, of Vortex Tubes:

3200 Series are used when Cold Fractions above 50% are desirable.  This provides maximum refrigeration…high flows and temperature drops that are optimal for many spot cooling applications such as tool cooling, setting hot melt adhesives, quick cooling of soldering/brazing, etc.

3400 Series are used for lower Cold Fractions (below 50%) and generate VERY cold air flow…as low as -50°F.  Some common applications for these are cryogenic lab sample cooling, circuit testing, or freeze seals in certain piping systems.

Temperature drops are dependent only on supply pressure and Cold Fraction setting. These values apply to any Vortex Tube, regardless of size/model.

Cold Fraction is adjusted by turning the Hot Air Exhaust Valve to let more, or less, hot air out, as shown in this short video:

What flow do I need?

Both the 3200 and 3400 Series Vortex Tubes are offered, from stock, in twelve distinct models of each series.  These are defined by the compressed air consumption, and the cold air flow is determined by the Cold Fraction setting.

Small Vortex Tubes come in three Models for each series, and consume 2, 4, or 8 SCFM when supplied with compressed air @100 psig.

Medium Vortex Tubes come in five Models for each series, and consume 10, 15, 25, 30, or 40 SCFM @100 psig.

Large Vortex Tubes come in four Models for each series, and consume 50, 75, 100, or 150 SCFM @100 psig.

Converting a Vortex Tube to a different Model (in the same size class) is as easy as changing the Generator (and the Taper Sleeve, for the Small Vortex Tubes):

The Generator and Taper Sleeve (*Small VT’s only) are changed by removing the Cold Cap.

So, for example, if you have a Model 3210 (10 SCFM consumption, 1,000 Btu/hr rated cooling) set to an 80% Cold Fraction, supplied with compressed air @100 psig & 70°F, it’s making a 16°F cold air flow of 8 SCFM.  If your situation calls for more flow, you can change the Generator…for example, if you convert it to a Model 3240 (40 SCFM, 2,800 Btu/hr rated cooling) – leaving the Cold Fraction at 80%, you’ll now get 32 SCFM of 16°F air.

What if you need colder air?  You can convert this same Medium Vortex Tube to a Model 3440 (40 SCFM consumption, max cold temperature) by changing the Generator again…and if you lower the Cold Fraction to 20%, it’ll make a -53°F cold flow of 8 SCFM.

Powerful and versatile, EXAIR Vortex Tubes are suitable for a wide range of applications requiring a consistent and reliable flow of cold air.  For help in selecting the right one for your needs, give me a call.

Russ Bowman
Application Engineer
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Vortex Tube Kits Make Heat Removal A (Cool) Breeze

I recently had the pleasure of discussing a cooling application with a customer.  The caller was familiar with our Cabinet Cooler Systems, and wanted to incorporate the same technology into a spot cooling application.  Problem was, he wasn’t sure about exactly how much cold air flow, and at what temperature, would suit his needs best…this was on a brand new mold (for plastic parts) that had just arrived.  His idea was to order a few different Vortex Tubes, and experiment with them.

I agreed that trying a few different Vortex Tube models would be a quick and easy way to find a solution, but I had a quicker and easier way: the Model 3930 Medium Vortex Tube Cooling Kit.  This gave him all the Generators that fit the Medium Vortex Tube, allowing him to make any medium Vortex Tube model he desired.  He would also be able to adjust the Cold Fraction to get the most effective temperature drop as well.

EXAIR Vortex Tube Cooling Kits come with all parts necessary to effect a wide range of cold air flow & temperatures.

With the Vortex Tube in place, it was very easy to configure the optimal cooling…as he decreased the Cold Fraction (to get colder air) he replaced the Generator (to get higher air flow.)  His application (cooling molded plastic parts) was satisfied with a Model 3225, set to a 70% Cold Fraction…this gave him 17.5 SCFM of cold air flow, at temperature of around 0F (a 71F drop from their compressed air supply temperature, which is around 70F.)

Is an EXAIR Cooling Kit right for your heat removal application?  If you’d like to find out,  give me a call.

Russ Bowman
Application Engineer
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