## Controlling Temperature & Flow on a Vortex Tube

Vortex Tubes are unique items that use an ordinary supply of compressed air to create two streams of air, one hot and one cold.  We can drop the temperature by as much as 129oF (71.1oC) below inlet temperature on the cold end. It can also be raised as much as 195oF (107.9oC) above the inlet temperature on the hot end.  And this can be done without any moving parts, motors, or Freon. Compressed air would be the only input.  In this blog, I will cover how to adjust the Vortex Tubes and the resulting effects.

The cold air flow and temperature are easily controlled by adjusting a slotted valve located at the hot air outlet.  Opening the valve (turning it counterclockwise) reduces the cold air flow rate and lowers the cold air temperature.  Closing the valve (turning it clockwise) increases the cold air flow and raises the cold air temperature.  So, how does this apply to cooling?

To go a little deeper, we have to consider cold air temperature and cooling capacity.  Cooling capacity is the rate at which heat can be extracted.  The higher the cooling capacity, the faster the heat is removed.  This deals with temperatures and mass air flow.  Like stated above, the colder the air temperature that we create with a Vortex Tube, the less cold air is produced. The two are inversely related.  So, we have to find a balance between the temperature and cold air flow. You can find this rate by using Equation 1:

Equation 1:

H’ = 1.0746 * Q * (T2 – T1)

H’ – cooling capacity (BTU/hr)

Q – cold air flow (SCFM)

T2 – Final temperature (oF)

T1 – cold air temperature (oF)

With a Vortex Tube, the temperature difference is based on the inlet pressure and Cold Fraction.  The Cold Fraction is the amount of compressed air entering the Vortex Tube that will blow out of the cold end.  The remaining portion of the air will travel out of the hot end as heated air.  We have a chart below that shows the temperature drop on the cold air side and the temperature rise on the hot air side.

Here’s an example.  If we use a model 3240 at two different Cold Fractions, we can see the difference in cooling power.  At 100 PSIG (6.9 Bar), the model 3240 will use 40 SCFM (1133 SLPM) of compressed air.  If we look at two different Cold Fractions: 20% Cold Fraction and 70% Cold Fraction, we can calculate the cooling capacities by Equation 1.  In setting some criteria for our example, we will be using 70oF (21oC) compressed air at 100 PSIG (6.9 Bar).  Also, we will have a target temperature of 95oF (35oC).

Example 1:  At a 20% Cold Fraction and 100 PSIG, the Vortex Tube will generate a cold air temperature drop of 123oF.  So, with a 70 oF inlet air temperature, the cold air temperature will be 70 oF – 123 oF = -53 oF.  The amount of cold air at 20% Cold Fraction is 0.2 * 40 SCFM = 8 SCFM.  Now that we have this information, we can calculate the cooling capacity.

H’ = 1.0746 * 8 SCFM * (95 oF – (-53 oF)) = 1,272 BTU/hr.

Example 2:  At a 70% Cold Fraction and 100 PSIG, the Vortex Tube will generate a cold air temperature drop of 71oF.  So, with a 70 oF inlet air temperature, the cold air temperature will be 70 oF – 71 oF = -1 oF.  The amount of cold air at 70% Cold Fraction is 0.7 * 40 SCFM = 28 SCFM.  Now that we have this information, we can calculate the cooling capacity.

H’ = 1.0746 * 28 SCFM * (95 oF – (-1 oF)) = 2,889 BTU/hr.

As you can see, Example 1 will give you a much colder air stream, but the cooling capacity is 56% less than Example 2.  Or, in other words, in one hour, the Vortex Tube that is set at 70% Cold Fraction can remove 2,903 BTU of heat from an object.  While the same Vortex Tube set at 20% Cold Fraction, which is much colder, will only remove 1,279 BTU of heat.

In the above examples, we used 95oF as the target temperature for our application. If the target temperature changes, then so does the relative cooling power generated by a vortex tube. We take this into account when we are performing calculations to determine which model and setting for cold fraction would be best for your application.

EXAIR offers a wide range of sizes and cooling capacities with our Vortex Tubes for different applications.  They can be used to cool parts, set materials, and regulate temperatures in environmental chambers.  They provide an instant and reliable flow of cold air at different temperatures.  In this blog, I showed the difference between cold temperatures and the effect of cooling capacity.  If you have an application that requires cooling, you can contact an Application Engineer at EXAIR, and we will be happy to run through these calculations to help you select the correct model.

John Ball
Application Engineer
Email: johnball@exair.com

## 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?

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 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

## 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
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