Tag: cold fraction

Controlling Temperature and Flow in a Vortex Tube

Published on by John BallLeave a comment

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 freon or moving parts is a phenomenon.  This phenomenon is the Vortex Tube.  It 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 inside.  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 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 Vortex Tube Performance Chart

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 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 size of the Vortex Tube that is used, the temperature drop and rise will follow the chart below.

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, flow and temperature can be used to calculate the correct Vortex Tube size for your application.  If you need help 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

How Do I Change the Air Flow and Temperature of a Vortex Tube?

Published on by Russ BowmanLeave a comment

If you’re a regular reader of the EXAIR Blog, you likely know that you can get cold (and hot) air from a Vortex Tube. You probably also know that there are ways to get more or less flow, and higher or lower temperatures. Today, I wanted to write about vortex tube operation. First, let’s recall the basics:

The unique physical phenomenon of the Vortex Tube principle generates cold air instantly, and for as long – or short – a time as needed.

To change the cold (or hot) air flow AND temperature (you can’t do one without the other), all you have to do is manipulate the Control Valve, or Hot Valve, as it’s oftentimes referred to because of its location at the ‘Hot’ end of the Vortex Tube. Essentially, as you open it, more hot air exits, meaning there’s less air to go to the ‘Cold’ end. By increasing the ‘Hot’ flow, more kinetic energy (in the form of heat) is carried away. And, since more energy (heat) is given off, the energy (heat) in the cold flow decreases as well, so you get colder air…and less of it as the hot flow increases. We can use the data in EXAIR’s Vortex Tube Specification and Performance Tables to calculate the cold (and hot) flows and temperatures at different positions of the Hot Valve. Let’s say we have a Model 3210 Vortex Tube that uses 10 SCFM when supplied at 100psig:

Let’s assume the compressed air supply temperature is 70°F and the Hot Valve is open wide enough to allow 40% of the Model 3210’s 10 SCFM (or 4 SCFM) worth of compressed air consumption out. That means that 60% (or 6 SCFM) are going to go out of the cold end. We call this condition a 60% Cold Fraction:

And, at that 60% Cold Fraction, the cold air is going to be 86°F colder than the supply of 70°F, which means that the 6 SCFM of cold flow is going to be -16°F. If the Hot Valve is opened further, to allow 5 SCFM out the hot end (and hence the other 5 SCFM will go out the cold end), that 5 SCFM of cold flow will now be 100°F colder than the 70°F supply, or -30°F. That’s as low as you can go with a 3200 Series Vortex Tube…they have a Cold Fraction range of 50-80%.

Now let’s say you want even COLDER air. You can simply replace the generator (shown to the left) in the Model 3210 to make it a 3400 Series Vortex Tube. If you replace its 10-R Generator with a 10-C Generator, you’ll now have a Model 3410, and you’ll be able to adjust your Vortex Tube to the 20-50% Cold Fraction range. The difference between R and C-style generators is the center hole size. The hole sets up proper internal pressure conditions to work better in each temperature range. In short, the generator type optimizes the temperature drop for each working condition.

You’re still working with a compressed air consumption of 10 SCFM, so, while the air gets colder, the flow decreases. At a 30% Cold Fraction, for example, you’ll get -48°F air (70°F – 118°F), but only 3 SCFM (30% of 10 SCFM) of the total flow going in.

If you need a -48°F net air temperature, but cannot accommodate such a reduction in flow, using a generator with a higher consumption rating is how you get around such an issue. If you were to replace that 10-C Generator with a 30-C, now it is a Model 3430 with triple the original flow of model 3410. Readjustment of the hot valve would be necessary to get back to a 30% Cold Fraction. That means the air flow will be the same temperature (-48°F) but it’s going to be 9 SCFM (instead of 3 SCFM.)

All you need to change the Cold Fraction of an EXAIR Vortex Tube is a flat-head screwdriver. If it’s something you’re going to be doing more frequently, our Adjustable Spot Coolers have a Temperature Control Knob that works the Hot Valve and may be a better choice for an application.

This is from the Vortex Tubes and Spot Cooling Products section of Catalog 35. The first graphic at the beginning of this blog is what you’ll find on page 200. You’re welcome.

The Adjustable Spot Coolers also come with three different generators, so you can get the different flows at the same temperature, or vice versa, as described above. If you have an application requiring cold (or hot) air flow, on demand, you’re looking for an EXAIR Vortex Tube. For help picking the right one, give me a call.

Russ Bowman, CCASS

Application Engineer
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What’s So Great About The Adjustable Spot Cooler?

Published on by Russ BowmanLeave a comment

EXAIR Adjustable Spot Coolers are ideally suited to a number of applications that need a flow of cold air on demand. Different applications will require more or less flow, and higher or lower temperatures. Let’s say you’re trying to cool an object to ambient temperature with a Model 3825 Adjustable Spot Cooler supplied with compressed air at 100psig and 70°F, but it’s not cooling the object as fast as you’d like. You can:

  • Increase the supply pressure, if possible. With a 100psig inlet pressure to the Adjustable Spot Cooler, you’re getting 20 SCFM of cold flow, at 16°F (assuming it’s set to an 80% Cold Fraction, which is usually ideal for spot cooling.) If you can get it to 120psig, you’ll increase the cold flow from 20 SCFM (80% of the 25 SCFM it’s using at 100psig) to 23.4 SCFM (80% of the 29.3 SCFM it’ll use at 120psig). And it’ll be colder (it’ll produce air with a 55°F temperature drop @120psig, vs 54°F @100psig).
EXAIR Vortex Tube Performance Chart – this is where the above – and below – ‘facts & figures’ come from.
  • Decrease Cold Fraction. Depending on the object’s size, material(s) of construction, amount of surface area available for heat transfer, etc., you could improve the cooling rate with lower temperature air, even if there’s less of it. If it’s particularly small in relation to the cold air flow pattern, a portion of that cold air flow might pass by without removing any heat at all. So, decreasing the temperature of the cold air that IS working will increase your rate of heat transfer. This is done by turning the knob of the Temperature Control Valve:
Turning the knob (2) counterclockwise opens the Temperature Control Valve, letting more of the supply flow (1) out of the hot end (3), and less of it to flow to the cold end (4). This also lowers the temperature of the cold air flow.
  • Change the Generator. The Adjustable Spot Cooler comes with a 25 SCFM Generator installed, but 15-R and 30-R Generators are included as well. Replacing the 25 SCFM Generator with a 30 SCFM Generator makes it use 30 SCFM @100psig, but you’ll increase the cold flow from 20 SCFM to 24 SCFM, assuming you leave it set at an 80% Cold Fraction. Keep in mind, though, that you can lower the Cold Fraction to get the same amount of cold flow as you were getting from the 25 SCFM Generator, but now it’ll be colder. If you open the Temperature Control Valve to a 70% Cold Fraction, you’ll reduce the cold flow to 21 SCFM of cold flow, but now it’ll be -1°F.

Everyone here at EXAIR wants you to get the most out of our products. If you’d like to find out more about how to do that, give me a call.

Russ Bowman, CCASS

Application Engineer
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Vortex Tubes: What is Cold Fraction?

Published on by John BallLeave a comment

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 of physics that has been referred to by many names including Ranque Tube, Ranque-Hilsch Tube and Maxwell’s Demon.  The modern name is Vortex Tube.  It can generate cold air to a temperature as low as -50 oF (-46 oC) simply by spinning compressed air at high RPM.  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.  To read more about the type of generators, you can find this here: Maximum Effort!!! The Two Types of Vortex Tube Generators. The generator controls the amount of compressed air that can enter the Vortex Tube as well as initiating the spinning of the air inside.  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 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 changes.  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 size of the Vortex Tube that is used, the temperature drop and rise will follow the chart below.

EXAIR Vortex Tube Performance Chart

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, flow and temperature can be used to calculate the correct Vortex Tube size for your application.  If you need help 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