How to Best Apply Vortex Tube Cooling

So, you have found yourself with a little bit of a conundrum. You need to cool a part but don’t know where to start and there are so many different options to choose from. In most cases when it comes to cooling with compressed air there are two different paths you can take. First is using a large volume of air at room temperature to blow across the surface area of the product. The other option is to use cold air from a vortex tube to drop the part’s temperature. In most case a large volume of air can be used to cool things down to relatively cooler temperatures; think cooling a cup of coffee using your breath. The issue you run into is when the temperature of the room air gets closer to the temperature you want to achieve in the end. In other words, when the temperature difference between your cooling air and your desired end temperature is small there is less cooling taking place with that same volume of air.

Mini Spot Cooler cooling down a bit used in milling plastic

This can be explained by looking at the cooling power formula:

Btu/hr = 1.0746*(CFM)*(Delta T)

In this case the Delta T is the difference between the temperature that you want to cool the product down to and the temperature of the air. This means the smaller the delta T is the higher the CFM flow will need to be to counteract the effect of the temperatures are so close to one another. Here are some examples of cooling a product and you are providing 1000 CFM of air to cool it.

Btu/hr = 1.0746*(1000 CFM)*(150F – 130F)

                Btu/hr = 21,492 Btu/hr

Btu/hr = 1.0746*(1000 CFM)*(150F – 100F)

                Btu/hr = 53,730 Btu/hr

As you can see the closer the Delta T is to 0 the less Btu/hr you get. Getting this kind of CFM flow is easy if you use something like EXAIR’s Super Air Knife or Super Air Amplifier. These systems take a small amount of compressed air and entrain the surrounding ambient air to increase the volume to a large blast. Take a look at model number 120022 which is the 2” Super Air Amplifier, this unit can produce 1,023 CFM while only using 15.5 CFM at 80 psig. But when you get close to cooling the temperature down to that room temperature or below it gets much harder; which only means that the temperature of the air being used to cool needs to be dropped. Dropping the air temperature can only be accomplished by using outside means like air coolers or in this case EXAIR’s Vortex Tubes and Spot Coolers.

EXAIR Air Amplifiers use a small amount of compressed air to create a tremendous amount of air flow.

Vortex Tubes and Spot coolers have some limitations. Generally they are not thought of products that produce large volumes of air (even though we make them up to 150 SCFM). And they are best suited for smaller areas of cooling, spot cooling, if you will. However, EXAIR Vortex Tubes do have one key feature that can help compensate for the lack of volume. LOW TEMPERATURE! The vortex tube can produce temperatures lower than 0F while stile retaining a good portion of air volume from the inlet.

Sub-zero air flow with no moving parts. 3400 Series Vortex Tubes from EXAIR.

For example, lets look at model number 3240 running at 100 psig with 70% of the air from the inlet exiting the cold side. At 100 psig the 3240 will use 40 SCFM at the air inlet and will have a temperature drop of 71F. If the compressed air has a temperature of 70F that means you will be seeing a temperature of -1F. Also, when using the 70% cold fraction you will see only 28 SCFM flow out of the vortex tubes cold side. Now let’s plug those numbers into the cooling power formula.

 Btu/hr = 1.0746*(28 CFM)*(150F + 1F)

                Btu/hr = 4543 Btu/hr

As you can see, using a small amount of compressed air you can still net you a good amount of cooling if the temperature is lower. All in all, the best option for cooling products down to temperatures that are above ambient temperatures is something that can produce a large volume of air. For anything that requires cooling the product down to temperatures around ambient temperature and below, use a vortex tube.

If you have questions about our Air Amplifiers and Vortex Tubes, or would like to talk about any of the quiet EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR or any Application Engineer.

Cody Biehle
Application Engineer
EXAIR Corporation
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Boundary Layer: Laminar and Turbulent flow

Fluid mechanics is the field that studies the properties of fluids in various states.  Fluid dynamics studies the forces on a fluid, either as a liquid or a gas, during motion.  Osborne Reynolds, an Irish innovator, popularized this dynamic with a dimensionless number, Re. This number determines the state in which the fluid is moving; either laminar flow, transitional flow, or turbulent flow.  For compressed air, Re < 2300 will have laminar flow while Re > 4000 will have turbulent flow.  Equation 1 below shows the relationship between the inertial forces of the fluid as compared to the viscous forces. 

Equation 1: 

Re = V * Dh / u

Re – Reynolds Number (no dimensions)

V – Velocity (feet/sec or meters/sec)

Dh – hydraulic diameter (feet or meters)

u – Kinematic Viscosity (feet^2/sec or meter^2/sec)

To dive deeper into this, we will need to examine the boundary layer.  The boundary layer is the area that is near the surface of the object.  This could refer to a wing on an airplane or a blade from a turbine.  In this blog, I will target pipes, tubes, and hoses that are used for transporting fluids.  The profile across the area (reference diagram below) is a velocity gradient.  The boundary layer is the distance from the wall or surface to 99% of the maximum velocity of the fluid stream.  At the surface, the velocity of the fluid is zero because the fluid is in a “no slip” condition.  As we move away from the wall, the velocity starts to increase.  The boundary layer distance measures that area where the velocity is not uniform.  If you reach 99% of the maximum velocity very close to the wall of the pipe, the air flow is turbulent.  If the boundary layer reaches the radius of the pipe, then the velocity is fully developed, or laminar. 

Boundary Layer Concept

The calculation is shown in Equation 2.

Equation 2:

d = 5 * X / (Re1/2)

d – Boundary layer thickness (feet or meter)

X – distance in pipe or on surface (feet or meter)

Re – Reynolds Number (no dimensions) at distance X

This equation can be very beneficial for determining the thickness where the velocity is not uniform along the cross-section.  As an analogy, imagine an expressway as the velocity profile, and the on-ramp as the boundary layer.  If the on-ramp is long and smooth, a car can reach the speed of traffic and merge without disrupting the flow.  This would be considered Laminar Flow.  If the on-ramp is curved but short, the car has to merge into traffic at a much slower speed.  This will disrupt the flow of some of the traffic.  I would consider this as the transitional range.  Now imagine an on-ramp to be very short and perpendicular to the expressway. As the car goes to merge into traffic, it will cause chaos and accidents.  This is what I would consider to be turbulent flow.      

EXAIR Digital Flowmeter

In a compressed air system, similar things happen within the piping scheme.  Valves, tees, elbows, pipe reducers, filters, etc. are common items that will affect the flow.  Let’s look at a scenario with the EXAIR Digital Flowmeters.  In the instruction manual, we require the meter to be placed 30 pipe diameters from any disruptions.  The reason is to get a laminar air flow for accurate flow measurements.  In order to get laminar flow, we need the boundary layer thickness to reach the radius of the pipe.  So, let’s see how that number was calculated.  

Within the piping system, high Reynold’s numbers generate high pressure drops which makes the system inefficient.  For this reason, we should keep Re < 90,000.  As an example, let’s look at the 2” EXAIR Digital Flowmeter.  The maximum flow range is 400 SCFM (standard cubic feet per min).  In looking at Equation 2, the 2” Digital Flowmeter is mounted to a 2” Sch40 pipe with an inner diameter of 2.067” (52.5mm).  The radius of this pipe is 1.0335” (26.2 mm) or 0.086 ft (0.026m).  If we make the Boundary Layer Thickness equal to the radius of the pipe, then we will have laminar flow.  To solve for X which is the distance in the pipe, we can rearrange the terms to:

X = d * (Re)1/2 / 5 = 0.086ft * (90,000)1/2 / 5 = 5.16 ft or 62”

If we look at this number, we will need 62” of pipe to get a laminar air flow for the worse-case condition.  If you know the Re value, then you can change that length of pipe to match it and still get valid flow readings.  From the note above, the Digital Flowmeter will need to be mounted 30 pipe diameters.  So, the pipe diameter is 2.067” and at 30 pipe diameters, we will need to be at 30 * 2.067 = 62”.  So, with any type of common disruptions in the air stream, you will always get good flow data at that distance. 

Why is this important to know?  In many compressed air applications, the laminar region is the best method to generate a strong force efficiently and quietly.  Allowing the compressed air to have a more uniform boundary layer will optimize your compressed air system.  And for the Digital Flowmeter, it helps to measure the flow correctly and consistently.  If you would like to discuss further how to reduce “traffic jams” in your process, an EXAIR Application Engineer will be happy to help you.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

EXAIR’s Super Air Amplifier Amplification Ratio’s, Explained

Much like the popular song from decades ago that was about “money for nothing”,  EXAIR can provide you with “air for free”.  What we mean by this is that when you choose to use our Super Air Amplifiers, you will produce a large volume of air while only requiring a small amount of compressed air. This is because Air Amplifiers amplify total output flow up to 25 times by entraining (pulling in) ambient air.

So just how does the EXAIR’s Super Air Amplifier do this?   By utilizing our patented design (Patent # 5402938) that incorporates a special shim to maintain the air slots precisely.  The compressed air is released toward the center of the Super Air Amplifier  which creates a constant, high velocity outlet flow across the entire cross sectional area.  This

SAA How It Works

The amplification occurs by entraining most of the ambient air from the back of the Super Air Amplifier. Another small volume of air is added again as the air exits the Super Air Amplifier further increasing the amplification.

SAA Blog 1Super Air Amplifiers that have outlet diameter’s of 3/4″ (19mm), 1 1/4″ (32mm), 2” (51mm) and 4” (102mm) are supplied with a .003” (0.08mm) shim which is ideal for most applications, however there is the optional .006” (.15mm) and .009” (.23mm) if more air volume and force is needed. The 8” (203mm) Super Air Amplifier comes standard with a .009” (.23mm) shim and for increased performance we offer an optional .015” (.39mm).  The chart below explains how to determine the total output flow and air consumption at different operating pressures for each Super Air Amplifier model.

SAA Blog 2

When you need “air for free” or more accurately stated, to get all you can from every SCFM of compressed air you produce, put the EXAIR Super Air Amplifier to work in your facility!

If you would like to discuss the EXAIR Super Air Amplifier or any of EXAIR’s Intelligent Compressed Air® products, give us a call as we would enjoy hearing from you.

Erik Kuhnash
Application Engineer
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Customizing Air Amplifiers

EXAIR’s line of Air Amplifiers can be found in a multitude of different applications across the world. They solve problems as simple as blowing debris off parts to exhausting fumes or circulating air. The Air Amplifier comes in two different styles either the Super Air Amplifier or the Adjustable Air Amplifier. Super Air Amplifiers come in a stock Aluminum Body with a diameter that ranges from ¾” to 8”. This differs from the Adjustable Air Amplifier which comes in either type 303 Stainless Steel or Aluminum and are Sized from ¾” to 4”.

The main difference between the Super Air Amplifier and the Adjustable Air Amplifier is the fact the Super Air Amplifier has a shim inside of it that sets the gap for the air flow. The standard shim thickness for the Super Air Amplifier in sizes of 3/4″ to the 4″ is 0.003” which is suitable for most applications. These shims can be exchanged for a thicker shim of thickness of either 0.006″ or 0.009″. The 8″ Super Air Amplifier is the only air amplifier that comes with a standard stock shim of 0.009″ and can be exchanged for a 0.015″ shim if needed.

Flanged Stainless Steel Adjustable Air Amplifier
Sanitary Flanged Adjustable Air Amplifier

Even though there is a wide variety of sizes and materials for the Stock Air Amplifiers they may not meet a customer’s specific application or need. Over the years EXAIR has produced many different custom Air Amplifiers for a customer’s specific need and the images throughout this blog are just a few of what we have done.

High Temp Air Amplifier

• Depending on the environment certain specific materials may be required like the food industry which requires specific Stainless Steel for various applications. One customer had a special PTFE plug made for the Adjustable Air Amplifier to help pull a sticky material through the process. The PTFE helped prevent the material form depositing on the Amplifier.
• For applications were mounting may be an issue, special attachments have been made to assist. For instances were an Amplifier may need to be mounted to a pipe a custom Stainless-Steel Adjustable Air Amplifier with a class 150 raised face flanges.
• Applications that are in a hot environment may require a special high temperature version which has be developed to operate in areas up to 700°F. The High Temperature Air Amplifier was so widely sought after that we turned it into a stock item. It is commonly used in large roto-molds and ovens to circulate air in order to maintain consistent temperatures.

Adjustable Air Amplifier with PTFE Plug Installed

No matter what your application needs are EXAIR will to work with you to create any custom Air Amplifier that fits your specific application needs.

If you have any questions about compressed air systems or want more information on any of EXAIR’s products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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