Convective Heat Transfer: How Do We Use It?

Vortex Tubes have been studied for decades, centuries even. These phenoms of physics and the theory behind them have been discussed on this blog before. The fact of the matter is there is still more to be discussed on how to correctly select the model that may be needed in your application. The reason being, there are different flow rates and an option for maximum refrigeration or maximum cold temp. The tendency is to say, well I need to cool this down as far as possible so I need the coldest air possible, give me the maximum cold temperature. That isn’t always the best option and we are going to discuss how to best determine which will be needed for your application. The first step, is to call, chat, or email an Application Engineer so that we can learn about your application and assist with the implementation of the Vortex Tube or spot cooling product for you. You may also want to try and take some initial readings of temperatures. The temperatures that would help to determine how much cooling is going to be needed are listed below:
Part temperature: Part dimensions: Part material: Ambient environment temperature: Compressed air temperature: Compressed air line size: Amount of time desired to cool the part: Lastly desired temperature:
With these bits of information, we use cooling equations to help determine what temperature and volume of air will best suit your needs to generate the cooling required. One of the equations we will sometimes use is the Forced or Assisted Convective Heat Transfer. Why do we use convective heat transfer rather than Natural Heat Transfer? Well, the air from EXAIR’s Intelligent Compressed Air Products® is always moving so it is a forced or assisted movement to the surface of the part. Thus, the need for Convective Heat Transfer.
Calculation of convection is shown below: q = hc A dT Where: q = Heat transferred per unit of time. (Watts, BTU/hr) A = Heat transfer area of the surface (m2 , ft2) hc= Convective heat transfer coefficient of the process (W/(m2°C), BTU/(ft2 h °F) dT = Temperature difference between the surface and the bulk fluid (compressed air in this case) (°C, °F) The convective heat transfer coefficient for air flow is able to be approximated down to hc = 10.45 – v + 10 v1/2 Where: hc = Heat transfer coefficient (kCal/m2 h °C) v = relative speed between the surface of the object and the air (m/s) This example is limited to velocities and there are different heat transfer methods, so this will give a ballpark calculation that will tell us if we have a shot at a providing a solution.  The chart below is also useful to see the Convective Heat Transfer, it can be a little tricky to read as the units for each axis are just enough to make you think of TRON light cycles. Rather than stare at this and try to find the hidden picture, contact an Application Engineer, we’ve got this figured out. convective_heat_transfer_chart
1 – Convective Heat Transfer Chart
Again, you don’t have to figure any of this out on your own. The first step to approach a cooling application is to reach out to an Application Engineer, we deal with these types of applications and equations regularly and can help you determine what the best approach is going to be.
Brian Farno Application Engineer BrianFarno@EXAIR.com @EXAIR_BF
1 – Engineering ToolBox, (2003). Convective Heat Transfer. [online] Available at: https://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html [02/10/2021]

Cooling Parts? Super Air Amplifiers May Be For You

Super Air Amplifier Family

When working with a cooling application, many customers will immediately look to the Vortex Tube and Spot Cooling product lines. While this may be the best solution for some applications, cold air is not always the best method that we have available for cooling. EXAIR’s Super Air Amplifiers are very effective at reducing the temperature of a part without requiring cold air, when the temperature differential between the Super Air Amplifier’s airflow and the temperature of the part is significant. Due to their ability to entrain large amounts of ambient air, we can move a lot of volume of air across the surface of the part and quickly lower the temperature.

I like to compare this to blowing on a hot cup of coffee just as it’s been brewed. The temperature of the air coming from your mouth is around 98.6°F, the same as your body temperature. Coffee can be as hot as 185°F when fresh. Due to the temperature differential between your breath and the hot coffee, we’re able to achieve a reasonable amount of cooling just by simply blowing across the surface. Typically, when the target temperature of the hot part or material needs to be around ambient temperature or higher, a Super Air Amplifier can be a good choice. 

While many applications utilize the outlet flow of the Super Air Amplifier to blow off, clean or cool a part or material, the ability of the Super Air Amplifier to entrain large amounts of ambient air can also be utilized to convey light materials or to draw in dust, smoke, or fumes from the surrounding environment. As the plugs on the exhaust side of the Super Air Amplifiers come in sizes of ¾”, 1-1/4”, 2”, 4”, and 8” the exhaust flow can be ducted with standard size hose.

EXAIR’s Super Air Amplifiers utilize a patented shim design to maintain critical positioning of component parts. This allows a precise amount of compressed air to be released at exact intervals toward the center of the Super Air Amplifier. This creates a constant, high velocity outlet flow across the entire cross-sectional area. Free, ambient air is entrained through the unit, resulting in high amplification ratios. The balanced outlet airflow minimizes wind shear to produce sound levels far lower than other similar air movers.

Patented Super Air Amplifier Shims

Super Air Amplifiers are supplied with a .003” thick shim that is ideal for most applications. Flow and force can be increased by replacing the shim with a thicker .006” or .009” shim. The flow of air is also controlled by adjusting the input pressure supplied to the amplifier. Higher pressures increase both the force and flow, while lower pressures decrease both force and flow. All Super Air Amplifiers are available in kits that come with a shim set as well as a suitably sized pressure regulator and auto-drain filter.

EXAIR has a solution for you if you need to move A LOT of air. Reach out to an Application Engineer today if you have an application that you believe could be served with a low-cost, simple solution!

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@EXAIR.com
Twitter: @EXAIR_TD

Air Nozzles and Air Jets: An Overview

One of the simplest solutions to lower your air consumption and noise level when it comes to compressed air is to switch your open tubes or pipes and liquid nozzles which are being used for air applications to an engineered compressed air nozzle. EXAIR’s Engineered Air Nozzles and Jets provide a simple solution for a wide variety blow off and compressed air applications and can solve a multitude of process problems efficiently. These applications can include simple blow offs, cooling, part ejection, and much more.

Super Air Nozzles:
Super Air Nozzles are one of the more versatile of all of EXAIR’s Engineered Air Nozzles. They come in many different sizes from a tiny size of M4 threads and 13 millimeters long to the largest with  1-1/4 NPT threads which has a 2″ hex and is almost 5″ long. These are usually used for standard blow off applications that replace open pipes to reduce your air consumption and noise. The force values vary from 2 ounces to 23 pounds of force. 

Another variation of the Super Air Nozzles is the Flat Super Air Nozzles; these nozzles create a small flat curtain of air at a high force to provide a wider blow off area for smaller NPT sized nozzles. The 1” and 2” Flat Super Air Nozzle also have replaceable shims that allow you to adjust the force coming out of the nozzle by increasing the amount of air that is used.   

EXAIR Air Nozzles

Back Blow Air Nozzles:
Back Blow Air Nozzles are designed in a way that blows that makes it easy to blow out the inside of pipes. The Back Blow Air Nozzles have holes around the outside diameter pointed back that creates a cone of air around the air inlet port. This makes it easy to dislodge clogs in pipes that you don’t want going back into the machine and for blowing out liquid and debris from the inside. They are also commonly used with EXAIR’s Chip Shield as to prevent any particles from flying back and hitting the user. Back Blow Air Nozzles come in three sizes: M4, ¼”, and 1” and can be used on inside diameters ranging from ¼” to 16”. 

EXAIR Back Blow Air Nozzles

Super Air Nozzle Clusters:
Super Air Nozzle Clusters use a number of the ¼” Super Air Nozzles to create one nozzle that has a wider cone and larger force. Clusters are usually used in wide area blow off but can also be used for part cooling and part reject as they do supply a wider area of force. Super Air Nozzle Clusters are sized by the number of nozzles in the cluster; the three sizes that we offer are 4-nozzle cluster (3/8” NPT inlet), 7-nozzle cluster (1/2” NPT inlet), and the 12-nozzle cluster (1” NPT inlet). 

EXAIR Super Air Nozzle Cluster

Air Jets:
Air Jets amplify the total volume of air into a high velocity stream of air. This makes it very useful for blowing off/drying applications and cooling applications due to the higher volume of air flowing through the unit. Air Jets come in two variations which are the High Velocity Air Jet and the Adjustable Air Jets. The High Velocity Air Jet uses a 0.015” shim that allows the air to escape the unit at a high velocity laminar flow to entrain the surrounding ambient air; this can be adjusted down using the shim kit which includes a 0.006” and 0.009” shims. The Adjustable Air Jet allows the user to easily adjust the air gap using the micrometer gap indicator. 

EXAIR Air Jets

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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Controlling Temperature and Flow on a Vortex Tube

Vortex Tube uses an ordinary supply of compressed air as a power source, creating two streams of air, one hot and one cold – resulting in a low cost, reliable, maintenance free source of cold air for spot cooling solutions.

One of the features of the Vortex Tube is that the temperature of the cold air and the cold air flow rate is changeable. The cold air flow and temperature are easily controlled by adjusting the slotted valve in the hot air outlet.

Vortex Tube Hot Valve Adjustment
Hot Plug Adjustment

Opening the valve (turning it counterclockwise) reduces the cold air flow rate and the lowers the cold air temperature.  Closing the valve (turning it clockwise) increases the cold air flow and raises the cold air temperature.

VT Adjustment Table

As with anything, there is a trade off – to get higher a cold air flow rate, a moderate cold air temperature is achieved, and to get a very cold air temperature, a moderate air flow rate is achieved.

An important term to know and understand is Cold Fraction, which is the percentage of the compressed air used by the Vortex Tube that is discharged through the Cold End.  In most applications, a Cold Fraction of 80% produces a combination of cold flow rate and and cold air temperature that results in the maximum refrigeration or cooling output form a Vortex Tube.

For most industrial applications – such as process cooling, part cooling, and chamber cooling, maximum refrigeration is best and the 32XX series of Vortex Tubes are preferred.  For those applications where ‘cryogenic’ cooling is needed, such as cooling lab samples, or circuit testing, the 34XX series of Vortex Tube is best.

To set a Vortex Tube to a specific temperature, simply insert a thermometer into the cold air exhaust and adjust the hot valve.  Maximum refrigeration, at 80% Cold Fraction, is achieved when the cold air temperature drop is 50°F (28°C) from the incoming compressed air temperature. See the video posted here for measuring and lowering and the cold air temperature.

For those cases when you may be unsure of the required cold air flow rate and cold air temperature to provide the needed cooling in an application, we would recommend an EXAIR Cooling Kit.  The Cooling Kit contains a Vortex Tube, Cold Air Muffler, Air Line Filter, and a set of Generators that will allow for experimentation of the full range of air flows and temperatures possible.

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EXAIR Vortex Tube Cooling Kit

To discuss your application and how a Vortex Tube or any EXAIR Intelligent Compressed Air Product can improve your process, feel free to contact EXAIR, myself, or one of our other Application Engineers. We can help you determine the best solution!

Jordan Shouse
Application Engineer

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