Heat Transfer – How Energy Can Move

Heat. One word can bring to mind so many different things from cooking to sun tanning. But what is heat and how does it move. Heat is essentially a form of energy that flows in the form of changing temperatures; this form of energy will flow from high to low. When you describe something as being hot, you are actually describing that the item in question has a higher temperature than your hand thus the thermal (heat) energy is flowing from that object to your hand. This phenomenon is what is referred to as heat transfer. Heat transfer can be observed all the way down to the atomic scale with the property known as specific heat. Every molecule and atom can carry a set amount of energy which is denoted by specific heat; this value is the ration of energy (usually in Joules) divided by the mass multiplied by the temperature (J/g°C).

Energy moving through atoms in an object

But how does this heat move from object to object? On the atomic scale, the atoms are storing the energy which will cause electrons to enter into an excited state and rapidly switch between shells. When the electron returns back to a lower shell (closer to the nucleus) energy is released; the energy released is then absorbed by atoms at a lower energy state and will continue until the thermal energy is equal between the two objects. Heat has four fundamental modes of transferring energy from surface to surface and they are as follows:

Advection
Advection is the physical transport of a fluid from point A to point B, which includes all internal thermal energy stored inside. Advection can be seen as one of the simpler ways of heat transfer.

Conduction
Conduction can also be referred to as diffusion and is the transfer of energy between two objects that have made physical contact. When the two objects come into contact with each other thermal energy will flow from the object with the higher temp to the object with the lower temp. A good example of this is placing ice in a glass of water. The temperature is much lower than the room temperature therefore the thermal energy will flow from the water to the ice.

Convection
Convection is the transfer of thermal energy between an object and a fluid in motion. The faster the fluid moves the faster heat is transferred. This relies on the specific heat property of a molecule in order to determine the rate at which heat will be transferred. The low the specific heat of a molecule the faster and more volume of the fluid will need to move in order to get full affect of convection. Convection is used in modern ovens in order to get a more even heat through out the food while cooking.

Radiation
Radiation is the transfer of thermal energy through empty space and does require a material between the two objects. Going back to the how thermal energy is released from atoms; when the electron returns to a lower energy shell the energy is released in the form of light ranging from infrared light to UV light. Energy in the form of light can then be absorbed by an object in the form of heat. Everyone experiences radiation transfer every day when you walk outside; the light from the sun’s radiation is what keeps this planet habitable.

EXAIR’s engineered compressed air products are used every day to force air over hot surfaces to cool, as well as dry and/or blow off hot materials. Let us help you to understand and solve your heat transfer situations.

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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The picture “Energy Transfer – Heat” by Siyavula Education is licensed under CC BY 2.0

Convective Heat Transfer: How Do We Use It?

Vortex Tubes have been studied for decades, close to a century. These phenoms of physics and the theory behind them have been discussed on this blog before. Many customers gravitate toward Vortex Tubes when needing parts and processes cooled. The fact of the matter is there is still more to be discussed on how to correctly select the which product may be needed in your application. The reason being, area, temperatures, and air flow volumes play a large role in choosing the best product for cooling. The tendency is to say, well I need to cool this down as far as possible so I need the coldest air possible which leads to the assumption that a Vortex Tube will be the right solution. 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 other 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
Visit us on the Web
Follow me on Twitter
Like us on Facebook