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]

What’s an EXAIR?

Sometimes taking customer’s phone calls remind me of an Abbott and Costello bit (but I have to be Costello). Conversations can feel a bit like twenty questions. Instead of opening with mineral, vegetable, or animal, customers call in wanting more information on an “EXAIR”.  For our brand manager and marketing department, it is a clear sign that what they are doing is working, but to me can be a bit confusing.

Before you start thinking I don’t know my product, please remember an “EXAIR” can be quite few things. We make the broadest variety of problem solving end-use compressed air products for industry which equates to many possibilities of what an “EXAIR” may be. Is it an Air Nozzle, an Air Knife, an Air Wipe, an Air Amplifier, an Atomizing Spray Nozzle, a Safety Air Gun, a Static Eliminator, a Vacuum Generator, a Line Vac, an Industrial Vacuum, a Vortex Tube, a Cold Gun, or a Cabinet Cooler?   Unfortunately, with no moving parts to wear out, our products sometimes will outlast their labels, so the customers don’t have anymore information. Then, I have to ask what the product does.

So I ask the customer, “does the EXAIR blow off, vacuum, clean, dry, cool, convey, evacuate, coat, divert, dust, float, open, lift, purge, or spray?”

And then I wait for the customer’s detailed and eloquent response…”It works”, they sometimes say. But most of the time they respond with all of the details or enough to determine what product they have. In, in the end, an “EXAIR” is generally a Cabinet Cooler or a Vortex Tube (though it may be any of the above selection) – and we won’t complain that our company name can be so closely associated with our products.

We have so many products because compressed air is so versatile and useful.  We have taken our expertise in compressed air and used it to solve numerous problems for our customers. This is not as easy, as it sounds.  First, you need to know how well our compressed air products can perform. Second, you need to know what kind of performance the customer needs to get the job done. For instance when working on a Cabinet Cooler sizing exercise: A customer has a control box that is 24″ tall by  36″ wide by 12″ deep.  This box is reaching temperatures that cause the electronics to fail. Generally, this temperature is going to be between 110 degrees Fahrenheit to 130 degrees Fahrenheit. The temperature in the plant was 95 degrees Fahrenheit, when it failed.  The customer would now like a Cabinet Cooler System to protect his enclosure from future temperature failures.

To calculate the heat load of the electronics, first we need to calculate the surface area in square feet. In the example above that would be 22 square feet. Second, we need to calculate the temperature differential between the outside and the inside of the cabinet.  The maximum temperature differential is 130 F – 95 F, which is 35 degree differential. With the temperature differential chart from our website, we can calculate the BTU/HR per square foot.

Temperature Conversion Table

For our example, it would be 13.8 BTU/HR/ft^2. Multiply this by our surface area. Our Cabinet Cooler needs to cool at least 303.6 BTU/HR. Our 4308 Cabinet Cooler System would be a good cabinet cooler for this enclosure. It can cool 550 BTU/Hr. It is rated for a NEMA 12 enclosure to prevent dust and oil from entering the cabinet.

To help the customer, you have to first ask the right questions. Most of these questions are listed on the Cabinet Cooler Sizing Guide on our website. What is the internal air temperature in the cabinet? What is the ambient air temperature? Are their any fans in the cabinet? What is the NEMA rating for the Cabinet? Sometimes it is best to speak with an Application Engineer to know for sure you have your bases covered.

Dave Woerner
Application Engineer
DaveWoerner@EXAIR.com
@EXAIR_DW