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]

360° Air Wipe Comparison for Extrusion, Hose, Cable, Pipe and Wire Blowoff or Cooling

When it comes to blowing off extrusions, cables, pipe, tubing, hose, or wire, the EXAIR Super Air Wipe is an ideal product that often stands alone in this field. From time to time I will receive a call asking about a block style blowoff for wire or cabling, maybe even small extrusion, and the Super Air Wipe is always there to provide a true 360° blowoff compared to what the customer is used to.

See, the block style blowoffs are similar to the SAW in they both offer a clamshell design that will open and close into place around the product to prevent the user from having to “thread the needle”.  Also, they both operate off compressed air. The means by which they deliver the air is where the stark contrast in performance begins.

First, let us take a look at how these block style blowoffs deliver the air to the product being blown off. Generally, inside of these, there are a couple holes on each half of the block that are drilled at an angle to deliver the air onto the surface of the product. These block style products have to be kept fairly close in diameter to the product so that the air contacts the surfaces and they are for the most part offered in smaller sizes only.

Now, how does the Super Air Wipe deliver the air to the product being blown off?  The air enters (1) into an internal chamber that fills (2) and then dispurses through a continuous gap (3) on each half of the wipe that is set by a shim. The shim can be interchanged with different thicknesses to give a course adjustment to the volumetric flow of air. The air then follows the Coanda profile (4) of the Super Air Wipe to maximize entrained air (5) and exits at a 60° included angle (6) to impact the surface of the part at a full 360°.

Stock Super Air Wipe Product OfferingBlock Style air wipes also have a tendency to operate very loudly, often exceeding 90 dBA and more. This will affects personnel directly and can also affect their communication when attending to a processing line.  Super Air Wipes operate at 82 dBA with 5″ and smaller diameters. Up to 89 dBA for diameterss from 6″-11″.

The Super Air Wipe is available in eleven sizes from stock in aluminum (up to 11″ dia.) while the stainless steel configurations are available in five sizes from stock (up to 4″ dia.).  If aluminum or stainless steel don’t fit the requirements of an application, custom sizes and materials are always available with short lead times.

Tight-fitting locations are ideal for a Super Air Wipe install.

If you notice that the block style air blowoff is leaving streaks or maybe the physical space requirements of the block are too much to fit into your production line, please contact an Application Engineer and let us help you determine which Super Air Wipe is going to be right for your needs.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

 

 

Bifurcation Of Air – The Wonders of Science That Is The Vortex Tube

EXAIR has provided the benefits of vortex tube technology to the industrial world since 1983. Prior to that, French scientist George Ranque wrote about his discovery in 1928 calling it the tube tourbillion. But it wasn’t until German physicist Rudolf Hilsch’s research paper in 1945 on the wirbelrorhr or whirling tube, that the vortex tube entered the minds of commercial engineers. Nearly 60 years later, EXAIR is a leading provider for cooling products utilizing vortex tube technology.

More than 2,000 BTU/hr in the palm of your hand!

EXAIR Vortex Tubes produce a cold air stream down to -50° F and are a low cost, reliable, maintenance-free (there are no moving parts!) solution to a variety of spot cooling applications. These applications span a wide variety of industries and include cooling of electronic controls, soldered parts, machining operations, heat seals, environmental chambers, and gas samples. We’re always finding compelling new cooling opportunities for the vortex tubes.

How a Vortex Tube Works

So how does it produce the cooling stream? Compressed air is plumbed into the side port of the Vortex Tube where it is ejected tangentially into the internal chamber where the generator is located. The air begins flowing around the generator and spinning up to 1 million RPM toward the hot end (right side in the animation above) of the tube, where some hot air escapes through a control valve. Still spinning, the remaining air is forced back through the middle of the outer vortex. Through a process of conservation of angular momentum, the inner stream loses some kinetic energy in the form of HEAT to the outer stream and exits the vortex tube as COLD air on the other side.

The adjustable control valve adjusts what’s known as the cold fraction. Opening the valve reduces the cold air temperature and also the cold airflow volume. One can achieve the maximum refrigeration (an optimum combination of temperature and volume of flow) around an 80% cold fraction. EXAIR publishes performance charts in our catalog and online to help you dial into the right setting for your application, and you can always contact a real, live, Application Engineer to walk you through it.

EXAIR manufactures its vortex tubes of stainless steel for resistance to corrosion and oxidation. They come in small, medium and large sizes that consume from 2 to 150 SCFM and offer from 135 to 10,200 BTU/hr cooling capacity. Each size can generate several different flow rates, dictated by a small but key part called the generator. That generator can be changed out to increase or decrease the flow rate.

While operation and setup of an EXAIR Vortex Tube are easy, its performance will begin to  decrease with back pressure on the cold or hot air exhaust of over 3 PSIG. This is a key  when delivering the cold or hot airflow through tubes or pipes. They must be sized to minimize or eliminate back pressure.

The Vortex Tube is integrated into a variety of EXAIR products for specific applications, like the Adjustable Spot Cooler, the Mini Cooler, the Cold Gun Aircoolant System and our family of Cabinet Cooler Systems.

If you would like to discuss your next cooling application, please contact an Application Engineer directly and let our team lead you to the most efficient solution on the market.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

Six Sigma and The Compressor Room

Throughout my undergrad courses as well as during my professional career I have encountered Six Sigma or Lean Manufacturing in many facilities.  There is at least one component to the theory that can be implemented into any facility with a compressor room. That component is the practice of the 5 S’s.

The 5 S’s of Lean Manufacturing come from the Japanese terms  listed below with their English translations:

Seiri – Sort (Organize)
Seiton – Set in Order (Orderliness)
Seiso – Shine (Cleanliness)
Seiketsu – 
Standardize
Shitsuke –  Sustain (Discipline)

These 5 points can aid in keeping any air compressor room in a facility efficient, safe, and effectively supplying the company with compressed air. How you may ask.

Sort – Keeping a compressor room as originally laid out and preventing it from being a catch-all for items that have nothing to do with the compressed air system. This can easily happen when it is actually a room that has unused floor space in a small facility. By keeping the area clean and free of unrelated materials, maintenance and troubleshooting can be done quickly. Clear labeling of anything kept in the room is also ideal to make items easily identified.

Set in Order – To deliver the air in a single path/direction as well as keeping equipment in locations where they can be easy to maintain and clearly labeled eases the troubleshooting and understanding of how the system is laid out. Rather than having a spaghetti bowl of piping running all around the room to different components it is wiser to keep a flow that matches the process. From the compressor(s) to the receivers, dryers, filter, and regulators, out to the point of use. This shouldn’t be a tangled web of piping that introduces air to a process which bypasses key components such as the dryer or receivers.

Block diagram of a compressor room layout.

Shine – The compressor room shouldn’t be a dirty grungy area. The compressor pulls the air in from this environment. Any exposed components easily collect airborne debris. By keeping the equipment clean again makes labels easy to read and a clean machine is always easier to perform maintenance and sometimes even troubleshoot. If there are puddles of oil or other liquids on the floor and no surfaces are clean then any leak may not be easily spotted.

Standardize – The layout and processes used within the room should be repeatable. Maintenance tasks should be performed on a schedule, per a process that doesn’t allow for much differentiation on methods and end results. This mitigates errors and is always the desired result when focusing on lean manufacturing. LOWER THAT DELTA!

Sustain – This is sometimes the hardest part of any process. Getting the program up and running, starting with a fresh build is always the easiest.  Everything is fresh, new and you want to keep it shiny. Years later the desire to dust and maintain piping as well as keep receiver tanks and floors clean isn’t always at the top of the desired list.  It should always be a priority because cleanliness also promotes safety and reduces overhead by lowering downturns due to housekeeping related failures.

If you want to discuss how we can help lean out your compressed air usage, maintenance costs, and help to standardize the use of compressed air in your facility, contact an Application Engineer today.

Brian Farno
Application Engineer – Green Belt Certified
BrianFarno@EXAIR.com
@EXAIR_BF