Troubleshooting Vortex Tube Performance

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This Vortex Tube was not operating properly when initially connected to compressed air

One of the fun parts of Application Engineering at EXAIR is explaining the operation of Vortex Tubes to our customers.  Sometimes they’re described as a “reverse tornado” inside of a tube, spinning a pressurized airstream and converting it into a hot and cold flow.  Other times we describe it through the generation of two vortices with differing diameters, and the difference in diameters results in one vortex shedding energy in the form of heat.

But, no matter the way we explain their operation, we always stress the importance of proper compressed air plumbing.  If the compressed air piping/hoses/connections are not properly sized, performance problems can arise.  (This is true for any compressed air driven device.)

This fundamental came to light when working with one of our customers recently.  They were using a medium sized Vortex Tube to provide spot cooling in an enclosed space, but were not seeing the flow and temperature drop they knew to be possible with an EXAIR Vortex Tube.  And, after looking at installation photos of the application, the root cause was quickly spotted.

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The red arrow in the bottom right corner of this image shows the beginnings of a reduction in compressed air supply.

I noticed what looked to be a very small hose connected to the inlet of the Vortex Tube in the image above.

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In this additional image, the small compressed air line is in full view. This was the root cause for performance problems in this application.

After further inspection of another photo, the small diameter tube was in full view.  This small hose serves as a restriction to compressed air flow, which in turn limits both flow and operating pressure of the downstream devices.  What that meant for this application, was poor performance from the Vortex Tube, all stemming from this reduction in piping size.

When looking to find the root cause of a performance issue with a compressed air driven unit, things aren’t always as easy as they were with this application.  A visual inspection is always a good idea, but if everything looks correct, here is a list of troubleshooting steps to consider:

  1. Check for quick-disconnects in the plumbing system.  Quick-disconnects are great from an operator’s perspective, but they can wreak havoc on compressed air flows due to small inside diameters and air volume restriction.
  2. Determine the operating pressure at the device.  This is imperative.  In order to make proper decisions to correct the performance concern, good information is required.  Knowing what is happening at the device is crucial for proper understanding.  There may be 100 PSIG at the main compressed air line, but only 60 PSIG at the device due to plumbing problems. A pressure gauge at the inlet of the compressed air product can provide this information.
  3. Check that the compressed air system has enough volume to properly supply the device.  A compressed air driven unit without the correct volume of compressed air is just as bad as having a lack of pressure.
  4. Check for leaks.  The US Department of Energy estimates that 20-30% of compressor output in industrial facilities is lost as leaks.  If your system and devices aren’t operating as they’re supposed to, check for leaks.  They may be contributing to the poor performance.  (Don’t know where your leaks are coming from?  Use our Ultrasonic Leak Detector!)

Fortunately for this customer, after improving the size of this tubing performance was on par with our published specifications and this customer was back in operation.  If you have a question about how to improve the utilization of the compressed air devices in your application, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

The Effect of Back Pressure on a Vortex Tube

Vortex tubes have been considered a phenomena of Physics and boggled minds for many years.  To give a brief run down of how the Vortex Tube works please refer to Figure 1 below.

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Figure 1

As seen above, the control valve is determining the amount of air allowed to escape the hot end and sets the cold fraction.  A cold fraction is the percentage of air that exits the cold side versus the hot side. The cold fraction and operating pressure sets the temperature drop on the cold end and temperature rise on the hot end, as well as volumetric flow out of both ends. The control valve is not the only variable that can alter the cold fraction of the Vortex Tube though.

In Figure 1 and the performance chart below, there is no restriction on the hot end or the cold end outlets. No restriction means no back pressure and the cold air has the easiest path to the area needing cooling. Back pressure can directly affect the performance of a Vortex Tube.  As little as 3 psig of back pressure can begin to alter the temperature drop or rise on the Vortex Tube.  This is due to the fact that Vortex Tubes operate off an absolute pressure differential.  If the outlets have a restriction on them then they are not discharging at atmospheric pressure, 14.7 psi. What kind of items can cause back pressure and can the performance with a back pressure on the outlet be determined?

Back pressure is created by implementing any form of restriction on the hot or cold outlet. This may be undersized tubing to deliver the cold air or a valve that has been installed to try and control the volume of air being blown onto the process as well as many other possibilities.  The best rule of thumb to eliminate back pressure is to keep the tubing on an outlet the same cross sectional dimension as the outlet on the Vortex Tube and try to keep the tubing as short as possible.

If back pressure cannot be prevented, the performance variance of the Vortex Tube can be calculated and possibly compensated for. The variables that are needed to do so are the inlet air pressure of the vortex tube and the amount of back pressure that is being seen on the outlets. If this is different from the hot end to the cold end both will need to be known.  If these are not known they can be measure by installing a pipe Tee and a pressure gauge. This may need to be a sensitive pressure gauge that measures even relatively low psig. (1-15 psig)

Once these variables are known, we want to look at an absolute pressure differential versus the back pressure differential. For example, the Vortex Tube is a operating at 100 psig inlet pressure, 50% cold fraction and 10 psi of back pressure.  We look at the pressure differentials and can use Algebraic method to determine the inlet pressure supply that the tube will actually perform at.

(100 psig + 14.7 psia) / (10 psig + 14.7 psia) = X / 14.7 psia
4.6437 = X / 14.7
X= 14.7 * 4.6437
X = 68.2628
(Values have been rounded for display purposes)

So if there is a 10 psig back pressure on the outlet of a Vortex tube operating a 100 psig inlet pressure the tube will actually carry performance as if the inlet pressure was ~68 psig.   To showcase the alteration in performance we will look at just the temperature drop out of the cold side of the Vortex Tube. (Keep in mind this is a drop from the incoming compressed air temperature.)

Vortex Tube Performance Data
Vortex Tube Performance Chart

As shown in the performance chart above, if the Vortex Tube was operating at 100 psig inlet pressure and 50% cold fraction the temperature drop would be 100°F.  By applying a 10 psi back pressure on the outlet of the Vortex Tube the temperature will be decreased to ~87°F temperature drop.   This will also decrease the volumetric flow of air exiting the Vortex Tube which can also be calculated in order to determine the cooling capacity of the Vortex Tube at the altered state.  Keep an eye out for a follow up blog coming soon to see that calculation.

Brian Farno
Application Engineer Manager
BrianFarno@EXAIR.com
@EXAIR_BF

I Just Saved 15% Of My Compressed Air Usage In 15 Minutes

I’ve been pretty entertained over the past few weeks with all the new commercials from a certain insurance company that boasts their ability to save you 15% in 15 minutes.  Well sure enough one of their competitors has now flipped that on them and turned it into almost a joke.  Their competitor now claims to save the same amount in half the amount of time.  While they never say the name it is still making sure that you get the point they are better than their competition.

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While the commercials are funny and I do get a kick out them it is still just talk until you actually try them out.   Here at EXAIR we have our own competitors who will claim their flow is one less SCFM than a similar EXAIR product, or maybe 1 dBA quieter.   The fact of the matter is, they can say whatever they want to in advertisements, catalogs, on websites, or even blogs.   What matters is the actual performance of the products as well as the level of service you receive from the company before and after you purchase.  We’ve said it before, and we will say it again, the proof is in the pudding.

EXAIR will not only provide you with enough (if not more than enough) information before you purchase a product, we will also then stand behind the product with a 30 day guarantee.  To top all of that off, we make ourselves available through phone, email, fax, mail, live chat, or even stop in our facility.  The point is we are here to help and we aren’t going to disappear after you buy the product.

Note: Depending upon your current SCFM use and total capacity – you may be able to save 15% by just twisting on some engineered air nozzles in place of open blow offs. This could actually be performed in less than 15 minutes – you get the picture…

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

Troubleshooting 101: Super Air Knife

Yesterday, I was working with a customer on troubleshooting a Super Air Knife. He had brought the knife into EXAIR’s demo room so I was able to verify a few items very easily.  When trouble shooting air knives there are no moving parts, so it is very small list of items to check.

  1. Check the Air Supply

  2. Check the plumbing

  3. Check the inside of the Air Knife for debris

The customer had a 36″ Super Air Knife ,and he was seeing some weak spots in the air flow as well as a gradient in flow from one side of the knife to the other.  The first thing I did was to install a pipe tee with a pressure gauge in both ports on the bottom of the knife.  This would allow me to monitor the pressure we were supplying to the knife to calculate the air consumption and ensure the our piping was not starving the knife for air.

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Feeding the knife with equal pressure from both sides, is necessary for any air knife 24 inches or longer. The customer immediately noticed that the flow from the knife lost any sort of gradient, once it was fed in (2) locations. Still the air knife exhibited a spot in the flow where air velocity significantly decreased.  Since we were getting correct pressure and supplying enough air, we decided to remove the cap from the Super Air Knife.  Under the cap we found a variety of debris and one dreaded piece of PTFE plumbing tape. The plumbing tape was suppose to prevent air leaks throughout the compressed air system, but a piece had become lodged in the air gap of the Super Air Knife preventing air flow through a small portion of the Super Air Knife.  As you can see, once we followed a few simple steps to ensure proper installation of the Super Air Knife, it was quick and easy to narrow down what caused the lack of performance. This is yet another reason to make sure you have clean and dry compressed air, as well as use a point of use filter separator.

Dave Woerner
Application Engineer
Davewoerner@EXAIR.com
@EXAIR_DW