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.

How_A_Vortex_Tube_Works
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

Outside Your Comfort Zone?

Over the past several months I have found myself more and more outside of my comfort zone throughout the day.  This feeling has been declining over the past few months, however, as the situations which cause discomfort seem to come up more and more every day so my comfort zone is constantly expanding. I like comfort zones, but also understand that being outside of them helps me to learn and gain new experience. I bring this up because I had a customer come in yesterday so they could be shown a demonstration of an EXAIR Super Air Knife.

Now, customers coming in is not a problem at all, I spoke with him last week and we discussed the application.  This time, it was outside of his comfort zone so he wanted to come in.  Well, when I got the call I had a visitor I grabbed a single business card and walk out to find that there were four gentlemen waiting to see me, not just one.  Still, not a problem. I showed them to our fully stocked demo room and we proceeded to discuss their application.  They were hoping to make the environment their employees work in a little cleaner.  They had new down draft work benches which had three sides on it.

The problem the team was having is that all their operators were using hand-held grinders to deburr parts as they were manufactured.   The downdraft table was added to help prevent the dust and debris from getting all over the operators, however it wasn’t working good enough.  So they started looking and found EXAIR Super Air Knives.  They didn’t believe that a Super Air Knife would move enough air and still be quiet enough to have an operator sitting at the station, so they drove down to our facility and I showed them all the benefits that a Super Air Knife has.  This was all based off a 6″ Aluminum Super Air Knife I had handy.  I then swapped the stock .002″ thick shim out with a .001″ thick shim.  They were amazed at how quiet the Super Air Knife was (with either shim) and how the flow of air was enough to disturb and direct dust but not over powering and blowing parts off the table. LSAN I could definitely see that they were impressed by the simplicity of working with the Super Air Knife and the performance it achieved. But alas, they were still trying to figure out how a 48″ would work, so I went straight out and got a 110048 off the shelf and hooked it up for them.  That was all that they needed in order to really get the wheels in their heads spinning into overdrive.   They all left with my contact information and catalogs in tow but I didn’t hear them stop talking about the possibilities until they were in the car.

The fact of the matter is that they were outside of their comfort zone and had no concept of how you could make compressed air blow in a laminar sheet to help contain dust in a down draft work bench.  Once they saw how easy the Super Air Knife was to hook up and mount they were instantly back into their comfort zone of making their employees happy and safe. If you have some applications using compressed air and you are well out of your zone, contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF