Sometimes Back Pressure is Good, Sometimes it is Bad

As the title of this blog indicates, I’m going to discuss situations in compressed air applications where a condition that we refer to as back pressure is good and where it is downright problematic.
Back Pressure Good – When we talk about Air Nozzles, Air Knives and Air Amplifiers, we always tell the customer to insure they have proper operating pressure at the inlet of our equipment. To do this, install a pressure gauge on a pipe tee, right to the inlet of our equipment to monitor the actual “working pressure” or as can be termed “back pressure” created by the product connected to the end of the pipe while it is operating. If the “back pressure” remains the same or drops by only a few PSIG, then all is good and the compressed air supply can deliver sufficient volume of air without pressure dropping. In other words, the Air Nozzle is able to create enough restriction to flow that the pipe can manifest enough pressure right behind the nozzle. This is exceptionally important to have good performance from the product connected to the pipe as maintaining proper input pressure and volume flow is what we are concerned with in these cases. The two water photos below represent the difference between no back pressure and back pressure situations with the result shown in the water flows.

garden-hose
A Nozzle with NO back Pressure
finger%20in%20nozzle
A restriction causes back pressure and more force at the nozzle

Back Pressure Bad – The situations where having back pressure is a bad thing is when it is observed in a Vortex Tube. The ideal situation for a Vortex Tube while operating is to have as high a pressure as is reasonable on the inlet (let’s say 100 PSIG), and zero “back pressure” on the outlet. The reason that you want to do this is to enhance the effectiveness of the Vortex Tube’s ability to generate cold air at its outlet. The more extreme the pressure differential across the Vortex Tube, the more extreme the temperature drop becomes.
The following photo shows a set up where by the cold output flow from the vortex tube is being restricted. The customer means well in that they want to measure the output pressure and also divide the cold air out over two areas were the cooling effect is needed. The problem is that the tubing is undersized and the ends, where the cold air hoses are inserted into cooling fixture, are blocked off and do not allow for sufficient flow.

VT

To demonstrate mathematically the effect of back pressure on a vortex tube, I have the following example: Assume a vortex tube has 100 PSIG at the inlet. A pressure gauge inserted into the cold flow measures 15 PSIG “back pressure”. What is the effective input pressure in this condition?
100 psig + 14.7 psia = absolute pressure ratio of 3.86
15 psig + 14.7 psia
Effective input pressure is determined as follows:
X + 14.7 = 3.86            x = 42 psig effective input pressure
14.7
At first glance you would not think the back pressure could have such a devastating effect on inlet pressure, but it does and this affects the performance of the vortex tube negatively as well.
So, what is the point of the discussion? Make sure you have plenty of pressure right at the inlet to all your compressed air equipment. And when operating a Vortex Tube, make sure the exhaust is free and clear to escape for best performance.

Neal Raker, Application Engineer
nealraker@exair.com

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