# Air velocity from a 5mm pipe

At first, I will try to describe my problem.

I have a compressed air at 0.4 MPa (4 bars) that goes trough a control valve and a flow meter into two 5 mm pipes.

Flow meter shows the value of 200 normalized liters/min (100 l/min into each pipe). According to a simple equation Q=Area*velocity this should mean that the velocity of the air coming from the pipe should be around 82 m/s (maybe I am incorrect and it doesn't work like this).

My problem is that the speed is nowhere near this value. I didn't measure it with a proper tool but I know what It should feel like to have outlet speed around 80 m/s. I guess the speed is no more than 20 m/s (the air speed is similar to the speed of air you are able to blow from your mouth with pursed lips).

My cfd simulation also showed the speed around 80-90 m/s with inlet flow of 100 l/min. So where can be the problem? I tested two different flowmeters, both with similar results. Is my calculation wrong or is there something wrong with the flowmeters? Flowmeter were just ordinary cheaper ones.

• How do you know what it should feel like? Sticking your hand out the car window is a very different feel from feeling what is coming out a 5mm pipe. – Jon Custer Sep 25 at 12:33
• Are your flow meters measuring standard liters/min or actual liters/min? Should you be using a pressure correction on the flow meters? Did you use the the diameter instead of the radius to calculate the exit pipe cross-sectional area (this would produce an error of 4x)? – David White Sep 25 at 23:24

In short, you may be over-simplifying the situation, because flow of air through valves/piping cannot always be assumed to be incompressible. If compressible effects are relevant, then you cannot rely on $$Q=Av$$, because the density is not constant. In that case, your calculations and CFD would both need to account for the effects of compressibility.