A method to calculate the amount of air leaving a shop air line

[RyanMH]

Hello everyone. I am trying to find a way to calculate how much air is flowing through a shop air line that is connected to an air compressor on one end and open to the atmosphere on the other. I do not have a method to measure this air (currently), and I know only the following:

-The compressor is large enough to keep up with the amount of air draining from the tank
-The pressure at the regulator (at the end of the line) is 40 psig
-Atmospheric air is 0 psig
-The diameter of the hose outlet is 9 mm
-Pressure drop through the regulator will be neglected

Does anyone know of a way to calculate air flow based on what little information I have given?

 

[zdas04]

How long is the 7 mm line?

There is no direct solution to the problem. You have to estimate the pressure drop in the flow line to determine the pressure at the outlet of the pipe. Then with that pressure you can calculate a mass flow rate out the end. It will be different than the mass flow rate you calculated the first step so you try again with the second mass flow rate to get a new dP. That gives you a new rate out the end (but hopefully the two numbers are closer together). Repeat until the two flow rates are close enough.

The important thing to keep in mind is that you can only do incompressible flow calcs if the downstream pressure is > 90% of upstream pressure (i.e., if dP in the 7 mm line is more than 5 psig) then the flow is "compressible" instead of "incompressible" and the incompressible flow correlations (like Weymouth, Darcy Weisbach, or AGA Fully Turbulent) don't work. Another limitation is that the incompressible flow assumption fails if the velocity in the little pipe is more than about 0.6 Mach (some authors claim that you should limit incompressible to less than 0.3 Mach, but that doesn't really improve results materially).

David

 

[1gibson]

Anything else running off the compressor at the same time?

Start a watch when the compressor kicks off, stop when it kicks on again. Note tank pressures at start/stop.

How much pressure did your tank lose over that period of time? From there, getting average mass flow should be simple.