by 84-1074663779 » Sat Apr 09, 2005 9:16 pm
Mouse is spot on, it is a theoretical ideal, that you may not be able to achieve. But it is a pretty good starting point for most of us.
It assumes the upstream air is undisturbed and non turbulent. Fitting a thin sharp edged orifice flush with the top of your bench over the test hole comes pretty close to the ideal.
The same identical orifice fitted INSIDE the bench used as the measurement orifice can behave quite differently. Turbulent air will not want to go through that hole quite as readily, and the flow coefficient will be less.
If you pointed a high pressure air hose straight at that orifice, a lot more air will go through than otherwise would.
Flow conditions upstream of the measurement orifice will effect the results significantly, and they can change in either direction. That is the problem.
An excellent test is to make two identical orifices, place one flat on top of the bench, and use the other as the measurement orifice located exactly as it will be in the final configuration.
In a perfect world the differential pressure drops should be identical. How much different they are will give you a few clues as to the internal flow conditions inside your bench.
If the largest measurement orifice works properly at maximum flow, the smaller orifices will always work properly too. It is very convenient to have predictable pressure versus flow, you can then plan a range of orifice sizes to give convenient flow ranges.
If flow through the measurement orifice is "wild" you can still calibrate your bench, but all the sizes will be all over the place, and if flow conditions change slightly, so will the calibration. To get stable consistent results "taming" the flow through the measurement orifice is a very important design goal.
Others have different ideas, but I like to have a very large plenum space upstream of the measurement orifice, place the orifice in the centre of a broad flat area well away from the walls, and keep the up stream air velocities as low as possible. This plenum space can simply not be too large.
Another way is to place the orifice in a long strait pipe, but the flow will then be "wild". Flow and pressure drop will not correspond to the very simple orifice formula above. It can still work, but the results are far less predictable.