If the application demands an exact and certain flow and pressure, then I think you would be wise to get the application engineers involved.
If close counts then this following information will be useful.
For the following orifice flow chart, I researched numbers from quite a few sources, and the figures shown are an average of those.
I can understand folks being puzzled about estimating pressure drop, as I was intially as I did my research. One of the sources says that the air is measured in SCFM, another one says that it is CFM, and still another uses ACFM. So, even the experts, among whom I do not count myself, are not able to provide consistent information.
The chart assumes that the air supply is 90 / 100 PSI, the air temperature is 70 deg. F., and there is virtually no pressure drop in the line as the orifice opens immediately to atmosphere from the compressed air supply.
Once you've selected what you think will be the ideal supply line I.D., one that will provide the flow capacity that your air tools or air circuit will require over the distance the line must travel, you then must consider estimating the pressure drop in your air system, to make sure you have allowed enough extra flow capacity for the line losses you will experience.
For example, aged black pipe used for air lines may contain an abundance of rust imparting a very rough surface to the inner diameter of the pipe. This will significantly increase the pressure drop, not to mention allowing chunks of rust to travel along with your compressed air, from time to time.
If there is considerable moisture in the mains, this too will increase pressure drop. The flowing compressed air has to overcome increased friction between it, the water, and the inner surface of the air pipe.
Every elbow, every Tee and even every coupling cause turbulence in the air as it passes, and this increases the pressure loss. The loss through fittings is calculated by estimating how much longer the air line would be to correspond to the type and size of fitting.
Each size, and each type of fitting, has a different pressure drop flow factor. Further, any compressed air supply valves, depending on their type, and size, and shape, can significantly influence pressure drop in the compressed air.
If I had 20 fittings from my compressor to my application, that would be the equivalent of an additional 100' of air line added to the total air line length I would use to figure my pressure drop over that distance.
In the larger fitting sizes, the pressure drop caused by the air fitting is less, but for simplicity's sake, I would still use the factor of 5 feet of air line length for every fitting in the air circuit.
One is, the higher the flow rate at a constant pressure, the greater the pressure loss per length of pipe.
The second, the lower the inlet pressure with a constant flow, the greater the pressure loss per length of pipe.
And last - regardless of the flow or the pressure, the smoother the I.D. of the pipe, the lower the loss over a given length of pipe.
When it comes to counting my pay check, my math skills are as good as anyone's. However, when it comes to formulas, when it comes to understanding the symbols and sequences for applying numbers in that formula, then I admit to being somewhat of a "mathophobic"!
If you would prefer to do the compressed air pressure-drop math yourself, then feel free to visit the (www.jegasho.net/rotron%20pages/orifice%20flow%20calculation.htm) page for a breakdown of the mathematical steps necessary to figure out the air flow through an orifice.
www.kaeser.com/Online_Services/Toolbox/Pressure_drop/default.asp offers an on line chart into which you can put the parameters of your air lines, and it will calculate pressure drop for you.
Need more info on pressure drop? If the guidelines and info here don't suffice, you may be forced to get an engineer or a professional fluid power expert to help. Good luck!