Estimating Pressure Drop

Estimating Pressure Drop is fairly straightforward if the end application for the compressed air is not a critical one.

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 info will apply.

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, as I was 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.

Air Through An Orifice

As a guide to help you determine pressure drop, first check the air flow through the orifice that is the same size (or close) to the I.D. of your selected air line. The purpose is to ensure that the expected air flow rating will be more than enough to run your application(s).

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, then consider the pressure drop in your system to make sure you've allowed enough extra flow capacity for the line losses you will experience.

Air Line Characteristics Matter

The characteristics or the condition of your air lines will affect any flow figures.

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.

If there is considerable moisture in the mains, this too will increase pressure drop, as the flowing compressed air has to overcome increased friction between it, the water, and the inner pipe surface.

Quick Guidelines

The following guidelines will help you guestimate pressure drop without using complex formulas:

Don't Forget The Fittings

Don't forget your compressed air fittings!

Every elbow, every Tee, are causing 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.

Make It Simple

To make it simple, and to err on the side of caution, if I were estimating pressure drop for the fittings in my supply air line, I would factor in 5' (five feet) of extra air line for every fitting, to allow for fitting pressure drop. 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.

Commonly Accepted Rules

There are a few commonly accepted rules about pressure drop.

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.

Be Sure...

The only way you can be sure of the actual pressure drop is to do the math for your particular system, or use one of the commercially available software packages written for that purpose.

When it comes to counting my pay cheque, my math skills are as good as anyone's. However, when it comes to formulae, 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 this site for a breakdown of the mathematical steps necessary to figure out the air flow through an orifice:

www.jegasho.net/rotron%20pages/orifice%20flow%20calculation.htm

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!