Estimating Pressure Drop

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Estimating Pressure Drop

Estimating Pressure Drop is fairly straightforward if the end application for the compressed air isn't critical.

If the application demands an exact certain flow and pressure then you would be wise to get the application engineered.

If close counts, for the following orifice flow chart, I researched numbers from quite a few sources, and the figures shown are an average of those.

It was a bit confusing since one of the sources says that the air is measured in SCFM, another one says that it's CFM and still another uses ACFM. So, even the "experts" aren't able to provide consistent numbers.

As a guide, first check the air flow through the orifice that is the same size (or close) to the I.D. of your selected air line. Ensure that the specific 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's virtually no pressure drop as the orifice opens immediately to atmosphere.

Once you've selected what you think will be the ideal supply line I.D. to provide the flow capacity that you 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 of your air lines will affect any flow figures.

For example, aged black pipe used for air lines will contain an abundance of rust imparting a very rough surface to the inner diameter of the pipe which 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 formulae:

If your air line is up to 3/4" I.D., then assume a loss of up to 10% of your available air pressure for every 100' of air line

If your air line is between 1" - 2" I.D. then assume 5% loss per 100' of air line

For air line 2 1/4" and up assume a 3% pressure drop per 100' of air line and you should be safe

Don't Forget The Fittings

Don't forget your fittings! Every elbow, every 'T', by causing turbulence is the air as it passes, 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.

To make things complex, each size and each type of fitting has a different pressure drop flow factor. And any supply line valves, depending on their type, can have widely varying pressure drop factors.

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.

In the larger fitting sizes, the pressure drop is quite a bit less, but for simplicity's sake, I'd still go with the 5' of equivalency.

Commonly Accepted

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 the 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. So once again, the numbers shown are guides.

To 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 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: Please click here.

Or, here is 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!