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Novice June 29, 2004 16:08

Gas flow through a 90 deg Elbow
I am a mechanical engineer, who does flow calculations occasionally. I have been asked to estimate the loss of pressure of a gas flowing through a 90 deg. elbow. Is there any ready formala for doing this, and is there any references somebody can help me. Thanks a lot for your assistance.

abhi June 29, 2004 19:59

Re: Gas flow through a 90 deg Elbow
The Diffrence Between pressure inlet and pressure outlet will be total pressure loss.

Novice June 29, 2004 21:12

Re: Gas flow through a 90 deg Elbow
My problem is I only know the inlet pressure, and may not have an oppurtunity to plug a pressure gauge after the outlet. I have to predict the pressure loss before the equipment is built. Any input is greatly appreciated.

Tim June 30, 2004 07:36

Re: Gas flow through a 90 deg Elbow
Hi yes you can use an approximate loss coefficient approach.

The pressure head loss (h) can be approximated as...

h = (k*u*u)/(2g)

as h = dp/(rho*g) you can re-arrange to get

dp = k. 0.5*rho*u*u

I assume you know or can calculate the density and average flow velocity. The value of k can be found listed in tables for differend bends/valve/etc.

a 90 degree elbow has a k coefficient of 0.9


loss of pressure = 0.9 * dynamic pressure

dp = 0.9 * 0.5* density * velocity squared

This is described fully in the book 'mechanics of fluids' by B.S. Massey . ISBN 0-412-34280-4

alternatively you can calculate the friction factor from a moody chart and use a l/d approximation to find the pressure loss

hf = f . (l/d) (vel squared/2g) Darcy weisbach equation


Good luck


Novice June 30, 2004 10:43

Re: Gas flow through a 90 deg Elbow

Thanks a lot, appreciate your help.

James Date July 4, 2004 08:29

Re: Gas flow through a 90 deg Elbow
Have a look in:

Fluid Flow - Internal Flow Systems

2nd Edition by D.S. Miller

This has pretty much everything you need to know about doing pipe flow calculations.

Regards James

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