pressure drop 1

[njlee]

Hi all,

I have a fluid question and hope you can help me.

If the air flow rate and tube (inlet and outlet) ID remain constant, is it true that the pressure drop across a member will remain the same no matter what the pressure at the system is operating at?

eg. for constant flow rate and tube ID, the pressure drop across a member is 0.5 psi and the pressure at inlet is 2 psi. I think the pressure drop will remain at 0.5 psi even if the inlet pressure is 10 psi.

thank you!
lee

 

[israelkk]

No. This is true for incompressible fluid only where the flow rate is a function of the square root of the pressure difference. In a compressible fluid (gases) it is more complicated. As long as the flow is not chocked meaning when Inlet pressure is less than double the outlet pressure the flow is function of inlet pressuredivided by the outlet pressure. When it is choked the flow is a function of the inlet pressure only. Therefore, only in choked flow condition the pressure drop will be constant if the flow and the tube diameter and legth remain constant/

 

[zdas04]

Israelkk,
Not quite right. Even in choked flow a change in upstream pressure will change the flow rate. There is no condition where a dP by itself in compressible flow will always give you either the volume flow rate or the mass flow rate.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[israelkk]

zdas04:

You are correct! however, as I mentioned, if the flow rate is constant and the tube diameter is constant the result is that the upstream pressure in a choked flow condition is constant. Meaning: if you take the same tube and play with the upstream pressure to receive a defined flow rate than the upstream pressure will always be the same assuming the temperature and the outlet pressure are the same. I was just refering to njlee phrase "eg. for constant flow rate and tube ID, the pressure drop ..." where njlee specifically stated that the flow rate is kept constant with the same tube diameter.

 

[quark]

To my knowledge, volume flow rate in choked flow is constant with increasing pressure but mass flow rate varies.

http://www.air-dispersion.com/usource.html

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[zdas04]

Quark,
Volume flow rate is a velocity times an area. In choked flow, raising the upstream temperature increases the speed of sound and thereby increases the volume flow rate. The Volume Flow Rate for choked flow is constant for varying upstream pressure, but varies with temperature.

One short-hand statement of the flow equation is q=C_prime*(dP*P/T)^0.5 where C_prime is a function of the fluid and piping characteristics. None of these terms except the constant wil be result in anunchanging volume flow rate under conditions that vary one of the other terms.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[quark]

Zdas!

You are quite right. I overlooked temperature effect. Thanks for the correction.

Regards,