4 replies to this topic

[chemks2012]

Dear all,

 

Another query from me please.

 

I want to estimate line size for the compressed air flowrate of 25Nm3/min at 7barg.

 

25Nm3/min equates to about 882scfm. I am using the second chart provided in the link below which says 'Applied Pressure 100psig' which equates to 7barg.

 

http://www.engineeri...ssure-d_36.html

 

If I wish to have pressure drop of 1psi/100ft, I can have 3" line.

 

My query is:

 

I want to cross check the velocity through 3" at this flowrate. The actual flowrate at 7barg is about 3m3/min. Am I correct in assuming that the velocity should be checked at 3m3/min and NOT at 25Nm3/min?

 

Thanks in advance for your help.

 

KS

Edited by chemks2012, 05 September 2013 - 10:58 AM.

[Sathya R]

velocity is m/sec; assuming the temperature is 30oc then the velocity will be 12.2m/sec in 3" 80 sch pipe.

[chemks2012]

Satyanarayana,

 

Thank you very much for your reply.

Yes, I am getting about the same answer which I calculated based on actual flowrate.

I am assuming you have also calculated based on actual flowrate of 3m3/min and not on 25Nm3/min otherwise velocity is ridiculously high.

Please correct me if I am wrong.

Thanks

KS

[gegio1960]

chemks,

just to clarify the fundamentals behind the calculation:

- your velocity and pressure drop calculation shall be made at operating conditions, ie operating pressure and temperature in your system, and for the actual internal diameter of your pipe;

- you didn't specify your operating temperature, Sathya has assumed it at 30°C;

- you've been not very clear in providing the operating pressure: 7 barg is almost equal to 100 psig, but it is a little bit high for compressed air systems (eg 5 - 6 barg). You'll have higher pressure drops and velocities at lower pressures;

- the normal flowrate, in Nm3/m, enters in the calculations only for being converted to actual conditions;

- Sathya has assumed sch 80 for your pipe: it is a conservative assumption since these systems (low P, low T) are usually made with STD schedule (ie 40). The graph you've linked is based on sch 40;

- the graph only fails to specify the reference operating temperature, but this is acceptable, since the instrument air systems are supposed to operate at ambient temperatures (eg 20-40 °C) and this variation has no big impact on the pressure drops.

I tried to summarize the various calculations in the attached sheet.

Regards

 

Attached Files

•  Air_Flow.pdf   14.86KB   86 downloads

[chemks2012]

gegio1960

 

Thanks for your reply, explanation and attachment.

Yes, all do make sense. Thanks.

 

The operating pressure for our system is is 20deg C.

Yes, we intend to install Sch. 40 line.

 

KS