Calculating volume required for a pressure value ( for duct design)

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Discussion Overview

The discussion revolves around calculating the volume required for duct design to maintain a minimum pressure of 12 psi at a specific section of the duct. Participants explore the application of fluid dynamics principles, including Bernoulli's principle and the ideal gas law, in the context of duct sizing and airflow calculations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant outlines a duct system with various sections and mentions the need to calculate the volume flow rate required to maintain a pressure of 12 psi at section 4, using Bernoulli's principle.
  • The same participant expresses difficulty in determining the number of moles ('n') for air when applying the ideal gas law, given the pressure condition.
  • Another participant suggests using the relationship Q = AV, assuming the velocity is derived from Bernoulli's equation, to help with the calculations.
  • A later reply emphasizes the importance of consulting SAE AIR1168-1 for relevant guidelines applicable to the duct design process.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the method to calculate the volume required for the duct design, with ongoing questions about the ideal gas law and the necessary parameters for calculations.

Contextual Notes

Participants mention assumptions such as laminar flow, incompressibility, and inviscid conditions, which may limit the applicability of their calculations. The need for specific values, like the number of moles for air, remains unresolved.

Who May Find This Useful

This discussion may be useful for engineers and students involved in fluid dynamics, duct design, and HVAC systems, particularly those interested in the application of theoretical principles to practical design challenges.

kirankanimetl
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I am calculating sizes of duct ( section1= constant area pipe, section2= transition to a rectangular cross section , section3= constant rectangular cross section upto 0.5m, section4= similar to section3, section5= similar to section1) and fan flow rate for a given pressure(minimum of 12 psi at section4) which is required to be maintained. for which I have started calculating from section4 and working through to the inlet by adding losses and pressure changes( bernoullis principle) and finally obtaining volume flow rate required.

I am stuck at calculating volume of section4 from ideal gas (PV=nRT)for the given pressure as I do not know 'n' value for air. Also I have considered fluid to be inviscid, incompressible and the flow regime to be laminar for simplifcation.

Can some body help me with this?
 
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If you have the velocity (guessing obtained from Bernoulli's equation) and the area, then
Q = AV
 
Hi,

Thanks for your initiation, but I guess you havn't understood my question,
To start calculations, I am working from rectangular section of the duct where there is a minimum pressure requirment of 12psi is nessecary. But to start calculations assuming the pressure to be 12psi gauge ( static+dynmic pressure) and without having duct sizes(which has to be calculated), i need to evaluate volume of the rectangular duct from ideal gas law (PxV= nxRxT)for a static pressure of maximum 12psi. After which velocities and flow rates can be calculated for each sucessive duct sections from bernoullis and continuty eq.

But I need to know number of moles i.e. 'n' value for air at STP or appropriate conditions.

And as I have previously specified I have assumed Flow to be laminar, incompressible and inviscid for simplification and I will later optimize this design through CFD solver.

Thanks again
Kiran
 
I would highly recommend that you get a copy of SAE AIR1168-1, specifically sections 1A and 1B. It is very applicable to what you are designing for.
 

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