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?