1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does T and P effect the mass flow rate of air through a pipe?

  1. Jul 5, 2012 #1
    Hi all,

    I have a question regarding the mass flow rate of air through a pipe. On paper, it looks like temperature and pressure effect the mass flow rate but I can't rationalize it in my head.

    Here's what I got:

    Goal: Calculate the actual mass flow rate of lets say, CH4 through a pipe
    Given: 10 SCFH (standard conditions are defined as 60F, 1atm)
    MW of CH4 = 16.04lb/lb-mol
    T=250F
    P=114.7psia

    My calculation:

    ACFH=10SCFH*(14.7psia/114.7psia)*((250+460)/(60+460))= 1.715ft^3/hr
    Molar volume=(RT)/P=0.143ft^3/mol

    Mass flow rate = molar volume/ACFM*MW=192.37lbs/hr

    However, I am told that the mass flow rate doesn't depend on T and P and the calculation should be as follows:

    Mass flow rate = SCFH/397*MW = 0.504lbs/hr

    The 397 is the molar volume at STP (60F, 1atm)

    Who is right in this case? Does the mass flow rate depend on T and P?

    Thank you for your time and comments.
     
  2. jcsd
  3. Jul 5, 2012 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    It all hangs on how SCFH is defined. My reading of online material (never heard of this before today) is that it is effectually a measure of numbers of molecules (and thus, of mass). That is, 10 SCFH means that the number of molecules (or corresponding mass) passing each hour is the number that would occupy 10 cu feet if held under the stated standard conditions.
    On this understanding, SCFH does not depend on temperature and pressure. The ACFH does.
     
  4. Jul 5, 2012 #3
    Hm, you could be correct. I still have a hard time with it.

    Mass flow rate is defined as density*velocity*cross sectional area of the pipe.

    The density will change at various temperatures and pressures. But if it is the total of moles coming out for a time, t, why would they call it SCMH (standard cubic moles per hour), not SCFH (standard cubic feet per hour)?
     
  5. Jul 5, 2012 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You mean, why wouldn't they call it that? Or rather, why not just MPH, moles per hour (don't need the 'cubic' part with moles, and don't need 'standard' any more). I agree it's not the most logical unit, but you can see how this might have grown out of engineering practice. Why do loggers use the Hoppus foot?
     
  6. Jul 6, 2012 #5

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    SCFH is a very strange unit. I'm more familiar with STP. I assume..

    SCFH would be equivalent to "Sandard Temperature and Pressure per Hour"

    Otherwise where does the unit of time come from in

    Mass flow rate = SCFH/397*MW
     
  7. Jul 6, 2012 #6
    I think that SCFH means "standard cubic feet per hour". The unit should be ft^3/h.
     
  8. Jul 7, 2012 #7
    Right. It's ft^3/hr at STP (Standard temp and pressure)

    I'm still unsure. Wish I could see some experimental data.
     
  9. Jul 7, 2012 #8
    Maybe I'll address the problem this way:

    Covert 50 SCFH (standard temp and pressure. 60F, 1atm) to lbs/hr (250F, 8atm).

    Assume an ideal gas.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does T and P effect the mass flow rate of air through a pipe?
  1. Flow through pipes (Replies: 3)

Loading...