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

[sadowski777]

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 let's 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.

 

[haruspex]

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.

 

[sadowski777]

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)?

 

[haruspex]

why would they call it SCMH (standard cubic moles per hour),

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?

 

[CWatters]

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

 

[nasu]

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

I think that SCFH means "standard cubic feet per hour". The unit should be ft^3/h.

 

[sadowski777]

I think that SCFH means "standard cubic feet per hour". The unit should be ft^3/h.

Right. It's ft^3/hr at STP (Standard temp and pressure)

I'm still unsure. Wish I could see some experimental data.

 

[sadowski777]

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.