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

In summary, the mass flow rate of air through a pipe is determined by the molar volume of the gas, the air temperature, and the air pressure.
  • #1
sadowski777
6
0
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.
 
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  • #2
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.
 
  • #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)?
 
  • #4
sadowski777 said:
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?
 
  • #5
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
 
  • #6
CWatters said:
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.
 
  • #7
nasu said:
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.
 
  • #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.
 

1. How does temperature affect the mass flow rate of air through a pipe?

The temperature of air affects its density, which in turn affects its mass flow rate. As the temperature of air increases, its density decreases, resulting in a lower mass flow rate through a pipe. This is because at higher temperatures, air molecules have more energy and move faster, taking up more space and decreasing the number of molecules that can pass through the pipe in a given time.

2. Does increasing the pressure affect the mass flow rate of air through a pipe?

Yes, increasing the pressure of air will increase its density and therefore its mass flow rate through a pipe. This is because at higher pressures, air molecules are closer together and take up less space, allowing more molecules to pass through the pipe in a given time.

3. Is there a specific relationship between temperature, pressure, and mass flow rate of air through a pipe?

Yes, there is a direct relationship between temperature, pressure, and mass flow rate of air through a pipe. As temperature increases, pressure decreases, and as pressure increases, temperature decreases. This relationship can be described by the ideal gas law, which states that pressure and temperature are inversely proportional when volume and amount of gas are held constant.

4. How do changes in the diameter of a pipe affect the mass flow rate of air?

The diameter of a pipe can greatly affect the mass flow rate of air. A larger diameter pipe will have a higher mass flow rate, as it allows for more air molecules to pass through in a given time. Conversely, a smaller diameter pipe will have a lower mass flow rate due to the limited space for air molecules to flow through.

5. Can the shape of a pipe affect the mass flow rate of air?

Yes, the shape of a pipe can have an impact on the mass flow rate of air. A smooth, straight pipe will have a higher mass flow rate compared to a pipe with bends or rough inner surfaces. This is because a smoother pipe allows for less resistance to the flow of air, resulting in a higher mass flow rate.

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