CFM out of pressurized vessel

[rofloligist]

I am trying to calculate the volume flow rate out of a pressurized vessel. The pressure in the tank is being held constant at 120 psi by a compressor and contains air. There is a 1 inch outlet on it. I have calculated the mass flow rate by assuming choked flow and using:

Q = C A sqrt (g k rho P ((2/k+1)^(k+1/k-1)))

where

C is discharge coefficient (assume about 0.75)
A is outlet area
g is gravity
k is ratio of specific heats (1.4)
rho is density
P is pressure in the tank (120 psi)

I got a mass flow rate of 0.549 lb/s leaving the tank. How do I convert this to CFM? Simply dividing by density? How do I know the density at the outlet?

 

[zdas04]

If you want actual cubic feet per minute, then you divide the mass flow rate by density. Density is at atmospheric pressure and ambient temperature. ρ=P/R[sub]air[/sub]/T with P and T in absolute units. If what you really want is standard cubic feet per minute, then instead of local atmospheric pressure and ambient temperature use standard temperature and pressure.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[rofloligist]

At standard atmospheric conditions, density of air is 0.0749 lbs/ft[sup]3[/sup]. Therefore my CFM should be 0.549/0.0749 = 7.3 CFM. Is this correct? I am trying to buy an inline air heater for a work project and need to know the CFM flowing through it. Thanks dude!

 

[LittleInch]

I've not heard zdas04 referred to a "dude" before ..

You've mistaken seconds for minutes - 0.549 lbs/ SEC divided by 0.0749 = 7.3 CFS = 438 CFM at whatever pressure 0.0749 relates to so if this is standard then it is 438 scfm. Sounds a bit more like it for a 1" hole with 120psi behind it...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

Dude. Be careful of your terminology. LittleInch has showed you the right way to do it. If you want "CFM at standard conditions" then just write SCFM.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[rofloligist]

thanks for the help guys