ACFM to SCFM conversion problem 1

[sirkingsley]

I am having trouble determining the proper equation to convert from actual air flow (acfm) to standard (scfm). The way that most sources say to make this conversion is a manipulation of the ideal gas law that looks like:
scfm=acfm*(Pact/Pstd)*(Tstd/Tact)
It makes sense to me, but on the literature for a certain flow meter we use, this conversion has the pressure and temperature ratios under a square root sign:
scfm=acfm*sqrt((Pact/Pstd)*(Tstd/Tact))
The rep from this company told me that it is derived from the Ideal Gas Law AND Bernoulli's Equation, and that Bernoulli is required since the air is in motion. I don't understand where Bernoulli would factor into this, since air is compressible. Any help anyone could offer would be appreciated.

 

[zdas04]

That makes more sense.

David

 

[dcasto]

sorry for the diversion, but we called PV=nRT the "pervert" equation in OP's class.

 

[BigInch]

That's just a typical equation used for calculating gas flowrate across any flow resisting, element given the differential pressure. Its Q at any pressure, just that the flowrate the meter is reading is at a pressure of 0.25 psig, therefore they are just ratioing that to Pg.

In other words, both are basically the same equation. Although vezos has printed the equation corrected for standard pressure, there being the a pressure of 0.25 psig, ie. Pb = 14.69 psia + 0.25 psig and also shows the correction for a typical natural gas specific gravity of 0.6 / G, G being that of air = 1.00. Air flow is assumed in the OP's equation.




**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

When its corrected to a base pressure, Pb = meter reading pressure, or some other base pressure you care to define in a sales contract, effectively you have converted to STANDARD cf.

If you want to divide by time, go ahead and you get cubic feet, or multiply by time and get a flowrate.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Latexman]

dcasto,

Two curiousities.

What's "OP's class"? The only thing I can think of is Unit Operations.

Why "pervert"? Emil Clapeyron is credited to be the first person to write PV=nRT in 1834 from Boyle's, Charles', Guy-Lussac's, and Avogadro's Laws. Is there something about Emil we might like to know about?

Good luck,
Latexman

 

[SNORGY]

I would think that if you have a reasonable idea of the RMM and the ACFM of the gas, the conversion to SCFM via the consideration of mass would be somewhat straightforward, as the Z-factors for various temperatures and RMMs are charted in GPSA.

Regards,

SNORGY.

 

[BigInch]

Yes it is, just the point here is this application is a low pressure meter where z factors are = 1.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[hydtools]

Here is the Compressed Air and Gas Institute(CAGI) formula:

http://www.cagi.org/toolbox/formulas.htm

Ted

 

[hydtools]

Here is what Lake Monitors uses to correct their scfm meter reading actual conditions to scfm at their meter calibration conditions where the meter reads directly in scfm:

http://www.lakemonitors.com/pneumatic_flow.htm

Ted

 

[dcasto]

Yeah, I think it was unit ops where we used the pv rt equation, notice the letter oreintation

 

[Latexman]

Good one!

Good luck,
Latexman

 

[vzeos]

"That's just a typical equation used for calculating gas flowrate across any flow resisting, element given the differential pressure......"

The minimum capacity equation for elevated pressure does not calculate the flowrate across any flow resisting element. It calculates the minimum flowrate that the meter can be expected to give an accuracy greater than 1%. Since the meter infers the gas flowrate from the gas stream velocity it is necessary to adjust the minimum allowable flowrate when metering at pressures greater than 0.25 psig because the gas stream velocity deminishes as the metering pressure increases and we need to maintain at least a minimum velocity. The metering pressure of 0.25 psig is an industry standard for catalog capacities.


I should point out that the turbine meter maximum capacity equation for elevated pressure P_g does not contain any square roots nor specific gravity. This equation is similar to the pressure factor adjustment for displacement meters:

Q_max @ P_g = Q _max @ 0.25 psig * ((P_g + P_a)/ P_b) * S
where P_g = gauge press., P_a = atmos. press., P_b= base press., S = Compressibility Ratio.)

 

[joeflow]

The rep. confused the ACFM/SCFM conversion with an equation that is used to convert an observed variable area flowmeter reading at nonstandard conditions into an ACFM reading.


SCF = ACF [(Pact*Tstd*Zstd)/(Pstd*Tact*Zact)]

Actual volumetric flow =
(Observed Flowmeter Reading)*SQRT{ [(Pact*Tstd*Zstd)/(Pstd*Tact*Zact)]}