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ACFM to Equivalent ACFM

ACFM to Equivalent ACFM

ACFM to Equivalent ACFM

(OP)
In converting the ACFM of a gas (say methane) to equivalent ACFM of air, shouldn't the densities be taken into account?  Our company ignores the importance of the difference in densities.

RE: ACFM to Equivalent ACFM

Interesting question.  Why would someone convert a volume flow rate of methane to an equivilent amount of air?

One way to calculate density is (this is the only place I've ever seen the characteristics of air explicitly used in Oil & Gas production and/or distribution):

rho=(P*SG)/(T*R(air)*Z)

but R(air)/SG(gas) = R(gas)

Could it be that you're confused and they're really converting ACFM to SCFM?  That conversion uses pressures, comprssibilities, and temperatures, but generally not density (when you do the math, the Gas Constant cancels so you don't need density, if you already have density for both flowing and standard conditions it works fine).

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

RE: ACFM to Equivalent ACFM

wsmith22:

I interpret your question as stating that you need to find the volume of air (at “actual” condition) occupied by a mass of air equal to the mass of methane occupying its related volume at the same actual conditions of temperature and pressure.  If that is so, then what you can do is use the handy Uconeer free software offered by Katmar (a member of this forum - and who I hope contributes to this thread).

You can search these forums to find the website address for Uconeer.

RE: ACFM to Equivalent ACFM

I would like to be able to help, but with the limited information available I simply do not know what wsmith is trying to achieve. His company could be selling compressors, valves, flaring equipment, bottled gases or many other things.

Wsmith, if you are serious about wanting help, give the full story of what you are trying to calculate and you will find that there are some very helpful engineers in this forum but very few competent mind readers.

RE: ACFM to Equivalent ACFM


To wsmith22, are you by chance referring to the equivalent-air method of rating fans ?

RE: ACFM to Equivalent ACFM

(OP)
Filter housings.  We test with air.  Customer gives gas flow in ACFM.  

I am confused because convert ACFM gas to SCFM gas.  Then use SCFM air data for sizing the filter housing?  It seems like the SCFM gas needs to be converted to SCFM air.

RE: ACFM to Equivalent ACFM

(OP)
I checked out Katmar's program.  Impressive.

Is the reason that you don't need to convert between methane and air because if you use the real gas law, the compressibilities are so similar that it is accurate enough to just use the ideal gas law?

RE: ACFM to Equivalent ACFM

WSmith,

I am still very confused about what you are trying to calculate. On the face of it, it sounds like what you are doing is wrong because the pressure drop for X scfm of air will be significantly different from the pressure drop for (the same) X scfm of methane under the same conditions of temperature and pressure.

Unless you are doing something like 25362 suggested and are using an "equivalent air" method, it makes no sense to me. This procedure would have to include correction factors for temperature, pressure and molecular weight in order to use air data to design for other gases. Is the procedure that you are using available for us to see on your company's web page?

regards
Harvey

RE: ACFM to Equivalent ACFM

(OP)
Harvey,

Yes, I am trying to find the equivalent air scfm for a gas scfm, such as methane.

I found this from a Parker website:

Equivalent air scfm = gas scfm*PS/PA*TA/TS*sqrt(s.g. gas)

where S = standard conditions
      A = actual conditions

Does this work?

RE: ACFM to Equivalent ACFM

WSmith,

This formula doesn't work for me. I do not like using formulas like this when there are perfectly reasonable ways of doing the calculation properly.

The short answer to your original question is that I think your company is putting itself at risk doing sizings this way. I agree with you that the density of the actual gas should be taken into account, but so should its pressure, temperature and viscosity.

If something is worth doing, then it is worth doing properly.

Harvey

RE: ACFM to Equivalent ACFM

wsmith22
You’re on the right track.  In the natural gas industry, we test components such as meters, regulators, filters and valves with air flow and then calculate the equivalent natural gas flow capacity at the same operating pressure and pressure drop.  You can do the computations in two steps, first you can convert the flow to standard conditions and then you can calculate the equivalent gas flow.

To correct to standard flow rate, Qs, you need to correct the flowing temperature Ta and pressure Pa to standard T - P conditions using the combined gas laws:

PsQs/TsZs = PaQa/TaZa

where the subscript a refers to actual conditions and the subscript s refers to standard conditions, Z is the compressibility.

Solving for Qs and noting that Zs=1 for Ps << 100 psia

Qs = Ts PaQa/TaZa Ps

Now, you can work out an equivalent gas flow by using a simple valve equation. You can write,
Qs-gas = Cv √[ (PaΔP)/ (TaGgas Zgas)]
Qs-air =  Cv √[ (PaΔP)/ (TaGair Zair)]

Cv, Pa, ΔP, and Ta cancel out when you take the ratio Qs-gas / Qs-air.  Also note that the flow is assumed to be turbulent so that viscosity does not come into play.

Qs-gas / Qs-air= √[1/(Ggas Zgas)] / √[1/(Gair Zair)]

Qs-gas / Qs-air= √[(Gair Zair)/(Ggas Zgas)]

Gair=1 by definition.  If  Pa << 100 psia then Zair and Zgas can be taken as 1.0 and

Qs-gas = Qs-air √[1/Ggas] or to calculate the flow rate of gas at standard conditions given the flow rate of air at actual conditions,

Qs-gas = Qa-airTs Pa /TaPs √[1/Ggas]

You can rearrange this equation to suit you purposes.

RE: ACFM to Equivalent ACFM

RGasEngineer:

I don't mean to nitpick, but it would be helpful if you defined all of your terms.  For example, I take it that your Q is volumetric flow rate rather than mass flow rate. I also take it that your G is gas specific gravity.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

RE: ACFM to Equivalent ACFM

mbeychok,

Sorry for any confusion.

Nomenclature:
Qs = Volumetric flow rate at standard conditions.
Ps = Standard pressure.
Ts = Standard temperature.
Zs = Compressibility at standard conditions.
Qa = Volumetric flow rate at actual conditions.
Pa = Actual pressure.
Ta = Actual temperature.
Za = Compressibility at actual conditions.
Zair = Compressibility of air at actual conditions.
Zgas = Compressibility of gas at actual conditions.
Q s-gas = Volumetric flow rate of gas at standard conditions.
Q s-air = Volumetric flow rate of air at standard conditions.
Q a-gas = Volumetric flow rate of gas at actual conditions.
Q a-air = Volumetric flow rate of air at actual conditions.
Cv = Valve constant
ΔP = Pressure drop across the component.

RE: ACFM to Equivalent ACFM

(cont.)
Ggas = Specific gravity of gas (density gas/density air).
Gair = Specific gravity of air (density air/density air  =  1.0).

RE: ACFM to Equivalent ACFM

(OP)
Thanks RGasEng.  That is what I was looking for.

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