ACFM to Equivalent ACFM

[wsmith22]

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.

 

[zdas04]

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

http://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

 

[Montemayor]

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.

 

[katmar]

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.

 

[25362]

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

 

[wsmith22]

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.

 

[wsmith22]

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?

 

[katmar]

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

 

[wsmith22]

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?

 

[katmar]

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

 

[vzeos]

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, Q_s, you need to correct the flowing temperature T_a and pressure P_a to standard T - P conditions using the combined gas laws:

P_sQ_s/T_sZ_s = P_aQ_a/T_aZ_a

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

Solving for Q_s and noting that Z_s=1 for P_s << 100 psia

Q_s = T_s P_aQ_a/T_aZ_a P_s

Now, you can work out an equivalent gas flow by using a simple valve equation. You can write,
Q_s-gas = C_v [&radic;][ (P_a[&Delta;]P)/ (T_aG_gas Z_gas)]
Q_s-air = C_v [&radic;][ (P_a[&Delta;]P)/ (T_aG_air Z_air)]

C_v, P_a, [&Delta;]P, and T_a cancel out when you take the ratio Q_s-gas / Q_s-air. Also note that the flow is assumed to be turbulent so that viscosity does not come into play.

Q_s-gas / Q_s-air= [&radic;][1/(G_gas Z_gas)] / [&radic;][1/(G_air Z_air)]

Q_s-gas / Q_s-air= [&radic;][(G_air Z_air)/(G_gas Z_gas)]

G_air=1 by definition. If P_a << 100 psia then Z_air and Z_gas can be taken as 1.0 and

Q_s-gas = Q_s-air [&radic;][1/G_gas] or to calculate the flow rate of gas at standard conditions given the flow rate of air at actual conditions,

Q_s-gas = Q_a-airT_s P_a /T_aP_s [&radic;][1/G_gas]

You can rearrange this equation to suit you purposes.

 

[mbeychok]

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)
.

 

[vzeos]

mbeychok,

Sorry for any confusion.

Nomenclature:
Q_s = Volumetric flow rate at standard conditions.
P_s = Standard pressure.
T_s = Standard temperature.
Z_s = Compressibility at standard conditions.
Q_a = Volumetric flow rate at actual conditions.
P_a = Actual pressure.
T_a = Actual temperature.
Z_a = Compressibility at actual conditions.
Z_air = Compressibility of air at actual conditions.
Z_gas = 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.
C_v = Valve constant
[&Delta;]P = Pressure drop across the component.

 

[vzeos]

(cont.)
G_gas = Specific gravity of gas (density gas/density air).
G_air = Specific gravity of air (density air/density air = 1.0).

 

[wsmith22]

Thanks RGasEng. That is what I was looking for.