Compressed Air CFM
Compressed Air CFM
(OP)
I am currently designing a HEPA filtered air system for a beverage filling machine. The goal is to complete an air change every 2 minutes within a 6.5 cubic foot compartment (not sealed and at atmospheric pressure). The SCFM would thus be 3.25
This is to be accomplished using a 1" inner diameter compressed air line (120PSI) passing through a 0-150 PSI regulator and then exhausting into the top of the fill compartment with open vents on the sides and bottom.
My question is how do I determine what pressure setting I restrict my regulator to in order to achieve my 3.25 SCFM?
(I can measure the length of the piping from the regulator to the fill compartment if necessary as well as the temperature.)
This is to be accomplished using a 1" inner diameter compressed air line (120PSI) passing through a 0-150 PSI regulator and then exhausting into the top of the fill compartment with open vents on the sides and bottom.
My question is how do I determine what pressure setting I restrict my regulator to in order to achieve my 3.25 SCFM?
(I can measure the length of the piping from the regulator to the fill compartment if necessary as well as the temperature.)





RE: Compressed Air CFM
You know the 120 psig inlet pressure, the outlet pressure and the flow. Flow across the regulator is Q = Cv (P1^2-P2^2), solve for Cv and set the regulator so that the Cv is the same. Find the Cv vs Position curve for the regulator and see what %Open gives you the Cv you need.
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RE: Compressed Air CFM
Q = Cv (P1^2-P2^2), a control valve characteristic?
Regards
RE: Compressed Air CFM
Please add a ^0.5 to the end of that.
I purposely left out the conversion factor, valve correction factor and compressibility effects, and don't mention you must check for non-critical flow.
Q = Cv * N1 * f_dP * ((P1^2-Pd^2)/Z/G/T)^0.5
Q = Standard Flow
N1 = 120/ºR/PSI
f_dP = Valve correction coefficient
P1 = Upstream pressure absolute
P2 = Downstream pressure absoute
Z = compressibility factor at upstream temp and avg press
G = specific gravity of gas relative to air
T = upstream temperature absolute
When not in critical flow.
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RE: Compressed Air CFM
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RE: Compressed Air CFM
BigInch, I believe I will have critical flow as I expect the drop in pressure to be greater than 50%.
I have found several different variations of the Cv computation formula and was wondering which is best. I found the follow equation from ht
Cv = q*[SG*(T + 460)]^(1/2) / [1360*(dp*po)^(1/2)]
q = free gas per hour, standard cubic feet per hour (Cu.ft/h)
SG = specific gravity of flowing gas gas relative to air at 14.7 psia and 60oF
T = flowing air or gas temperature (oF)
pi = inlet gas absolute pressure (psia)
dp = (pi - po)
po = outlet gas absolute pressure (psia)
RE: Compressed Air CFM
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RE: Compressed Air CFM
I'm just trying to find the simplest way of using that 120 psi compressed air line to achieve my required air changes.
RE: Compressed Air CFM
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RE: Compressed Air CFM
And is critical flow the same as choked flow? Because from what I understand about choked flow the volumetric/mass flow rate can be controlled independently from the downstream pressure and temperature.
RE: Compressed Air CFM
This does result in critical flow through the choke nipple, but so what? In spite of what has been said above, critical flow is a very precisely understood thing. With a known orifice and a known upstream pressure and temperature, critical flow has exactly one mass flow rate regardless of downstream pressure (until downstream pressure increases enough to stop the critical flow).
With a properly sized hole (around 0.056 inches) you will get exactly the flow rate you want without excessive noise or a lot of complexity. Just set a cheep Big Joe regulator to 65 psig and drill a hole in a pipe-sized billet and you'll have it. There is nothing magic about the 65 psig. I would play with the upstream pressure until I got to a hole size that matched a standard drill size (e.g. at 13 psig you would get your flow rate with a 1/8 hole). Big Joe regulators are regularly used to cut pressure from over 100 psig to under 20 psig for burners without problems.
David Simpson, PE
MuleShoe Engineering
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RE: Compressed Air CFM
Q = Cv * N1 * f_dP * ((P1^2-Pd^2)/Z/G/T)^0.5
Should't for non choked flow- adiabatic(and low mach no), the pressure drop be proportional to the square of flow?
Regards
RE: Compressed Air CFM
I2I
RE: Compressed Air CFM
I2I
RE: Compressed Air CFM
---KenRad
RE: Compressed Air CFM
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