Cv and acceptable leakage of air

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  • #18821
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    We have a butterfly style damper for air service on an industrial system that was certified by the manufacturer as ANSI/FCI70-2 Class IV, which allows for a leakage rate of 0.01% of damper capacity. The Cv of the damper is 32042 and if we were to do a water test, at 1 psi differential, we would expect the acceptance criteria to be 0.01% x 32042 = 3.2 USGPM. However, we will be doing our test with air and are somewhat confused as to how to determine the acceptance criteria in SCFM of air. It has been debated that Cv is unitless so as long as we are considering standard conditions that the same logic applies as shown with water earlier. That is, 0.01% x 32042 = 3.2 SCFM. When we test this logic using a Cv calculator online, with 32042 as an air flow input value at 1 psi differential, at standard conditions the resultant Cv is not 32042. We are trying to do the test and write our test procedure and any help to clarify our confusion would be appreciated.

    #20898
    admin
    Keymaster

    One way to calculate flow is to use the equation Q=22.48 x Cv x sqrt{(P1-P2)*P2/T/G} where : Q = flow rate @ 1 atm, 68 deg F, & 36% rel humidity, in scfm Cv=dimensionless flow coefficient P1=upstream pressure in psia P2=downstream pressure in psia T=upstream temperature in deg rankine (usually 528) G=specific gravity of air at one atmosphere, 68 deg F, & 36% rel humidity (usually = 1) If I use P1-P2=1 psi, P2=14.7 psi, T=528, I get a flow of 120,182 SCFM. Times this by 0.01% or 0.0001 the leakage rate would be ~ 12 SCFM. I hope this helps.

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