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Re: Cv and valves
> Date: Wed, 13 Sep 2000 14:35:01 -0700
> From: Dave Gomberg <gomberg at wcf_com>
> At 03:48 PM 9/13/00 -0400, Bob asked:
> >Micro-mite Forged Metering Valves: 1600 Series Cv = 0.0008
> >Is this Cv rating sufficiently low that it could be used straight off
> >the CO2 tank valve w/o a regulator?
Dave does some Rocket Science and concludes:
> For gases: CFM=22.7*Cv*sqrt((delP*P1)/((460+T)SG))
> Running straight off the tank, P1=delP=800, so it works out like
> Cv=3.6x10**-8 or .000000036
> That's very very small. You won't get there with a needle valve that you
> can afford. And it wouldn't be thermally stable anyway.
A. I would assume (perhaps incorrectly, as I am wont to do) the Cv spec for a
valve correlates to the the maximum flow the valve will support. For instance,
the Cv of a Nupro M series valve is 0.03 which correlates to a flow of 1.2 cubic
feet per minute at an inlet pressure of 35 psig. I am using such a valve and I
have it adjusted for a flow rate that seems, although not actually measured,
considerably less than 1.2 cfm. So I assume the rated Cv is the max and the
effective Cv gets real small as the valve is closed. So I would disagree with,
but can't conclusively disprove, the statement that "You won't get there with a
needle valve that you can afford." But I will eagerly nod in agreement with the
thermal stabilty thing.
B. I have some anecdotal evidence that such a thing may be possible, although
not recommended. Back when we were trusting young pups, we bought a Dupla CO2
"Starter Kit" which Daleco MBP graciously modified to use a U.S.-threaded 2.5
pound bottle instead of the amusing 160 gm bottle Dupla supplied. The starter
kit was composed of the bottle (no regulator), a needle valve connected directly
to the bottle, a bubble counter, a check valve, a diffuser and the Dupla
Continuous CO2 Test. The needle valve did indeed work although it was tricky to
get "just right" and was thermally unstable but not dramatically so. We used
this setup for a year before graduating to a single stage, dual gauged Air
Products 1102 regulator, Nupro B-4MG2 valved, Eheim canister filter reactored
five pound setup.
C. The needle valved 2.5 pound bottle often ran to empty (Doh!) but never
"dumped", as fas as I know.
George Booth in Ft. Collins, Colorado (booth at frii_com)