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Cv and valves
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?
For gases: CFM=22.7*Cv*sqrt((delP*P1)/((460+T)SG))
where delP=P1-P2
P1= input pressure (psi)
P2=output pressure
SG=specific gravity of the gas relative to air=1.0
T=temperature (F)
P1 is typically about 15, P2 about 2, delP about 13, SG about 1.5, T=70,
CFM (at 1L/day)=1/(30*24*60),
so Cv=1/(30*24*60*22.7*sqrt(13*15/530*1.5)) or about 1.8*10**-6, .0000018
THE CRITCAL POINT HERE IS VERY LOW FLOW.
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.
Just got a price quote on a Hoke valve for giggles. $107 for the
valve. Then you need Gyrolok fittings to get out of their proprietary
connection system, those are a couple of bucks a piece. Then you would
need an adapter from NPT to barb, those are a buck or so each. Then you
would need a barb and barb to get to the right size, that's another
buck. And a short piece of 1/4" hose to connect the big barb to the
reducer. And in the end you would have a great valve jury-rigged to
work. Not what I would call workmanlike. You see, our flow rates are
way, way, way too low to be anything like a close match to what is offered
commercially. But Hoke makes a great valve.
--
Dave Gomberg, San Francisco mailto:gomberg at wcf_com
NEW Planted Aquaria Magazine: http://www.wcf.com/pam
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