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Re: Needle valves in CO2 systems

> Date: Tue, 02 Jan 2001 07:24:01 -0800
> From: Dave Gomberg <gomberg at wcf_com>
> Subject: Needle valves in CO2 systems
> 
> I have heard many on this list claim great value for the use of a needle 
> valve in a pressurized CO2 system.  Attempting to check it out, I was quite 
> disappointed.
> 
> First some numbers:
> 
> Flow L/day               1     = 2.315E-05CFM
> Input pressure, psi     30
> Output pressure         15      15 delP
> Specific Gravity         1.5       Re air=1.0, CO2=1.5
> Temperature, F          70
> Cv=CFM/(22.7*sqrt((p1*delP)/((460+T)*sg)))
> Cv=     1.355E-06  =  0.0000013554
> 
> Now assuming we set the regulator for 30psi and want 15psi out of the 
> valve, with a flow of one liter per day, we need a valve operating at a Cv 
> of .000001
> 
> I have not yet found such a valve. 

snip...
> 
> Does anyone else have a proposed valve for use in this application?
>

Well, Dave, I didn't run any numbers, but I have found that some junk Fabco
needle valves I had laying around work rather well at about the numbers you
suggest. [Model FC-32, Fabco-Air, Gainsville FL, to be exact.]

It may not help the group, much, because I have no idea if they are
available. I bought a whole bunch for about $0.50 apiece several years ago
at a local industrial surplus house. [I used to do a lot of air bearings.]

They are compact, with a brass body about 1" square and 1/4" thick. Inlet
and outlet are standard 10-32 threaded airline bayonet fittings, and about
half were the small size for the final tubing run to the tank. :-) A knurled
nob allows me to adjust a 25-30 psi inlet pressure to allow about 2 or 3
bubbles/second into my Eheim check-valve diffuser combos. One has to be
careful to run the CO2 the right direction, as they do have some kind of
reverse-flow bypass if you get them backwards. Big arrow on the side makes
it easy. :-)

The small flat body will also make it easy to attach to a
temperature-regulated aquarium wall, if they prove to be temperature
sensitive (most are, somewhat).

I suggest you forget the numbers, and look around for a source of the tiny
needle valves used throughout industry for small pneumatic controls. Fabco,
Clippard, etc. all make such things, but I doubt if they spec what you want.
You'll have to do what I did. Try it out, and try to make it goof up. 

I left for my Xmas trip, before getting a chance to see how my setup handles
an honest "end-of-tank-dump" situation. Just diddling the regulator pressure
convinced me it will handle it OK, tho.

You buy the sushi, and I'll tell you where the better surp stores are in the
South Bay Area. :-)

Wright


-- 
Wright Huntley, Fremont CA, USA, 510 494-8679  huntleyone at home dot com

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