Re: Avoiding Dumping CO2

[Wright Huntley <jwwiii at pacbell_net>]

Date: Thu, 23 May 2002 15:17:11 -0500
From: zxcvbob <bob at area51online_net>
Subject: Re: Avoiding Dumping CO2


> 
> 
> I just love it when someone decides to show off their ignorance by
> ridiculing something they don't understand.  It elevates the ignorance
> (which is excusable) to arrogant stupidity (which is not).
> 
> Best regards,
> Bob
> 


Well said, Bob, but...

Part of the problem is a total failure to understand fluid flow, which 
is a truly complex phenomenon. Part of the problem is using terms like 
"sonic," which is somewhere between quite wrong and poorly defined. [I'd 
tend to buy "supersonic."]

The real process is the transition from laminar flow to turbulent. The 
exact cause of the turbulence can be caused by many things, but the 
velocity of sound isn't particularly important in most cases. The fluid 
velocity, density, the surface over which it runs, Reynolds Number <VBG> 
and other things are more important.

At risk of oversimplifying a complicated subject, let me try to describe 
the CO2 flow in the following way.

Laminar (smooth) fluid flow tends to be fairly linear. If the pressure 
(regulator psi) increases by a given proportion, the amount of volume 
flow increases by the same percent through any orifice with laminar flow.

Double the psi and the number of bubbles per minute doubles. Well, 
roughly, anyway.

The absolute last thing you want in your *flow-rate* regulation system 
is laminar flow.

The amount of energy being expended to force the volume of fluid through 
the orfice is linear with pressure until turbulence starts (laminar flow 
stops). At that flow level, the turbulence eats great gobs of energy, 
and the increases in pressure beyond that feed the turbulence (make it 
greater) with only minimal increase in flow rate. Supersonic velocity 
*can* be the cause of the turbulent flow, but it is not in the vast 
majority of situations I have encountered.

Excess turbulence heats the valve and makes things unstable if the valve 
is not stable to begin. There are good valves and bad valves. Test.

Once flow is turbulent, the orifice is essentially a *flow-rate* 
regulator with only minor sensitivity to inlet pressure. Trying to 
adjust the flow with that inlet pressure is not a really good idea, 
IMHO. Set the inlet pressure to the needle valve to be plenty high 
enough to assure 100% turbulent flow (typically 10-30 psi), and set the 
needle valve (now in full turbulent flow) to set the rate of CO2 
injection into the tank.

OK Bob, now that I have contributed my own "arrogant stupidity" I think 
I'll put my:

Flame Shield <UP>

Wright


-- 
   Wright Huntley -- 650 843-1240 -- 866 Clara Dr. Palo Alto CA 94303

               The truth that is suppressed by friends
                 is the readiest weapon of the enemy.
                                 -- Robert Louis Stevenson