Tank Gas Management ... a new approach

[pat w]

Edit: Design updated and more detailed writeup in Post #18

I've been following two topics of late; the discussion begun by Tom on the gas concentration differences between canister system and wet/dry systems and their relative characteristics regarding O2:CO2 ratios, and the regrettable news about Gerry gassing a large number of his fish due to an understandable mistake. These and general nagging concerns of my own over the “gas” balance in my own tank prompted this exercise.

First let me state that this is by no means a condemnation of the methods currently in use in planted tank gas management. It is however an attempt to add to the current dialog and explore possible improvements and perhaps a new direction.

I've always felt the the meticulous metering of CO2 was a possibly weak aspect of the hobby. The very length of the Dual Stage Regulator topic is testament to the attention this particular area attracts. The topic is littered with very creative methods of achieving levels of control that would not otherwise be available without the outlay of large sums of money. The fact that the normal cost would be out of the reach of the average person says to me that the difficulty of metering gas into our tanks with the level of control needed is prohibitively complex and perhaps a new approach may be indicated. In searching for a better method I explored rotometers as perhaps a better way. There was some promise, but the meters which would give control and feedback at the very low flow levels were again too costly.

Had I been looking for a low water flow solution there were a number of comparatively economical rotometer choices. So I asked myself … What could be done to move away from trying to control very low gas flows and go instead to controlling low or even moderate water flows? These are some of the items I came upon:

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1.The water metered into the tank would need to have a CO2 concentration that was predictable and constant.

2.It should be non-dependent on things such as end of tank dumps and needle valve variations to allow for the use of less expensive and less precise equipment.

3.It should also assist in the balance of night time O2 levels.
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The next post will be an offering (for discussion) of a Tank Gas Management Reactor that I'd like to discuss as a possible direction.

Pat

[pat w]

Say hello to Frank, my new design for a Tank Gas Management Reactor.



The general idea is to circulate water while feeding enough CO2 to take it as close to full CO2 saturation as possible. I attempt to achieve that with the generation of three flow paths.



First the Primary Saturation Flow, which is begun by the Circ Pump and is passed back down to the bottom level to feed the pump again; very simple. Directly out of the pump the water is channeled into a nozzle into the narrower venturi tube. A gas feed line is inserted much like a mazzei to introduce CO2 or O2 into the flow at point “B”. A filter sponge or similar substance would be placed into the flow path to block the bubbles of gas from passing into the lower level insuring a solid water stream into the venturi.



Second is the Induced Saturation Flow. This is the flow of water drawn from behind the venturi and should be restricted to the upper level with the clear PVC. This should be the only path for undissolved gas bubbles and the bubbles should circulate continuously. A disc of fine screen material might be placed in the path to chop the bubbles into a fine mist to promote easy circulation.

Last is Pass Through Flow which is induced by whatever pump you use to pump water out of your tank, through the reactor, and back to the tank. The flow will, in actuality, be a mixed pattern as it is mixed with the water circulating within the reactor itself. Before anyone raises the question; yes the flow will eventually make it out of the reactor. I've seen many similar systems at work where flow is passed through a system while a separate pump recirculates and mixes the contents of the system. If you use a filter to supply this flow you will need to supply a additional bypass path with a another ball valve to restrict bypass flow and force the desired MAXIMUM flow through the reactor. This MAXIMUM will later be metered down to regulate the gas delivery to the tank.



CO2 is supplied by the usual CO2 tank/regulator/solenoid however the demands on these components will be greatly reduced. It is delivered to the reactor in relatively brief bursts that will produce a sizable CO2 blanket at the top of the large clear vertical PVC tube with the “A” on top. The “A” is a float switch which takes over the job of switching the CO2 solenoid on the regulator. As CO2 is used the volume of gas in the collection tube will reduce and the water level will rise. The float switch will then turn on the solenoid to replenish the blanket. No more end of tank worries from less expensive regulators and the need for a needle valve is greatly reduced or eliminated entirely. At night the 3-way valve is switched off and instead of CO2, air is injected into the venturi to supply the needed O2. (not shown is a needed check valve) The size of the CO2 collection chamber will allow for the measurment of the actual CO2 delivery rate in milliliters per minute by mesuring the rate of rise in the water level.



The final flow back to the tank is regulated by any flow restricting valve and then to any of several low cost flow meters.

More to come on theory and operation.

Pat

[pepetj]

Can't wait to read in extensive the rationale of your interesting proposal. How long have you had a prototype working? What have you measured? If I get your idea right it means that your proposed system may noticeably extend the life time of our pressurized CO2 cylinders meaning longer service time per cylinder before refill is needed?

Pepetj
Santo Domingo

[pat w]

pepetj,

No prototype as yet. Only theory and design, so far. I'll be getting the parts together over time.

Can't say I'd expect any improvement in economy over a more conventional reactor design. The main focus was to move away from regulating low levels of gas flow and trying to use fluid flow instead.

More later,
Pat

[pat w]

Basic Theory of Operation

I’ve based my design on some premises that may or may not be valid. Some are dependant on others so if one of the earlier ones fails there will be a sort of domino effect.

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1. The venturi will draw gas which is not under pressure into the side port. This one is pretty safe as it is used widely in other areas.

2. As the gas to water ratio of the Induced Saturation Flow increases this gas draw will decrease until finally it stops completely while there is still a mix of water and gas in the flow path. This is the big one. If this fails the whole deal is off as eventually the induced flow would be in a state of vapor lock and there would be no difference between this reactor and any other.

3. The foam at “C” will stop the flow of gas to the inlet of the pump but not reduce water inlet pressure enough to cause cavitations. This may be addressed with some sort of baffle system that would rely on the floating of the bubbles and the sinking of the water, but not so much if the bubbles are chopped up too finely.

4. The Induced Saturation Flow will continue to dissolve gas until ‘saturation’ is achieved (Assuming that Pass Through Flow is closed). This should reduce the gas to water ratio and allow more gas to be drawn in until item 2 comes into play.

5. When Pass Through Flow is open an equilibrium will be obtained between the saturation level and the gas to water ratio; dependant on the Pass Through Flow Rate.
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As the system runs with Pass Through closed (or the supply pump turned off) there should be an increase of gas in the Induced Flow area, but as the gas levels here rise the compressibility of the gas in the mix should reduce the suction allowing less and less gas in until it eventually stops. This assumes that there will be none of the gas entering solution. As gas is dissolved, the gas in the mix will decrease to a point that will allow make up gas to be drawn in.

Now introduce Pass Through Flow. The new water in the system will reduce the gas to water ratio and will be at a lower saturation level allowing for more of the gas to enter solution and subsequently more gas being drawn into the venturi to make up for the loss. The natural tendency of the system to move toward equilibrium should cause the rate of the gas drawn into the venturi to be exactly the same as the rate the gas is dissolved and delivered via the outlet to the tank. This could be tested by placing a bubble counter on the outlet of the CO2 collection tube and observing the action.

A manual switch could be placed in parallel to the float switch in the CO2 collection tube and used to pre-charge the tube with an excess of CO2. This would allow the user to time the rise of the water level in the tube. This should give a solid metric on the amount of CO2 being delivered to the tank as opposed to simply a rate.

At night the 3-way valve will cycle and room air will begin to mix in the induced flow. In a short time the CO2 should be well below potentially hazardous levels. Cycling back just before lights on would bring CO2 levels back up.

Comments and/or Critiques Please ...
Pat

[pat w]

Float switches; nice one for $11.24
http://www.mcmaster.com/#liquid-level-switches/=9fit3g

3-way valve
http://cgi.ebay.com/1-4-Electric-Sol...item43a21816a0

And if you have a smaller tank and a need to go with a lower Pass Through flow rate than 50 GPH Dwyer Visi-float has you covered, including the metering valve, for $46 and change in brass; $56 in stainless which replaces the Tom Aquatics flowmeter and the ball valve.
http://www.gogenlab.com/products/pro...wmeter-4-scale

Pat

[andrei_0]

I think the water is more likely to flow like this:CO2-A.jpg

[pat w]

I think the water is more likely to flow like this:CO2-A.jpg

I guess I needed to do more than just describe the venturi operation. I plan to build up a nozzle that will direct flow into the opening of the narrower section causing an induced flow in the direction previously indicated.

Perhaps this will help.



The nozzle will be narrow enough to fit inside the "T" and allow a good amount of flow around it to be drawn into the narrow section of pipe. This should cause the flow at the other end to split; some going down to make up for the suction of the pump; the other turning up to makeup for the flow induced by the venturi flow.

Thanks for the comment though as now that I see the nozzle in place it looks as if the gas feed line might be better placed in the reducer at the end of the nozzle.


Pat

[pat w]

Here's another design. Same idea, push the CO2 concentration in the reactor as high as possible and then regulate the level in the tank by controlling the water flow rate back to the tank. The CO2 supply can be in a separate reservoir as long as there is an expansion path for the water back to the tank and the reservoir is lifted so the water preasure doesn't force the CO2 out and into the reactor. It's important that the venturi draw regulate the flow of CO2 into the reactor. I'm still working on this part.

The image below is just the saturation reactor without the CO2 reservoir.

[Left C]

Would a flow rate indicator like this Fisher 10A2235A model be useful to you? It can measure from 0.05 to 0.60 gpm of water or 0.10 to 1.30 scfm of air. There are over 10 of them and they are not very expensive. http://0041f2c.netsolhost.com/Ratosight10A2235A.pdf
http://cgi.ebay.com/FISCHER-PORTER-R...item56325e3666