CYA for Salt Chlorinator Pools II

I'd like to bring this thread back to its original topic so I'm putting my response in a new Saturation Index thread where I hope to hear from you and Ben (and others).

Richard

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Originally Posted by tphaggerty

The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

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I hope this doesn't duplicate something said farther below, where I haven't read yet.
[Edit]Sure enough, it was beat to death. Sorry to waste the bandwidth[/Edit]

I have a SWG and have seen CC chlorine levels above zero. It has been after a swim party or other event that put a good load on the pool. My solution was simply to add some bleach, and let things circulate over night.

This does not deny that some form of superchlorination may be occurring in the cell, but it also shows that water not having gone through the cell recently is still at risk.

[Edit]I think I will try and up my CYA based on what I read here. I was keeping it lower because I didn't understand how it would help. Now that I can grasp a reason why it is a good thing I will follow through.

In reading all of this I had a couple of thoughts.

First, my aquarite turbo cell has what looks like plastic mesh around the plates. This should spit out lots of vortices to both slow the water down and mix it a lot near the plates, suddenly that makes sense. High turbulence should move the chlorine away from the plates more quickly. Clever mechanical tricks are always a good thing.

Second, someone with a two speed pump should be able to test whether slower flow is more or less efficient somehow. This would not be an easy test since fairly accurate estimates of flow rate would be needed, but worth considering.
[/Edit]

Sentient

My comments in bold.

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Originally Posted by Sentient

I have a SWG and have seen CC chlorine levels above zero. It has been after a swim party or other event that put a good load on the pool. My solution was simply to add some bleach, and let things circulate over night.

This does not deny that some form of superchlorination may be occurring in the cell, but it also shows that water not having gone through the cell recently is still at risk.

I would guess as much though with sufficiently high levels of chlorine (independent of whether there was an SWG) the CC should get reduced between a somewhat slow breakpoint and a somewhat faster exposure to sunlight. At least that's the current thinking (subject to change, of course). Ironically, the typically higher CYA levels and somewhat lower chlorine levels found in most SWG pools means that achieving breakpoint takes even longer outside of the salt cell (that is, it takes longer to get breakpoint from the chlorine in the pool because the HOCl levels are relatively low). Fortunately, the salt cell itself has high chlorine levels so eventually after many turnovers most of the water has gone through areas of high chlorination to achieve breakpoint. Again, my speculative theory.

First, my aquarite turbo cell has what looks like plastic mesh around the plates. This should spit out lots of vortices to both slow the water down and mix it a lot near the plates, suddenly that makes sense. High turbulence should move the chlorine away from the plates more quickly. Clever mechanical tricks are always a good thing.

Yup, creating all sorts of vortices and somewhat turbulent flow (what I would call "controlled" turbulence since normal uncontrolled turbulence causes all sorts of problems) helps sweep away the generated chlorine away from the plates. Sounds like a good design in your system.

Second, someone with a two speed pump should be able to test whether slower flow is more or less efficient somehow. This would not be an easy test since fairly accurate estimates of flow rate would be needed, but worth considering.

I would suspect that the SWG manufacturers designed their systems for typical flow rates and that slowing down the flow would result in lower efficiency. Anyway, I think we've all pretty much concluded that SWG systems have a lot going for them regardless of the specifics of the (electro)chemistry that is involved.

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Slower flow provides better efficiency. Think of it as longer contact time of the combined chlorines and the "shocking effect" of the salt chlorine generator cell. This is evident in the Pool Pilot's manifold assembly, the Hayward TurboCell body, the various other 2"inlet and larger body designs.
The larger body allows the water flow to actually decrease, thus giving you the slower flow - Higher Efficiency.
The problem is that at low flow, you need to make sure that there is still sufficient flow to activate the flow switch, or evacuate all the air with Gas Trap systems.

Hope this helps.

I think we misunderstood each other.

When I said that slowing the flow would lower the efficiency, I meant slowing it down from the pump side with the already slowed down flow that is in the cell itself. In other words, the cell has already slowed things down for higher shocking effect and optimum efficiency with higher CYA to bind the chlorine, but if you were to slow down the pump a lot, then wouldn't there be a buildup of chlorine at the plates because there wouldn't be enough flow to sweep it away?

In other words, I figured that the salt cell had already designed into it the proper amount of slowdown for high efficiency and having a flow that is much higher (wouldn't build up as high a localized chlorine level to shock) or much lower (would build up too high a chlorine level near the plates to be efficient) would be less optimal.

Richard

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Originally Posted by chem geek

I think we misunderstood each other.

When I said that slowing the flow would lower the efficiency, I meant slowing it down from the pump side with the already slowed down flow that is in the cell itself. In other words, the cell has already slowed things down for higher shocking effect and optimum efficiency with higher CYA to bind the chlorine, but if you were to slow down the pump a lot, then wouldn't there be a buildup of chlorine at the plates because there wouldn't be enough flow to sweep it away?

In other words, I figured that the salt cell had already designed into it the proper amount of slowdown for high efficiency and having a flow that is much higher (wouldn't build up as high a localized chlorine level to shock) or much lower (would build up too high a chlorine level near the plates to be efficient) would be less optimal.

Richard

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So in a two speed system, one could theoretically expect higher "cell shock" at low speed, or more overall chlorine getting into the pool for a given run time and duty cycle at high speed. I expect the difference is probably not very noticable by the average user. They will still just set the duty cycle control maintain the desired chlorine level as they always have.

In any event it is all very intersting!

Well, I believe I have figured out why the salt cell seems to do a much better job of superchlorinating a portion of the pool water -- both in disinfecting (killing bugs) and in oxidizing (getting rid of combined chlorine) than simple addition of chlorinating liquid or bleach. When I described earlier how adding chlorinating liquid or bleach was similar to what was done in a salt cell in that there are high levels of chlorine as the concentrated chlorine (125,000 ppm for 12.5% chlorinating liquid) gets diluted in the pool water, I didn't account for the large differences in pH between the two methods. The pH of chlorinating liquid or bleach is very high and though this gets reduced as it is diluted, it is still somewhat high until the dilution is quite broad. This means that most of the chlorine is in the form of the hypochlorite ion (OCl-) which is a much less powerful oxidizer and disinfectant than hypochlorous acid (HOCl).

If I run through some dilution calculations (using standard pool values of pH 7.5, TA 100), I find that the disinfecting chlorine level even with no CYA present only reaches a peak at a dilution of 195:1 and a disinfecting chlorine (HOCl) level of 14.2 ppm. With 30 ppm CYA, the peak is reached at a dilution of 112:1 and a disinfecting chlorine level of 7.1 ppm. At 80 ppm CYA, the peak is reached at a dilution of 75:1 and a disinfecting chlorine level of 3.5 ppm.

In other words, the strong alkalinity of chlorinating liquid or bleach severely cuts down its disinfection and oxidation ability. Of course, the hypochlorite ion (OCl-) is very high (my earlier posts' calculations of dilution would apply mostly to this hypochlorite ion concentration), but is at least 30 times less effective an oxidizer and disinfectant (and possibly worse than that).

Contrast this with what goes on in an SWG cell. At the plate where the chlorine gas is generated, the chlorine gas combines with water to form hypochlorous acid and hydrochloric acid so is a very strongly acidic process. This means that almost all of the chlorine is in the form of the powerful oxidizer and disinfectant hypochlorous acid (HOCl). It is only in the area in the middle between the two plates where the alkalinity from the other plate combines with the acidity to have net alkalinity similar to chlorinating liquid. So, near the acid environment of the chlorine generating plate, water will indeed be exposed to very high levels of disinfection and oxidation -- i.e. will be superchlorinated. Of course, this will be a small subset of the total water flow through the cell, but that is still much better than what happens when simply adding chlorinating liquid or bleach to a pool.

So to simulate the salt cell through adding chlorinating liquid or bleach, one would have to add acid to lower the pH of the pool, then add the chlorine, then add base to raise the pH back up. This is obviously impractical and would swing the pH too much and have other side effects. (Also remember that you never want to add concentrated acid to concentrated chlorine as that produces large amounts of chlorine gas which is quite poisonous).

So that explains why the SWG pools seem to require less FC than non-SWG pools, at least for the pool water that gets circulated through the cell. There are still some users' pools that seem to need higher levels of FC (say, 5-6 instead of 3) to prevent mustard/yellow algae, but that may be due to biofilms stuck to pool surfaces. Regular (once a week) brushing might help to force algae and other microorganisms from pool surfaces into the general body of pool water that will eventually go through the superchlorinated portion of the salt cell. However, if growth rates of algae are high, then even regular brushing might not keep up with such growth rates so the higher chlorine levels in the body of pool water would be the only way to keep the algae at bay.

Richard

Novice here, so please have a little patience with my question. I have a new salt water pool, approx 27,500 gallons, in-ground, with Goldline/Aqualogic equipment. The problem I am having is my Free Chlorine level has been practically zero. PH and all other levels are fairly normal.

After intial shocking using the chlorinator, I ran the CYA at about 30ppm. I used bleach a couple times to get the FC level up, but now have started to rely on the chlorinator.

After hearing that salt water pools require higher CYA, a couple days back I added stabilizer (using tied up socks in the skimmer to desolve the stuff). Today the CYA read 70 ppm (where I absolutely cannot see the black dot.) But my FC level is still pretty much zero.

Do I need to wait a couple more days for the CYA level to take effect, or is there something else I need to do. Chlorinator level of the spa is 10% and pool is at 50%. I'm thinking I may need to shock again. Weather here has been cold, with some rain.

Any suggestions or advice will be appreciated.


Mike

I posted some questions/answers here:

http://www.poolforum.com/pf2/showpos...77&postcount=3