CYA for Salt Chlorinator Pools II

[mas985]

After thinking about this a bit more, I wondered what the actual chlorine ppm would be leaving the cell given the production rate of the cell. To find out, I started with the maximum production level of my Aqualogic cell which is 1.45 lbs/day of chlorine (assumes 100% setting running 24 hours/day). Thus the production rate in GPM would be;

1.45 lbs/day * .125 gallons/lbs * 1 day / 24 hrs * 1 hr / 60 min = 1.26e-4 GPM

For my pool, the water flows at about 90 GPM (estimated from pump head curve, psi and suction in. mg.) so the maximum ppm leaving the cell would be 1.26e-4/90 or 1.4 ppm continuously. This is of course is added to the existing ppm but still it is not exactly shock level.

So if it is true that the cell does indeed have a very high level of chlorine in the cell, then the cell must either produce chlorine that is very unstable and reverts back to salt shortly after production and/or, very little of the water going through the cell is actually between the plates of the cell (ie. reduce volume of water, increased CL ppm).

Looking at the interior of my cell, I noticed that the plates only take up about half of the area of the cell which means about half or a bit more of the water never touches the cells but goes around them. Still this would only double or triple the ppm level in the cell.

So the only plausible explanation for a very high chlorine levels in the cell is that the chlorine gas is very unstable, which I think someone has already pointed out. It would seem that most of the chlorine reverts back to salt before or shortly after exiting the cell.

Therefore, the only explanation I can gather for the need of high CYA, is that during this temporary phase of high chlorine, the high CYA may prevent some of this unstable chlorine from reverting back to salt. So the CYA may have two jobs, one for UV and another for stabilizing the chlorine in the cell.

Also, from what I understand, chlorine has two jobs. First is for sanitation which kills all of the nasty bugs in pool water. From what I have read, this does not require a very high residual to accomplish so even 60-80 ppm CYA can usually accomplish this fairly quickly or quick enough.

The second job of chlorine is oxidation which removes all of the other stuff such as sweat, suntan oil and algae which is a big one. Higher levels of chlorine are required to accomplish this. However, it is not necessary to remove this stuff in seconds or minutes but is sufficient to remove in hours or even days.

So it would seem that the high CL in the cell does much of the oxidation, which can be accomplished over hours or day, and the CL residual is responsible for killing the bugs, which can be accomplished with a low CL and a fairly high CYA.

The only anomaly to this whole theory is that my residual chlorine did not increase with an increase in CYA which I would have expected. I am willing to bump it up to 70 just to make sure there is not something else going on.

Sorry for the long thought experiment but I would like to fully understand the SWG process and as Waste has pointed out, so far, we have not seen a comprehensive explanation. This is my first attempt at one but I am sure it has some flaws.

[waterbear]

After thinking about this a bit more, I wondered what the actual chlorine ppm would be leaving the cell given the production rate of the cell.

(Deleted long duplicative quote -- use links if needed Waterbear; don't requote the W...H...O...L...E L...O...N...G thing! PoolDoc)

If your hypotheses about the oxidation is correct then monitoring the CC at lower CYA and then again at higher CYA would be a telling factor!

[chem geek]

I started commenting on appropriate CYA levels in an SWG pool in the CYA / Pool Pilot Question thread and I was pointed to this thread which has been very informative (and has me eat crow).

Obviously with my technical interest I would like to get to the bottom of why higher CYA levels are needed in an SWG pool. Assuming the data on the breakdown of free unbound chlorine vs. chlorine tied up in CYA is correct, then the amount of extra work (time) the SWG needs to be on at 40 ppm CYA is only 12% higher than at 80 ppm CYA with 3 ppm FC (it's a 45% increase in work or time going from 80 to 20 ppm CYA). Since most people see a much bigger jump than that (to maintain the same FC level), either the chlorine breakdown data from sunlight is wrong or CYA is needed in the SWG process to improve its efficiency.

It may very well be that what was said earlier on this thread is what is happening. Namely, that rather high localized concentrations of chlorine (HOCl, OCl- from dissolved Cl2) are generated and that with CYA these high concentrations get tied up quickly before they have a chance to breakdown. I find this somewhat surprising as a decent flow rate should dilute the concentrations rather quickly, but physical flow rates are usually not nearly as fast as most chemical processes (more on that later in this post).

The best way to test this theory would be with a pool that is covered to minimize the breakdown from sunlight to remove that factor from the experiment (also try not to use the pool during the experiment). Since the chlorine demand would be lowered, the chlorine generator cell should be set to run less frequently (lower percentage) to maintain a constant FC level, but the cell should be run at the normal power level used in a typical uncovered pool. Start out with a CYA level of 40 ppm or below -- way below, such as 20 ppm, would be even better. Record what percentage of time the cell needs to be on to maintain a constant FC level (say, 3 ppm). Then add CYA to the recommended range of 80 ppm. Adjust and record the percentage of time the cell needs to be on to maintain a constant FC level. Report back to this thread your results.

The above experiment was sort of done by some members of this forum (and posters to this thread) but I don't believe a cover was used so the variability in sunlight is a problem since this is a huge factor in chlorine breakdown.

I am still puzzled by the prevention of algae in an 80 ppm CYA 3 ppm chlorine pool since that only produces 0.015 ppm HOCl at a pH of 7.5 which is enough for disinfection, but presumably not enough for preventing algae (in Ben's experience). Though some have said that the higher chlorine concentrations in the SWG cell may kill algae, that would not be true for any algae that adhered to the plaster. So maybe the SWG cell kills free-floating algae and that usually that is good enough.

A post from "chem geek" would be incomplete without some technical analysis to scare the bejesus out of 90% of the forum, so stop reading at this point if you're (rightfully) frightened by chemistry.

I actually have the rate constants for the conversion of HOCl into the chlorinated cyanurates (i.e. the take-up by CYA). My rough calculations show two things. First, that a level of CYA of around 50 ppm may be the concentration where the rate of take-up by CYA of incrementally generated chlorine in the cell is equal to the generation rate of this chlorine. This lends support to the theory that the CYA helps the chlorine generation by preventing the buildup of unbound chlorine (i.e. HOCl and OCl-). Second, the incremental concentration generated by the cell is rather small so this build-up theory only works if there is a very high local concentration at the generation site (the electric plate) and that is certainly plausible. However, this whole business about superchlorination is a bit overblown since the calculations show that it would take nearly 100 turnovers of the pool water to shock it with the equivalent of 10 ppm. On the other hand, a shock of 1 ppm takes only 10 turnovers and 0.1 ppm only one. Therefore I don't believe the theory of the "supershock" is the appropriate one because not enough water is exposed to 10 ppm or higher levels of chlorine, but free-floating algae even with Ben's chart is prevented by 0.05 ppm HOCl.

Richard

[Poolsean]

Richard,

I can tell you from personal experience, I was the R & D director for AutoPilot at one time, that the concentration of chlorine within the cell is WAY in excess of 10 ppm. It was more along the line of 80 ppm, as determined by MANY dilutions of the water sample directly from BETWEEN the electrodes.

Remember, chlorine gas is produces with quickly gets absorbed by the water yielding a purer sodium hypochlorite. This certainly is sufficient to treat the water at a shock dose, regardless of the flow rate through the cell. However, we've found that by slowing down the water flow, you allow more "contact time" to process the water and rid the combined chlorines, and kill the "bad stuff". Basically, it increases the efficiency of the electrolytic process and killing effect.
Sorry I'm not a chemist to be able to present ionic formulas and osmosis reactions, but being a very hands on manager at AutoPilot, I've seen, repaired and resolved many salt generator issues as well as pool water chemistry related issues (with and without salt sytems). I'm no expert by any means, but I do have experience and a little (dangerous) knowledge to get by.

[chem geek]

Sean,

I wasn't clear with the amount of chlorine levels in the generator. I was talking about the normal operation that generates the incremental chlorine for the pool and referring to what some people were claiming was a natural superchlorination in the cell during this process. I was also referring to the entire cross-section of area that the water flows through and not just an area near the plates (which has incredibly high concentration of chlorine).

I believe what you are talking about is a superchlorination mode which runs the cell at a much higher power level. If you ran at this level during the duration of one pool turnover, then the pool itself would get to that 10 ppm level. Is this what you are talking about? I apologize for my ignorance about the mode you have for your cells.

Richard

[Poolsean]

No apologies, but thanks for "slappin me in the face" and making me aware that I sounded like a sales guy. That's the last thing I want to sound like here as it irritates Ben if I do.

The normal operation, when you inspect anyone's cell, is for the water to pass through and around the electrodes, or blades. Let's face it, the chlorine is only produced between the blades so the space around them does not get the same concentration of chlorine. However, it is quite high between the blades AND still quite high surrounding the blades too. Oddly, if you test the water coming out of the cell, you're prone to only get 1-5 ppm higher than what's tested in the body of the pool. You may see a similar result if you take a water sample from the closest return line to the pool equipment.
Now, the design of the cell is going to be rather different from manufacturer to manufacturer. If you inspect the various designs, you'll see different dimensions of blades, spacing between the blades, solid vs mesh, % ruthenium oxide coatings on the blades, and power (milliamps/sq cm) to the blades. So when you asked for more details, it's not easy...besides, most of this is proprietary information.

What I am taking about is not just under superchlorination mode, this is under normal operation. Besides, most Superchlorination mode does not increase the power to the cell, it simply extends the time the cell is energized. I believe there is only one manufacturer that claims to increase the output to 125%.
Most manufacturers cycle their cells on and off, during the day. Some run the cell continually, for a % of the pump run cycle (I'm not sure how this is determined without having to enter your physical pump run time). Some constantly energize the cell and fluctuate the power to the cell between a low output, up to a high output. Perhaps this last one is the model you're describing? Those are typical of Australian systems that also require higher salt levels to operate (4000 - 6000 ppm).

Hope this helps

[Tredge]

I've been fascinated by this discussion and I'm continually trying to understand.

Are you saying that because the water "between the blades" is 80+ppm chlorine while the water exiting the cell is only 1-2ppm higher than the pool itself that the high CYA acts as a transport of that high chlorine? Sort of a way to get it into the pool?

That logic makes sense to me.....what Doesnt make sense is when Ben's best guess chart comes into play.

The chlorine in the pool, regardless of where it came from, has the same chemical properties as any other chlorine.

In my experience, when I ran my SWG at 80ppm CYA, my chlorine maintained at a lower power setting....as suspected. However, Algae on the walls and in the pool ran out of control and I fought it all season.
Ever since I lowered my CYA to 35-40ppm, I've not had to shock my pool once and haven't had a hint of "slimy walls" or the onset of algae.

There could be other factors at play since my tests were over the course of 2 seasons...