Designing new salt system - any suggestions?

[miro_gt]

Hi all, my first post here as I found this forum last night

I've been in the pool industry over 10 years now so I've seen/worked with/know of quite a variety of equipment. As I got some free time before the summer hits, I decided to produce my own salt system due to the various problems that occur with other brands. In this task I would aim for low cost reliable system, rather than fancy and expensive alternatives.

To give you an example of what the system should perform like, I would use one of the known good systems in the market - the Autopilot controller with one SC-60 cell (their biggest for residential purposes) for up to 50k gallon pool. That combo you can buy for about 1,450 USD as per google shopping search of the lowest prices.

Pros of my system:
- will cost considerably less, at about 750 USD top, including the cell.
- will produce more chlorine, up to twice as much as a single SC-60 cell, as it will have twice the electrode area of a SC-60 cell.
- the cell could eventually and most likely be enough to cope with bigger than 50k gallon pools, or 50k gallon pools with heavy bathing load.
- will not use impulse power supply so will not overheat or experience other power failure.
- will use less electronics so fewer parts could possibly fail.

Cons of my system:
- will not look as good, more like an industrial type metal box or two, with the cell being as 3" PVC pipe with 2" line connections.
- will not be so flexible with control options - most likely there would be on/off switch, output level knob, retractable fuse, and active light.
- will not have feedback so you will have to test the water to see if you need to adjust the output for the desired level.
- will be heavy somewhat, at least more than equivalent controller and one cell from other manufacturers.
- will not be certified but I'll look into that later.

About the cell: I plan on using thick stainless steel electrodes from the highest grade commercially available stainless steel - 316. However, as all metal parts in salty water, it will degrade over time so eventually you'll have to replace the cell with new one in couple years. Based on this fact I decided on using thick electrodes so that the cell would last much longer. To give you an example, I plan on using 1/16 inches thick steel plates, which are a good 3 to 4 times thicker than what you have in a SC-60 cell. I think they use titanium alloy for their plates but those are not that much better than SS 316 and do fail as well over time. On the bright side of the things, my cell would be cheaper as I plan on price of 250 to 300 USD top for replacement cell. My hope is that it will last up to 5 years before needing replacement. SC-60 cells we usually replace on every 2 to 3 years, rarely they last 4.

Let me know what you all think about this project. I plan on making one prototype to use it at some pool over this summer, and if all looks good then I'll mass produce. Being an el.engineer I can make the system more fancy with adding LCD display, more control options, ORP and PH feedback for automatic control, etc. but all that will make it more expensive and the low cost objective will be gone.

Suggestions for design improvements are always welcome

 

[bobodaclown]

I think competition in the industry is always a good thing. Currently I'm running a compupool CPSC48 its for pools up to 60,000 gallons and you can pick it up for $750 and a replacement cell is running about $450. It's a pretty bare bones system. Their cell is stated to have a 10,000 hour life with reversing polarity.

They also produce a generic SC-60 replacement cell.

I'm not associated with any company or such just a frugal owner.

 

[miro_gt]

I'm also not associated with any particular brand in any way, but the service technician from our company went to work in another pool company where they decided to buy bunch of those compupool systems for the pools that they have due to the good price. While it brought them business as the customers got excited for getting salt system, the reality showed bunch of problems with the units - controllers were overheating, and many of them melted the cell where the cable connects to it. In short, he had to replace many of those units, and this happened this past summer.

this was for commercial pools though, I guess for residential ones that system may perform OK.

also this system requires close to 3,000ppm salt to generate chlorine, and this is because the electrodes in the cell are too far apart. The Autopilot can work at salt levels from 2,400 upwards, same as what my system will use since it will have similar distance between the electrodes. Or at least that's the plan. I can make those even closer so that say 2,000ppm salt would be enough, but sometimes calcium buildup (or other debris) can clog the gaps between those and that's no good.

thinking about build ups, I plan to use AC current rather than DC. While I would loose performance the system will prevent such buildups due to the constantly changing polarity of the electrodes, which would also ensure equal wear out. The performance drop would be compensated with the higher surface area (twice as big) and with somewhat more powerful current. That is still within 12 Volts so pretty safe. If the performance drop is too big then I guess I'll go with DC and will build in changing polarity function, but as of now I dont plan to.

also, my output will be regulated with time intervals rather than regulating the power that goes to the cell. What that means is that if you set output at say 10% then my system will be generating chlorine for 6 minutes and then will wait for 54 minutes before it activates again. The same setting for the other systems would mean that those would be working non-stop at 10% of their generating capabilities. Well non-stop until they reach certain level of chlorine in the water, or at least that was their plan, however since they dont have PH control their system would stop generating at high PH thinking that chlorine is high enough, and we all know PH will raise when chlorine is generated from salt. I've seen additional PH controllers sold to connect to Autopilot system to make it work correctly, however those add to the cost and the PH probes last up to 2 years.

my system will be more ugly looking for sure, hehe, too bad many people go for the look of things.

 

[duraleigh]

miro_gt,

Check your PM's (just under and to the left of "troublefreepool")

 

[miro_gt]

I must note that the purpose of this thread is not for me to sell some units over the forum, but rather to gather information of what people would want in a salt system, as well as design suggestions. Also to see if the price would be attractive somewhat for what the system would offer.

 

[chem geek]

Welcome to TFP!

You can't use AC for efficient electrolysis. The frequency of reversal is too high so what ends up happening is that the reactions you are trying to accomplish reverse (undo) themselves too frequently. So when you generate some chlorine atoms to form chlorine gas, the polarity reverses and they go back to becoming chloride ions again. The reason is that when you are generating at your first polarity, the concentration of the products gets very high at the cell plate (i.e. the atoms don't migrate very far) so when you reverse polarity they simply reverse their reactions. Remember that the likelihood of a reaction is based on the thermodynamics while the reaction rate is based on the activation energy (overvoltage), but both are dependent on the concentration of reactants and products.

If you wait longer before reversing polarity, then there is far less buildup of products that can reverse their reactions. Chlorine gas that is generated dissolves into the water and is swept downstream by the water flow. When one reverses polarity in a DC system, there is very little chlorine gas in the water to reverse back into chloride ions and even the amount of hypochlorous acid in the bulk water is very low compared to its level when one has opposite polarity and is generating chlorine. The same is true for hydrogen gas generation.

You also shouldn't use stainless steel for your plates, primarily because they are not an inert electrode and dissolve iron and chromium into the water. I don't know where you got the idea that they last as long as carbon, titanium or platinum. In fact, we've seen several reports of stainless steel corrosion from stray voltages that are far smaller than what you are likely to use for your electrolysis.

As for regulating the power levels, I presume you intend to do that by varying the voltage, but that won't work well since you need to supply a sufficiently high voltage for the electrolysis reactions to start and if you go too much higher in voltage then you can produce too many side reactions that will cut down on your efficiency. If you want to vary your output without varying on/off time (which, by the way, what is wrong with that?), you can have multiple separately connected plates so that you use only some of them. This doesn't give you continuous variability which is one reason why the on/off time is typically used -- it's a far simpler approach. Another continuous variability approach would be to vary the distance between the plates, but that also sounds complicated.

Though you can increase the rate of chlorine output by reducing the distance between the plates or by increasing their surface area, this does not allow you to lower the salt level by very much unless you are willing for efficiency to drop considerably. The lower the salt level, the more oxygen gas will be produced instead of chlorine gas. One can often get away with this tradeoff in a spa where 1500-2000 ppm is enough for a salt cell to still produce enough chlorine for the small water volume, but the only way you can efficiently use low 1000-2000 ppm chloride levels in a pool would be to use specialized coatings or materials that significantly lowered the activation energy (overvoltage) for chlorine and/or raised the activation energy for oxygen.

Also, you should read the thread SWG - Amps Volts and bipolar cells to learn more about the importance of specialized coatings that lower the activation energy (overvoltage) of the chlorine reaction letting one have a lower salt level for chlorine production (or, equivalently, increasing the efficiency of that production at the same salt level to maximize production of chlorine gas over oxygen gas) and that allow for longer plate life. You can also read that thread to learn about bipolar cells (i.e. passive plates) that allow one to have high surface area in a compact volume (basically like having multiple electrolytic cells in series in an efficient manner).

 

[miro_gt]

Great discussion you had in that thread, thank you for posting it

I guess I'm going to switch to DC as the only way that makes sense then. I would try with using impulses at first as if I were to supply stable DC then I will have to fit capacitors in the AC to DC circuit, and in hot weather those might compromise reliability. Then I will work out switching polarity option of some sort. I do know some systems switch on every 4 hours and others on 2 hour intervals based on non-stop output.

The control of my output will be based on time ON and time OFF, as it will be somewhat easier to produce. This means that I will be supplying constant voltage but the current would vary based on water chemical content (besides the salt level itself). It will have sufficient chlorine production nonetheless.

My plan is to feed around 15 to 20 amps to the cell, and this is over twice more than what other systems would feed.

I chose SS 316 based on reports I've seen on that one performing in seawater. The steel itself has incubation period after which it starts to show deterioration. Based on this I increased the thickness of the electrodes so that those would last longer. Also, salt concentration in the pool would be much less than seawater, so all that should increase the cell life substantially.

 

[chem geek]

I chose SS 316 based on reports I've seen on that one performing in seawater. The steel itself has incubation period after which it starts to show deterioration. Based on this I increased the thickness of the electrodes so that those would last longer. Also, salt concentration in the pool would be much less than seawater, so all that should increase the cell life substantially.

Those reports are talking about PASSIVE corrosion, but using the metal as an electrolytic plate will produce ACTIVE corrosion at a MUCH faster rate. It is similar to taking a metal such as copper and connecting it to a positive voltage intentionally dissolve the copper into water. You are mixing up two completely different methods of action. The passive corrosion refers to corrosion from oxygen as an oxidizer attacking the metal in the presence of water, but when you connect the metal to electricity (voltage) as you do in electrolysis, then that is an entirely different matter. If the thermodynamics (including overvoltage) favor turning solid metal into metal ions vs. producing oxygen gas from water, then that is what will happen. That is why electrolysis is done using relatively inert metals or substances such as carbon, titanium or platinum (or boron-doped diamond if one wants more hydroxyl radicals and is not focused as much on chlorine).

Note that not only is iron easily electrolyzed, but so is chromium:

Cr(s) --> Cr²⁺ + 2e^- ..... E₀ = +0.913V
Fe(s) --> Fe²⁺ + 2e^- ..... E₀ = +0.447V

It isn't that chromium doesn't corrode, but rather that it DOES corrode by reacting with oxygen to form a passivity layer of chromium oxide (Cr₂O₃). Using stainless steel as a cathode (hydrogen gas generation plate) is one thing, but using it as an anode (chlorine gas generation plate) is quite another. See this link for a typical complaint when using stainless steel as an anode. The eventual introduction of chromium and iron into the water is not good as these can stain and chromium can be further oxidized (by chlorine or at the anode) into more dangerous forms (i.e. hexavalent chromium, though is mostly an issue only via inhalation while California has a drinking water public health goal of 0.02 ppb). Carbon, Titanium and Platinum are far less likely to corrode and even if they do they are far less likely to cause metal stains. Read this paper that describes how 316 stainless steel is inappropriate for use as an anode in a proton exchange membrane fuel cell. Basically, the passivity layer cannot reform quickly enough to prevent continued electrolytic conversion of the stainless steel.

There is no question that the stainless steel will not corrode as fast as iron or copper, but it will still corrode faster than the complex metals and coatings used in current saltwater chlorine generators. They don't spend all that time and money on those materials for nothing. If it were so easy to use stainless steel without side effects, they would be doing it.

 

[miro_gt]

what about galvanized steel then? Would that be somewhat better choice?

I can order titanium plates instead but that would triple the cost.

while I do understand what you're saying, the practical side of the things is that we still replace cells on every 2-3 years. There would be no difference if after this time the cell is half eaten if it were to be made from SS or not. As far as the metal dissolving into the water I dont think it would be that big of a problem considering the time over which it would do so and the amount that would get dissolved vs. the pool volume (say 20k gallons). I could be wrong though so I'll have to test with my prototype to see how everything turns out.

I do have practical experience with cell dissolving like that actually in a pool that was about 25 to 30k gallons. The system there was ChlorKing and used one big cell, such as in this picture: http://www.saltpoolspecialists.biz/imag ... alunit.jpg . I dont know when it was installed but I used it for 2 years with great success, and on the 3rd year there were visible chunks of the plates dissolved already. At that time it was hardly generating anything but we didn't replace the cell as it was too expensive for the customer, therefore the pool switched to chlorine tablets.

on the other hand I have an autopilot cell sitting right next to me that would also not generate chlorine, yet the plates inside look fine from what I can tell. So far with the special coatings as it would still not work anymore, thus was replaced with new one that performs fine ATM.

so my plan is to make cheap cell that would outperform the competition for its lifetime, which I consider to be about 3 years with heavy usage.

 

[chem geek]

what about galvanized steel then? Would that be somewhat better choice?

That would be even worse in terms of life. Galvanizing puts a layer of zinc over the steel where the zinc corrodes to form zinc oxide and zinc carbonate. If used as an anode in electrolysis, it will likely corrode in a matter of hours to days at the most. Stainless steel will last much longer, but it doesn't do well against corrosion when used as an electrolytic anode in the presence of chloride ions and of course those are needed since you want to produce chlorine.

Have you checked pricing of graphite anodes? They are often used in the chlor-alkali industry which is very similar to what you are doing except their chloride concentrations are far higher as they use brine. Unfortunately, they too will corrode and originally they used lead-containing coatings to inhibit such corrosion but lead, of course, was seen to be problematic. Titanium coated with platinum was then used for a while in swimming pools and then later noble metal oxide coatings. At least when graphite corrodes, it mostly produces carbon dioxide. They might need to be replaced more frequently, but they are relatively inexpensive.

Anyway, I doubt that you will find something better than what is being used today but you can experiment and see what happens. Just be careful about the possible regulatory implications or side effects for the material that you use. Also note that the life of current technology SWG cells has increased and isn't 3 years as you describe. One can readily buy an oversized cell that lasts roughly proportionally longer, but does not cost proportionately more to the pool owner.

 

[mas985]

Without the Iridium Oxide or Ruthenium Oxide coating, wouldn't the preferred reaction be Oxygen instead of Chlorine? I thought the coatings not only protect the cells but also act as a catalyst.

 

[chem geek]

Yes, the coatings can help to produce chlorine vs. oxygen at lower salt levels by changing the activation energy known as overvoltage in electrolysis (acting as a sort of catalyst, as you describe). I don't know the ratios at 3000 ppm if one were to use stainless steel or graphite. The early saltwater chlorine generators developed in Australia used higher salt levels closer to 6000 ppm, possibly because they needed to for efficiency given the coatings/materials used at that time.

This paper describes problems with AC ripple for a DC source and how this problem is a function of frequency so implies the need for smoothing the voltage. Note also that "the anodic corrosion rate of platinum rises from about 1 to 2 µg A^-1h^-1 to 50 or higher µg A^-1h^-1, for chloride concentrations varying from near 30 g l^-1 (seawater) to around 2 to 3 g l^-1." Note Figure 2 that shows how much better the anode potential for chlorine production is when using noble metal and mixed metal oxide on titanium compared to graphite and platinum electrodeposited onto titanium.

This paper gives some history of coating development in the chlor-alkali industry and shows in Figure 6 the electrode potential difference between oxygen vs. chlorine as a function of coating weight percentage. As noted in that paper, "The potential difference is effectively a method of stating that an electrocatalyst must have low chlorine overpotential and high oxygen overpotential for chlorine to be selectively released from brine solution (especially from dilute brines)." That would be something else for miro_gt to look at. Try different electrodes in 3000 ppm salt water and measure the rate of production of chlorine. You may find that you can't generate enough chlorine (i.e. you generate mostly oxygen) without using the more expensive coatings.

By the way, note in this link that Chlor-King uses 5000 ppm. Also, there is no indication that their electrode stack uses stainless steel so I would not assume that.

 

[Strannik]

Trying to reinvent the wheel, eh?

Given how long the technology's been around there isn't much room for improvement left.
If I were you, I'd look into direction of reducing salt content to the one of tap water (similar to the Ecoline range that we supply).

From the experience, perfecting just the coating technology alone takes years of experiments and a fat wallet.
Doing small scale experiments just doesn't work, as the technology you get might not scale very well to allow manufacturing in commercial quantities. And that's assuming that you get your hands on the formula for the coating mix. If you have to figure that one out, might take even more.

 

[miro_gt]

^ not really trying to reinvent something, but I just see a spot where I could fit perfectly, i.e. cheap and reliable for its life time system that would outperform most out there atm for the given price range. One thing that I see is that most controllers out there are using impulse power supplies. I personally think that's a big NO if one is to seek reliability in harsh environments, i.e. hot pump rooms with possible chemical vapors such as from acid/water mixture. Another thing is the relatively small cells that are commonly in use.

so I'll give it a shot and will see what happens.

speaking of the design, I had to upgrade the power supply that I was planning to use due to couple components that would induce voltage drop before the power would get to the cell, thus further limit the current. I'm not to the testing point yet but if needed I could now bump the supply voltage into the ~20V range.

P.S. Thank you [chem geek] for the guidance that you provide, very helpful so far

 

[Strannik]

what's your targeted market? is it commercial or residential?

 

[miro_gt]

the product will go commercial first, though will not be retail sold at the beginning. I just know of couple people in different pool companies that can pass it on.

I'm thinking the chance for good market over the residential area is small as the competition systems are sufficient for small pools with lighter usage. Besides, my system would have more of an industrial look, or in other words ugly :| , so people would naturally look elsewhere. That's why I asked if you all think that somebody would consider it based on performance for the price.

 

[Poolsean]

Once you've designed something that works, make sure you get UL and NSF50 before you try to sell it commercially. All states require these approvals. Some states may also require additional testing.

 

[Strannik]

With commercial the main thing is reliability, and it takes a while to build up your name in the market. You'd also need to make sure your system can integrate with various control systems that are usually used in commercial pools (even if it's simple on/off type of control).

 

[miro_gt]

thank you for the tips.

I can build my control circuit so that it would have a switch for external/internal control. It will be different cable for the external control as most ORP/PH controllers have 5A fuse on their outputs to control feeders, and there's a chance that my system when initially started could draw close to that from the grid for a moment, so if plugged directly to such a controller the result may not be good (for the controller). I know for sure my power supply will draw more than 5A at 110V input if driven at 100% of its ratings, but the plan is to use only half of its capabilities (or less).

speaking of the regulations for certification, what is the voltage limit that can be used over the cell (or pair of electrodes) for generating chlorine?

 

[chem geek]

speaking of the regulations for certification, what is the voltage limit that can be used over the cell (or pair of electrodes) for generating chlorine?

I don't think there are specific regulations regarding the voltages for SWCGs since they are self-enclosed (i.e. you can't put yourself or anything in between the plates). Some bipolar SWCG systems are around 25V with 2 active and 4 passive plates so 5V for each surface pair.