Running a SWG at 100% and Cell Life

I am not convinced that higher flow rate (>energy cost) would be a benefit over the reduction in cell life. Especially since we really don't know how much of a reduction in cell life it actually may be.

It's entirely plausible that the flow rate could vary with duty cycle and change the way the plates wear out. Any gas bubble that forms near the surface will, in effect, act as a non-conductive coating. But is there a substantial enough difference to matter and thus incur the higher energy cost of running a pump at higher speed ... I agree with Mark that it's hard to say without any real data.

So basically we are back to square one?

Here is my new system I'll test out:
12 hour run (8am-8pm), relying on solar cover heating, so running during day will hopefully improve sun heat...
RPM: 1800, power ~ 300W, flow ~ 28 GPM, total power 3.6 KW*hr/day
SWG Cell: 35% (Aquarite T-15)
target FC in morning ~ 4/5 ppm

ajw22 said:

All of the SWG's from the major US manufacturers use an ON/OFF cycle time to control generation %.

SWG Generation Cycle Times - Further Reading

www.troublefreepool.com

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If my Circupool Core 35 (installed for a 17,500 gal pool) uses duty cycle to control generation, then it must use very short duty cycle pulses, because I can clearly seen a nearly instantaneous color and visual change in the water flowing between the input and output sides of the generator's clear plastic view ports when I change the power setting. Slightly less immediately evident is the change in the quantity of Cl bubbles emitted from the returns.

JoyfulNoise said:

Late to the party … love the discussion so far. In my defense, I was making meatballs all day for an evening party so that took precedence. Being of Italian heritage, cooking meatballs and sauce takes precedence over all else. If you need CPR, it’ll have to wait until the meatball mix is finished.

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Sounds fair.

I have seen generators that use a mesh type material for the plates and guess that this is an attempt to spread current density more evenly across the plate maybe? Is there any benefit over mesh vs plates?

I have looked at an ultra low salt SWG (~1000ppm salt). Are these more likely to have a risk of shorter life span then a standard one that operates at ~4000ppm salt?

When it comes to down time and remineralization our cells control the pumps where if the cell is on the pump is on and the only exception is either cell or pump failure. The cell will still reverse polarity during operation but when both are switched off the cell will ‘sit in its own juices’ until its turned back on again. The cell is not allowed to turn off and be flushed with pool water prior to the pump switching off. Is there any benefit to flushing the cell prior to switching the pump and cell off?

SoDel mentioned that we my have cells that control amps and although I’m not away of anything specific all the SWG’s I’ve pulled apart have smallish % control knobs which I assume is just volt control. Ive always been under the impression that its a lot harder to control amps then volts and would think that the manufacturers are more likely to do whatever is cheeper.

In a real world sense if you’ve ever nursed a non self cleaning cell to the end of its life they always scale around the edges first which I guess is evidence of uneven current distribution.

I do turn my pump off for 5 minutes per day to force it to reprime. I schedule the cell to turn off 15 minutes prior to flush it. Unfortunately OmniLogic pretty much ignores the chlorinator schedules. Meh. Flushing the cell to both cool it and flush it prior to turning the pump off makes sense to me. One day, just another update over the horizon, I may get it to work .

AUSpool said:

The cell is not allowed to turn off and be flushed with pool water prior to the pump switching off. Is there any benefit to flushing the cell prior to switching the pump and cell off?

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My Viron is doing pre- and post-purges with the pump running and the cell unpowered. There is a 5 minute pre-purge at the beginning of a programmed cycle (and also when starting the system manually) to ensure priming, and a 2 minute post-purge to flush the cell before shutting down (this doesn't happen when shutting down manually, so I usually set the output to 0 before turning it off manually).

Even my old Hurlcon did these pre- and post-purges, so I thought they would be standard for all Astralpools.

AUSpool said:

When it comes to down time and remineralization our cells control the pumps where if the cell is on the pump is on and the only exception is either cell or pump failure.

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Just to make that clear to our US friends: Our SWGs have a controlled powerpoint for the pump to be plugged in.

Getting back to the current density thing, would this not also suggest using the least concentration of salt that would still allow the cell to reliably function?

SoDel said:

Getting back to the current density thing, would this not also suggest using the least concentration of salt that would still allow the cell to reliably function?

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I don’t think so. Matt can correct me if I’m wrong, but the chlorine production is dependent only on current once you meet the minimum required voltage.

But to drive current, you have ohms law. Lower conductivity water would require a higher voltage for the same current. So the lower the salt levels, the higher the voltage you need to drive the required current. I’m assuming the higher voltage would have some sort of negative life effects on the plates of the cell.

I think you’re assuming a constant current power supply. I don’t think they work that way. It’s an unregulated linear supply (so there is increased voltage drop with increased current draw, but the higher voltages observed at lower salt levels are just a function of finite available supply current thus more drop at higher salt levels). I think the cell deterioration still has current per unit area in the equation.

SoDel said:

I think you’re assuming a constant current power supply. I don’t think they work that way. It’s an unregulated linear supply (so there is increased voltage drop with increased current draw, but the higher voltages observed at lower salt levels are just a function of finite available supply current thus more drop at higher salt levels). I think the cell deterioration still has current per unit area in the equation.

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Fair point, I was. Considering that constant current power supplies are so cheap these days, it seems almost unfathomable that someone wouldn't use one.

However, a constant current power supply would allow regulating chlorine production without doing on/off cycles at full output. The % generation setting in that case would just set the current going to the cell.

SWG's don't regulate current precisely. They apply a voltage via a duty-cycle and whatever current you get is set by the plate area and the solution conductivity. The only thing a power supply will regulate is the maximum current output (clamping) to avoid burning out the supply board. There is a level of sophistication assumed in these things that is not the reality. Voltage difference per plate has to be kept below a minimum value or else you will favor the electrolysis of water (splitting water into hydrogen and oxygen) and diminish the production of chlorine gas.

Also, the issue isn't so much current density but the distribution of current across the plate. It is not homogeneous. Electrical current tends to "crowd" at the edges and at "hot spots" on the plate rather than uniformly being distributed. Remember that the current density is related to the electric field distribution (charge moves along electric field lines) and the electrical field around these pates is not uniform. There tends to be a greater density of electric field lines at the edges and the flow of the electrolyte causes a "bulging" of the field in the direction of the flow. This is why plate edges deteriorate first and why you see scale formation greatest at the out flowing edges of the plates.

Yes, I get that but let’s take a snapshot in time and space on, let’s say an area at the edge of a plate where there is an expectation of more deterioration. At that spot and at a snapshot in time, there will be a current as a function of salinity and what else is going on over the rest of the plate at that instant (Kirchhoff’s laws, etc.). Reduce salinity and we reduce that current, and presumably, cell wear at that spot, for that instant. Scale that up to the quasi-random stuff going on all over the plates all the time, I’d still expect to see a trend of reduced deterioration with reduced salinity. But is the reduced level of wear at that spot and at that time commensurate with the reduced amount of chlorine production — in other words, does it make any sense to try to keep salinity as low as we can, or would the reduced wear just be an indicator of reduced chlorine production?

SoDel said:

do turn my pump off for 5 minutes per day to force it to reprime

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Interesting, why. I thought the most wear on the pump was during priming. I've been running my vs pump at some rate 24/7 on that basis...

Jejunum said:

Interesting, why. I thought the most wear on the pump was during priming. I've been running my vs pump at some rate 24/7 on that basis...

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Can’t give a good reason other than as a fail safe. If for some reason it loses prime or is losing prime (unnoticed small suction side side leak, loose pump cover, whatever), at least once every 24 hrs it will fill the strainer and catch up. Wear on the pump? It will outlive me

SoDel said:

It will outlive me

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Really? I thought that this basically need rebuilding every 7 years - best case. Or are you a centenarian?

Jejunum said:

Really? I thought that this basically need rebuilding every 7 years - best case. Or are you a centenarian?

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Ok, an implicit question about pump life. I’ll bite.

Some have opined that the wet end can last basically forever if well cared for, e.g.:

As for the motor, bearings and seal, it will wear out when it wears out. I haven’t done any destructive longitudinal testing as far as priming or not once a day. Likewise, I can’t find any data showing material accelerated wear as a result of priming. If you have any, please share.

I can find data showing that running a pump too slow to prime but fast enough to heat will cause it to heat up, possibly hot enough to melt which will destroy the pump, which, in my mind, gives me a rational reason to cause it to fully prime once per day, whether it needs it or not. Again, if you have contrary data, I’d like to know.

SoDel said:

As for the motor, bearings and seal, it will wear out when it wears out. I haven’t done any destructive longitudinal testing as far as priming or not once a day. Likewise, I can’t find any data showing material accelerated wear as a result of priming. If you have any, please share.

I can find data showing that running a pump too slow to prime but fast enough to heat will cause it to heat up, possibly hot enough to melt which will destroy the pump, which, in my mind, gives me a rational reason to cause it to fully prime once per day, whether it needs it or not. Again, if you have contrary data, I’d like to know.

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Yeah I doubt the pump itself will wear out, but like you said the seals will.

I'm not an expert here, but the way these pump seals work is they use a thin film of the fluid they are pumping (water) as the lubricating surface the spinning part of the seal and the fixed part of the seal. Running the pumps dry for too long will damage the sealing surface and lead to leaks. Leaks lead to bearings dying. Saw this happen on my hot tub where a leak on the seal for a long time caused the bearing to die. I could tell it was a long time due to the rust on the pump end of the motor that didn't exist on the other side of the motor.

That said, although I never got around to doing it, this leak just killed the bearing, and with a little clean up work I'd be able to replace the bearing and seal and bring the pump back to normal working condition. I bought a new pump and my plan was to repair the old one to have on hand to swap when the second hot tub pump had a seal leak/bearing issue. Then I could swap it out and fix the old pump at my leisure. I no longer have the hot tub, but I kept the pump. I'll probably find a project to use the motor in at some point in the future.

My gut feeling aligns with you, that you should run the pump on priming speed when you kick it on for a bit in case there was a small leak that let air in, before dropping back down to your normal low speed. In theory nothing has leaked and running at low speed is fine, but if some did than you won't be pumping any water and having air churning up in the pump head could lead to increased seal wear. Still, I don't personally see that leading to pump death at low speed unless the motor isn't air cooled and instead is water cooled, but you also would not be circulating any water which would pool issues over a period of days, especially if you have a SWCG which is no longer producing chlorine due to lack of flow.