Does a copper heat exchanger care more about LSI or pH?

[QPSUtah]

This is strictly a theoretical question, given that in most scenarios to achieve a proper LSI you're generally going to have a TA and pH that will be fine for your heater. Acknowledging that, does a copper heat exchanger care more about LSI or pH? Seems that LSI is mostly geared toward plaster longevity.

As an illustration, if my CA was very low, say 50, water temp of 90, TA of 100, CYA of 10, and Salt at 3400, pH is 7, the LSI is -1.38. I would guess that is very hard on the plaster, as the plaster wants to give calcium back to the water, but will it also be hard on the heat exchanger? The heat exchanger doesn't want to leach calcium back into the water. It doesn't have any.

Or is "etching" (negative LSI) water, etching to all materials and surfaces regardless? Or is there a strict pH band that should be maintained for your copper heater regardless, even though the LSI could theoretically still be balanced.

 

[ajw22]

PH affects wear in copper heat exchangers.

LSI/CSI affects calcium scale accumulation in heat exchangers.

Two different problems.

Which one do you want to discuss?

 

[PoolStored]

The CSI/LSI is solely an index used to predict how calcium carbonate scale will react in water ... it says NOTHING about metal corrosion.

There are other empirically derived indices that are used to PREDICT when corrosion MIGHT be possible. The Pukorius Index (PI) and Ryznar Index (RI) are used in this regard. But remember, these indices only tell you what COULD happen, not what will happen.

Low pH is very corrosive.

 

[JamesW]

The copper is mostly protected by a thin film of copper oxide (patina), which is most affected by pH.

This builds up slowly over time and it requires a good pH.

2Cu + O₂ -- 2CuO.

Before the buildup of the protective layer, chlorine oxidation can be more aggressive and you need to always minimize the amount of chlorine as much as possible by following the FC/CYA rules and avoiding any need to SLAM.

Because the copper in the copper oxide is already oxidized, it is not affected by oxidizers like oxygen or chlorine.

However, if the pH is low, the copper oxide patina is dissolved and the elemental copper is now susceptible to oxidation by chlorine and oxygen.

CuO + 2H⁺ --> Cu²⁺ + H₂O.

Calcium carbonate scale can also provide a barrier to oxidizers like chlorine and oxygen, so CSI can help if it provides a thin layer of calcium carbonate.

The problem is trying to develop a thin layer without triggering a thick layer of calcium carbonate.

Any calcium carbonate layer provides a base layer for more calcium carbonate to attach to and you can get runaway scale buildup if the CSI is high enough.

So, I would say that pH is most important and CSI might be helpful as long as you can avoid a heavy scale buildup.

 

[QPSUtah]

PH affects wear in copper heat exchangers.

LSI/CSI affects calcium scale accumulation in heat exchangers.

Two different problems.

Which one do you want to discuss?

This is what I had assumed. So regardless of the LSI, you should maintain a pH (and perhaps TA) floor to protect the copper heater (pH of 7 or above) while still maintaining a good LSI balance (below 0.30) to prevent scale in the heater.

 

[JamesW]

The copper in the top left is elemental copper.

The brown has a protective layer of copper oxide.

The green is copper carbonate.

Ideally, you want the copper to turn a chocolate brown with no green.

The heat exchanger starts as elemental copper and you want to develop a brown copper oxide layer over time.

This requires a good pH.

A CSI of 0.0 might help if it forms a very thin layer of CaCO₃ without triggering a thick layer.

A thin layer makes a full scaling event more likely if the CSI goes too high.

Green copper carbonate is unstable and indicates poor chemistry.

Really old copper plumbing in good condition will be dark brown with no green.

 

[ajw22]

This is what I had assumed. So regardless of the LSI, you should maintain a pH (and perhaps TA) floor to protect the copper heater (pH of 7 or above) while still maintaining a good LSI balance (below 0.30) to prevent scale in the heater.

Correct.

The pH floor, as you put it, protects the copper patina scale layer that protects the copper from wear. Low pH can quickly strip that protective layer off. You also need at least 200 CH for vinyl and fiberglass pools with a gas heater to create that protective layer, although the pool surfaces do not need the CH.

 

[JoyfulNoise]

Pourbaix (speciation) diagrams help when discussing metal/aqueous solution interactions. Here’s a sample of a calculated copper diagram when chloride is present -



Most pool water is oxidizing so it has a positive potential (Vshe > 0), typically +500mV or more. The X-axis is pH. The slanted dotted lines represent the region where water is stable and not breaking down into hydrogen and oxygen gas.

So if you look around the pH value of 7 and run a vertical line up to 0.5V you’ll see that copper remains stable as a chloride or mixed oxy-/hydroxychloride. That’s the patina that @JamesW was talking about. However, if you start to drop the pH to lower values, you’ll see that copper cations (dissolved copper) becomes the dominant and stable species. That’s when copper metal starts to etch. Saturated LSI conditions just ensure that the water has lots of carbonate in it so that the pH cannot drop very quickly and there’s plenty of anions around that copper can form a stable film with. But even if the LSI were “low” but the pH was in the right range, copper metal would simply form an oxide layer since that is the stable species.

 

[JoyfulNoise]

Just as an addendum to what I posted, that diagram is simply "instructive" in that it is only constructed for copper and water and some chloride. It's also be calculated at a temperature of 5 degrees C. Pool water is much more complicated in the sense that there are a lot of different dissolved chemicals, all of which are capable of interacting with the copper and changing what appears on the diagram. Temperature also has a significant effect as well. So in a very complicated aqueous solution, all of those regions depicted could change size a shape a bit as well as there being other species and regions present that might account for things like metal phosphates forming, or metal-cyanurate complexes. Obviously the diagram doesn't show any carbonates because that wasn't part of the calculations, so a new diagram could be constructed with the effects of added carbonate species.

In short, a copper metal heat exchanger will be fairly stable and lasts long time as long as it isn't chemically abused by the owner of the water body. The biggest killer of a copper heat exchanger are pool and spa owners that use highly acidic and oxidizing chemicals, like trichlor, in inappropriate or dangerous ways. Inline trichlor puck chlorinators are absolutely the worst kind of abuse one can add to a plumbing system as the chemical stew that builds up inside it is just flat-out dangerous.

 

[JamesW]

In my opinion, a pH in the 7.8 to 8.3 range is generally the best choice as it reduces pH rise due to carbon dioxide offgassing and it protects the CSI at lower TA levels.

In a heat exchanger, the temperature of the water can be substantially higher when the heater is on.

The water temperature can be as high as 134 degrees without tripping the HLS.

So, the CSI will be higher in the heat exchanger when the heater is firing.

 

[QPSUtah]

Correct.

The pH floor, as you put it, protects the copper patina scale layer that protects the copper from wear. Low pH can quickly strip that protective layer off. You also need at least 200 CH for vinyl and fiberglass pools with a gas heater to create that protective layer, although the pool surfaces do not need the CH.

Super interesting. Great point.

 

[QPSUtah]

In my opinion, a pH in the 7.8 to 8.3 range is generally the best choice as it reduces pH rise due to carbon dioxide offgassing and it protects the CSI at lower TA levels.

In a heat exchanger, the temperature of the water can be substantially higher when the heater is on.

The water temperature can be as high as 134 degrees without tripping the HLS.

So, the CSI will be higher in the heat exchanger when the heater is firing.

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View attachment 633401

So higher pH with lower TA to narrow the pH band? Do you then keep a somewhat elevated chlorine level to compensate for the higher ph? And its affect on “active” vs “inactive” chlorine?

 

[PoolStored]

So higher pH with lower TA to narrow the pH band? Do you then keep a somewhat elevated chlorine level to compensate for the higher ph? And its affect on “active” vs “inactive” chlorine?

No. The FC/CYA relationship works just fine, even with pH 7.8 to 8.0...most of the FC, % wise is bound, with CYA in the water. Always follow this...Link-->FC/CYA Levels

HOCl goes from maybe 2% to about 1.2% from pH of 7 to 8 at CYA of 30. Target range of FC at 30CYA is 4-6ppm. At 4ppm and 1.2% HOCl is .048ppm. Bacteria dies at at .011 (as set by the WHO). Algae likely dies at .02, but our best info says .05. Plenty to maintain enough killing power. Stay in the range = Good to go!

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

 

[ajw22]

So higher pH with lower TA to narrow the pH band?

Pool pH can be kept anywhere between 7.0 and 8.0. Anywhere within that range is equally good.

Since low pH is more damaging than high pH, the practical pH floor should be 7.2. pH excursions into the mid-8s do nothing.

Do you then keep a somewhat elevated chlorine level to compensate for the higher ph? And its affect on “active” vs “inactive” chlorine?

pH anywhere between 7.0 and 8.0 has no practical effect on chlorine effectiveness. You do not need to compensate chlorine level for the pH. Don't over complicate things and tie together pH level with chlorine level.

 

[QPSUtah]

Pool pH can be kept anywhere between 7.0 and 8.0. Anywhere within that range is equally good.

Since low pH is more damaging than high pH, the practical pH floor should be 7.2. pH excursions into the mid-8s do nothing.



pH anywhere between 7.0 and 8.0 has no practical effect on chlorine effectiveness. You do not need to compensate chlorine level for the pH. Don't over complicate things and tie together pH level with chlorine level.

So the chart we all see about effect of pH on HOCl (below), practically, doesn't make a difference? I guess in my mind I was thinking, okay, at a pH of 7, it's 75/25, or whatever, so if your FC is 4 PPM you have 3 PPM of active/effective chlorine and at 8, if it's reverse, you would only have 1 PPM of active/effective chlorine. Of course CYA changes all of this...

 

[JoyfulNoise]

That chart doesn’t take into account the role of CYA as a chlorine buffer. When you add CYA to the water, the effects of pH are drastically reduced.

 

[JoyfulNoise]

See this post and the associated charts -

Post in thread 'Pool Water Chemistry'
Pool Water Chemistry

 

[QPSUtah]

That chart doesn’t take into account the role of CYA as a chlorine buffer. When you add CYA to the water, the effects of pH are drastically reduced.

Correct, but even in a pool without CYA, would you still ignore trying to adjust FC according to pH? Just doesn’t make a big enough difference even in a CYA free pool?

 

[JamesW]

The FC/CYA ratio already takes into account the effect of pH on the level of HOCl.

Maintain a FC?CYA of 7.5% to 10% and you should be good regardless of the pH.

 

[QPSUtah]

In my opinion, a pH in the 7.8 to 8.3 range is generally the best choice as it reduces pH rise due to carbon dioxide offgassing and it protects the CSI at lower TA levels.

In a heat exchanger, the temperature of the water can be substantially higher when the heater is on.

The water temperature can be as high as 134 degrees without tripping the HLS.

So, the CSI will be higher in the heat exchanger when the heater is firing.

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View attachment 633401

What is considered “low” vs “high” TA? Keeping the TA at 50 is a little counterintuitive, though the LSI math makes sense, as a supposed drawback to “low” TA is fluctuating pH, and the supposed benefit of “high” TA is resisting changes to pH. But perhaps I just need to reframe what is high vs low TA.