Heater Scaling Problems

[SF POOL BOY]

I have two pool heaters that continue to scale inside the heat exchanger tubes. They are 2 million btu heaters which are forced air. The manufacture recomends a minimum inlet temp of 130 with a 30 degree delta T. They started out (new) with a automatic mixing valve to regulate the inlet temp. It was slow acting so the temps would climb to around 150 before the valve would start to back down the temps. It would maintain a delta T of about 37. I run the pool water a little on the corrosive side -.4 to try to keep the heaters from scaling. I really would rather keep my pool water closer to zero as it is a brand new concrete pool.

Does any one have any recomendations or advise they would be willing to share?

Water Chemistry.. PH 7.1 Alk. 70 CH 350 TDS 2300 Pool Temp 83

I have swapped out the automatic mixing valve with a ball valve so that I can better regulate the heater temps to keep them as low as possible. It runs 130 in and 160 out now.

[chem geek]

The scaling really has nothing to do with the temperature of the forced air, but rather with the temperature at the interior surface of the heat exchanger tube where it makes contact with the pool water. I would suggest increasing your water flow rate which should reduce the temperature increase of your water and would carry away heat a little more efficiently possibly slightly lowering the temperature of the interior surface of the heat exchanger. Obviously, you may have some limits to flow rate due to pipe sizes since velocity is limited and it may cost you more to pump at a higher GPM through the system.

By the way, your saturation index is actually more like -0.56 when using an accurate formula (you can use The Pool Calculator for a reasonable estimate, but don't forget to put in the TDS into the "Salt" level. Also, you didn't give a Cyanuric Acid (CYA) reading -- is it 0? If it's not zero, then your saturation index is even lower.

In normal copper or cupro-nickel or even titanium heat exchangers with gas heaters (which have much higher temps on the input side), there is usually around a 30F temperature difference between the interior of the heat exchanger in contact with the water vs. the incoming water temperature itself -- this is assuming normal flow rates such as 40-50 GPM in a 250,000 BTU/hr. input (200,000 BTU/hr. output) gas heater. So the calculations for the effective saturation index for water in contact with the heat exchanger is 30F higher so usually results in a target saturation index of the bulk water of -0.2 so that it's 0.0 at the heat exchanger surface.

Richard

[Mark357]

A few questions:

1) Why does the manufacturer recommend an inlet temp of 130 degrees?

2) Why a delta T of 30 degrees?

3) What is causing your TDS to be so high?

4) How big is your pool? It must be huge to need such large heaters.

5) What is the humidity like in your area?

6) What is the maximum flow rate of the heaters?

The saturation of calcium carbonate is inversely proportional to the temperature. This means that the higher the temperature the less calcium carbonate it can hold. At the temperatures you are running inside of your heaters it will be very difficult to avoid scaling. The only reason I can think of to have such a high inlet temp is to avoid condensation from the hot air contacting the cooler heat exchanger.

I would recommend lowering the water inlet temp as much as possible while avoiding condensation. I don't see any reason to try to get a delta T of 30. I would drop it as much as possible unless there is a good reason not to. If you could get your inlet temp closer to 100 and your delta T to 10 to 20 you would greatly reduce your scaling problems.

You are right to be concerned about the plaster of your new concrete pool. My formula for the life of plaster is

30/X = Life of plaster. X = 10^Y + 2.4Y. Y = SI below 0.0.

X= 10^0.4 + 2.4 x 0.4 = 3.47. Life of plaster = 30/3.47 = 8.64 years.

It is important to know the makeup of your TDS as that is probably contributing to your scaling. You may need to do something to reduce the TDSs. Possibly a partial drain and refill or maybe even a water softener to treat the water. You can add a sequestering agent to help keep minerals suspended.

A question for Richard: You say "but don't forget to put in the TDS into the "Salt" level". If it is salt then it would lower the SI, but if it is not salt then won't it raise the SI? Doesn't the makeup of the TDS determine the effect the TDS has on the SI?

[SF POOL BOY]

A few questions:

1) Why does the manufacturer recommend an inlet temp of 130 degrees?

2) Why a delta T of 30 degrees?

3) What is causing your TDS to be so high?

4) How big is your pool? It must be huge to need such large heaters.

5) What is the humidity like in your area?

6) What is the maximum flow rate of the heaters?

The saturation of calcium carbonate is inversely proportional to the temperature. This means that the higher the temperature the less calcium carbonate it can hold. At the temperatures you are running inside of your heaters it will be very difficult to avoid scaling. The only reason I can think of to have such a high inlet temp is to avoid condensation from the hot air contacting the cooler heat exchanger.

I would recommend lowering the water inlet temp as much as possible while avoiding condensation. I don't see any reason to try to get a delta T of 30. I would drop it as much as possible unless there is a good reason not to. If you could get your inlet temp closer to 100 and your delta T to 10 to 20 you would greatly reduce your scaling problems.

You are right to be concerned about the plaster of your new concrete pool. My formula for the life of plaster is

30/X = Life of plaster. X = 10^Y + 2.4Y. Y = SI below 0.0.

X= 10^0.4 + 2.4 x 0.4 = 3.47. Life of plaster = 30/3.47 = 8.64 years.

It is important to know the makeup of your TDS as that is probably contributing to your scaling. You may need to do something to reduce the TDSs. Possibly a partial drain and refill or maybe even a water softener to treat the water. You can add a sequestering agent to help keep minerals suspended.

A question for Richard: You say "but don't forget to put in the TDS into the "Salt" level". If it is salt then it would lower the SI, but if it is not salt then won't it raise the SI? Doesn't the makeup of the TDS determine the effect the TDS has on the SI?

[SF POOL BOY]

A few questions:

1) Why does the manufacturer recommend an inlet temp of 130 degrees?

2) Why a delta T of 30 degrees?

3) What is causing your TDS to be so high?

4) How big is your pool? It must be huge to need such large heaters.

5) What is the humidity like in your area?

6) What is the maximum flow rate of the heaters?

The saturation of calcium carbonate is inversely proportional to the temperature. This means that the higher the temperature the less calcium carbonate it can hold. At the temperatures you are running inside of your heaters it will be very difficult to avoid scaling. The only reason I can think of to have such a high inlet temp is to avoid condensation from the hot air contacting the cooler heat exchanger.

I would recommend lowering the water inlet temp as much as possible while avoiding condensation. I don't see any reason to try to get a delta T of 30. I would drop it as much as possible unless there is a good reason not to. If you could get your inlet temp closer to 100 and your delta T to 10 to 20 you would greatly reduce your scaling problems.

You are right to be concerned about the plaster of your new concrete pool. My formula for the life of plaster is

30/X = Life of plaster. X = 10^Y + 2.4Y. Y = SI below 0.0.

X= 10^0.4 + 2.4 x 0.4 = 3.47. Life of plaster = 30/3.47 = 8.64 years.

It is important to know the makeup of your TDS as that is probably contributing to your scaling. You may need to do something to reduce the TDSs. Possibly a partial drain and refill or maybe even a water softener to treat the water. You can add a sequestering agent to help keep minerals suspended.

A question for Richard: You say "but don't forget to put in the TDS into the "Salt" level". If it is salt then it would lower the SI, but if it is not salt then won't it raise the SI? Doesn't the makeup of the TDS determine the effect the TDS has on the SI?

[SF POOL BOY]

Thanks for the replys guys, Ill try to answer some of these questions.

The Heater Rep recomends 130 minimum inlet temp to prevent condesation on the outside of the heat exchanger. The heater will actually trip out on an error code when temp falls below this point for more than 15 minutes.

I mis spoke about the 30 degree delta T when they were first set up the T was about 36- 37,

I'm not sure why the TDS is this high, we have two smaller pools also and they are only around 900

The pool is 420,000 gallons

I am located in central IL. so we have fairly high humidity through the middle of the summer.

I don't know the Max flow rates for the heaters, I do know they are cuper nickle although. I think I have a good amount of flow because if I increase it any more I am unable to maintain my 130 degree inlet temp. And the heater will trip off.

Last year the pool was built and put into service It was never really cleaned out properly of all the construction debri in the piping, gutters, surge tank ect. Also after the pool was filled they were still saw cutting concrete around the pool for two weeks. We had really cloudy water almost all summer. Little by little I cleaned and flushed out all that I could and then we changed the filter media. This seemed to help with the cloudyness.

Could it be that I am still seeing some of this construction debri this season? Maybee this would explain the high TDS

I am certainly not an expert or an engineer but I really don't see how these heaters could work in any pool situation without having scaling problems.

I am consulting with a pool tech from Spear corp and he thinks we should install a heat exchanger to get the pool water out of the heaters all together, Having a glycol solution on one side and pool water on the other.

I think this is a great idea because it would allow me to keep my pool water closer to 0 SI to protect the pool.

Can anyone see a downside to doing this or know of anything that I am not considering?

Do you guys have any experience with a setup like this for a pool?

Thanks

[Mark357]

Using a dual heat exchanger will be less efficient, but it may be a good solution if there is not too much loss of efficiency. Did the pool tech from Spear corp. indicate how much efficiency would be lost with the dual heat exchanger setup?

[SF POOL BOY]

Using a dual heat exchanger will be less efficient, but it may be a good solution if there is not too much loss of efficiency. Did the pool tech from Spear corp. indicate how much efficiency would be lost with the dual heat exchanger setup?

No he did not, I will check with him to see if he knows or can figure for me. I do think that even with a loss it would be well worth it in the long run for the sake of the pool and the heaters though.

In a perfect pool chemical situation what would the life expectancy of the concrete be. SI at 0 I know you gave me the formula but still a little over my head.

Thanks

[Mark357]

30/X = Life of plaster. X = 10^Y + 2.4Y. Y = SI below 0.0.

X= 10^0.0 + 2.4 x 0.0 = 1.00. Life of plaster = 30/1.00 = 30.0 years.

X= 10^0.1 + 2.4 x 0.1 = 1.50. Life of plaster = 30/1.50 = 20.0 years.

As you can see just going 0.1 negative cause a 10 year reduction in plaster expected life. This is my formula based on my experience and my opinion. I believe that it is a fairly accurate formula assuming everything else is done right. Most people will tell you that 15 years is good for plaster. Under ideal conditions plaster can last for 30 years. Water that is saturated with calcium carbonate will not dissolve very much plaster. It will still dissolve some due to various factors. This is also based on my opinion of what a good plaster finish is vs. what is a poor plaster finish. It is also important to avoid scaling. Therefore, you should also avoid going too high on the SI. Usually less than 0.3 is best.

[Mark357]

Here are a few more notes related to your situation:

I noticed that you mentioned "saw cutting concrete around the pool for two weeks". Concrete dust is one of the worst things you can get into your system. When the dust went through the system it reached high concentrations. If the dust was vacuumed through the system it reached super high levels. If the heater was running while the levels were high that would cause scaling.

The dust should have been vacuumed to waste, and not through the filter. If it was vacuumed through the system that would explain the cloudiness. I would imagine that your flow rate is probably in the 700 to 900 gpm range with a 15 to 20 HP pump and a correspondingly sized sand filter.

1) What are the specs on your system and equipment? Pump, filter, flow rate etc?

Here are a few more things you can do:

1) Get an extra 3/4 or 1.0 HP pump and use it to vacuum to waste instead of using the filtration system to vacuum. This will avoid getting so much dust and junk in your system. It will also reduce your TDS as it will remove about 1 inch of pool water per vacuum.

2) Add a 120 GPM DE filter to the output of the sand filter to get more of the finer particles out of the water. You can set it up with a few 2 inch Jandy "never-lube" valves so that it is easy to use. You would divert about 100 GPM from the output of the sand filter and through the DE filter using a flow meter to set the correct flow. By using the existing flow rate there is no extra pump to buy or install. By using sand filtered water you will get a very long operational time out of the DE. This is not a common thing to do, so don't expect anyone to immediately understand what you want, or why you're doing it.

[SF POOL BOY]

Here are a few more notes related to your situation:

I noticed that you mentioned "saw cutting concrete around the pool for two weeks". Concrete dust is one of the worst things you can get into your system. When the dust went through the system it reached high concentrations. If the dust was vacuumed through the system it reached super high levels. If the heater was running while the levels were high that would cause scaling.

The dust should have been vacuumed to waste, and not through the filter. If it was vacuumed through the system that would explain the cloudiness. I would imagine that your flow rate is probably in the 700 to 900 gpm range with a 15 to 20 HP pump and a correspondingly sized sand filter.

1) What are the specs on your system and equipment? Pump, filter, flow rate etc?

Here are a few more things you can do:

1) Get an extra 3/4 or 1.0 HP pump and use it to vacuum to waste instead of using the filtration system to vacuum. This will avoid getting so much dust and junk in your system. It will also reduce your TDS as it will remove about 1 inch of pool water per vacuum.

2) Add a 120 GPM DE filter to the output of the sand filter to get more of the finer particles out of the water. You can set it up with a few 2 inch Jandy "never-lube" valves so that it is easy to use. You would divert about 100 GPM from the output of the sand filter and through the DE filter using a flow meter to set the correct flow. By using the existing flow rate there is no extra pump to buy or install. By using sand filtered water you will get a very long operational time out of the DE. This is not a common thing to do, so don't expect anyone to immediately understand what you want, or why you're doing it.

[SF POOL BOY]

Pool specs are as follows:

2 30 HP pumps throttled to achieve a flow rate of 2250 through the sand filters (6) 1850.

We do have a couple of pool vacs which we have always vacumed this to waste which eliminated some of the debri. A lot was drawn into the system though on the surface and what was already in the piping surge pit ect. We did manage to clean most of this up through out the year last season. Then changed the filter media as a last step. At start up this year we flushed out most of the piping before filling also.

Kinda sounds to me that the only reasonable option for us would be the heat exchanger plan regardless of a loss of eficiency if any.

Definitly can't sacrifice the entegrity of the pool just to heat the water.

Thanks for your thoughts..

[chem geek]

30/X = Life of plaster. X = 10^Y + 2.4Y. Y = SI below 0.0.

X= 10^0.0 + 2.4 x 0.0 = 1.00. Life of plaster = 30/1.00 = 30.0 years.

X= 10^0.1 + 2.4 x 0.1 = 1.50. Life of plaster = 30/1.50 = 20.0 years.

As you can see just going 0.1 negative cause a 10 year reduction in plaster expected life. This is my formula based on my experience and my opinion. I believe that it is a fairly accurate formula assuming everything else is done right. Most people will tell you that 15 years is good for plaster. Under ideal conditions plaster can last for 30 years. Water that is saturated with calcium carbonate will not dissolve very much plaster. It will still dissolve some due to various factors. This is also based on my opinion of what a good plaster finish is vs. what is a poor plaster finish. It is also important to avoid scaling. Therefore, you should also avoid going too high on the SI. Usually less than 0.3 is best.

Mark,

His original numbers were the following:

Water Chemistry.. PH 7.1 Alk. 70 CH 350 TDS 2300 Pool Temp 83

which give a very negative saturation index (-0.56) which I agree is a problem for plaster. So why is he getting so much scaling since his saturation index is so low? If he were to raise the pH to avoid dissolving plaster, wouldn't that make his scaling problem even worse?

Richard

[Mark357]

Richard,

There are several problems that are contributing to the scaling. The first is the extremely high water temperatures in the heat exchangers. It seems that they were going as high as 160 to 170 degrees. That makes the current SI +0.2 to +0.3. Normally, water temperatures in a heat exchanger are much lower.

Most likely the pH has been well over 7.1 many times, which increases it even further. The OP began going negative SI after finding scaling and as an attempt to control it. At a pH of 7.7 the SI in the exchanger will be 0.8 to 0.9. The OP notes that the pool was recently built. New plaster gives off lots of calcium carbonate and hydroxides. The O.P's pool is on the order of 115 ft X 115 ft. That is a lot of plaster, which would have generated a lot of dust.

It is also likely that the alkalinity has been higher in the past.

The next issue is the high TDS at "TDS 2300", which I believe are from all of the concrete sawing and construction that the OP mentioned. Concrete that is cut with a diamond blade saw is a very fine powder. It can cause many scaling issues. It is a very bad thing to get into your pool and into your filtration system. The 2300 level is the current level; it was probably much higher earlier while all of the construction work was being done. Concrete being cut generates a lot of dust which gets into the filtration system and causes the TDS to spike to very high levels. I think that the OP needs to get these levels to below 1000.

You are also subtracting the TDS value from the SI equation. The make-up of the TDS and what effect you're calculating determines whether it is subtracted or added. In this case, I think it should be added. If you add the TDS, you get closer to + 0.4 to +0.5. Add on 0.6 for the times that the pH was at 7.7 and you get +1.0 to + 1.1. We need to know the saturation index of each component of the TDS and the Index of the combined components. Scale can be many other things than just calcium carbonate.

Add all of these factors together and you are going to have major problems.

Note: I take care of concrete pools with heat exchangers where I run an average range of 0.0 to 0.25 positive SI with no scaling problems. Also, without actually visiting the job site it is difficult to know what other factors are relevant that the operator might not know to include in their description.

[chem geek]

Mark,

I must not be understanding his heat exchanger at all. It sounded like forced air was heated and then exchanged heat with the water. The inlet, which I presume means the water inlet, has 130F temperature air while the outlet has up to 170F air temperature since I presume the heat exchanger has the air and water going in opposite directions as that's the normal way such exchangers work. That does not mean, however, that this is the temperature of the water or even the metal touching the water. Depending on the flow rate of the water and it's efficiency of turbulent mixing, the temperature at the water/metal boundary can be quite a bit lower than that of the air temperature on the other side. In a normal gas heater, for example, the temperature of the flame and the metal it touches or is near is very, very high yet the temperature on the other side of the metal heat exchanger touching the water is much, much lower and usually around 30F higher than the bulk water temperature.

Air Out 130F <------------------------ Air In 160-170F

....... Heat Exchanger Tube Surface touching Air (air temp or lower)

....... Heat Exchanger Tube Surface touching Water (water temp or higher)

Water In (80F or so?) -----------------------------> Water Out (In temp + 10F?)

I can understand if his water parameters were higher at other times that the scaling would occur so that makes sense and that what was posted was mitigating that, but he said that they continued to scale anyway. So that's why I suggested a higher flow rate of the water, if possible, to remove the heat over a larger volume of water which would mean a lower temperature at the heat exchanger surface touching the water and potentially less scaling.

The TDS effect on the saturation index isn't two-way. It's not dependent on what the TDS is composed of, at least thermodynamically. The TDS is a rough proxy for the ionic strength of the water and that changes the activity coefficients for the chemical equilibrium in one direction only. Higher TDS is higher ionic strength and makes the saturation index more negative. The composition of TDS comes into play for kinetics where higher magnesium, for example, can disrupt the rate of formation of scale, but it doesn't change the thermodynamics which is whether scale can form at all. The presence of plaster dust can accelerate the rate of formation of scale, but only if the saturation index is positive -- the presence of plaster dust doesn't change the index itself. If the index were negative, then the plaster dust would tend to dissolve. If the index is positive then the plaster dust can act as seeds to speed up formation of scale. The index by itself gives no indication as to the rate of either dissolving plaster nor forming scale.

So for your saturation index formula predicting plaster life, you would of course use the TDS in the saturation index calculation, but would not adjust the saturation index with the content of the TDS, but would rather add other factors to your formula that would help determine the rate of dissolving plaster. That is, no component of TDS is going to change water from dissolving plaster to scaling or vice versa -- the components of the water would only change the rate of such dissolving or scaling. The TDS overall, to the degree that it accurately predicts ionic strength, does affect the saturation index. Generally, higher TDS is almost always composed of salt, sodium chloride, since chlorine usage over time had chlorine convert to chloride while sodium is added either from chlorine product (i.e. sodium hypochlorite) or from a base (sodium carbonate). If the total hardness is very high, then the magnesium will count double towards conductivity but counts a factor of 4 towards ionic strength so the TDS will underestimate ionic strength meaning that very hard water will have a greater tendency to dissolve plaster or less of a tendency to scale PLUS the magnesium ion itself slows down scale formation (not sure if it affects the plaster dissolving rate).

The Taylor watergram unfortunately does not have a TDS component. It is a snapshot for "normal" pool water saturated with calcium carbonate -- typically 500 ppm or so. It needs to be adjusted for high TDS water such as in SWG pools with around 3200 ppm. The saturation index formula does have TDS as a component but the traditional pool industry factors for TDS aren't correct (Wojtowicz writes about this here in Swimming Pool Water Balance Part 7) and I also have the more accurate derivation at the bottom of my spreadsheet). The traditional formula underestimates the ionic strength effect of TDS so high TDS water has a more negative saturation index than the traditional formula -- for SWG pools, the accurate saturation index is about 0.1 lower than the traditional estimate (that is, the traditional estimate has an SWG pool -0.08 lower compared to a non-SWG pool while the accurate index has an SWG pool -0.22 lower).

Richard

[Mark357]

Richard,

I think that the heat exchanger is a set of finned copper tubes that pass over the natural gas fire in the heater. The pool water goes through the tubes and the heat from the fire heats the air in the heater; the air then passes over the heat exchanger and out of the heater. The OP meant that the water going into the heat exchanger was 130 F and it was coming out at 167 F. The temperature of the air that contacts the heat exchanger is probably well over 500 F. I think that the OP's use of the term "forced air" caused some confusion. I think that he meant that the heater had a fan on the exhaust that creates air movement through the heater rather than relying on just the air flow caused by hot air rising. This is becoming a standard on most new heaters.

"The presence of plaster dust can accelerate the rate of formation of scale, but only if the saturation index is positive"

Most likely the SI was very positive in the heat exchanger. The OP is considering switching to a dual heat exchange process that uses two heat exchangers. The first one will be in the heater and the second will then exchange heat with the water in a second exchanger.

" The TDS effect on the saturation index isn't two-way. It's not dependent on what the TDS is composed of,"

Calcium is a component of TDS. If you increase calcium, you increase TDS. Calcium is a component of TDS that increases the SI. Carbonates and Bicarbonates are a component of TDS, they also increase the SI.

"Generally, higher TDS is almost always composed of salt, sodium chloride,"

In this case the TDS is almost definitely not salt. Most likely the primary components of the TDS are Tricalcium silicate and Dicalcium silicate, which form Calcium silicate hydrate + Calcium hydroxide; and Tricalcium aluminate (forms calcium aluminate hydrate), Tetracalcium aluminoferrite and Gypsum. All of these will contribute to the total level of dissolved solids that the water can hold.

"I can understand if his water parameters were higher at other times that the scaling would occur so that makes sense and that what was posted was mitigating that, but he said that they continued to scale"

Most likely the OP only recently switched to a negative SI after the heaters scaled again. He does not say that they have scaled after switching to a negative SI.

Another issue with high TDS is the increased conductivity of the water. If there is any type of stray current going through the water and the heat exchanger is acting like a negatively charged anode it will attract positively charged cations, like calcium, like what happens in a salt cell. If the heat exchanger is acting like a positively charged anode it will be corroded at an accelerated rate.

Using your model it seems like you can just continue to add dissolved solids indefinitely. At some point the water becomes saturated and can not hold any more.

Mark.

[chem geek]

OK, I misunderstood since I didn't think the water was that high in temperature. Why would pool water entering into this heat exchanger be 130F? That's confusing to me.

Yes, I know that calcium (measured from CH) and carbonate (measured from TA, CYA and pH) are components of the saturation index and also contribute to TDS. What I meant was that the formula for saturation index also has TDS as an additional component (in addition to CH, TA, CYA, pH and temperature) and that is for estimating ionic strength which changes chemical equilibria. I misunderstood what you said about TDS since it sounded like this was a factor outside of the saturation index. It was the following paragraph that confused me.

The make-up of the TDS and what effect you're calculating determines whether it is subtracted or added. In this case, I think it should be added. If you add the TDS, you get closer to + 0.4 to +0.5. Add on 0.6 for the times that the pH was at 7.7 and you get +1.0 to + 1.1. We need to know the saturation index of each component of the TDS and the Index of the combined components. Scale can be many other things than just calcium carbonate.

The above sounded like you can add or subtract the TDS part of the saturation index and that's not true, but if you meant that without knowing the CH and TA that you are guessing that the high TDS could mean high CH and TA, then that makes perfect sense, but the OP had explicit measurements of CH, TA, pH. As for scale being other than calcium carbonate, what other things (other than metal stains) precipitate out before calcium carboante?

The calcium silicates you describe, which are components of pool plaster, when dissolved in the water do form calcium carbonate if the water is under-saturated with calcium carbonate. The calcium silicates are differing proportions of calcium oxide with silicate and the calcium oxide can combine with dissolved carbon dioxide to form calcium carbonate. Also, calcium carbonate in aggregate (marble, limestone) can dissolve if the water is not saturated in calcium carboante.

CaO(s) + CO₂ ---> Ca²⁺ + CO₃^2-

CaCO₃(s) ---> Ca²⁺ + CO₃^2-

Now if you are talking about curing with newer plaster, then this produces calcium hydroxide as follows (using tricalcium silicate as an example)

2Ca₃SiO₅ + 7H₂O ---> 3CaO•2SiO₂•4H₂O + 3Ca²⁺ + 6OH^-

However, the net effect is a rise in Calcium Hardness (CH) and in pH. In other words, you don't have calcium silicate hydrate dissolved in water as part of TDS. It is not soluble to any great extent. The tricalcium silicate becomes calcium silicate hydrate, but this occurs inside the pool plaster (which is technically portland cement) and any "dust" in the water will get filtered out as it is not soluble and does not measure as TDS since it is not dissolved. The other substances you mentioned are also not soluble in water and likewise don't measure as TDS since they are not dissolved. [EDIT] Silicon Dioxide, SiO₂, is soluble up to 120 ppm but being a neutral species it won't get measured in a conductivity TDS test. This link gives CSH solubility where at CH levels around 300 ppm which is roughly 3 mM (calcium ion), the maximum silicon ion concentration before saturation is 200 µM or 0.2 mM which is fairly small, equivalent to 20 ppm if in the same units as CH. [END-EDIT] The net of what is in normal TDS pools around 500 ppm is calcium and bicarbonate. However, you don't get to 1000+ from just those two. You get that high from the sodium and chloride that come from chlorine sources as well as from magnesium from the fill water which is a component of Total Hardness (along with calcium). There may be a small level of other ions such as sulfates. At a pH of 7.5, TA of 100, CYA of 30, and CH of 300, the TDS of these adds up to around 525 ppm (TA and CH are measured as ppm CaCO₃ and not as ppm of their actual components so that's why these things don't directly add up). Any higher TDS is almost all sodium chloride salt, the largest exception being magnesium, but over time with chlorine addition it is sodium chloride that builds up fastest. All sources of chlorine produce 0.8 ppm salt for every 1 ppm FC because chlorine converts to chloride when broken down in sunlight or oxidizing ammonia or an organic (bleach, chlorinating liquid, and lithium hypochlorite add an additional 0.8 ppm salt). So at 2 ppm FC per day, that's an increase of 36 ppm salt per month (which will be reduced by dilution from splash-out or backwashing) from some sources of chlorine and double that from most hypochlorite sources.

[EDIT] Evaporation and refill will increase TDS from what is in the fill water which is typically CH, TA and Magnesium. However, even evaporation rates (shown here) of 1/3" per day would increase pool water TDS by fill water TDS over about half a year and fill water TDS is generally low unless it's well water and in any event it's mostly measurable as CH and TA (so again, any extra TDS not accounted for by CH and TA and CYA is sodium chloride salt and magnesium). [END-EDIT]

Though it is true that TDS is proportional to conductivity, stray current or voltage will not plate out calcium. In a salt cell the reason that the plate that generates hydrogen gas gets scale is because the pH near that plate goes way up due to the following reaction where the scaling has nothing to do with the currents or voltages directly:

2H₂O + 2e^- ---> H₂(g) + 2OH^-

You can absolutely have very high TDS if it is composed of salt. After all, that's how SWG pools have 3200 ppm TDS since the salt level as measured by the chloride test is 3000 ppm (though that is chloride, not all is balanced by sodium as there is some calcium and magnesium in the water as well). The ocean has far higher TDS than that (about 35,000 ppm; also see this link for normal ocean and river composition). TDS does not fill up readily except from substances that aren't very soluble such as calcium carbonate. So if you mean you can't fill up water with more and more calcium (CH) or carbonate (TA and high pH), yes that is true. But you can certainly continue to add chlorine to the pool and build up sodium chloride salt to very high levels and it will not get saturated.

Richard

[Mark357]

"Why would pool water entering into this heat exchanger be 130F? That's confusing to me."

The OP noted:

"The Heater Rep recommends 130 minimum inlet temp to prevent condensation on the outside of the heat exchanger."

Condensation can be a concern if there is high humidity and the hot air is contacting a cold heat exchanger. 130 degrees F is unnecessary.

I think that the scaling can be attributed to these primary factors:

1) Excessive water temperatures in the heat exchanger. (See notes)

2) High SI at the time of scaling. Estimate of +0.4 in pool and +1.0 in the heat exchanger.

3) High Total Dissolved Solids in excess of 1,000 for a non-salt pool. High levels of TDS can cause corrosion and scaling even though high TDS will also increase the solubility of the calcium.

4) Large amounts of concrete dust from saw cutting getting into the system.

5) Large amounts of Plaster dust getting into the circulation system.

Recommendations:

1) Limit water temperature to less than 120 F in the heat exchanger.

2) Maintain a Saturation Index between 0.0 and 0.2

3) Reduce Total Dissolved Solids (Excluding Salt) to less than 1,000.

3) Vacuum to waste whenever heavy dust or debris needs to be vacuumed.

4) Add DE filtration to the system to remove finer particles than the sand filters can remove.

5) Add a "Stain and Scale" control chemical.

6) Maintain Calcium below 240 ppm.

Note: When calcium is dissolved in water heat is given off (exothermic), therefore its solubility is inversely proportional to temperature. In addition to the high temperature in the heat exchanger causing a direct solubility effect on the Calcium Carbonate, the high temperature also has three indirect effects.

(1)Carbon dioxide is less soluble in hot water; therefore it comes out of solution at 160 F in the heat exchanger. Example: At 140 F the water can only hold 44% of the carbon dioxide level that it can at 85 F. Or water at 85 F can hold 2.24 times as much dissolved carbon dioxide as water at 140 F. This raises the pH in the exchanger, which lowers the solubility of the calcium carbonate.

(2) As the pH is raised, the equilibrium between carbonate and bicarbonate shifts towards carbonate. [1]

The increased level of carbonate combines with the calcium to form calcium carbonate. The reduction of the bicarbonate shifts the equilibrium of calcium bicarbonate with Calcium carbonate and carbon dioxide towards the calcium carbonate. CaCO3 + CO2 + H2O <-> Ca(HCO3)2

(3) The solubility of calcium carbonate is proportional to the level of dissolved Carbon Dioxide in the water. As the carbon dioxide level drops, the water can hold less calcium carbonate.

(4) As water is heated, the equilibrium between bicarbonate ions and carbonate ions is shifted toward carbonate. Water at 176 F will have 2.4 times the carbonate concentration as it will at 77 F. [1] The increased level of carbonate combines with the calcium to form calcium carbonate. The reduction of the bicarbonate shifts the equilibrium of calcium bicarbonate with Calcium carbonate and carbon dioxide towards the calcium carbonate. CaCO3 + CO2 + H2O <-> Ca(HCO3)2

[1] http://www.advancedaquarist.com/issues/mar2002/chem.htm

[Mark357]

A few more questions about your setup:

1) Is there an auxiliary pump that just pumps the water through the heater? It seems like it would be necessary for the mixing valve to work properly.

2) What is the total water flow rate through the heater?

3) What is the mix ratio? How much water in gpm comes from the pool system and how much is recirculated 160 F water from the heater outlet??

4) You say that you have 6 filters. What is the design flow rate of each filter?