Temperature vs. Calcium/Magnesium Solubility

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Taxonomy

• Cooling Boiler & Wastewater
• Industrial Water Treatment
• Industrial Water Treatment
• Industrial Water Managment
• Cooling Systems

6 Answers

1. It is critical that chemical oversimplification never be applied. Every system is unique. Systems may have what would appear to be the same issues. That does not mean they are the same. Many years ago this issue was driven home for me. There was a cooling system that had been treated extremely successfully for many years. A new unit was built along side that was an exact duplicate of the original. Within 2 weeks, the water in the new system was turbid. Within 6 weeks, the primary air aftercooler was experiencing heat transfer issues. The unit was taken down and the aftercooler was fouled. How could that be? The water was the same, controls were the same, the chemicals were the same? Flowrates the same, H/E design the same? Everything you could identify was the same. There was only one thing that was different.  The cooling tower sump on the new unit was 2x the old one. This would not seem to be as big a deal as it was. The solution was to partition the new basin. Problem went away. 

2. Exothermic heat of dissolution makes a salt to have temperature inverse solubility. Typical  example are Cal carbonate , Cal phosphate, Cal sulfate, Mag silicate etc.

3. I tend to agree with most of what has already been stated, however one can be "fooled" by ignoring important anions in the water, the balance of bicarbonate with carbonate, etc., how much silica is dissolved in the water (not merely suspended), all have a role to play.  I have witnessed waters that are marginally stable with respect to scale indices still produce a scale that has nothing to do with calcium carbonate, and the water was not saturated with respect to magnesium silicate.  The critter that forms is a complex ferric calcium-magnesium aluminosilicate, possibly arising from silt particles (as in clay) that were present in the water as SDI.  A bugger to be rid of too, requires something like concrete remover if going the chemical route.  I suspect cold scale removal by CO2 or Nitrogen systems (CONCO), might be very successful on scales such as this.

   2 Comments

   1. The principle of cryo-blasting the scale is that it happens rapidly enough with dry ice powder or with liquid nitrogen, that the scale cools first, does not particularly transfer heat from the metal to cool it much, and therefore shrinks drastically first, and becomes a  powder that the gases leaving the tube carry with the flow.

   2. very interesting post by James Stewart. Can you please elaborate on cold scale removal that is mentioned. I am learning about this the first time. Thank you.

4. It depends on what else is present in the water. If there is a lot of nitrate, the solubility of both will or can be large. The same is true with chloride. carbonate will decrease both as temperature increases due to invert Ksp's (solubility products). 

5. Dear James McDonald, PE, CWTCWT, it is the same like in any water. But, you must declare what you mean in term "industrial system", as Ca & Mg solubility also depends on the total dissolved solids. 

6. Calcium and Magnesium exhibit inverse solubility with relation to temperature. As the temperature increases, the solubility decreases significantly. In most open cooling water systems, a decent rule of thumb for maximum allowable skin temperature on a heat exchanger is 130F. Above this, the likelihood of calcium and magnesium precipitations increases substantially. 

    

   Again this is also dependent on concentration of the calcium and magnesium species in the bulk solution.