Problem 5.10: The chapter opens with a story about 2000
From Chapter 5 Problem Set. Level 3: System Integration.
Question
The chapter opens with a story about 2,000 pounds of scale depositing on cooling tower fill after an acid feed failure. Using carbonate equilibrium, explain the chain of events: what left the water, what shifted, what formed, and why pH was the gatekeeper.
Answer
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When acid feed stopped, CO₂ stripping continued unchecked. As CO₂ left, equilibrium shifted: carbonic acid decomposed, bicarbonate consumed H⁺, and pH rose. Once pH passed ≈8.3, carbonate was no longer suppressed - it was permitted to exist. With calcium already present and 60,000 sq ft of wetted surface area, CaCO₃ precipitated naturally and relentlessly. pH did not cause the scale - it gave carbonate permission.