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. As pH climbed through the 8s, the carbonate fraction increased sharply, making calcium carbonate precipitation increasingly favorable. With calcium already present and 60,000 sq ft of wetted surface area, CaCO₃ precipitated naturally and relentlessly. pH did not cause the scale, but it gave carbonate permission.