Problem 12.3: A facility manager notices that the

From Chapter 12 Problem Set. Level 1: Conceptual Mastery.

Question

A facility manager notices that the conductivity controller is holding its setpoint perfectly, and concludes that the cooling tower chemistry must be under control. Explain why this conclusion might be wrong. What diagnostic would you use to verify the mass balance, and what would the results tell you?

Answer

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Conductivity tracks total dissolved solids but does not distinguish between species. If calcium carbonate is precipitating as scale, its contribution to conductivity disappears – but the controller does not know this. Makeup continues entering, the conductivity setpoint holds, and the true concentration of scaling species drifts from what conductivity reports. The diagnostic is to compare cycles of concentration calculated from a soluble tracer ion (chloride) against cycles calculated from a scaling-risk ion (calcium). If chloride-based COC is significantly higher than calcium-based COC, calcium is leaving the water as scale. The divergence quantifies how much.