Problem 12.6: A tower operates at 5 cycles based on

From Chapter 12 Problem Set. Level 2: Applied Thinking.

Question

A tower operates at 5 cycles based on conductivity. Lab results show:

MAKEUP: Chloride = 48 mg/L; Calcium = 160 mg/L as CaCO₃

TOWER: Chloride = 245 mg/L; Calcium = 640 mg/L as CaCO₃

1. Calculate the chloride-based COC and the calcium-based COC.
2. Is the mass balance intact? If not, what percentage of calcium is leaving the water as scale?
3. What would you recommend as the next diagnostic step or treatment response?

Answer

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Chloride COC = 245 ÷ 48 = 5.10. Calcium COC = 640 ÷ 160 = 4.00. The mass balance is not intact. Chloride-based COC exceeds calcium-based COC, meaning calcium is leaving the water as scale. Expected calcium at 5.10 cycles = 160 × 5.10 = 816 mg/L as CaCO₃. Actual = 640 mg/L. Calcium lost = (816 − 640) ÷ 816 = 21.6% precipitating as scale. Next steps: evaluate LSI/RSI at current conditions, check condenser approach temperature for fouling evidence, review scale inhibitor residual and dosing, and consider whether cycles need to be reduced or inhibitor chemistry adjusted.