Problem 3.10: A cooling towers makeup water reads 400 Scm

From Chapter 3 Problem Set. Level 3: System Integration.

Question

A cooling tower’s makeup water reads 400 µS/cm. The circulating water reads 1,600 µS/cm. Estimate the cycles of concentration and the circulating TDS. If the makeup contains 80 mg/L calcium, what is the approximate circulating calcium, and why does the answer matter more at the heat exchanger surface than in the basin?

Answer

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Cycles = 1,600 ÷ 400 = 4 cycles. Circulating TDS ≈ 0.7 × 1,600 = 1,120 ppm. Calcium ≈ 80 × 4 = 320 mg/L. Calcium carbonate has inverse solubility, so the hottest surface in the system is where ΔG turns over first. The basin may stay clear while the exchanger tubes scale.