Problem 12.10: You are called to evaluate a cooling tower
From Chapter 12 Problem Set. Level 3: Optimization.
Question
You are called to evaluate a cooling tower that is consuming significantly more makeup water than expected. The conductivity controller is functioning and holding tower conductivity at 2,400 µS/cm. Makeup conductivity is 600 µS/cm. Based on the calculated cycles, the makeup meter shows water consumption approximately 30% higher than the mass balance predicts. Identify at least three possible explanations for the discrepancy and describe how you would investigate each one.
Answer
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Tower cycles are 2,400 ÷ 600 = 4 cycles. If the tower is truly holding 4 cycles, then excessive drift or a tower-water leak would not normally explain makeup above the predicted value. Those losses remove concentrated tower water, so they act like uncontrolled blowdown. The conductivity controller should respond by reducing controlled blowdown, and total makeup should remain governed by evaporation and cycles. If the concentrated losses became larger than the required blowdown, tower conductivity would fall below setpoint.
More consistent explanations are: (1) Evaporation is higher than assumed. The heat load, condenser water flow, tower range, or operating hours may be higher than the values used in the calculation. Investigate by verifying condenser water flow, entering and leaving condenser water temperatures, tower range, and chiller load. (2) The makeup meter is over-reading. The meter may have an incorrect multiplier, scaling error, installation issue, or mechanical problem. Investigate by checking the meter setup and comparing it against a portable flow meter or controlled fill test. (3) Some metered makeup water is not actually reaching the tower. A branch line, hose bib, washdown connection, or other user may be connected downstream of the meter. Investigate by tracing the piping and isolating branch lines. (4) A makeup-line leak may exist downstream of the meter but upstream of the tower. This would increase metered water use without changing tower conductivity. Investigate by closing the tower makeup isolation valve and checking whether the meter continues to move.