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Case Study: Cooling Tower Control Problem

By James McDonald, PE, CWT Originally Published: CSTN - September 2002

A client had a large, three-cell cooling tower system that was having problems maintaining conductivity. The cooling tower had two possible makeup water sources. The primary makeup source was tertiary RO water from wastewater treatment at 175 µS. The secondary makeup source was well water at 480 µS. The operators noticed that when the tower is running on well water, they were able to maintain the proper conductivity of 1,700 µS in the cooling tower, but when the tower is running on RO water, they are only able to maintain a conductivity of 700 µS even with the blowdown valve completely closed. What was happening? Before you continue with this article, look at the figure below and take the time to consider what

could explain this. Why was the cooling tower unable to maintain the conductivity setpoint on RO water but could on well water?

Tertiary ROMakeup

Well WaterMakeup

CoolingTower

Blowdown

Process

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Faulty Conductivity Control? The conductivity controller was able to properly maintain conductivity on well water. The conductivity controller was working fine.

Blowdown Valve Leaking? There was an air gap between the blowdown line and the drain. The blowdown valve was not leaking past.

Uncontrolled Water Loss? The answer was that there was an uncontrolled water loss somewhere. This water loss was bigger than blowdown required when the cooling tower was on RO water makeup, but smaller than the blowdown required on well water.

Was It Drift or a Leak? We inspected the cooling tower and saw no drift or overflow from the tower that would explain this sudden change in operation. There must have been a leak, but how big of a leak? Using the cooling tower operational parameters, we calculated just how big the leak was.

Operational Parameters • Recirculation Rate: 11,227 gpm

• ∆T: 10 °F

• Evaporative Cooling: 90% (tropical climate)

• Conductivity Setpoint: 1,700 µS

• RO Makeup Conductivity: 175 µS

• Well Makeup Conductivity: 480 µS The evaporation rate was:

Evap = 11,227 * 10 * 0.9 / 1000 = 101 gpm

Under normal operation, the cooling tower cycles would have been:

CyclesWell = 1700/480 = 3.54

CyclesRO = 1700/175 = 9.71

The cycles they were currently running on RO water was:

CyclesRO Current = 700/175 = 4

The amount of blowdown required under normal operation was:

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BDWell = 101 / (3.54 - 1) = 39.8 gpm

BDRO = 101 / (9.71 - 1) = 11.6 gpm

The blowdown required when they currently ran on RO water was: BDCurrent RO = 101 / (4 - 1) = 33.4 gpm Notice that this number is smaller than the blowdown required for well water but much higher than the blowdown required for RO water under normal operation conditions. This clearly shows why the cooling tower could maintain the conductivity setpoint on well water but not on RO water. Since the blowdown valve was completely closed when they were currently running on RO water, this meant that there was a 33.4 gpm leak somewhere. A sizeable leak indeed!

Finding the Leak The cooling tower system was used to cool several rather complicated processes with many heat exchangers, so finding the leak was not going to be simple. I had to leave the facility before the leak was found, but the water management associate in charge had planned to question the operators in the area for possible leaks. If this didn't work, red dye was going to be added to help track the leak down.