Fundamental cooling tower issue

January 05, 2017 0 Comments

A company operates a 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 microS. The secondary makeup source was

well water at 480 microS. The operators noticed that when the tower is running

on well water, they were able to maintain the proper conductivity of 1,700

microS in the cooling tower, but when the tower is running on RO water, they are

only able to maintain a conductivity of 700 microS 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?

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

ΔT: 10 °F

Evaporative Cooling: 90% (tropical climate)

Conductivity Setpoint: 1,700 microS

RO Makeup Conductivity: 175 microS

Well Makeup Conductivity: 480 microS

The evaporation rate was:

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

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

Well Cycles = 1700/480 = 3.54

RO Cycles = 1700/175 = 9.71

The cycles they were currently running on RO water was:

RO Cycles Current = 700/175 = 4

The amount of blowdown required under normal operation was:

Cooling Towers 222

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.

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