BACKGROUND

The chemical process industry, petroleum refining,

utilities, and many other industries, use large quantities

of water for cooling.

There is an increased emphasis being placed upon

the re-use of cooling water by means of cooling towers.

The cooling effect is obtained by the evaporation of a

small fraction of water and heat exchange with the air

passing through the cooling tower. The problems

encountered in cooling systems are not usually with

the equipment, but with the water. As the water evap-

orates, the dissolved solids concentrate. These

impurities in water cause scale and corrosion in the

heat exchange equipment.

PROCESS

There are many variations in cooling towers and heat

exchange design. A common feature is the control of

the water quality with the use of continuous pH and

conductivity measurement while maintaining a given

set of conditions. This is done to further minimize

corrosion and protect the equipment. Continuous

recirculation of the cooling water causes the concen-

tration of the impurities to increase. The relative

concentration of the impurities in the water is

measured by a contacting conductivity sensor, such

as a Model 400 sensor. A conductivity controller initiates

the opening of a blowdown valve when the conductivity

becomes too high. This causes a demand for make-up

water, which is less concentrated in impurities, and

thus lowers the conductivity.

Most of the impurities in cooling water are alkaline,

which is indicated. The alkaline impurities, especially

calcium carbonate, are less soluble at high pH values.

Therefore, a small quantity of sulfuric acid is added to

the circulating water to lower the pH value and thus

prevent the formation of solids (scale). The ideal pH

sensor for this application is the general purpose

Model 3900. The Langelier Index is a factor obtained

from calcium hardness, alkalinity, pH, conductivity,

and temperature, and indicates scaling potential.

For cooling water containing a high level of suspended

solids, a toroidal conductivity sensor such as the

Model 228 and a fouling-resistant pH sensor such as

Model 396P are recommended.

Corrosion and scaling are further minimized by the

addition of chemical scale and corrosion inhibitors.

Inhibitors are fed based upon one of three methods: on

acid demand, on opening the blow down valve, and on

operating the make-up water valve. Inhibitors are fed

on acid demand when the acid pump runs and/or when

the inhibitor pump runs. On opening the blow down

valve, the inhibitor pump adds the inhibitor to another

part of the system and precisely balances the loss of

inhibitor by blowdown.

The warm water and air also produce an ideal environ-

ment for biological growth. To control algae and slime

growth, biocides (i.e., chlorine or bromine) are added

on a time basis, such as a given quantity once per day,

twice per week, once per week, or by one of the above

methods. Chlorine levels can be monitored using the

chlorine sensor Model 499ACL.

Dispersants are added to prevent coagulation or

flocculation of suspended solids (dust, living microor-

ganisms, dead cells, etc.). Dispersants are added in

the same manner as inhibitors.

Ozone treatment is a powerful alternative to chemical

treatment and will reduce operating costs significantly

while increasing safety. Unlike most chemicals, ozone

has a half-life of only 20 minutes and will not be found

in blow down water. The dissolved ozone sensor

Model 499AOZ is intended for continuous measure-

ment of dissolved ozone between 0 and 10 ppm. Use

of ozone treatment has seen a growing interest for

cooling tower control and is a cost-effective solution for

many applications.

Cooling Water Control

Theory

Application Data Sheet

ADS 43-006/rev.D

November 2010

INSTRUMENTATION

For high solids or coating applications, use the following sensors:

Model 400 Conductivity Sensor

• Fast and easy start up with predetermined

factory cal constant (not for 1055)

• Rugged titanium electrodes

• Versatile mounting configurations for

screw-in, retractable, or flow-through

Model 3900 General Purpose

pH/ORP Sensor

• Extended sensor life provided by

double junction reference.

• Maximum chemical resistance

provided by rugged polyphenylene sulfide body,

completely sealed to eliminate sensor leakage

• Optimum versatility with various mounting options.

Model 499ACL Free Chlorine

Sensor

• Measures free chlorine without

sample pretreatment or reagent

• Automatic correction to 9.5 pH

• Easily replaceable membrane with no special tools

required

Model 499AOZ Dissolved Ozone

Sensor

• Rugged construction

• Automatic pressure equalization

maintains correct membrane tension

• Quick cable to sensor disconnect — Variopol option

Model 228 Toroidal Conductivity

Sensor

• Toroidal measurement principle greatly

reduces sensor fouling problems.

Model 396P pH/ORP TUpH Sensor

• Polypropylene reference junction and helical

pathway mean longer sensor life in process

solutions containing heavy solids.

Model 1056 Analyzer

• Multi-Parameter Instrument – single

or dual input. Any combination of

pH/ORP/ISE, Resistivity/Conductivity,

Chlorine, Oxygen, Ozone, Turbidity,

Flow.

• Large Display – easy-to-read process

measurements.

• Seven Languages: English, French, German, Italian,

Spanish, Portuguese, and Chinese.

• HART and PROFIBUS DP Digital Communications.