COOLING WATER TREATMENT

Most of the water employed for industrial purposes is used for cooling a product

or process. The availability of water in most industrialized areas and its high heat

capacity have made water the favored heat transfer medium in industrial and

utility type applications. Direct air cooling is finding increasing use, particularly

in water-short areas but is still far behind water in total numbers of applications

and total heat transfer loading.

During recent years, the use of water for cooling has come under increasing

scrutiny from both environmental and conservational points of view and as a

result, cooling water use patterns are changing and will continue to do so. For

example, many systems pass cooling water through the plant system only once

and return it to the watershed. This creates a high water withdrawal rate and adds

heat to the receiving stream. On the other hand, cooling towers permit reusing

water to such a large extent that most modern evaporative cooling systems reduce

stream withdrawal rates by over 90% compared to once-through cooling. This

substantially reduces the heat input to the stream but not to the environment,

since the heat is transferred to the air.

These changes in cooling water system design and operation have a profound

impact on the chemistry of water as it influences corrosion, deposition, and fouling

potential in the system. This chapter reviews the industrial operations which

use water for cooling purposes, the problems of corrosion, scale, and fouling in

these systems and how these problems affect plant production through loss of heat

transfer, equipment failures, or both. In addition, various cooling water treatment

concepts are examined and the control procedures required for their success are

discussed.

HEATTRANSFER

Heat transfer is simply the movement of heat from one body to another, the hotter

being the source and the cooler the receiver. In cooling water systems, the

product or process being cooled is the source and cooling water the receiver.

Cooling water usually does not contact the source directly; the materials are

usually both fluids, separated by a barrier that is a good conductor of heat, usually

a metal. The barrier that allows heat to pass from the source to the receiver is

called the heat transfer surface, and the assembly of barriers in a containment

vessel is a heat exchanger.

In many industrial heat exchangers both the source and receiver are liquids. If

the source is steam or other vapor that is liquefied, the heat exchanger is called a

condenser; if the receiver is a liquid that is vaporized, the exchanger is called an

evaporator.

The simplest type of heat exchanger consists of a tube or pipe located concentrically

inside another—the shell. This is called a double pipe exchanger (Figure

38.1). In this simple exchanger, process liquid flows through the inner tube

and cooling water through the annulus between the tubes. Heat flows across the

metal wall separating the fluids. Since both fluids pass through the exchanger only

once, the arrangement is called a single-pass heat exchanger. If both liquids flow

in the same direction, the exchanger is parallel or cocurrent flow; if they move in

opposite directions, the exchanger is a countercurrent type.

Progressing from this exchanger, more sophisticated units are designed to

improve the efficiency of the heat exchange process. Figure 38.2 shows a shelland-

tube exchanger. Process fluid and cooling water could be located on either

side of the barrier.

Another simple heat exchange device is the jacketed vessel, with cooling water

passing through the space between the double walls of a chemical reaction vessel,

removing heat from the process. This design is like a thermos bottle, but in this

case, the double wall is used for heat removal instead of insulation. Plate-type

heat exchangers, somewhat resembling plate-and-frame niters, are used in many

chemical process industries because of their compact design and availability in a

wide range of materials of construction.

Removing Undesirable Heat

Once the water completes its job and cools the source, it contains heat that must

be dissipated. This is accomplished by transferring heat to the environment. In

once-through systems cool water is withdrawn, heated, and returned to a receiving

stream, which subsequently becomes warmer. In this type of system each

pound (0.454 kg) of cooling water is heated I

removed from the source.

In open recirculating systems, water is evaporated; this phase change from liquid

to gas discharges heat to the atmosphere instead of to a stream. Evaporating

water dissipates about 1000 Btu per pound (555 cal/kg) of water converted to

vapor. When evaporation is used in the cooling process, it can dissipate 50 to 100

times more heat to the environment per unit of water than a nonevaporative system.

(This is explained in more detail in a later section of this chapter.)

0F (0.560C) for each Btu (0.252 cal)

Sensible Heat Transfer

The two most common ways heat is transferred from process fluid to cooling

water in the heat exchange process are conduction and convection. Heat flows

from a hot fluid through a heat exchange surface to the other side by conduction.

Heat is then removed from this hot surface by direct contact with cooling water

i.e., by conduction. Subsequently this heated water then mixes with other cooler

water in a heat transfer process called convection.

The five factors controlling conductive heat transfer are:

1. The heat transfer characteristics (thermal conductivity) of the barrier.

2. The thickness of the heat transfer barrier.

3. The surface area of the barrier.

4. The temperature difference between the source and the cooling water (the driving

force).

5. Insulating deposits on either side of the barrier.

Of these five factors, the first three are inherent in the design of the exchanger.

Items 4 and 5 are operational characteristics that change depending on the conditions

of service. Deposits on either side of a metal barrier have a lower thermal