Cooling water is used directly or indirectly to cool
chemical products, steel products, etc.
Generally, cooling water is used indirectly to cool process fluids
(liquids and gases) through heat exchangers. This system is called an indirect
cooling system. Solid products are often
cooled directly by spraying water through a nozzle. This system is termed a direct cooling system
and is used in plants handling solid products, such as iron works and food
factories.
Cooling water systems are classified as shown in Table 3.1
The demand for cooling water has a
great influence on the entire industrial water supply. To reduce the water requirements for cooling systems,
the changes have been made, such as changing from once through to recirculating
cooling systems and the reduction of blowdown water quantities by higher cycle
operation of open recirculating cooling systems.
Such effective utilization of the water causes concentration of
dissolved solids, resulting in frequent problems from corrosion, scale and
slime. Various cooling water treatment chemicals
are used to prevent these problems. The development of these chemicals and the
related technology makes possible more effective utilization of cooling water. Recent developments in treatment have made it possible to prevent the loss of
thermal efficiency in heat exchangers even under lower water
flow rates and higher outlet water temperatures. Application of advanced cooling water treatment
technology provides "energy saving operation'' of
cooling systems by reducing the speed of cooling tower fans and
decreasing the number of water circulating pumps.
Cooling water treatment chemicals can cause environmental
pollution because they are discharged with the cooling water. Therefore, nonpolluting chemicals have been
developed.
The cooling water treatment chemicals and technology
described here are mainly concerned with open recirculating cooling water
systems.
3.1 Outline
of Cooling Water Systems
3.1.1
Types and Features of Cooling Water Systems
(1)
Open recirculating cooling water
system
Figure 3.1 shows a typical water
flow in an open recirculating cooling system. In the open system, water which
has been heated by flowing through heat exchangers is cooled in a tower by
partial evaporation and the release of the latent heat of evaporation. In this way, the water is recirculated and
reused. This system is widely used to
cool products, refrigerants, etc., in oil refineries, petrochemical and other
plants. Fresh water is usually used in
this system.
(2)
Closed recirculating cooling water
system
In a closed recirculating
cooling water system, the water which is heated by passage through heat exchangers
or other equipment is cooled in a secondary cooler using seawater, air, or the
cooling water from the open recirculating cooling water system. It is then
recirculated and reused .
Usually water losses are low in
this type of system and concentration of dissolved solids by evaporation is not
a problem. The closed system includes
cooling systems for engines, bearings, air conditioners, etc. Figure 3.2 shows an example of the water flow
in a typical closed system.
(3)
Once through cooling water system
In a once through cooling
system, the water after being heated is discharged directly into a receiving
stream without recirculation. As this system
needs a large amount of water, seawater and underground water have been used. However, the once through cooling systems are
being used less frequently since the utilization of underground water is being
regulated due to ground subsidence. Figure
3.3 illustrates the water flow in this system.
This system may be used for
cooling secondary coolers in closed cooling water systems. Thermal power plants frequently install once
through systems using seawater because they require large amounts of cooling
water for the steam condensers.
3.1.2
Equipment and Materials in Cooling Water
Systems
(1)
Cooling tower
Cooling towers are used to bring
warmed water into contact with cool air (precisely, air of wet bulb temperature
below that of the water being cooled) to evaporate some portion of the water and
to release the latent heat of evaporation in order to lower the water
temperature.
Cooling towers can be classified
into two major types. One is the natural draft type where air is supplied by
natural convection and the other is the mechanical draft type where air is
supplied by fans (Fig. 3.4).
The mechanical draft type
includes forced draft and induced draft types. Also, they are classified as counter flow or
cross flow according to the flow direction of water and air. In the counter
flow type, air moves vertically upward through the packing, counter to the
downward fall of water. In cross flow
towers, air flows horizontally across the downward fall of water. Each type has its advantages and disadvantages,
and the most suitable type is selected depending on such conditions as
installment site and tower capacity.
To be continued in Part 2...
