A big amount and various types of pumps are used in process of power plant. Below is a typical arrangement with main pumps shown in each process.

① Vertical axial flow/mixed flow pump, Split casing pump

② Vertical mixed flow pump/vertical turbine pump

③ Condensate pump

④ Boiler feed pump

⑤ Slurry pump

⑥ Vertical mixed flow pump/vertical turbine pump/vertical sump pump

⑦ Split casing pump

⑧ Desulfurization pump

Fig. 3.1 Flow diagram of an open recirculating cooling water system

A typical flow diagram of an open recirculating cooling system is shown in Figure 3.1. In this sys- tem, cooling water removes heat from process flu- ids passing through heat exchangers and the tem- perature is risen. The hot water is cooled in a cool- ing tower by the partial evaporation and the release of the latent heat of evaporation. Thus the water is recirculated and reused. This system is widely used to cool products, refrigerants, etc., in petro- leum refineries, petrochemical and other plants. Fresh water is usually used in this system.

(2) Closed recirculating cooling water system Figure 3.2 shows an example of flow diagram of a closed cooling water system. In this system, the water heated by passing through heat exchangers or other equipments is cooled by secondary cool- ers using seawater, air or the cooling water from an open recirculating cooling water system. It is then recirculated and reused.

The water loss is usually small in this system and the concentration of dissolved solids by evapora- tion scarcely occurs. The closed system includes cooling systems for engines, bearings, etc., chilled water systems for air conditioning and so on.

(3) Once through cooling water system In an once through cooling water system, the water after being heated is discharged directly into a receiving stream without recirculation. As this system requires a large amount of water, it is being

Fig. 3.2 Flow diagram of a closed recirculating cooling water system

Fig. 3.3 Flow diagram of an once through cooling water system

used less frequently except the system of seawater supply. Figure 3.3 illustrates the flow diagram of this system.

This system is used for cooling products in facto- ries or closed recirculating cooling water with the secondary coolers. In Japan, thermal power plants frequently install once through systems using sea- water because they generally locate near sea- coasts.

3.1.2 Equipments and Materials in Cooling Water Systems

(1) Cooling towers

Cooling tower is used to cool hot water for the recycling. In the cooling tower, hot water contacts with cold air and some portion of the water evapo- rates. As the result, the hot water is cooled by releasing the latent heat of evaporation.

Cooling towers are 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 fan. The mechanical draft type includes forced draft

Fig. 3.4 Types of cooling towers