Cooling tower types

Heat rejected to the environment by most industries is of Httle consequence. Cooling flows of air or water are deployed over equipment or through heat exchangers and the relatively small quantities of heat are dissipated to the surrounding air. Small cooling towers, often of the evaporative type, have become ubiquitous in an industrial faciUty. 

Performance of a given type of cooling tower is governed by the ratio of the weights of air to water and the time of contac t between water and air. In commercial practice, the variation in the ratio of air to water is first obtained by keeping the air velocity constant at about 350 ft/(min ft of ac tive tower area) and varying the water concentration, gaL/(min ft of tower area). As a secondaiy operation, air velocity is varied to make the tower accommodate the cooling requirement. 

FIG. 12-14 Sizing chart for a coiinterflow induced-draft cooling tower, for induced-draft towers with (1) an iipspray distributing system with 24 ft of fill or (2) a flume-type distributing system and 32 ft of fill. The chart will give approximations for towers of any height. (Ecodyne Carp.)... 

Figure 12-16 illustrates the type of performance curve furnished by the cooling-tower manufacturer. This shows the variation in performance with changes in wet-bulb and hot-water temperatures while the water quantity is maintained constant. 

Recent developments in cooling-tower fan energy management also include automatic variable-pitch propeller-type fans and inverter-type devices to permit variable fan speeds. These schemes involve tracking the loaa at a constant leaving-water temperature. 

Figure 30-lC is distinctly different from the first two in the type of SO2 control processes used and the sequence of the particulate matter and SOj controls. It is a promising approach for up to 90% SO2 control of western United States coal, and there is a single waste product. Other features include the collection of particulate matter at temperatures below 90°C and the possibility for spray dryer cooling tower water integration. This. system may or may not include a catalytic NO unit. 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

Another type of crossflow cooling tower is the wet-dry tower, which consists of a normal crossflow tower over which a few air coils are placed. The hot water is first cooled by an air cooled heat exchanger and then drops to the wet cooling tower where more cooling is obtained by the evaporative mechanism. Figures 5 and 6 provide examples. In contrast, deck-filled towers contain tiers of splash bars or decks to aid in the breakup of water drops to increase the total water surface and, subsequently, the evaporation rate. 

Figure 5. Single structure type wet-dry cooling tower.

The wet deck or surface, is the heart of most cooling towers. Generally, it takes the form of PVC (polyvinyl chloride) plastic film type surface. Water is made to spread out on this surface maximizing it s contact area with air to encourage evaporation. It consists of individual vacuum formed sheets with proprietary patterns of ridges. 

Cooling tower fan motors must be properly selected for long life and trouble free operation. In addition, it is important to match the motor s enclosure to the application. Motors in draw through applications that are mounted directly in the air stream, for example, experience tough duty and must be of the totally enclosed type. Motors are categorized as follows ... 

Gear boxes are used on just about all large field erected cooling towers and many factory assembled towers- all of the propeller ty e fan, draw-through type. Almost all are right angle gear drives with the input shaft horizontal and the output... 

The quantity aAp is defined separately for each type of cooling tower. It depends on many variables jet pressure, jet division, airflow velocity, and others. The total energy balance for a cooling tower is (see Fig. 4.19)... 

Regenerator The hot and cold fluids pass alternately through a space containing solid areas/particles, which provide alternately a heat sink and a heat source. An example of the rotating-type matrix is a cooling tower. 

Most cooling towers are built of redwood or cypress. However, special conditions and atmospheres dictate other types of construction. 

The fill illustrated in Figures 9-1 lOA and B is typical of many cooling tower heat transfer evaporative cooling surfaces. The wooden splash type is the oldest in terms of length of usage, while the film types (some fabricated of plastic) have been in service about 40 years [148]. 

Figure 9-11 OB. Representative generic types of fiii for cooling towers. Used by permission of the American Institute of Chemical Engineers, Beyer, A. H., Chemical Engineering Progress, V. 89, No. 7 (1993) all rights reserved.

Gears should be specified as American Gear Manufacturer s Association (AGMA) requirements for cooling tower service in order to ensure an adequate minimum service factor rating of 2.0. The spiral bevel type is probably used a litde more often than the worm gear. It is also cheaper. When gears are used with induced draft applications, the... 

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