Types of cooling tower
Cooling towers may be broadly classified as (i) induced draught, (ii) forced draught and
(iii) cross flow.
Induced-draught towers consist of a tall casing with a fan at the top, rather similar to that shown diagrammatically in Figure 11.1. The casing of the tower is packed with a material, the purpose of which is to provide a large wetted surface for evaporative cooling. Many different materials are in use, ranging from wooden slats to plastics. The performance of a cooling tower does not depend on the type of filling used but rather on its arrangement and on the uniformity and effectiveness of the water distribution. A constant air velocity through the filling is also important in securing good performance.
The induced-draught tower has the advantage that any leakage will be of ambient air into the tower, rather than vice versa. This means that there is less risk of the nuisance caused by water and humid air leaking outwards to the vicinity than arises in other towers. An important advantage is that humid air and drift (if any) leaving the tower is discharged at a fairly high velocity, with some directional effect. There is thus much less risk of short — circuiting, and the influence of wind pressure is reduced. It is true, however, that the fan and water, being in the stream of humid air leaving the tower, are more liable to corrosion, but adequate anti-corrosive protection should minimise this disadvantage.
Forced-draught towers, as their name implies, push air through the tower. Figure 11.3 illustrates this. Leakage is out of the tower but the fans and motors are not in the humid airstream. Short-circuiting is something of a problem, and forced-draught towers should, therefore, not be used in a restricted physical environment.
Fig. 11.3 Forced draught cooling tower.
Although the height of a tower is of importance in producing a long path for contra-flow evaporative heat exchange to take place, the cross-section of the tower is also important because the larger its value, for a given height, the greater will be the wetted surface area available. It follows that the volume occupied by a tower dictates its cooling capacity, rather than height alone. A choice is thus offered in choosing a tower: a tall tower or a short tower may be adopted, but in either case the volume will be about the same. Because of their unsightly obtrusion on skylines, cooling towers are sometimes criticised by architects and town planners. There is, then, sometimes considerable pressure to reduce their height, and low silhouette towers are chosen.
The cross-draught tower provides a low silhouette, as Figure 11.4 shows. Such towers are really a combination of induced-draught and cross-draught, but are designated crossdraught, nevertheless.
Fig. 11.4 Cross-draught cooling tower.
The casings of towers may be of metal, fibreglass, wood or concrete, or a combination of these. It is best to use the least corrodable material possible, both for the casing and the fill. For this reason, and also because of the opportunity it presents of giving the tower an architecturally harmonious anonymity, cooling-tower casings and ponds are sometimes erected by the builders; only the fill, the fan and the motor, and the water piping, being installed by the mechanical services’ sub-contractors.
The design of towers is continually undergoing change, much of it effective. As a result, towers of great compactness are sometimes possible. One such type of tower uses a rotating water header coaxial with the induced-draught fan. This, coupled with the use of a cellulose fill, rather like corrugated cardboard, which presents a very large surface area, produces a uniform distribution of water and constancy of airflow.
An indirect cooling tower is sometimes used. This is similar to the arrangement in Figure
11.1 But the tubes shown crossing the middle of the tower convey cooling water for the condenser, not refrigerant. The cooling water provided is then not polluted by the spray water and the airstream in the cooling tower. On the other hand, heat transfer between the water in the piping and the stream of air and spray water outside the piping is less efficient.
There are also forms of tower that do not use mechanical draught. One of these uses the density difference of the warm humid air within the tower and the cooler air outside. Its application is for power stations and it is not used for air conditioning because of its bulk. Another type of tower that sometimes does find application in air conditioning uses water jets to induce airflow in parallel with the water stream. Such towers are either downflow or crossflow. Although they are relatively quiet and free from vibration, air velocities into and out of the towers are low and they are consequently susceptible to changes in wind speed and direction. They are best suited to cases where some variation in the cooling water flow temperature is tolerable.
To provide a small margin, that covers operation beyond design conditions, it is good practice to select cooling towers to operate in a wet-bulb that is one half to one degree above the design value adopted for the rest of the system.
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