SECTION II STRUCTURAL COMPONENTS
H. Fill (heat transfer surface)
The single most important component of a cool-ing tower is the ill. Its ability to promote both the maximum contact surface and the maximum con-tact time between air and water determines the eficiency of the tower. And, it must promote this air-water contact while imposing the least possible restriction to air low. Maximum research and devel-opment effort goes into the design and application of various types of ill, and technological advances are cause for celebration.
Most reputable cooling tower manufacturers design and produce ill speciically suited to their distribution, fan, and support systems; developing all in concert to avoid the performance-degrading effects of a misapplied distribution system, or an air-impeding support structure. Those who are less meticulous will adapt commercially available com-ponents (ill, fans, driveshafts, distribution systems, etc.) into the shape and appearance of a cooling tower, relying upon the laboratory ratings of these components to remain dependable in less-than-laboratory conditions.
The two basic ill classiications are splash type (Fig. 70) and film type. (Fig. 71) Although either Figure 69 — Mechanical equipment mounted on torque tube, before installation of fan blades. Note retaining guards for
driveshaft.
Figure 70a — Splash type fill: wood splash bars. Figure 70b — Splash type fill: plastic splash bars.
Figure 71 — Film type fill.
type can be applied in crosslow or counterlow coniguration, counterlow towers are tending to-ward almost exclusive use of the ilm ills. Crosslow towers, on the other hand, make use of either type with equal facility, occasionally in concert.
Splash type fill breaks up the water, and inter-rupts its vertical progress, by causing it to cascade through successive offset levels of parallel splash bars. Maximum exposure of the water surface to the passing air is thus obtained by repeatedly arresting the water’s fall and splashing it into small droplets, as well as by wetting the surface of the individual splash bars. (Fig. 17)
Splash ill is characterized by reduced air pres-sure losses, and is not conducive to clogging.
However, it is very sensitive to inadequate support.
The splash bars must remain horizontal. If sagging occurs, the water and air will “channel” through the ill in separate low paths, and thermal performance will be severely impaired. Also, if the tower is not level, water will gravitate to the low ends of the splash bars and produce this channeling effect.
Long term performance reliability requires that the splash bars be supported on close centers, and that the support materials be as inert as practicable.
Of the various support mechanisms presently in use, iber reinforced plastic grid hangers are recog-nized as having the longest history of success, with PVC coated wire grids also enjoying considerable use. In utilizing coated carbon steel grids, however, care must be exercised to assure that the splash bars will not abrade the coating, exposing the wire to corrosion.
Treated wood lath (primarily Douglas Fir) pre-dominated for many years as splash bar material, and continues to be extensively used because of its strength, durability, availability, and relatively low cost. (Fig. 70a) Currently, however, plastics have gained predominance. They may be injection mold-ings of polypropylene, or similar materials which can be compounded for resistance to ire; or they may be extrusions of PVC (Fig. 70b), which inher-ently has a low lame spread rate. Stainless steel or aluminum splash bars are occasionally used in steel framed towers where totally ireproof construction may be mandatory.
Film type fill causes the water to spread into a thin ilm, lowing over large vertical areas, to
pro-mote maximum exposure to the air low. (Fig. 71) It has the capability to provide more effective cool-ing capacity within the same amount of space, but is extremely sensitive to poor water distribution, as well as the air blockage and turbulence that a poor-ly designed support system can perpetuate. The overall tower design must assure uniform air and water low throughout the entire ill area. Uniform spacing of the ill sheets is also of prime importance due to the tendency of air to take the path of least resistance.
Because the ill sheets are closely spaced in the highest performance ill designs, the use of film fill should be avoided in situations where the cir-culating water can become contaminated with debris. A diverse range of clog resistant ill designs are available, with a progressively lower perfor-mance capability increasing with fouling resistance in general.
Film ill can be made of any material that is ca-pable of being fabricated or molded into shaped sheets, with a surface formed as required by the design to direct the low of air and water. Because PVC is inert to most chemical attack, has good strength characteristics, is light in weight, has a low lame spread rate, and can be easily formed to the shape required, it is currently the most popular ma-terial.
I. DRIFT ELIMINATORS
As a by-product of the cooling tower having pro-moted the most intimate contact between water and air in the ill, water droplets become entrained in the leaving air stream. Collectively, these solid water droplets are called “drift” and are not to be confused with the pure water vapor with which the efluent air stream is saturated, nor with any drop-lets formed by condensation of that vapor. The composition and quality of drift is that of the circu-lating water lowing through the tower. Its potential for nuisance, in the spotting of cars, windows and buildings, is considerable. With the tower located upwind of power lines, substations, and other criti-cal areas, its potential as an operating hazard can be signiicant.
Drift eliminators remove entrained water from the discharge air by causing it to make sudden changes in direction. The resulting centrifugal force separates the drops of water from the air, deposit-ing them on the eliminator surface, from which they low back into the tower. Although designers strive to avoid excess pressure losses in the movement of air through the eliminators, a certain amount of pressure differential is beneicial because it assists in promoting uniform air low through the tower ill.
Eliminators are normally classiied by the number of directional changes or “passes”, with an increase in the number of passes usually accompanied by an increase in pressure drop. They may consist of two or more passes of spaced slats positioned in frames (Fig. 72) or may be molded into a cellular conigura-tion with labyrinth passages. (Fig. 73) Some towers
Figure 72 — Two-pass “herringbone” drift eliminators of wood construction.
Figure 73 — Three-pass “cellular” drift eliminators of PVC construction.
Figure 74 — Drift eliminators molded integrally with fill sheets.
Figure 75 — Casing extended to handrail height. Figure 76 — Crossflow air inlet face. (Note apparent lack of louvers.)
that utilize ilm type ill have drift eliminators molded integrally with the ill sheets. (Fig. 74)
Since drift eliminators should be as corrosion resistant as the ill, materials acceptable for ill are usually incorporated into eliminator design, with treated wood and various plastics (predominantly PVC) being most widely used.
In the decade of the 1970s, concern for the pos-sible environmental impact of drift from cooling towers stimulated considerable research and de-velopment in that ield and, as might be expected, signiicant advances in drift eliminator technology occurred. Currently, the anticipated drift levels in smaller, more compact towers will seldom exceed 0.008% of the circulating water rate. In larger towers, affording more room and opportunity for drift-limiting techniques, drift levels will normally be in the region of 0.001%, with levels of 0.0005%
attainable. (Sect. V-H)