Getting Your Fill
Not all cooling tower fills will meet your application's performance and environmental requirements, so choose carefully. Here's a primer to help you out.
Changes in cooling tower components have been at a pace that can only be considered glacial when you realize that the first evaporative cooling systems date as far back as 2000 B.C. and the first thermal designs for industrial systems date to the late 1800s. However, in the last 10 years, there have been significant increases in cooling tower fill and drift-eliminator designs driven by the power, refining, chemical, petrochemical and food processing industries. This renaissance was motivated by a need for improved performance, improvements in material and manufacturing techniques, and attention to environmental and OSHA concerns.
There are two broad categories of fills: splash and film. Splash fills typically are used where water quality is poor and where fill fouling can occur (table 1). Splash fills work by breaking up the hot circulating water into small droplets that create an increased surface area, which allows for both convective and evaporative cooling. Typical splash fills have about half the thermal performance of film fills. The lower thermal performance is due to the splash fill's inability to equal the surface area of film fills coupled with the higher pressure drop of splash fills.
Splash fills may be considered the original fill, and their design types are numerous (figure 1). The designs can be grouped into two categories. The first is splash-fill profile designs that include extruded "V" bars, flat bars, convex profiles and net shapes. The second category is grid packs, which usually offer somewhat higher performance but still function mostly as splash fills because they break up the hot water into smaller droplets.
The most commonly used cooling tower fills are film fills. They form a thin layer of water over the fill surface and drive cooling performance by having a large surface area of water film in contact with the cooling tower air combined with lower pressure drop (air-side pressure drop). The KaV/L (a measure of the rate of evaporative and convective cooling reported as a nondimensional number) and pressure drop combine to create the relative thermal performance of the fill.
The measurements are critical in evaluating film fills. Similarly designed fills do not equate to similar performance. Variations in surface structure (microstructure), materials of manufacture, leading-edge squareness, quality of manufacturing and quality of final assembly of the fill pack play a major role in overall thermal performance. It is important to have a fill that has documented thermal data on a well-designed test cell.
An example of thermal data that does not reflect the fill's actual performance was documented on an inexpertly manufactured cross-fluted 30-mm spaced fill. It had a reported KaV/L rating similar to a design having 12-mm spacing and about 2.2 times greater surface area. The reason for the very high KaV/L was realized when the test cell was visited. The cell included a 6.6' (2 m) high spray zone with many high-pressure spray nozzles and a 32.8' (10 m) high rain zone. The reported high KaV/L was achieved not due to any significant contribution by the fill but by the major contribution to the cooling process from the spray and rain zone. While it is important to have accurate thermal data, it is equally important to minimize the spray- and rain-zone effects in the test cell.
Film fills are classified into four categories (figure 2):
Cross-fluted fills for counterflow or crossflow towers.
Vertical-offset fills for counterflow towers.
Vertical-flow fills for counterflow towers.
Crossflow standoff fills for crossflow towers.
Cross-fluted designs provide high thermal performance and have alternating fill sheets at 60o angles to one other, creating redistribution of water at each sheet interface. They also offer improved water distribution in the direction of the fill pack. For optimum water distribution, many experts in the area of fill recommend that the fill installation be done in alternating layers, each at a 90o angle to the adjacent layer.
The results of many field applications indicate that alternating layers do not always add to fouling potential. Fouling can occur anywhere in the fill section with lower potential at the bottom and top. Many results indicate there is no appreciable difference in fouling potential between one continuous pack and multi-layer packs because fouling potential is dictated by fill design. Besides offering improved distribution, multi-layer fill packs can be inspected easily. A drawback of using one continuous fill pack is that if fouling occurs, the weight gain makes fill removal difficult.
For an air-travel height of 6' (1.8 m) in a counterflow tower, it is recommended to have three 2' (0.61 m) fill-pack heights with the immediate layer at 90o to the top and bottom layer. This arrangement increases water mixing and helps ensure even distribution of air and water, optimizing tower performance.
Vertical-offset fills are a newer design. Like cross-fluted fills, there are redistribution points throughout the fill pack. However, the majority of the water film travels in a vertical path, which keeps the water-film velocity high and reduces potential fouling. The water-film velocity inhibits the growth of bio-film, a key ingredient to fouling (figure 3). Combining the larger flute openings and the higher water-film velocity makes the vertical-offset fills less prone to fouling. Thermal performance of a fill is a combination of its KaV/L and pressure drop. Even though the vertical offset design typically has slightly lower KaV/L compared to similar cross-fluted fills, the lower KaV/L is offset by its lower pressure drop, which gives it similar overall thermal performance to cross-fluted fills.
Vertical-flow fills are designed to address poor water-quality applications. The design directs the water in a vertical path, and microstructure and capillary features allow for water mixing and lateral water distribution (figure 4). The higher water velocity through the fill reduces fouling potential. A typical vertical-flow fill has 10 percent to 15 percent lower thermal performance compared to a cross-fluted or vertical-offset fill. However, combining a vertical-flow fill with a high performance fill such as cross-fluted or vertical-offset on the top 1' (0.30 m) will improve overall performance. Using a high performance fill as the top layer of a vertical-flow design likely will not create additional fouling. The spray effect on the top layer keeps the water-film velocity high and reduces fouling potential in the top layer.
Crossflow standoff fills are used in crossflow towers with air travels of 3 to 8' (0.9144 to 2.438 m). These fills are installed by hanging the sheets, or bottom, supporting the assembled fill packs. The fill sheets are separated by molded-in standoffs (spacers) and have a sheet-surface structure designed to distribute the falling water evenly over the surface. Some end users find the hanging sheets create a maintenance headache due to sheets breaking off the hangers, which produces thermal performance and/or replacement problems. Bottom-supported fills eliminate this issue. Crossflow standoff fills provide an opportunity to upgrade splash fill crossflow towers to achieve potential thermal improvement of 25 percent to 40 percent. Also, because the standoff fills have clear open spaces between the sheets, fouling is less of a concern unless high fiber content or an unusual process contamination are present.
After selecting the fill design, key elements in specifying the cooling tower fill include materials of construction, material thickness, fill rating, glued or non-glued installation, water temperature and testing.
Materials of Construction. The Cooling Technology Institute provides a material standard CTI Standard 136 for rigid PVC for cooling tower service. This quality of PVC has been the most accepted cooling tower fill material because it is durable, flame resistant, stable in various water conditions and easily formed. Cooling tower fill is exposed to outdoor elements; therefore, it is recommended that rigid PVC be compounded with UV inhibitors.
Material Thickness. The most commonly used material thickness is 0.010" (0.25 mm). For applications where there is excessive wear due to maintenance conditions or unique spray impingement, the material thickness of 0.015" (0.38 mm) should be considered. Always specify thickness after forming. If only a thickness is specified, it may be interpreted as starting gage, which would give you significantly thinner material.
Rating. Request both KaV/L and pressure drop data with test cell information.
Non-glued Fill. Consider using non-glued, mechanically assembled fill, which has two important advantages. First, if the fill packs are to be assembled at your site, the environmental and safety issues of gluing packs may prohibit field assembly. Second, unglued mechanically assembled systems address long-term environmental problems. ISO14001 directs the use of best available technology to protect the environment.
Temperature. Specify the highest circulating-water temperature to which the fill will be exposed. CTI Standard 136 specifies a heat-deflection temperature of 160oF (71oC) for PVC, which is adequate for most applications. However, high temperature PVC, PP and other plastics also may be considered.
Testing. Specify that the fill is tested for ASTM E84, the standard for flame spread. The minimum fill rating should be 20 or less.
Drift eliminators are another aspect of cooling tower systems that should be considered. They are designed to contain the circulating water in the cooling tower system. Excessive drift from a cooling tower can cause localized equipment corrosion, environmental contamination, and health and safety problems such as the spread of Legionella.
There are two basic designs of drift eliminators: blade and cellular (figure 5). Both work by separating droplets from the airstream through inertial impaction. The straight-line path of a droplet in the airflow will impact on the drift eliminator surface, form a water film and drain back into the wet section of the cooling tower.
Drift eliminator performance is measured by drift tests that express drift loss as a percent of the recirculation rate. Initially, a sensitive-paper test was designed for drift loss measurement, and it is relatively accurate at high drift rates. However, with the required lower drift loss rates, CTI has adopted the new heated bead isokinetic (HBIK) test that directs the sampling of cooling tower exhaust air and retains the dissolved minerals leaving the tower. Measuring the dissolved minerals and stoichiometrically comparing the result to the dissolved minerals in the recirculating water achieves a direct measurement of drift loss. The selection of a drift eliminator requires the consideration of the targeted drift rate needed, combined with an acceptable pressure drop.
The cellular drift-eliminator design provides lower drift rates because it has more surface-for-droplet impaction. The current best technology for the cellular design offers 0.0005 percent drift loss. The drift rate depends not only on the drift eliminator but also relates to its proper position in the tower, which must be above the spray area so no spray impacts the drift eliminator. A mapping of the airflow should show a consistent air velocity throughout the plenum. The air velocity should not exceed the drift eliminator design maximum. The drift eliminator should be installed so that there are no openings, or gaps, through which droplets can bypass.
The fill and drift eliminators should be regarded as the heart of a cooling tower. The best approach to improving cooling tower performance is to contact a manufacturer or experienced rebuilder and explain your needs. The Cooling Tower Institute (www.cti.org) is a good source for these companies. PCE