How often do you think about your cooling tower or the fill that provides the cooling engine for your process? Unfortunately, if you’re like many plant operators, your cooling tower is but one piece of equipment in your large facility, and its ranking on your priority list is probably lower than many other expensive and more intricate pieces of equipment in your plant.
One of the challenges this lack of priority can yield is a lack of in-depth knowledge for a critical piece of the puzzle: the cooling tower. Just how critical it is becomes evident in the summer months, during the hottest days of the year, when production must be unexpectedly scaled back because the return cooling water from the cooling tower is so high it’s negatively impacting plant efficiency.
Whether you have an existing cooling tower that needs some work done, or you’re looking to purchase a new cooling tower to meet a plant capacity expansion, one of the choices you’ll have is the fill around which the cooling tower is built. Choosing the right fill for your process and installation is an important decision since it can impact your plant’s operation for years to come.
How Fills Work
Before discussing fills it’s important to understand the basic function of a cooling tower. In general, cooling towers are fairly simple devices. The goal is to take water heated by your plant’s process and cool it down, removing the same quantity of heat that your process added to it so that you can re-use the water in a recirculating loop.
The cooling tower is essentially just a box built to contain the water to be cooled. Because the water is cooled via the evaporative heat and mass transfer of water, the box has components that enhance that mechanism, such as air-moving equipment and components that improve the interactions of air and water, namely “fill.”
There are many types of fills available to cooling tower repair/rebuild and manufacturing companies. Some basic examples are splash fills and film fills.
- Splash fills derive their name via the mode of cooling they provide: water splashes on fill surfaces, which increases the air/water contact.
- Film fills are so named because of the water film that forms on the surfaces of the fill. The generation of this very thin film of water on the fill’s surface provides a very high amount of surface area for air/water contact, and as a result, film fills can offer the most amount of cooling for a given volume amount. Since film fills came into existence in the 1960s, fill manufacturers have developed a wide variety of different fill types and designs to address different water quality challenges that plant operators face.
Water Quality a Key Factor in Fill Choices
Due to the potential large volume of material, fill can be a substantial purchase. However, choosing the wrong fill can have an even greater impact on your bottom line due to reduced efficiencies and lost production on a day-to-day basis, and a need to replace fill many years sooner than expected. The key to choosing the right fill for your plant lies in your water quality, matching that water quality with a fill design that is appropriate, and utilizing a companion water treatment program that complements both.
There are three main factors that are evaluated when looking at choice of fill: Total Suspended Solids (TSS), Biological Activity/Control (via Total Aerobic Bacteria [TAB] plate counts), and Oil/Grease content in the circulating water. These can also be supplemented by calcium, magnesium, and silica evaluations related to the scaling potential of the water; ammonia, sulfide, and nitrate evaluations related to the nutrients available to promote biogrowth; and information on the process being cooled, make-up water source, and circulating water cycles of concentration.
The three main factors are important because they characterize the solids available in the water to potentially plug the fill (TSS) and the potential items (biofilm growth and oils/greases) that would adhere the solids to the fill. The supplemental items help to provide a more complete picture of the potential difficulties for the water treatment program to keep dissolved solids from precipitating out and causing scaling issues, to keep biogrowth under control, and to understand what potential contaminants could be introduced to the system.
Common Macro-structure Film Designs
Fill designs vary in order to allow for different water qualities to circulate through the fill without negatively impacting the performance of the fill or causing structural issues to the tower from excessive weight gain. One of the most basic design parameters used to account for this is the macro-structure of the flute corrugation. Common macro-structure designs include cross-fluted, offset-fluted, and vertical-fluted corrugations. These designs are found in film fills, trickle fills, and modular splash fills.
To prevent plugging of fills, the most important factor for fill design is water velocity. When the velocity of the water is higher, there is less of a chance for suspended solids to settle out on the surface of the fill, and there are greater shear forces applied to any matter that has adhered to the fill’s surfaces. For this reason, the fills that are more resistant to fouling have vertical designs that maximize the force of gravity on the flowing water.
The impact of a vertical macro-structure applies to not only film fills, where it is widely acknowledged as a benefit for low fouling fills, but also to trickle fills and modular splash fills. In order to clarify this a little more, an understanding of the terms “trickle fill” and “modular splash fill” is required.
A trickle fill derives its cooling via water trickling along the fine strands that comprise the members of a trickle fill. As in a film fill, the trickling water forms a thin film along the many fine strands, but since the overall physical structure is not a solid sheet, there is much less surface area available to which fouling can adhere.
Another inherent aspect of trickle fills is the fact that the intersections of the fine strands provide numerous locations for the trickling water to split and thus reduce the water velocity as it moves through the pack. While that aspect of trickle fills generally helps their cooling performance when the pack is new and clean, it greatly increases the propensity for them to foul. The result is a fill that gives the impression of being a low-fouling product since it is somewhat see-through, but which is easily misapplied when the low water velocity of the trickling water is ignored. When that happens, the cooling performance can suffer drastically due to the greatly reduced airflow that results from foulant build-up in the fill.
Modular splash fills, on the other hand, are fills that cool in the same manner as typical bar or grid type splash fills: droplet generation. Splash fills, including modular versions, are typically the most fouling- forgiving types of fills available. This can be true even for modular splash fills that resemble trickle fills but include vertical macro-structure geometries and droplet generation points throughout the pack. While the efficiency of an “as new” modular splash fill may, on paper, not quite reach that of a true trickle fill, if installed in a tower that has fouling potential, the cooling efficiency in the installation can beat that of a fouled trickle fill in a short period of time.
For film fills, the tradeoff has historically been balancing cooling performance with fouling resistance. Fills with the highest efficiency are cross-fluted film fills that provide a lot of surface area for water film coverage and constant mixing and redistribution of both the water and the air throughout the fill section. From a fouling perspective those features that provide great cooling lead to great tendencies to foul.
In an effort to compromise between cross-fluted designs and fully vertical-fluted designs with the goal of achieving a fill design that balances cooling performance and fouling resistance, several variations of offset-fluted designs have come to the forefront of product offerings in the past decade. With their ability to match common cross-fluted fill performance with greater fouling resistance, they have become the largest product offerings by volume, and they have many years of field-proven performance backing them up.
Innovation Leads to Higher Performance, Low Fouling
The latest developments in fill designs have yielded both a further refinement of an “old” fill design and also a completely new design – all in efforts to keep evolving fill designs toward greater performance and narrowing the gap between “high performance” and “low fouling.”
An example is Brentwood’s newest cross-fluted fill design. Called ThermaCross™, it is a refinement of a 19-millimeter (three-quarter-inch) cross-fluted product. It includes a patent-pending MicroBoost™ design that maximizes the air-water interface along with a more vertical macrostructure for increased cooling performance and improved fouling resistance compared to shallower-angled 19 mm cross-fluted fills. The improved cooling performance enables it to perform up to 13% better than standard 19mm-spaced products.
Another example is Brentwood’s patent-pending ShockWave™ fill, which incorporates an enhanced vertical flute design. As with traditional vertical-fluted designs, there is a clear sight path visible through the air travel depth of the fill pack. Unique to the design is the way the diamond tube channel promotes full mixing of the air as it moves through the fill section while the microstructure promotes full utilization of the sheet surface for water filming.
The combination of these features leads to much improved cooling performance over traditional vertical-fluted fills. Another feature of the fill is the wide sheet spacing of 25.4mm (one inch) per sheet. Not only are the openings larger to permit larger particles to pass through the fill, but the fewer number of sheets increases the water loading per sheet for a given water flow. The increase in water loading helps keep water velocity high, resulting in commensurate fouling resistance.
Ensuring Years of Reliable Performance
The key to proper fill selection, especially if the goal is to provide a fill that will last for years, is to match the fill type with the water quality circulating in the cooling tower system. Reputable fill designers will offer guidelines based on the water quality parameters to help ensure the selection you make will provide years of reliable performance. The three main keys are TSS, Biological Activity/Control, and Oil/Grease content in the circulating water.
It’s also important to factor in cost. When designing a new tower, total purchase price of the cooling tower is largely defined by the size of the tower. An often-overlooked ramification of this is that for the same required amount of heat rejection, a smaller tower will have higher efficiency fill in it than a larger tower. Thus, if making a purchasing decision based solely on upfront cost, you may end up with a tower that meets your design requirements for the first week it becomes operational. However, if the high efficiency fill in the tower is not appropriate for the water quality, then the up-front savings can easily be eclipsed by higher operating costs or lost production from your facility.
Looking at the long-term operating costs, or plant efficiency gains over time, is a better way to evaluate the total costs to your facility and can show the true benefits of giving a little bit of initial performance away to reap the rewards of consistent output over years of problem-free operation.
About the Author
Bill Miller, P.E. has worked for Brentwood for over 20 years. He is the global technical lead for Brentwood’s Cooling Tower product line and also oversees the company’s research and development as the Director of Application Engineering. Miller has published and presented several papers at the Cooling Technology Institute (CTI) and Electric Power Research Institute (EPRI) conferences, and is a member of multiple CTI committees related to cooling tower performance, fills, drift eliminators, and materials. He is also a member of ASHRAE. He has a Bachelor of Science in Aerospace Engineering from Pennsylvania State University, and is a licensed Professional Engineer in the Commonwealth of Pennsylvania.
Brentwood is a leader in the development, engineering, and production of plastic solutions for cooling towers. By offering the most complete line of internal polymer components in the industry, including fills and drift eliminators, Brentwood works with customers to ensure they receive the best-suited products for their specific projects and applications. For more information, visit www.brentwoodindustries.com/cooling-tower/.
All photos courtesy of Brentwood.
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