Cooling towers offer a proven technology for rejecting heat from condenser water and industrial processes. To maximize the operating cost savings, the fill media — the heat transfer surface over which the water flows — should be optimized for heat transfer and in good condition.
The fill is the heart of the cooling tower system. Efficient operation and the overall cooling system performance depend on three factors:
- Maximizing the surface area of the fill.
- Distributing the water across the surfaces evenly.
- Optimizing the thickness of the water film.
After years of operation, however, any cooling tower’s fill media can degrade to the point where it must be replaced. It can be difficult to evaluate the claims by various manufacturers about which type of replacement fill is the best choice. The fact that manufacturers promoting their own products may offer competing claims further complicates matters.
One good way to understand which fills are optimal for the application is through independent research. A side-by-side comparison conducted in a real-life operational situation provides an objective evaluation. The competing products could be observed for months or years to determine both how well they perform at the time of installation and how they stand up over time.
This is exactly what happened when a large professional sports arena in southern California replaced the fill in its cooling towers. (While the sports arena obviously is not a process application, the lesson learned here would apply to industrial process applications as well.)
At the facility, three cooling towers with a crossflow design had been in use for 18 years. While they were performing adequately, due to their age, the building staff decided the fill should be replaced as part of a long-term maintenance plan.
Hanging Fill vs. Block Fill
The facility opted to replace the fill pack in one cooling tower per year rather than taking all three towers offline at once. This staggered installation timeline provided an unusual opportunity to compare the performance of the new fill in the years immediately after it was installed.
In the first tower and in the second tower a year later, the existing cooling tower fill was replaced with a hanging-style replacement fill that matched the fill originally installed. When it came time to replace the fill in the third tower, the building owner decided to use a different product, a block-style fill.
The cooling tower has a crossflow design, meaning that the water flows vertically down the fill as air flows horizontally across it. Hot water from the system enters the cooling tower and is distributed over the fill (heat transfer surface). Air is drawn through the fill, causing a small portion of the water to evaporate. This evaporation removes heat from the remaining water, which is collected in the cold-water basin and returned to the system to absorb more heat. The cooling tower was designed to use hanging sheet fill. Long, continuous sheets with patterns embossed to increase the surface area are hung to allow smooth water flow from top to bottom.
The block-style replacement fill is a fundamentally different design. Blocks are comprised of corrugated layers of PVC sheets, with wavy sheets sandwiched between layers of flat sheets. The 12-by-12-by-42-inch blocks are stacked vertically.
FIGURE 1. A test protocol was developed to assess and compare the performance of the electrical bacteria deactivation system versus biocides alone on the bulk water bacteria count in the system and the measurement of the biofilm sensor signal.
Initial Hesitations about Block Fill
The cooling tower fill manufacturer raised concerns about whether the block-style fill would perform adequately, and whether it was compatible with the cooling tower design. The vertical spacing of the block fill is wider than the hanging fill, providing less physical surface area than the hanging fill. Reduced surface area results in less evaporative cooling.
Another concern was that water would not flow smoothly and evenly through the block fill. Within each block, where the corrugated layers are connected to each other, water flow can be impinged. Moreover, the transition between blocks significantly interrupts the flow.
In addition, the connections between the flat pieces and the corrugated layers, as well as the transitions between blocks, tend to trap solids. This can lead to scale buildup and fouling, resulting in unacceptable degradation in performance in just a few years.
Testing Plan to Compare Performance
After a discussion among the building owner, maintenance staff and installation contractor crew, the parties agreed the block fill would be installed. An independent testing company, American Air Balance Co., would be retained to conduct tests comparing the performance of the cell with the block fill to the performance of one of the cells with the hanging-style fill.
The thermal testing was performed shortly after the block fill was installed. The chosen time was during a peak usage time when all three cooling towers were running at full speed. When the two cooling towers were tested, Cooling Tower 1 (CT-1) with block fill performed at 72 percent of the original fill’s thermal capacity. Cooling Tower 2 (CT-2) with the hanging-style fill performed at 96 percent of the original fill’s thermal capacity.
The cell with block fill was tested again by the independent lab 18 months after installation and again at 28 months. The results showed an additional performance drop-off during that 28-month period.
FIGURE 2. The measurement of biofilm potential signal through the observation period is summarized.
First Cost vs. Total Cost of Ownership
This real-life experiment with side-by-side testing showed that the type of fill can make a big difference. The cooling towers in this case were designed to use hanging fill. Replacement with a third-party block fill significantly reduced evaporative cooling performance. After the testing showed performance deficiencies of the block fill, it was replaced with the hanging-style fill.
While the block fill may have had a lower initial purchase price, it ended up costing more in energy consumption, maintenance costs and shorter useful life than if the OEM replacement fill had been selected. The design and quality of the fill are critically important to the cost-effective, efficient and reliable performance of the evaporative cooling tower.
1. “Selection Program GPM” is a calculated prediction of how many gallons per minute of water a Series 3000 unit with the original fill could cool with a given air inlet temperature; water inlet and outlet temperatures; and power consumption. The “Selection Program GPM” in this table above states that each of CT-1 and CT-2 with the original fill has the thermal capacity to cool 2,799 gallons of water per minute from specific entering water temperature (°F) — 82.1, 91.2, 91.4, or 82°F — to 73°F at 60°F wet-bulb while consuming 35 kW of power. Each of these same towers with the original fill can also cool 1,984 gallons of water per minute from 82° to 71°F at 61°F wet-bulb while consuming 36 kW of power.
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