Is cooling tower design affected by the type of fill material used? Testing shows how the materials used for cooling tower components such as fill affect cooling performance.
To help ensure optimum cooling performance, manufacturers of cooling tower components provide cooling tower designers with thermal fill design data. This includes pressure drop (PD), based on water loading and airflow, plus thermal performance data in the form of KaV/L, where K is the mass transfer coefficient, a is the heat transfer area/unit volume, V is the active volume/plan area and L is the mass water loading rate. The KaV/L figures are developed for each fill in a test cooling tower.
When using supplied KaV/L performance curves, it is important to understand that the test cooling tower is being used (figure 1). The best test cell for providing data will minimize the spray and rain zones so that their contribution to cooling is minimized. This allows the cooling tower designer to understand the real contribution of the thermal fill. As an example, for the total cooling in an actual cooling tower, the spray zone can contribute 10 to 20 percent; the rain zone 10 to 25 percent; and thermal fill 55 to 80 percent.
As noted, cooling tower fill manufacturers provides pressure drop and KaV/L data for their thermal fill products. These data have been provided as an important tool for new tower manufacturers or rebuilders (companies upgrading existing cooling towers), allowing them to size systems based on the customer requirements and provide the heat rejection needed.
System observations and tests have shown that the material of construction affects the pressure drop and KaV/L. The pressure drop and KaV/L values reported are based on rigid PVC, meeting Cooling Technologies Institute Standard 136, polymer material standard. If another material is used, the performance may vary.
In the past, our company did not provide de-rating factors for alternate materials because it was felt that over time, the materials would condition to perform more closely to the most commonly used plastic fill material: rigid PVC. This article will review test data that supports the need to de-rate thermal fills made of plastics that have low surface energy.
Testing Evaluates Low-Surface-Energy Polymers for Cooling Towers
A new cooling tower that has rigid PVC fill will require an average of 30 days of operation before the tower is at optimum performance. This is mainly due to the fact that the thermal fill, which is responsible for up to 80 percent of the total cooling, needs time to condition (complete wetting of the fill’s surfaces). The thermal fill may have surface processing agents or no surface oxidation, which will impede complete film formation. Our recommendation to customers is to run the acceptance test for new or rebuilt cooling towers 30 to 60 days after the start of operation so the actual long-term tower performance can be measured.
Some residual processing films can reduce the water-film formation on cooling tower fills. In addition, the use of alternate plastics can affect the thermal fill performance through resistance to water film formation (figure 1). The effect is referred to as non-wetting, and it is seen in plastics that have low surface energy. Table 1 provides comparisons of surface energy of various materials. The group that indicates low-surface-energy polymers has such a low surface energy that water will bead, making good film formation difficult.
Tests were conducted comparing the KaV/L and pressure drop for the same cooling tower fill designs made from rigid polyvinylchloride (RPVC) and the second from polypropylene (PP) plastics. The RPVC has 38 percent higher surface energy compared to polypropylene plastic. This higher surface energy makes rigid PVC more receptive to water filming. Table 2 shows the typical material characteristics of each plastic tested.
For the KaV/L test data provided to customers, the fills are conditioned as recommended to represent how the thermal fill will perform after startup conditioning. Likewise, for the evaluation low-surface-energy plastic, the fill were conditioned in a test cell so that they would have thermal performance similar to an operating tower after 30 to 60 days of continuous operation. Each fill was the same design; the only difference was that one was made of rigid PVC and one of polypropylene. After the first test, the conditioning period was extended to measure the thermal fills’ improved performance.
It was believed that cooling thermal fills made from a low-surface-energy plastic such as polypropylene would develop surface oxidation and form mineral deposits on the fill, which will aid in water film formation. Therefore, many designers did not change the thermal performance data relative to the type of plastic being used. It also was identified that some KaV/L curves are being used with data that has been developed with a process of spray painting with a flat (non-glossy) paint as substitute for recirculating water conditioning. This process would provide good data if the final tested fill had a wetting characteristic similar to the actual field-conditioning process. Our test shows that flat painting of the fill surfaces will not provide similar results; therefore, the KaV/L curves developed in this process should not be used for cooling tower design.
Tests were conducted by the Czech Technical University in Prague. The first step was conditioning of the fill in a special conditioning tank built for that purpose. As shown in figure 2, all tests were conducted in a test cooling tower with a plan area of 3.94 by 3.94 feet (1.2 by 1.2 meters).
Test Results for Low-Surface-Energy Polymers
The first conditioned sample of rigid PVC and polypropylene fills represented conditioning of about 30 days of cooling tower operation. It showed that the rigid PVC fill had close to a 7 percent higher thermal performance than the propylene film for lower water loadings (figures 3 and 4). This was seen in the water streams over the polypropylene fill compared to the thin-water film over the surface of rigid PVC fill. This streaming or beading effect reduces the surface area of water exposed to the airflow and also can increase pressure drop, both of which reduce thermal performance.
The tests were conducted at various water loadings. The tests noted the highest thermal performance loss for polypropylene fill was in lower water loading of 2 to 3 gal/min/ft2 (5 to 7.5 m3/hr/m2). It has been observed that the filming effect is proportional to the water loading. Higher water loadings overcome the resistance to wetting of low-surface-energy plastics.
The extended conditioning showed an improvement in thermal performance of both the rigid PVC and polypropylene fill; however, it was noted that the improvement of the rigid PVC fill exceeded that of the polypropylene fill. Cooling tower recirculation provides for the conditioning of fills by:
• Removing any possible film of processing agents.
• Allowing for surface oxidation.
• Providing a thin layer of mineral deposit.
These effects will be influenced by the mineral content of the recirculating water. Low mineral hardness in the cooling tower will extend the time required to condition fill.
Figures 3 and 4 show a comparison of thermal performance of rigid PVC and polypropylene fill over time for two water loadings. The rigid PVC fill provides higher thermal performance compared to polypropylene fill for both the lower and higher water loading. The most significant variation is seen in the much higher thermal performance of rigid PVC fill compared to polypropylene fill for the lower water loadings.
Consequences for Mechanical Draft and Natural Draft Cooling Towers
Mechanical draft counterflow towers typically have high water loading in the range of 4 to 8 gal/min/ft2 (10 to 20 m3/hr/m2). These towers will have a smaller performance loss when using low-surface-energy plastic. The highest performance loss is experienced for low-surface-energy plastic if used in natural draft cooling towers which operate at low water loadings in the range of 2 to 3.5 gal/min/ft2 (5 to 8.75 m3/hr/m2). Natural draft cooling towers will show the largest performance loss when using low-surface-energy plastic.
In conclusion, the thermal performance data (KaV/L) used for cooling tower design must take into consideration the plastic material used for the thermal fill. This can be accomplished by presenting the data for each specific plastic material or by way of a de-rating factor for the specific low-surface-energy plastic.
When using a low-surface-energy plastic such as polypropylene compared to rigid PVC, depending on the design conditions, you may need to use more energy for the same heat rejection. Based on our tests, for a low-water-loading design, a loss of 4 to 9 percent on thermal performance is expected with low-surface-energy plastics. Mechanical draft towers designed to achieve low approach temperatures (the difference between the wet bulb temperature and the cooling tower’s exit water temperature) typically will operate at low water loadings. Also, it is typical for natural draft towers to be designed with low water loadings due to their low air-side pressure drop requirement for proper operation. For a natural draft tower, this loss of thermal performance will equate to a larger shell height and higher pumping power consumption for new tower designs. For existing systems, it can mean as much as a 1°F (0.5°C) hotter cold water temperature. This increase in cold water temperature will affect condenser operation in power production or other heat exchange functions in chemical, petrochemical, food processing or general industrial operations – and negatively affect productivity. Because natural draft towers are used on larger systems, the opportunity lost is in the millions of dollars. PC
James Wallis is a business development consultant, and Rich Aull is director of cooling tower application engineering for Brentwood Industries Inc., Reading, Pa., a manufacturer of cooling tower components such as fill. For more information from Brentwood Industries, call 610-236-1100 or visit www.brentw.com