Building CodesBuilding codes usually will note approved materials of construction and will reference fire-protection standards such as NFPA 214 and Factory Mutual. (The Factory Mutual approval guide for tested towers includes a list of component manufacturers and materials.) Building codes might also address noise, wind and seismic standards that will influence material decisions for a system's structural and housing components, as well as the fill and drift eliminators.
Corporate and Insurance Carrier GuidelinesCorporations set policies and practices to protect corporate assets, provide for the most efficient operations and attend to the health and safety of their employees and neighbors. Reviewing corporate practices and past experience can help you avoid future problems. For example, companies that are certified under ISO 14001 must consider the best possible practices regarding the environment. Cooling tower fill and drift eliminators that are assembled on the project site typically are assembled using glue, and many glues contain volatile organic compounds (VOCs). Selecting an assembly method that eliminates gluing such as mechanical assembly will address ISO 14001 considerations.
Your insurance carrier also likely has cooling tower guidelines. A common requirement is to use materials that are rated self-extinguishing per ASTM D635 or have a flame spread rating of 25 or less per standard E84. The Cooling Technology Institute (CTI) has set material standard 136 for cooling tower fill and drift eliminators (table 1). Wood and some plastic materials such as polypropylene (PP) do not meet these standards. Materials that do meet these standards include rigid polyvinyl chloride (PVC), some fiberglass-reinforced plastics (FRP), metal and ceramic.
When selecting materials, you should request specific test results and not rely on a supplier's claims of flame retardancy. “Flame retardant” is not a standard. In fact, materials such as PP fill that have been marketed as “flame retardant” have been observed to burn aggressively and far exceed the CTI flame spread standards.
Performance RequirementsThermal and Drift Performance.Material selection can affect the thermal performance of your fill and the drift rate of your eliminators. Materials with a lower thermal performance will reduce the ability of the tower to cool the water and also will increase the system's operating costs. Materials with a high drift loss will waste water and can cause corrosion, contamination and legionella transmission due to the amount of aerosol leaving the tower.
The thermal performance of a fill is measured by its convection/evaporative transfer characteristic (KaV/L) and pressure drop. The higher the KaV/L and the lower the pressure drop, the colder the water and the lower your energy costs. Plastics can be formed in engineered shapes and can provide a high surface-area-to-volume ratio and reduced airflow resistance. These benefits have made plastics the preferred material for fills and drift eliminators. However, different plastics will exhibit different thermal performance, structural capacity and long-term durability characteristics, even when using the same product design and material thickness. For example, PP is hydrophobic and will result in a lower KaV/L because the water does not film as effectively. Over time, PP fill will show improvement in KaV/L, but it takes typically six months or more to improve the performance close to that of rigid PVC fill. This lower performance can increase operation costs and reduce production, and often can cost companies millions of dollars per year. Using rigid PVC can provide a conservative 3 percent thermal improvement compared to PP. At a 500 MW power plant, this improvement could save a minimum of $250,000/year.
For drift eliminators, 0.0005 percent of recirculation is the “gold standard” for drift loss. This rate can be achieved with engineered plastics.
Water Quality.The cooling tower's required makeup water quality and airborne or process contamination limits typically affect the fill product design more than the material selection. However, material selection can be important in some situations. For example, if the circulation water contains high amounts of oil, the oil will bond to many fills, especially rigid PVC and PP. Other minerals and airborne contaminates also can bond with the oil and foul the fill. Wood and ceramic materials are the best material choices for these systems because they resist oil bonding.
Structural Loads.Every fill material and fill pack design has a particular load-carrying ability. Because plastic material deforms over time, both the current load-carrying ability and creep must be considered for the proper selection of material and thickness. For example, a 19 mm cross-corrugated fill pack using CTI standard 136 rigid PVC in a 10 mil (0.254 mm) thickness has a design load capacity of 700 lb/ft2(3,400 kg/m2). Considering a creep factor of 1.6 for a 20-year life, the design load of this fill would be 440 lb/ft2(2,150 kg/m2). Compared to PP, the PVC is more rigid and will carry approximately 50 percent greater load for the same thickness (figure 1). PP also has a higher creep factor of 2.5 for a 20-year life (figure 2).
Temperatures.Returning hot water is distributed to the cooling tower by a spray system. Only about 10 percent of the cooling occurs in this spray system, so the top layer of cooling tower fill will need to handle the hot water temperature after the spray zone (table 2). Key items to consider in this application are the hot water temperature, the duration of time at this peak temperature and the expected temperature at each fill layer. Materials suitable for high-temperature applications are high-temperature PVC, PP, stainless steel and ceramic. Typically only the top layer requires the higher temperature material.
Cold temperatures in some climates can also be a consideration. Both fills and drift eliminators can experience subzero temperatures and snow/ice loads in cooling towers during standby conditions. Materials that can be used in these severe applications include 20 mil (0.5 mm) PVC and high-impact acrylonitril butadiene styrene (ABS).
The materials used for your cooling tower fill and drift eliminators can affect the equipment's performance. By considering issues such as local building codes, corporate and insurance guidelines, and performance requirements, you can be sure to choose the right materials for your cooling tower. PCE
Jim Wallis is international director of Brentwood Industries Inc., Reading, Pa., a manufacturer of cooling tower fill and drift eliminators, and he is also the managing director of Brentwood Asia Ltd., Bangkok, Thailand. Richard Aull is engineering manager of Brentwood Industries Inc. For more information, call (610) 236-1100; visit www.brentwoodindustries.com or www.cti.org; e-mail firstname.lastname@example.org or email@example.com.