A cooling tower does its job day in and day out, often 24 hours per day. It operates in a hot, wet and corrosive environment. Keeping equipment failures to a minimum is paramount. Component selection and maintenance impact cooling tow
Structure and CasingMost large process cooling towers have a wood structure (typically Douglas fir or redwood) encased with fiberglass siding. The outer surface of the lumber is treated with a preservative chemical to prevent fungal attack that might lead to internal rot. All wet cooling tower lumber will be affected by fungus eventually. Untreated cracks in the lumber accelerate the process, and iron or ferrous steel fasteners used in the structure can lead to black rot.
Periodically treating the wood structure with a light organic solvent chemical preservative helps to extend the effective life of cooling tower structures, particularly in the plenum area. Tri-butyl tin oxide is one compound used for this treatment. In addition, it is important to perform periodic inspections of the wood structure for signs of rot. Surface probing with a pointed instrument is the most common test method.
Many new cooling towers are built with fiberglass structures. The material resists moisture and most chemicals found in cooling towers. Fiberglass casing generally is durable but lightweight, so heavier grades are recommended for longer unit life. Casing material also is available as pultruded fiberglass panels, including the top deck of the tower. Over several years, sunlight will cause the fiberglass casing to discolor and weather. Painting the casing's outside surface will significantly extend useful life.
Fill MediaInside the cooling tower is fill material that helps hot water evaporate. Splash bar fill is designed to break up water into small droplets as it falls through the tower. Splash bar fill works especially well if cooling water is dirty or contains organic material. In a freezing climate, it is important to prevent splashing water from freezing at the tower's air inlet. Return water temperature should not be allowed to drop below 50°F (10°C), and the outboard portion of fill should be flooded with extra water to keep ice under control. Heavy ice buildup can break inlet louvers and complete sections of fill.
Film fill is used in all types of cooling towers. Film fills typically are made from polyvinyl chloride (PVC) sheets arranged in a closely spaced pack. Hot water spreads out in a thin sheet on the fill and evaporates. Individual fill sheets may be embossed with corrugations or surface dimples to improve water distribution and increase heat transfer rate.
Film fills generally are not suitable for water that contains significant amounts of suspended solids or dissolved organic material. If the organic or solid loading is too high, fill will grow deposits of bacteria, silt and slime. If unchecked, this growth will progress rapidly until it completely plugs the fill. To prevent this, keep cooling water clean and free of organic material. Continuous treatment with biocides such as chlorine, bromine, ozone or a nonoxidizing biocide is recommended. The addition of side-stream filtration or cyclone-type separators also is helpful.
New film fills are designed to offer a slower clogging rate compared to previous de-signs. These low clog rate fills incorporate vertical, tubular passages that provide fewer spots for silt and slime to attach.
Plugged fill may harbor the Legionnella Pneumophila bacteria that causes Legionnaires' Disease. If small droplets from cooling tower drift are inhaled, a person may be exposed to the bacteria. Information about preventing Legionnaires' Disease is provided in American Society of Heating Refrigerating and Air-Conditioning Engineers Guideline 12-2000 Standard, and the Cooling Technology Institute's Legionellosis Position Statement WTP-148 (96).
Air-Moving EquipmentMost large process cooling towers have a propeller fan on top of the tower driven by an electric motor through a set of reduction gears. Variations on this theme are used but they tend to be on smaller commercial towers.
Following the maintenance instructions provided by the cooling tower manufacturer should extend the tower's useful life. Weekly visual inspections that include checking the reduction gear lubricant level and making note of any unusual sounds or vibrations often will catch a problem when it is small and easy to correct.
The cooling tower fan has the toughest job in the tower. While constantly pumping air through the tower with blade-tip speeds of more than 130 mph, the fan is pelted with small droplets of water carryover. Most large cooling tower fans are constructed of hollow fiberglass. Epoxy fiberglass resin has an inherent toughness against continuous attack by drift particles; even so, after several years in operation, leading-edge erosion is inevitable.
Blade repair can be performed with flexible overlays made of polypropylene or Neoprene. Tower operators should not wait until erosion eats completely through the glass fibers, or structural failure can occur. All blade-attaching hardware should be tightened to manufacturer's specifications. Fiberglass blade material can creep under constant-clamping pressure, and hardware can become loose over time. Tightness of blade-attachment hardware should be verified during annual maintenance checks. Good maintenance practices will prevent catastrophic fan blade failures.
Gear Box and Drive ShaftGear boxes are heavy duty and designed to last for years of continuous service, but they must be full of clean oil to achieve their design life. The weakest link in cooling tower gear drives is the high-speed shaft seal. Synthetic rubber lip-type seals typically are used to keep oil in the unit. These seals usually last three years before they become worn and start leaking. Dirt and heat can cause seal hardening and scoring of the gear-input shaft. Change the oil seal every few years or specify mechanical face seals that have hardened wear surfaces.
The long drive shaft connecting the motor and gear box requires little maintenance except the flexible elements at each end that allow for misalignment. If the shaft has stainless steel shim packs as flex elements, it should be checked periodically for signs of cracking. Stress cracks can develop after a few years and lead to serious failures. Replace the packs at both ends of the shaft if cracks are noted.
Elastomeric-type couplings and flexing elements are common as well. While typically lasting for more than five years, annual evaluation for unusual wear or deterioration of the elastomer material is advised. Carbon fiber-epoxy drive shafts are more expensive, but they are lightweight, smooth running and durable.
The MotorMost cooling tower motors today are the heavy duty, mill and chemical type. Some cooling tower motors are designed specially for cooling tower service. Unique shaft seals, lubricants and extra insulation are available to keep moisture away from critical motor surfaces. Because of the constantly wet cooling tower environment, the motor must have open drains at low points in the motor housing. If these drains plug, motor bearings may be forced to operate in water collected from condensation, and eventually they fail from rust.
A cooling tower has a tough job. The process depends on the cooling tower to operate continuously for years, with a minimum of maintenance and repair. Costly, unexpected cooling tower breakdowns can be avoided through researched component selection and proper maintenance.