Cooling towers are some of the most cost-effective cooling systems today. They remove unwanted heat from water by bringing cool air and warm water into contact, transferring or rejecting the heat via evaporation.
Wet and dry cooling towers differ in how they remove heat. Most cooling towers are wet, while air-cooled heat exchangers are dry. However, both designs use an axial fan to move air inside the tower, feature a covering to contain the fan and funnel the air into the fan and have plenums (specific spaces) to direct the air, allowing the heat to be transferred by direct or indirect contact. chiller refrigerator
Cooling towers are categorized as either crossflow or counterflow towers. In crossflow towers, the water and the air move almost perpendicular to each other, while in counterflow towers, the water and air move in opposite directions. In addition, cooling towers can be classified as induced-draft or forced-draft towers. In induced-draft towers, the fan, commonly at the top of the tower, pulls the air through the tower’s fills. Conversely, in forced-draft towers, the air is pushed toward the fill media from the tower’s base.
Cooling tower mechanical upgrades can significantly improve efficiency while increasing reliability and performance. Investing in fan and drive system upgrades can lead to major energy savings, reduce maintenance costs and extend the cooling tower’s life span. In this discussion, we will focus on three mechanical improvements:
System efficiency is one of the best ways to reduce energy costs and increase airflow for the cooling system to run at its best. Fan design should not be based on a “one size fits all” concept but rather a carefully designed airfoil custom-built for the cooling tower’s specific duty conditions. A low-drag airfoil shape designed with features such as high-blade twist, wide-chord width and superior finish will result in high efficiency levels.
The most efficient blade designs feature a seamless, hollow construction to ensure durability. They are made from lightweight, corrosion-resistant materials such as fiberglass-reinforced plastics from polyester or epoxy resin, depending on the application. The lightweight designs ensure a low moment of inertia, reducing wear and stress on the cooling tower’s motor, bearings and drive system.
Ultimately, a properly designed cooling tower fan with the design features outlined above will provide more air flow with less power consumption than the typical metallic fan blades supplied by many cooling tower manufacturers. It is common to realize power savings of 10%-40% with custom-designed blades.
Cooling towers come in all shapes and sizes and can utilize several forms of power transmission, including gear, belt and direct drives. Our focus will be on large-capacity cooling towers that require lower fan speeds and higher torque.
The most common method used in these larger towers is a mechanical gearbox, with a high-speed input from an electric motor that sits outside the turret and is connected via a drive shaft to the gearbox input shaft. The gearbox’s output shaft, which sits vertically, is directly coupled to the fan blade. These gearbox systems are supplied in various frame sizes in primarily single- and double-reduction configurations. Horsepower (hp) and reduction ratios vary depending on the drive size. Typical horsepower capacities range from 7.5 to 150 hp, with ratios varying from 5:1 to 70:1.
Cooling tower gearboxes are needed to drive the cooling tower fan blade, which develops airflow through the tower—introducing an exchange of heat. The fan drive application is often exposed to extreme environmental conditions with large temperature swings, moisture, chlorine and chemical exposures—depending on the tower’s design.
In the fan drive application, the gearbox’s output shaft is connected to the fan blade, typically in a vertical position. This creates a heavy thrust load on the shaft and increases the exposure of the output shaft seal to moisture and contamination. Heat loads on these gearboxes can be large, often requiring cooling fins in the gearbox housing to dissipate heat faster.
Key features improving drive efficiency
As is the case in most mechanical drive systems, there are plenty of opportunities to extend drive life and reduce downtime by paying attention to the simple things. One challenge with cooling tower fan drives is that the drive is not easily accessible, resulting in an “out of sight, out of mind” mentality. A preventive maintenance plan is the best way to mitigate unscheduled downtime and cooling tower failure.
Here are some proactive steps that can prevent unnecessary downtime costs:
Vibration analysis: Consider installing permanent vibration sensors that provide real-time, remote vibration data. Monitoring for trends can identify issues before failure occurs.
Regular lubrication and oil analysis during scheduled outages: Check for leaking seals, ensure proper lubrication levels and periodically analyze fluid for contaminants and wearing of internal materials.
Proper choice of lubrication: Consider synthetic lube oil over petroleum-based lube oil for longer changeout intervals and higher heat loads. Work with your gearbox supplier for the optimum lubrication choice.
Proper repair: Partner with an experienced repair provider to ensure that quality bearings, seals, materials and craftsmanship are used.
Other factors can affect overall cooling tower efficiency, such as feed water quality, water circulation and chemical usage. However, focusing on fan design and the drive system will provide the largest increase in efficiency and the quickest return on improvement investment.
cooler heat exchanger Scott Smith is division manager – West Shops at Motion. A certified fluid power specialist, he has over 35 years of experience in the fluid power and industrial distribution business, including more than 12 years with Motion. Smith holds a degree in manufacturing engineering from the Oregon Institute of Technology. For more information, visit motion.com.