Cooling towers
Considerations in fan design and mechanical drive systems.

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.

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:

  1. Energy reduction through fan efficiency
  2. Gearbox drive efficiency
  3. Extended equipment life

Energy Reduction Through Fan Efficiency

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.

Improved Drive Efficiency

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

Robust gearbox design with:

  • Pumpless lubrication in which oil is delivered to all locations using an oil slinger with an oil management system
  • Heavy casting to reduce noise and vibration
  • Vertical cooling fins in the housing to maximize thermal performance
  • Quality bearings—roller or tapered—sized to exceed minimum L10 life as the American Gear Manufacturers Association specifies
  • Double radial lip seals or mechanical seals to prevent oil leaks and reduce contamination
  • Marine-grade or epoxy paints to withstand moisture and chemical exposure to the housing
  • Proper gearbox mounting to eliminate vibration that can lead to bearing and gear tooth damage
  • Proper coupling and shaft alignment to reduce vibration and shaft loading, which can lead to premature bearing failure
  • Proper lubrication to reduce scaling, galling and high temperatures that can lead to gear and bearing failure

Extended Equipment Life

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.