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Cooling Towers

Cooling towers are installed next to, behind, or on top of large buildings to remove heat from water used in a chilled water mechanical refrigeration (air conditioning) system. The concept is simple, akin to perspiration. Latent heat of vaporization is removed from the water by blowing air over it. Typically, the temperature of the water is reduced by about 10 degrees F. before it returns to the refrigeration system.

Cooling Tower Maintenance Issues

Though simple devices, cooling towers require routine maintenance to perform optimally. Preventive maintenance should include:

  • Water Treatment and Corrosion Control: All cooling towers must have water treatment systems for the prevention of biological and chemical degradation of the equipment. Biologicals that can exist in a tower basin are legionella, dead animals, and algae. Chemicals that can build up in a tower basin or on tower wet surfaces are rust, residues from hard water, residues from chemical treatments (if not properly administered), and residues from rain. Sediments can also build up from windblown dirt and plant matter. Freezing protection is often a challenge for cooling towers that must run in the winter.

Water treatment for biologicals and rust is generally done automatically by a time-based chemical injector system (small pump and timer piped into the condenser water system) or manually by adding premixed chemicals through a small tank. Sediment is usually caught in strainers or settles in the basin and must be manually flushed out.

Freeze protection is accomplished by electric heaters in the pan or basin of the tower. Heaters generally aren’t operated while the tower is in use. The heat being rejected by the tower keeps the pan from freezing. You may see water freeze up on the "fill" or inside surfaces of the tower in winter. This can destroy the inner surfaces of the tower if not addressed. Freeze protection is very often in the piping to the tower as well. Heating cable is usually wrapped around the piping to and from the tower and then insulated.

  • Fan Motors/Bearings: Tower fans come in a variety of styles. Fan speed or quantity is used to control the temperature of the water leaving the tower, which is critical for the efficient operation of the chiller plant. Where there are multiple motors, they may be directly connected to a fan via the motor shaft or gearbox, or they may drive a fan through a belt drive. Gearboxes are pretty rugged, and maintenance on these is just like that for your car: check the oil. Belt drives are subject to a lot of wear and tear due to multiple starts and stops as well as weathering. Bearings also are exposed to the weather constantly and require regular checks for washed-out grease.
  • Cleaning: Most towers, except for winterized units, are shut down during the cooler weather (50 degrees and less). The tower will stay off until the warm weather returns. Just before this happens, the tower should be cleaned out and checked. The basin should be filled and checked for leaks that may have formed in the off-season, dirt that may have accumulated, and fill that may have been damaged by the weight of ice.

What We Inspect

When asked to perform a Property Condition Assessment, our review of building system components is necessarily limited. However, there are a number of things we do look at to help determine whether the system has been well maintained, whether the system is performing as intended, whether there are any safety concerns, and how future maintenance, repair, or replacement has been anticipated. The things we look at include:

Base and Mounts. We look for:

  • Broken vibration isolators between the tower housing and the supporting steel/concrete
  • Uneven deflection in the vibration isolators; (This may indicate broken or failing mounts)
  • Rubber motor mounts that may have weathered and dried out
  • Bolts in motor or bearing mounts that may have broken or are badly rusting

Noise and Vibration. When a tower is running, it should be pretty quiet. Noise will be produced when:

  • Fan wheels or propellers are out of balance
  • Metal panels or safety screens that are missing screws (and they always are) vibrate
  • Belts that have stretched too much cause vibration or squeal

Size v. Anticipated Load. The cooling tower should be able to reject at least as much heat as the chiller plant can transfer to it. If a tower is not sized properly, it would be discovered long before a PCA was done. A tower’s ability to reject heat is based on its ability to evaporate water. If a tower has all fans running and the leaving water temperature is not 10 to 12 degrees cooler than the entering water, there may be a loss of capacity. Cleanliness, proper water distribution inside, damaged fill, loose belts, or failed fan motors are potential culprits. When inspecting, we look for the steadiness of the fan speed. Speed fluctuations over a period of a few minutes reflect an out-of-tune control system or problems in flow control.

Evidence of Corrosion. We inspect for:

  • Corrosion of bolts that secure pumps, fan motors, gearboxes, or bearings to the tower
  • Rusting where thermostats or water level controllers enter the basins or pans
  • Rusting of the upper decks where the water is introduced to the tower
  • Leaks where piping joins the underside of the pan. (Note: leaks can be expensive. Treatment chemicals go down the drain which, can also have environmental penalties.)
  • Corrosion of enclosures and conduit.

Overspray. Overspray is caused by either too much air blowing through the tower and taking the water droplets with it or the entering water not being properly distributed on the cooling surfaces of the tower. Overspray can be felt by people entering or leaving the building, or the overspray carries water treatment chemicals that cause spots on parked cars. If the tower is otherwise okay, it’s possible to install screening over the discharge of the tower to break up the water droplets. Overspray is situational. You may not see it on a particular day because the tower is lightly loaded and only one fan is running at a low speed.

Makeup water. As towers evaporate water to produce cooling, water needs to be made up from the building’s city connection. In the tower basin/pan, there is usually a level control, like in a toilet tank, that keeps a certain amount of water in the system at all times. These devices can get stuck, corroded, or fouled. Water can be constantly running into the pan, through the overflow and out to the storm drains. This wastes treatment chemicals and water and could bring on environmental penalties.

Repair or Replace

When a cooling tower is old, consideration must be given to how long it can remain in service before maintenance costs exceed replacement costs. Fortunately, there aren’t a lot of moving parts in a tower. With proper maintenance, a tower can last up to 20 years. When repairs are needed, they are generally on motors, bearings, belts, controls, and gaskets. These repairs are all reasonable, but when a tower goes for long periods without corrosion and water treatment, the major structure of the tower begins to decay. Replacement of a tower requires electrical, mechanical, and rigging trades; downtime; permits; and plan review by the local authority. If the tower is on a roof, the rigging and permit process gets worse. If a helicopter is required and you’re near an airport, the FAA is also involved. It’s always advisable to try and repair the tower in place, even for major repairs. A new tower will cost about $50/ton (capacity) plus installation. Rebuilding can be more cost -effective, especially if sections of the tower can stay online while sections are being rebuilt. There are firms that specialize in this service. Criterium Engineers can help you find and work with these firms.

Volume 11, Number 1

April 2000

Copyright © 2000

The Engineering Advisor is intended to enhance your knowledge of technical issues relating to buildings.  For additional information on any subject, please feel free to call us.  Our commitment is to provide you with timely, accurate information.