Water-cooled heating, ventilating, and air-conditioning (HVAC) systems use equipment called cooling towers to remove heat from water used to cool a process or a building. Water-cooled systems provide significant benefits over air-cooled systems, including lower energy costs, smaller size, lower sound level, and reduced maintenance requirements. Cooling towers normally use just a small percentage of the total energy used by the cooling system. A well-maintained cooling tower enables the entire cooling system to perform more reliably and ensures that energy users get the most from their investment.

An improperly maintained cooling tower produces warmer cooling water, which reduces the efficiency of the chiller. Scheduled periodic maintenance on the evaporative cooling equipment prolongs the life of the tower and can result in significant energy savings. For example, if a chiller uses $20,000 of electricity each year, there can be an additional $500 to $700 per year for the extra electricity required for every degree increase in condenser water temperature. In addition, towers that are not properly maintained will have a shorter operating life, will more likely need costly repairs, and are less reliable. Poorly maintained towers can also cause variable frequency drives (VFD) to run at higher speeds than would otherwise be required, further increasing energy costs.

Manufacturers have designed cooling towers to reduce the difficulty of performing tower maintenance, realizing that maintenance that is too difficult tends to be neglected. Regularly scheduled maintenance, such as cleaning of suction strainers, inspection and lubrication of drive components, and proper water treatment is easy to do and is essential to the proper operation of the entire cooling system.

Strainers and Water Distribution Systems

To achieve optimum cooling tower performance, water must be distributed uniformly over the wet deck or coil. Uneven distribution is a sign of clogged distribution nozzles. Some manufacturers offer integral low-pressure drop strainers upstream of the hot water basins, which help prevent the nozzles from clogging. Suction strainers in the cooling tower basin keep loose debris from reaching the pump and clogging the distribution nozzles. Most manufacturers have designed strainers that can be accessed from outside the equipment, which allows easier cleaning without interrupting operation. Inspect and clean both the inlet strainer and the basin strainer as necessary.

Cold water basins should be sloped to guide water and debris towards the suction strainer. Other accessories such as sump sweeper piping can minimize the accumulation of dirt in the tower. Keeping the recirculating water clean makes the water treatment program more effective, reducing treatment costs. Additionally, system heat exchangers remain cleaner, reducing overall system maintenance.

Drive Components

The mechanical fan and drives systems have many components that need to be checked on a regular basis. Power must be off, and all disconnect switches locked and tagged out.

Fan shaft bearings should be lubricated after every 2,000 hours of operation or every three months.

In belt-driven systems, belt tension and drive alignment should be checked on a monthly basis. In gear-driven systems, the drive shaft and the coupling alignment should be examined as well as the gearbox oil level and quality according to manufacturer’s recommendations.

The vibration cutout switch (VCOS) must be operational and maintained according to the manufacturer’s guidelines.

There are many ways to reduce energy consumption further, including using variable frequency drives, which also provide sound control and reduce wear and tear on the fan drive system. Keeping the fan drive system in good working order helps to minimize energy consumption and ensures safe and reliable operation. (See the photo)

Proper Basin Water Level

Cooling tower water recirculates through the system. A small portion of the recirculating water is lost through evaporation and bleed or "blow down" operations (water that is drained out of the tower to prevent accumulation of solids in the basin). This water is replaced either by means of a make-up valve and float assembly or an electronic water level control and solenoid valve, which keeps the water level at a preset level. Some manufacturers provide easily accessible make-up assemblies to make inspection and maintenance easier. Tower operating and maintenance instructions provide this information. Proper water level is necessary to prevent air entrapment in the pump, which can lead to erratic operation and premature wear.

Water Treatment

Water treatment programs need to be designed for to control scale build up, protect against corrosion, and control biological contaminants such as Legionella pneumophilia, which is the bacteria that causes Legionnaire’s disease (LD).

A metering valve must be installed at the pump discharge to control the cooling tower’s bleed rate, The bleed rate should be adjusted to maintain proper water chemistry in the tower and to prevent the accumulation of impurities in the tower basin. A conductivity meter can reduce operating cost by continuously monitoring the water chemistry and automatically adjusting the bleed rate, which reduces both water and sewage cost. Biological control is easy to accomplish and essential for safe operation. Consult with a water treatment specialist familiar with the local area and follow the manufacturer’s guidelines for water quality.

Summary

Evaporative water-cooled systems are the best heat rejection solution for many applications. These systems offer flexibility, conserve water, and save energy. In most cases, cooling tower problems can be avoided or eliminated by following recommended maintenance procedures. There are many interrelated factors that must be considered, including periodic inspections, and the use of competent specialists such as certified water treatment service professionals. Careful attention to cooling towers through routine maintenance can reduce energy cost and avoid expensive problems and downtime while increasing the life expectancy of the tower.

Test Your Understanding

1. An improperly maintained cooling tower produces colder water, improving the efficiency of the chiller.

True

False

2. Maintenance that is difficult to perform will often be neglected.

True

False

3. Some important maintenance areas are:

a. drive components

b, water distribution

c. water treatment

d. all of the above

4. Variable frequency drives (VFD) help in reducing energy consumption and sound levels.

True

False

5. Improper water treatment can lead to Legionnaire’s disease (LD) and other maintenance problems.

True

False

Answers

1. False

2. True

3. d. all of the above

4. True

5. True