Cooling Towers

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Introduction

Cooling towers are heat exchangers that use water and air to transfer heat from air-conditioning systems to the outdoor environment. Most commonly, they are used to remove heat from the condenser water leaving a chiller. Cooling towers are usually located on rooftops or other outdoor sites. Because they are frequently out of sight, they are often neglected by operation-and-maintenance technicians, resulting in lower cooling-system efficiency. This document will help you adopt best practices for the efficient operation and maintenance of cooling towers.

Types of Cooling Towers

There are two basic types of cooling towers, open and closed (sometimes called direct and indirect).

Open (Direct) Cooling Towers

Open cooling towers expose the condenser water coming from the chiller plant directly to the atmosphere. This warm water is sprayed over a fill in the cooling tower to increase the contact area, and air passes through the fill. Most of the heat is removed by evaporation. The cooled water remaining after evaporation drops into the collection basin and is returned to the chiller's condenser.

graphic figure

Open (direct) Cooling Tower

Closed (Indirect) Cooling Towers

A closed cooling tower circulates warm water from the chiller plant through tubes located in the tower. In a closed tower, the cooling water does not come in contact with the outside air. Water that circulates only within the cooling tower is sprayed over the tubes and a fan blows air across the tubes. This cools the condenser water within the tubes, which is then recirculated to the chiller plant.

graphic figure

Closed (indirect) cooling tower

Key Components of Cooling Towers

This section explains how the components of a cooling tower work together.

Water Distribution

Hot water from the chilled-water system is delivered to the top of the cooling tower by the condenser pump through distribution piping. In an open tower, the hot water is sprayed through nozzles onto the heat transfer medium (fill) inside the cooling tower. Some towers feed the nozzles through pressurized piping; others use a water-distribution basin and feed the nozzles by gravity. In a closed-loop tower, the water from the condenser loop runs through tubes in the tower and is not exposed to the outside air. Water for cooling the tubes circulates only in the tower.

In the open tower, a cold-water collection basin at the base of the tower gathers cool water after it has passed through the heat transfer medium. The cool water is pumped back to the condenser to complete the cooling-water loop. In the closed tower, the condenser water cools as it moves through the piping in the tower and returns to the chiller plant.

Heat Transfer Medium (Fill)

Cooling towers use evaporation to release waste heat from an HVAC system. In an open tower, hot water from the condenser is slowed down and spread out over the fill. Some of the hot water is evaporated in the fill area, or over the closed-circuit tubes, which cools the water. Cooling tower fill is typically arranged in packs of thin corrugated plastic sheets or as splash bars supported in a grid pattern.

Air Flow

Large volumes of air flowing through the heat-transfer medium help increase the rate of evaporation and the cooling capacity of the tower. The cooling-tower fans generate this airflow. The size of the cooling-tower fan and airflow rate are selected to achieve the desired cooling at design conditions of condenser-water temperatures, water flow rate, and wet-bulb temperature.

Cooling towers may have propeller fans or squirrel-cage blowers. Small fans may be connected directly to the driving motor, but most designs require an intermediate speed reduction provided by a power belt or reduction gears. The fan and drive system operate in conjunction with the control system to control start/stop and speed. Variable-speed drives (VSDs), when added to the fan motors, control fan speed and more precisely regulate the temperature of the water as it leaves the tower.

Drift Eliminator

As air moves through the fill, small droplets of cooling water become entrained and can exit the cooling tower as carry-over or drift. Devices called drift eliminators remove carry-over water droplets. Cooling-tower drift becomes annoying when the droplets fall on people and surfaces downwind from the cooling tower. Efficient drift eliminators virtually eliminate drift from the air stream.

Safety Issues

Water Treatment

Cooling-tower water must be regularly treated, generally with chemicals, to prevent the growth of harmful bacteria, minimize corrosion, and inhibit the buildup of scale (mineral deposits) on the fill.

Maintenance Personnel

Cooling towers are often placed in precarious locations, and inspection ports can be located in awkward or exposed locations. This can create a hazardous working environment. Be sure to implement adequate fall-prevention measures and procedures. In addition, always follow lock-out and tag-out safety procedures.

Best Practices for Efficient Operation

Always consult the manufacturer's manual for the cooling-tower. Another excellent source of information and standards for cooling towers is the Cooling Technology Institute. Here are some recommendations for operating any cooling tower more efficiently:

Implement a preventive-maintenance program: This includes regular water treatment and maintenance of the mechanical and electrical systems. See the Maintenance Schedule for Cooling Towers, below for more information.

Reduce the temperature of water leaving the tower: The temperature of water leaving the cooling tower should be as cold as the chiller manufacturer will allow for entering condenser water. Newer chillers usually tolerate colder temperatures for water returning from the cooling tower. Check with your chiller manufacturer's representative or manual and set the entering condenser-water temperature (same as the leaving cooling tower temperature) as low as possible.

Operate cooling towers simultaneously: Direct water through all towers regardless of the number of chillers operating. Tower fans should be staged on as required. Operating the towers simultaneously will use less energy in most situations than staging towers individually. This strategy is particularly effective with VSDs on the fans. When a fan VSD reaches 40% speed (adjustable), the next fan stages on and operates in parallel, both now running at a minimum speed of 20%.

Balance water distribution between multiple towers (or cells within a single tower enclosure) and within each tower or cell. Water often flows down only one side of the tower, or one tower may have more flow than an adjacent tower. This increases the temperature of the water returning to the chiller and reduces the efficiency of the tower.

Consider a condenser water reset strategy: The temperature set point of the water leaving the cooling tower should be at least 5°F (adjustable according to the design) higher than the ambient wet-bulb temperature. If the Direct Digital Control (DDC) system has a wet-bulb temperature sensor, this can be done automatically. Otherwise the operator should consider manually adjusting the set point seasonally.

Close the bypass valve before starting the cooling-tower fans: Make sure the DDC control sequence prevents the tower fans from starting before the cooling-tower bypass valve is fully closed. If the bypass valve isn't fully closed, hot water leaving the chiller short circuits into the water returning to the chiller, adding unnecessary load to the compressor.

Trend log the temperature of the water leaving the tower: Use the trend logging capability of the DDC to track the temperature of the water leaving the tower. Higher than normal temperatures may indicate that the tower in not operating properly.

Best Practices for Maintenance

Inside an operating cooling tower is much like a hurricane. This harsh environment must be regularly inspected and maintained for best system performance.

Effective water treatment: Effective water treatment eliminates harmful bacteria and bio-film and controls scale, solids, and corrosion. Bleed or blowdown-the continuous flow of a small portion of the recirculating water to a drain to eliminate dissolved solids-is insufficient by itself to control scale and corrosion and is always ineffective in controlling biological contamination. A regular chemical-treatment program is always recommended for controlling biological organisms, scale, and corrosion.

Prevent scale deposits: When water evaporates from the cooling tower, the minerals that were dissolved in it are left behind as scale deposits on the surface of the fill. Scale build-up inhibits heat transfer from the water to the air, which reduces the fill's effectiveness. Excessive scale build-up is a sign of inadequate water treatment.

Prevent or clean clogged spray nozzles: Algae and sediment that collect in the water basin as well as excessive solids that get into the cooling water can clog the spray nozzles. This causes uneven water distribution over the fill and uneven airflow through the fill, which reduces evaporation. These problems indicate improper water treatment and clogged strainers. Kits are available to replace older, smaller distribution nozzles or troughs with large-orifice, clog-free designs.

Ensure Adequate Airflow: Poor airflow through the tower reduces the transfer of heat from the water to the air. Poor airflow can be caused by debris at the inlets or outlets of the tower or in the fill, loose fan and motor mountings, poor motor and fan alignment, poor gearbox maintenance, improper fan pitch, damage to fan blades, or excessive vibration. Reduced airflow due to poor fan performance can ultimately lead to motor or fan failure.

Ensure Adequate Pump Performance: A closed-loop cooling tower uses a pump to transport water over the tubes for evaporative cooling. Proper water flow is important to achieve optimum heat transfer. Loose connections, failing bearings, cavitation, clogged strainers, excessive vibration, and operating outside of design conditions result in reduced water flow, reduced efficiency, and premature equipment failure.

The table below provides a schedule for maintenance tasks.

Download this table as a Word document

Maintenance Schedule for Cooling Towers
Description Comments Maintenance Frequency
Cooling tower use/ sequencing Turn on/sequence unnecessary cooling towers Daily
Overall visual inspection Complete overall visual inspection to be sure all equipment is operating and safety systems are in place Daily
Fan motor condition Check the condition of the fan motor through temperature or vibration analysis and compare to baseline values Weekly
Clean suction screen Physically clean screen of all debris Weekly
Operate make-up water float switch Operate switch manually to ensure proper operation Weekly
Vibration Check for excessive vibration in motors, fans, and pumps Weekly
Check tower structure Check for loose fill, connections, leaks, etc. Weekly
Check belts and pulleys Adjust all belts and pulleys Weekly
Test water samples Test for proper concentrations of dissolved solids, and chemistry. Adjust blowdown and chemicals as necessary. Perform weekly for open towers and monthly for closed systems.

Weekly (Open)

Monthly (Closed)

Check lubrication Assure that all bearings are lubricated per the manufacture's recommendation Monthly
Check motor supports and fan blades Check for excessive wear and secure fastening Monthly
Motor alignment Aligning the motor coupling allows for efficient torque transfer Monthly
Check drift eliminators, louvers, and fill Look for proper positioning and scale build up Monthly
Inspect nozzles for clogging Make sure water is flowing through nozzles in the hot well Annually
Clean tower Remove all dust, scale, and algae from tower basin, fill, and spray nozzles Annually
Check bearings Inspect bearings and drive belts for wear. Adjust, repair, or replace as necessary. Annually
Motor condition Checking the condition of the motor through temperature or vibration analysis assures long life Annually

References

FEMP 2004. O&M Best Practices Guide 2.0.

FEMP 2002. Continuous Commissioning Guidebook for Federal Energy Managers.

ASHRAE Journal 2005. Maintaining Cooling Towers.

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