Hawaii Building Engineers Association

Cooling Tower Maintenance

COOLING TOWERS: Regular Care Saves Time, Money, Energy

by Glenn A. Babcock 

A cooling tower is a marvelous device that enables building owners and operators to take advantage of operating cost savings inherent in water-cooled systems. By conserving both energy and water, a well maintained tower enables the cooling system to perform reliably and cost-effectively at optimum efficiency. Contractors who maintain existing cooling systems can provide their customers a great service by implementing a comprehensive, routine cooling tower maintenance program.

Unfortunately, for many years the cooling tower has been the forgotten component of the water cooling system when it comes to maintenance. A newly installed cooling tower reliably delivers the design fluid temperature and flow rate. However, since its heat transfer operation creates a “hurricane-like” environment and is a natural “air-washer”, the cooling tower needs routine inspection and maintenance to continue performing as designed.

This article takes a brief look at routine maintenance, and suggests ways to help you improve your customers' cooling tower performance.

How They Work
Let's start with a look at how a cooling tower works. In an open circuit cooling tower, warm water from the heat source is evenly distributed directly over a heat transfer surface called “fill” or “wet deck,” while air is simultaneously forced or drawn through the tower. This causes a small percentage of the water to evaporate. The evaporation process removes heat and cools the remaining water, which is collected in the tower cold water basin and is returned to the heat source (typically a water-cooled condenser or other heat exchanger). Similarly, in a closed circuit cooling tower or evaporative condenser, the heat is rejected indirectly from a fluid or vapor in the coil section by spraying re-circulated water over the coil section, evaporating a small percentage of the water.

The temperature at which the cooled fluid is returned to the system measures tower performance. This temperature can vary depending upon the actual cooling load, water flow, airflow, and ambient temperature conditions.

To perform properly, all tower components must be kept clean and free of obstructions. The following sections describe how this is accomplished through routine maintenance. Maintenance-frequency will depend largely upon the condition of the circulating water and the environment in which the tower is operating.

Keep Strainer Systems Clean
Strainers in the tower provide a means of keeping debris out of the condenser system water loop. Strainers in the cold water basin outlet prevent debris from reaching the pump. Some towers feature low-pressure drop pre-strainers upstream of the hot water basin to prevent clogging of distribution nozzles. Inspect and clean both strainers as necessary. Some tower designs allow external access to the strainers, which enables maintenance to take place without the need to turn off the unit.

Maintain Even Water Distribution
The water distribution system should evenly distribute water over the wet deck surface or coil section via either a gravity distribution system or a pressurized spray system, using a series of nozzles. If water distribution is found to be uneven, the nozzles need to be checked. Clean clogged nozzles in accordance with the manufacturer's recommendations.

In a gravity distribution system, the nozzles can be externally accessed, visually inspected, and cleaned by removing the hot water basin covers on the fan deck. Some innovative spray distribution systems have nozzles and branches held in place by snap-in rubber grommets, which allow easy removal to clean and flush debris.

A well-designed cold water basin is sloped toward the strainer to keep dirt (which can accelerate corrosion) from accumulating. Keep these basins clean by occasionally flushing the dirt out of the system. Another way to accomplish this is to install accessories such as a sump sweeper piping system in conjunction with water filtration or separator devices. Water filtration saves maintenance costs by reducing the dirt in the cooling water system, which in turn reduces the time required to clean the cold water basins. It also reduces water treatment cost.

Water Conservation

                                                    

Watergy by definition means “connecting water with energy.” In simple sense it takes a certain amount of electricity to deal with the water that we use. The connection can be seen when we turn on the faucet for hot water and the hot water tank must reproduce the hot water that was used. The water going down the drain must be purified and the cost to do so is billed back to us in the cost of the electricity and maintenance to perform that task. So what saves us more money turning off our lights or turning off our faucets? Given a scenario of five minutes of a faucet running would be equal to the cost of lighting a 60 watt light bulb, if not more. Water on Oahu needs to be conserved and we can do our part, by reducing our waste.  The Environmental Protection Agency (EPA) has come up with a program called Water Sense.  The U.S. General Accounting Office (GAO) has estimated that, even assuming non-drought conditions, at least 36 states will suffer some sort of water shortage by 2013. On Oahu the water shortage is expected to affect us by 2010. In an effort to promote water efficiency and help protect the future of our water supply, the EPA has launched the Water Sense Program that has been in effect since 2006. The program has found that the reduction in water is driven by the cost of building new wastewater plants and other projects to meet the demand for water. Taxpayers and users of water would be the revenue stream to provide new facilities and products to reduce consumption.  Demand side management is one means to reach their goal through the reduction of use and not utilize taxpayer’s money to increase the infrastructure. Regardless, the price of water is on the rise. With an estimated 7% - 12% increase annually.