Cooling towers operate on the principle of evaporative cooling. As water evaporates, it absorbs a quantity of heat from its environment equal to the heat of vaporization, which represents almost 10 times more energy than that needed to raise the temperature of water 1°C.
The warm water from the discharge side of the chiller is pumped to a cooling tower, where it’s trickled down honeycombed panels to atomize the droplets. A fan draws air across the panels to accelerate the rate of evaporation and increase the rate of cooling. The more water that evaporates, the higher the temperature reduction per volume of fluid. (In very dry climates the fan is not needed at all since the rate of natural evaporation is sufficient to cool the water.) Both fan speed and volume of fluid can be varied to obtain the desired degrees of cooling to the condenser water.
Modern cooling towers have an automatic valve that replaces water lost to evaporation. Usually, the valve is activated by a float in the bottom of the tower basin that opens the valve to add water when the level drops. This mechanism should be adjusted properly so excess water is not added, only to be skimmed off at the overflow drain.
The mechanical drive for the tower fan also requires occasional maintenance. Direct-drive motors need balancing and lubrication. Belt-driven fans need belt tensing and replacement when worn.
Shaft-driven fans need lubrication, adjustment, and occasional balancing. The fan bearings in each case need lubrication, and the fan itself requires balancing so that any counterweight or blade damage does not destabilize the unit to the point of tearing up the bearings, then the motor. A properly-adjusted drive mechanism requires the minimum of energy to drive the fan and to provide maximum cooling to the circulating fluid.
The cooling-tower water loop is an open loop. The water is exposed to the atmosphere as it flows through the tower. This causes some unique water-treatment problems. Other fluid cycles have trouble with dissolved solids; the tower loop has to cope with suspended solids. These are solids of microscopic size, and larger, that enter the water stream. The solids can be blown dust or sand captured by the water, smoke or soot particles filtered from the air, or bacteria and microbes growing in the warm, oxygenated water. The latter can be controlled by careful and consistent treatment with chemicals such as chlorine; the suspended solids are a little more troublesome.
The suspended solids can clog the flow passages of the condenser bundle in the chiller or plug the cooling-tower cells and cause damage to the pump, valves, and pipe fittings. It’s therefore urgent to keep the solids content under control. Some facilities flush the system occasionally and replace the entire volume with clean water. This new fluid must be treated with antibacterial chemicals, though, an expense which may be prohibitive for larger systems or for those which collect solids quickly.
Suspended solids are a significant problem in industrial areas with soot and other pollutants in the air. Cooling towers in such environments often have a small filter that cleans a portion of each volume of circulated water. This secondary flow loop continually draws water from the return side of the cooling-tower circuit and returns the filtered water to the primary loop. The chemical treatment is not affected because the filter medium is inert and doesn’t react with the dissolved chemicals.
Proper control of dissolved solids reduces routine maintenance to the condenser loop equipment. It also minimizes flow-control problems, which in turn maintains the heat-transfer surfaces in the condenser side of the chiller and on the cooling-tower surfaces. It also tends to reduce the cost of chemical treatment since chemically active solids that enter the water can cause an imbalance in the water chemistry that must be corrected by additives.
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Excerpt from Energy Conservation Projects.