Cooling Towers


Two basic types of evaporative cooling devices are used. The first of these, the direct- contact or an open cooling tower, exposes water directly to the cooling atmosphere, thereby transferring the source heat load directly to the air. The second one called a closed-circuit cooling tower, consists of indirect contact between heated fluid and atmosphere, essentially combining a heat exchanger and cooling tower into one relatively compact device. The most rudimentary of contact devices is a spray filled tower that exposes water to the air without any heat transfer medium or fills. In this device, the amount of water surface exposed to the air depends on the spray efficiency, and the time of contact depends on the elevation aid pressure of the water distribution system. To increase the contact surfaces and time of exposure, a heat transfer medium, or fill, is installed below the water distribution system, in the path of the air. The two types of fill-in use are splash type and film-type. Splash-type maximizes contact area and time by forcing the water to cascade through successive elevations of splash bars arranged in staggered rows. Film-type fill achieves the same effect by causing the water to flow in a thin layer over closely spaced sheets, principally polyvinyl chloride (PVC), that are arranged vertically. Either type of fill can be used in counter-flow and cross-flow towers. For thermal performance levels typically encountered in air conditioning and refrigeration, a tower with film-type fill is usually more compact. However, the splash-type fill is less sensitive to initial air and water distribution and, along with specially configured, more widely spaced film-type fills, is preferred for applications that may be subjected to blockage by scale, silt, or biological fouling. Closed-circuit cooling towers contain separate fluid circuits. An external circuit, in which water is exposed to the atmosphere as it cascades over the tubes of a coiled bundle, and an internal circuit, in which the fluid circulates inside the tubes of the coil bundle. In operation, heat flows from the internal fluid circuit, through the tube walls of the coil, to the external water circuit and then, by heat and mass transfer, to atmospheric air. As the internal fluid circuit never contacts the atmosphere, it +can be used to cool fluids other than water and/or to prevent contamination of the primary cooling circuit with airborne dirt and impurities. Some closed-circuit cooling tower designs with cooling tower fill to augment heat exchange in the coil.

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