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Cooling Tower Fundamentals

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Cooling towers basically consist of some enclosure or shell housing a given height of some suitable packing or fill media. Hot water is doused on top of this packing either by pressurized nozzles or gravity systems such as open troughs or trays. As a fraction evaporates, thus removinglatent heat, the remaining flow trickles through the media and is finally collected in some sort of bottom basin or sump. In properly designed cooling towers, the small fraction that evaporates during this brief passage is enough to cool down the remanent flow to target temperature. There is also a small sensible heat loss due to the temperature difference between incoming hot water and air. 

The interacting air stream can be arranged to meet said trickling liquid in either counterflow (countercurrent) or crossflow or sidewise fashion. 

Mechanical-draft cooling towers can be further classified as either induced or forced draft units. Induced draft units induce a negative pressure or vacuum right below the fan and pull in air from frequently louvered window sides. In forced draft units one or more fans are located at the air inlet as determined and essentially blow through required airflow.

Induced draft designs insure an even, smooth air distribution over the tower's cross-section and if properly designed, e.g. larger diameter fans, can significantly lower required fan HP for a given thermal context. Forced-draft units risk significant thermal de-rating due to recirculation. In order to avoid this, some unfortunate forced-draft designs operate at the upper "thru fill" velocities, say 675 - 700 fpm only to find themselves requiring twice or thrice fan HP to load duty comparable, more engineered induced draft units specifically designed to operate at much lower "thru fill" velocity and selecting larger fan diameters. 

Cooling towers can be constructed in groups of cells.  Each independent partition or cell may have one or more fans and one or more distribution system.  Cells can be shut down if cooling water demand drops or cycled as needed, say for individual maintenance




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