Closed-Circuit Cooling Towers (Mechanical Draft). Both counter flow and cross-flow arrangements are used in forced- and induced-draft fan arrangements. The tubular heat exchangers are typically serpentine bundles, usually arranged for free gravity internal drainage. Pumps are integrated in the product to transport water from the lower collection basin to upper distribution basins or sprays. The internal coils can be of any of several materials, but galvanized steel, stainless steel, and copper predominate. Closed-circuit cooling towers, which are similar to evaporative condensers, are used extensively on water-source heat pump systems and screw compressor oil pump systems, and wherever the reduced maintenance and greater reliability of a closed-loop system are desired. Closed-circuit cooling towers also provide cooling for multiple heat loads on a centralized closed-loop system.
Indirect-contact towers require a closed-circuit heat exchanger (usually tubular serpentine coil bundles) that is exposed to air/water cascades similar to the fill of a cooling tower. Some types include supplemental film or splash fill sections to augment the external heat exchange surface area for instance, air flows down over the coil, parallel to the recirculating water, and exits horizontally into the fan plenum. Recirculating water then flows over cooling tower fill, where it is further cooled by a second air stream before being reintroduced over the coil.
Coil Shed Towers (Mechanical Draft). Coil shed towers usually consist of isolated coil sections (non ventilated) located beneath a conventional cooling tower. Counter flow and cross-flow types are available with either forced- or induced-draft fan arrangements. Redistribution water pans, located at the tower’s base, feed cooled water by gravity flow to the tubular heat exchange bundles (coils). These units are similar in function to closed-circuit fluid coolers, except that supplemental fill is always required, and the air stream is directed only through the fill regions of the tower. Typically, these units are arranged as field-erected, multi fan cell towers and are used primarily in industrial process cooling.