Cooling tower is a unit used for transfer of heat to atmospheric air carried by circulated water while cooling the manufacture products or working mediums (liquids, gases, condensed steams) in heat exchangers. They are used at reverse water supply systems at the enterprises of all industry types. In cooling towers water temperature is reduced due to its partial evaporation (mass transfer) and heat exchange with air. The principle of cooling tower operation lies in the following. Water is constantly supplied through drift eliminators (water distribution component) the way so that its supply to the entire surface of drift eliminators is uniform. During evaporation water which, if required, undergoes preliminary water treatment for the prevention of saline deposite and corrosion in the system comes out as droplets with air and reduces the temperature. Automated ventilators with variable frequency drives and regulated louvres are installed for the purpose of draft through a cooling tower.
Depending on the effective capability of air and water contact surface increase all cooling towers are divided into:
• Trickling - having horizontal bars placed at height of the fill from which water flows as rather big droplets.
• Film - the ones in which water flows on flat vertical corrugated plates or other checkers as a thin film.
• Spray (empty) - singled out by spraying water by means of nozzles.
Depending on the direction of airflow towards water in the fill cooling towers can be counter flow, crossflow and of mixed flow.
Cooling tower structure
Water distribution structures of cooling towers can be:
1) free-flow - as a system of molds from which water flows over rough edges of walls or flows through holes or pipes in the bottom;
2) forcing - as a system of pipes with nozzles. Fill is made in the form of horizontal bars of one- or multi-row louvres.
Depending on the type of draft creation cooling towers fall into:
• Fan cooling towers - the ones in which draft is created by means of suction or pressure fans (as usual, of axle type);
• Chimney type cooling tower - the ones which have natural draft;
• Open type cooling tower - the ones in the fill of which, separated by louvres from all sides, outer air comes by means of air force effect.
The temperature of cooled down water in the cooling tower is defined at other the same conditions by irrigation density [hydraulic load on square unit of the fill, usually in m3/(m2*h)] and water temperature drop. This temperature depends on meteorological conditions. The less irrigation density which defines the size of cooling tower but the cost of the cooling tower is bigger, the less is the differential of cooled down water temperature and wet bulb temperature which corresponds to the parametres of free air. Specified values of irrigation density in cooling towers of different types is 5-30 m3/(m2*h).
At low minus temperatures of outer air in order to avoid icing of exhaust windows, support beams and lower part of the fill the temperature is usually sustained at not lower than 10-12⁰ С in two ways: by means of reduction of airflow rate (by means of reduction of rotary velocity or disconnecting fans, covering the part of window area by shields); creation of hot air curtain (by means of irrigation density increase from the side of free air inlet).
Mineralized water is subject to treatment: complete evaporation with removal of contained solid salts; partial evaporation at which the concentrate is put into a special drainage basin, treated water is put back to the system. Sometimes the purge is significantly reduced and even dismissed by means of a proper treatment of feed water.