Cooling towers provide a means of removing residual heat from plant cooling water systems by a water to air exchange. Two basic methods of water circulation are once through and recirculation systems. In once through systems water enters the plant, picks up heat in the plant heat exchangers and/or main strem condenses, is cooled in the cooling tower and discharged. The recirculating system reuses the same water with only make up water suppliedto replace losses through evaporation and blow down. The 5 basic types of cooling towers: counterflow; crossflow; forced draft mechanical; natural draft hyperbolic; double crossflow are illustrated below.