lost to the atmosphere the continual addition of makeup water to the cooling tower would result in the accumulation of scale and salts within the circulating water system because loss through evaporation is essentially distilled water leaving the salts behind to avoid this undesirable buildup of scale and salts a portion of the water is extracted this portion termed 34 blowdown water 34 constitutes the third loss source since it is discharged to the external source of water supply the makeup water is thus equal in volume to the volume of water lost through evaporation and drift and the amount of water extracted as blowdown the extraction of blowdown water is usually operationally so designed that the accumulation of salts within the tower is held to an acceptable level this accumulation varies dependent upon system design representative operational numbers would be 200 parts per million of total salts contained in the makeup water compared to six hundred parts per million of salts contained in the water collected within the tower typically the salts are constituted of calcium and magnesium sulfates a mechanical draft tower is illustrated in fig1 a fan is used to create the draft of air through the tower fig2 illustrates an alternative configuration of a cooling tower termed a natural draft tower a natural draft tower is considerably taller than an equivalent mechanical draft tower.