The FUNCTION of a cooling tower is to reduce the temperature of an open recirculating cooling water stream so that the water can be reused. The main PURPOSE of a cooling tower is to conserve water.
A cooling tower is a heat rejection device that cools the water by evaporation. This is accomplished by recirculating water and passing ambient air through it. Heat energy is removed through this evaporation process, thus dropping the temperature of the remaining recirculating water. Approximately 970 BTU's are removed from the recirculating water for every pound of water that is evaporated. As the water evaporates in a cooling tower system, it leaves behind all of the dissolved solids that were present in the fresh makeup water used to replenish system losses (due to evaporation and system bleed). Remember that only water evaporates - any dissolved solids or suspended solids that remain behind in the water become more concentrated as additional water is evaporated. As the concentration of dissolved solids builds up in the recirculating water, it is said to be "cycling up". Here is an example: Our makeup water has a dissolved solids concentration of 500 ppm. If we allow the dissolved solids in this water to concentrate up in an open recirculating water loop to 1500 ppm, we would be running at 3 cycles of concentration.
What happens when some of the minerals that make up the dissolved solids reach their saturation point? We will investigate that situation in our blog next week.