Controlling the scale-forming tendencies of a cooling tower system is based on regulating the level to which minerals are “allowed” to concentrate through evaporation. Cycles management is one aspect of preventing mineral scale formation on heat exchange surfaces and is carefully coupled to other methods such as: proportionally-dosed application of chemical deposit-control agents, pH control, or even mechanical removal of minerals. Regulating cycles is accomplished by removing some of the concentrated cooling tower water through bleed-off and allowing it to be diluted with fresh “make-up” water while heat is rejected through evaporation. The amount of bleed-off required is based on calculating the allowable cycles of concentration, derived from the saturation limits of the most insoluble dissolved species. Control of cooling water bleed-off is normally done by continuously measuring conductivity of the cooling tower water and tripping a blowdown solenoid or motorized ball valve when a pre-programmed trip point is reached. Make-up water meters with electric contact heads can also be used to regulate bleed-off volumetrically (together with time and flow-rate) to fix necessary cycles of concentration.
Managing cycles and deriving the setpoint at which bleed-off occurs is a very important, balanced consideration. If the set point is too high, minerals will precipitate, and energy costs will rise. If too low, the precious resources water and treatment chemicals are wasted through unnecessary blow-down. So, it is essential that your treatment program be professionally designed, and that the potential obstacles to uninterrupted operation are eliminated. Your Metro water treatment consultant will establish permissible limits of cycles of concentration based on information derived from your facility, including a comprehensive analysis of the local water supply.
John D. Caloritis, CWT
The Metro Group, Inc.