By Brad Buecker
"Evaporation is utilized to its fullest extent in cooling towers, which are designed to expose the maximum transient water surface to the maximum flow of air – for the longest period of time.”1
For water to evaporate it must consume a large amount of energy to change state from a liquid to a gas.
Evaporation causes dissolved and suspended solids in the cooling water to increase in concentration. This concentration factor is (logically) termed the cycles of concentration (C). Cycles of concentration can be monitored by comparing the ratio of the concentration of a very soluble ion, such as chloride or magnesium, in the makeup (MU) and recirculating (R) water. Very common is a comparison of the specific conductivity of the two streams, particularly where automatic control is utilized to bleed off recirculating water when it becomes too concentrated.
Besides blowdown, some water also escapes the process as fine moisture droplets in the cooling tower fan exhaust. This water loss is known as drift (D). Where towers are well-designed, drift is quite small and can be as low as 0.0005 percent of the recirculation rate.2 Drift particulate minimization is very important, as regulations on particulate emissions from cooling towers continue to tighten. Leaks in the cooling system are referred to as losses (L).
1. J.C. Hensley, ed., Cooling Tower Fundamentals, 2nd Edition; The Marley Cooling Tower Company (now part of SPX Cooling Technologies, Overland Park, Kan.), 1985.
2. Personal conversation with Rich Aull of Brentwood Industries.
Power Engineering, July 2010