"Cooling Tower Heat Transfer 101"
By Brad Buecker
[Excerpts]
"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.
Figure 1
Figure 1 shows process conditions that could easily exist in a cooling system. We will calculate the mass flow rate of air needed to cool 150,000 gpm of tower inlet water to the desired temperature. We will also calculate the water lost by evaporation (go to link for full calculation). So, with an inlet cooling water flow rate of 150,000 gpm (1,251,000 lb/min), the calculated air flow is 1,248,000 lb/min, which, by chance in this case, is close to the cooling water flow rate. (Obviously, the air flow requirement would change significantly depending upon air temperature, inlet water temperature and flow rate, and other factors, and that is why cooling towers typically have multiple cells, often including fans that have adjustable speed control). The mass balance of water = 146,841 gpm. Thus, the water lost to evaporation is 3,159 gpm. A very interesting aspect of this calculation is that only about 2 percent evaporation is sufficient to provide so much cooling.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).
Reference:
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
