Evaporation, drift, and blowdown losses
The amount of water evaporated in a cooling tower depends entirely on ambient temperature and humidity. To illustrate, assume a constant building or process cooling load. The total amount of heat energy rejected to the airstream through the tower is also then constant. The total load on the air side of the tower is then equal to Q = 4.5 x cfm x (Ho - Hi). The sensible heat component is Qs = 1.08 x cfm x (To - Ti) and the latent component is Ql = 4.5 x 1050 x cfm x (Wo - Wi).Although the total heat transfer will always be the same, the sensible and latent components are always changing because the entering air temperature and humidity conditions are always changing. The amount of water evaporated is obtained from the latent component, or the difference in entering and leaving humidity ratio. At peak design conditions, the evaporation rate can be 1.0% to 1.5% of the total circulation rate depending on the range. Note that the evaporation rate is much less in colder temperatures because the sensible component increases as the outside air temperature decreases.
Water droplets that become entrained in the airstream as it passes through the tower are known as "drift." Most towers are designed with drift eliminators, which can usually reduce the amount of drift loss to about 0.05% to 0.1% of the total water circulation rate. So for calculating drift losses, an assumption of 0.1% is reasonable.
As water is evaporated by the cooling tower, it leaves behind dissolved solids and other impurities that could potentially cause scaling and corrosion. To reduce the number of concentrations (C) of these impurities, some of the water is bleed off, commonly referred to as "blowdown" or "bleed-off." Typically the C will range from 2 to 5. With a known evaporation rate and assumed drift, the amount of blowdown required can be calculated from:
C = (evaporation + drift + blowdown) / (drift + blowdown)
For a level of 2 C for example, with 1.0% evaporation and 0.1% drift, the blowdown rate would be 0.9%, almost as much as the evaporation rate. Good quality filtration and chemical treatment systems can reduce the amount of blowdown significantly by allowing for higher concentrations.
Example application
Hourly weather data is probably the best and most reliable tool available to accurately estimate cooling tower water consumption. The total cooling load can be calculated every hour based on an assumed load profile. With the entering air enthalpy known each hour, the leaving enthalpy, Ho, is calculated from the total load. Since we know that the leaving air is saturated, we can calculate the leaving humidity ratio, Wo, from Ho at 100% rh. Now we have the increase in humidity ratio. And the evaporation rate in Lbw/hr is then equal to cfm x 4.5 x (Wo -Wi).Let's look at a cooling tower serving a 1,000-ton data center in Washington. In order to emphasize the impact of weather on consumption, the cooling load was assumed to be constant, operating 24/7/365. Drift was assumed to be 0.1% of the total circulation rate and blowdown was calculated every hour based on 3 C. Note that even though the cooling load is the same year round, water consumption in the peak summer months can be 2 or 3 times as much as in the winter months. The reason, as explained earlier, is that the latent component of the total load decreases as the temperature decreases.
Another benefit of this type of analysis is that it can help in analyzing water and sewer costs by showing how the weather can directly affect water consumption on a year-to-year basis. Note that the consumption was much higher in DC during 2002, primarily due to last year's long hot and humid summer. An additional 1.3 million gal of makeup water was required compared to normal. Also, the peak gpd in 2002 was 93,168, compared to a normal of 84,000. ES
EDITOR'S NOTE: Some images associated with this article did not transfer to the Internet. To view the images, please refer to the print version of ES.
