Cooling Towers

The use of cooling towers represents the largest reuse of water in industrial and commercial applications. Cooling towers offer the means to remove heat from air conditioning systems and from a wide variety of industrial processes that generate excess heat. While all cooling towers continually reuse water, they can still consume 20 to 30 percent of a facility's total water use, losing water to evaporation and requiring regular "blowdown" to maintain the quality of cooling water. Optimizing operation and maintenance of cooling tower systems can offer facility managers significant savings in water consumption, on the order of 25 percent of cooling tower water use.

Cooling towers continuously recirculate warm water from a heat source, such as an air conditioning system or process equipment, to the cooling tower. In most cooling tower systems, warm water (or water to be cooled) is pumped to the top of the tower where it is sprayed or dripped through internal fill materials called wet decking. The wet decking creates a large surface area for a uniform thin film of water to be established throughout the tower. Air is blown through falling water over the wet decking to cause evaporation.

Fans pull air through the tower in a counterflow, crossflow, or parallel flow to the falling water in the tower. To provide the most efficient cooling, the air and water must mix as completely as possible. Cooling occurs in a tower by the mechanisms of evaporative cooling and the exchange of sensible heat. The loss of heat by evaporation lowers the remaining water temperature. The smaller amount of cooling also occurs when the remaining water transfers heat to the air.

Cooling towers lose water through two main processes; evaporation and blowdown. The rate of evaporation is about 1.2 percent of the rate of flow of the recirculating water passing through the tower for every 10 F decrease in water temperature achieved by the tower. Another rule of thumb for estimating the rate of evaporation from a cooling tower is as follows: evaporation equals three gallons per minute (gpm) per 100 tons of cooling load placed in the tower -- equal to 12,000 British thermal units (Btu) per hour of heat removed by the tower.

Blowdown is a term for water that is removed from the recirculating cooling water to reduce contaminant buildup in the tower water. As evaporation occurs, water contaminants, such as dissolved solids, build up in the water. By removing blowdown and adding fresh makeup water, the dissolved solids level in the water can be maintained to reduce mineral scale build-up and other contaminants in the tower, cooling condensers, and process heat exchangers. Optimization of blowdown practices, in conjunction with proper water treatment, represents the greatest opportunity for cooling tower water efficiency improvement.

The following are some opportunities to conserve water in cooling tower operations:

• When the dew point temperature is low, tower air induction fans can be slowed by using a motor speed control or cycled on and off, saving both energy and water evaporation losses.
• Drift is a loss of water from the cooling tower in the form of mist carried out of the tower by an air draft. A typical rate of drift is 0.05 to 0.2 percent of the total circulation rate. Reduction in drift through baffles or drift eliminators can conserve water, retain water treatment chemicals in the system, and improve operating efficiency.
• Water consumption of cooling towers can be reduced significantly by minimizing blowdown practices in coordination with an integrated operation and maintenance program. Blowdown is minimized by increasing the acceptable concentration ratio of constituents in cooling tower water before scheduling blowdown. Typical concentration ratios of 2 to 4 generally can be increased up to six or more without affecting cooling tower performance, reducing by one-third the amount of makeup water required.
• Managers may also consider installing a cooling tower treatment system as an alternative to blowdown. While such systems can save water, they may actually cost more in energy for the treatment system than the energy savings that can be realized by the attendant water savings. Ozone systems, which are commonly used to control biological fouling, have high capital and operating costs, which may offset the water savings.
• It is also important to note that while in some cases a reduction in water use may reduce energy demand, in others flow reduction may cause the cooling tower to operate inefficiently, thereby increasing energy use.