What is the difference between a "forced draft" and "induced-draft" cooling tower?

As the terms suggest, forced-draft cooling tower employ an air distribution system that forces air into the tower, while an induced draft tower operates by pulling air through the tower. Forced draft towers are characterized by fans positioned on the side of the cooling tower. Induced draft cooling towers utilize fans located on top of the cooling tower.

What is the difference between a "counterflow" and a "crossflow" cooling tower?

The crossflow or counterflow designation characterizes the orientation of the airflow within the heat-transfer media (fill) in the tower with respect to the direction of water flow. In counterflow towers air travels vertically upwards through the fill and makes intimate contact with water droplets falling down through the fill media. Hence the air and water travel in opposite directions. In crossflow cooling towers the air passes through the fill media in a horizontal direction, thus crossing the downward water flow. Counterflow towers are inherently more efficient than crossflow towers.

How does cooling actually take place inside a cooling tower?

There are two primary mechanisms by which water is cooled inside a cooling tower. Sensible heat transfer takes place when the incoming air temperature is lower than the temperature of the incoming water, thus heat from the water is absorbed by the colder air. If that were the only cooling that took place inside a cooling tower, the cold-water temperature would be limited by the ambient temperature. However, the bulk of the cooling that takes place inside the cooling tower (>80%) is driven by evaporation of the water itself. Evaporation requires energy (heat), so when water is evaporated within the fill media in a cooling tower, heat is removed with the water vapor and leaves in the exiting air stream from the top of the tower. The result is that the remaining water is cooled significantly, even to temperatures below the actual ambient temperature.

What is the entering wet-bulb temperature?

Simply stated the entering wet-bulb temperature (EWBT or WBT) is a measure of the level of humidity in the ambient air entering the cooling tower. In general, the higher the wet-bulb temperature, the more moisture that exists in the air. The wet-bulb temperature is a key parameter in the designing/sizing of a cooling tower, since it determines the degree to which more water can be evaporated. Cooling towers operating in high wet-bulb temperatures require a larger tower than those found in lower wet-bulb regions of the country.

Why is blowdown important?

Blowdown is the term given to water discharged from the cooling tower system to control the buildup of dissolved solids, such as salts or other impurities that occur in water as well as suspended solids that are "washed-out" of the entering air. As a pot of tea gets concentrated if it continues to boil, so the water in a cooling tower becomes concentrated with salts and other impurities as water evaporates. In addition to blowdown, the predominant loss of water from a cooling tower system is through the planned and desirable evaporation that takes place. When water is evaporated to pure water vapor, it leaves behind many impurities which redissolve in circulating water or even deposit on cooling tower internals. Make-up water is introduced to the system to compensate for water losses, but the circulating flow continues to become increasingly concentrated with these impurities as more water evaporates. If the dissolved solids level becomes too high, accelerated scaling can occur inside the cooling tower and reduce the efficiency and or capacity of cooling in the tower. Blowdown of the circulating flow is implemented to keep this dissolved solids level below that saturation level.

What is cooling tower drift?

In every cooling tower, there is a loss of water to the environment in the form of pure water, which results from the evaporative cooling process. This evaporated water leaves the tower in a pure vapor state, and thus presents no threat to the environment. Drift, however, is the undesirable loss of liquid water to the environment, via small unevaporated droplets that become entrained in the exhaust air stream of a cooling tower. These water droplets carry with them minerals, debris and microorganisms and water treatment chemicals from the circulating water, thus potentially impacting the environment. High drift losses are typically caused by fouled, inefficient or damaged drift eliminators, excessive exit velocities or imbalances in water chemistry.

Why is minimizing drift important?

Minimizing drift losses in a cooling tower reduces the risk of impacting the environment with potentially corrosive water treatment chemicals. Drift is usually responsible for damage to property near the cooling tower yard, i.e. buildings, cars, etc. Water use and chemical consumption are also reduced since more remains in the circulating flow, thus generating savings in operating costs. Last but not least, excessive drift losses pose serious health risks, not only because of the chemicals released, but because of microorganisms that can be transmitted through drift, most notably L. pneumophila, the bacterium that causes Legionnaires' disease.

How can drift be minimized?

Certainly, the most effective means of reducing drift is to install high-efficiency drift eliminators in your tower. The drift eliminators are your last, but most critical line of defense for mitigating drift. Maintaining a balanced water chemistry is also very important. Certain chemicals used specifically for cooling water treatment can reduce the water’s surface tension, thus interfering with the normal agglomeration of water droplets that occurs in the drift eliminators. The result is that water droplets are smaller and more easily entrained in the exiting air stream. There is no substitute for a well-maintained water treatment program. Finally, periodic inspection of spray distribution systems and drift eliminators is recommended. A clogged spray nozzle, fouled drift eliminator or even an improperly installed drift eliminator can cause excessive drift in a cooling tower.

What is Legionnaires' Disease?

Legionnaires' Disease is a potentially serious illness that affects the human respiratory system. It is contracted through the inhalation of air-born water droplets or mist containing viable Legionella pneumophila bacterium that occur naturally in most US water sources. Cooling towers provide a hot, humid environment that is an ideal breeding ground for the bacterium. Once a tower is infected, the bacteria are easily transmitted in the drift leaving the cooling tower. About 1,000 cases are reported each year in the US, though the CDC estimates that 10 – 15 times that number of cases may actually occur each year.

How can the transmission of Legionnaires' Disease be eliminated?

While there is no doubt that Legionnaires' Disease is a real and potentially serious health issue for cooling tower operation, the implementation of sensible equipment designs and the execution of appropriate operations, maintenance and housekeeping procedures can virtually eliminate the risk to human health. Tower designs that feature low drift losses, readily accessible/cleanable internals, and basins that promote the accumulation and removal of sedimentation are key to lowering the associated health risk. Periodic cleaning of the cooling tower internals is strongly recommended.

What is the governing body for cooling tower technology and where can I find more industry information on cooling towers?

The Cooling Technology Institute is the world-wide industry authority on cooling tower technology. There are Manufacturers, Owner/Operators, and Suppliers represented in the CTI organization, thus providing a truly independent perspective to the industry. The CTI guidelines and specifications are available at the CTI site.

Where can I find out more about the application of cooling towers within the HVAC system?

The ASHRAE Society is organized for the sole purpose of advancing the arts and sciences of heating, ventilation, air conditioning and refrigeration for the publics' benefit through research, standards writing, continuing education and publications. Here is the link to their site.