Cooling Tower

Cooling towers are used to remove waste heat from a process by releasing it to the atmosphere.  Cooling is necessary for a steam cycle to continue to operate due to equipment and thermodynamic limitations.  The heat from cooling the steam from the turbines from vapor to liquid in the condenser has to the atmosphere.  In some ways, a cooling tower is analogous to the radiator on an automobile.

Two types of cooling processes are generally used, direct and indirect cooling.  Although the two processes are different, they both accomplish the same task.  Depending on the application, one process may be more appropriate than the other.

Cooling Towers at MNSU
Cooling towers at MNSU [129].

 

Types of Cooling Towers

Industry uses cooling towers to reject waste heat to the environment. There are two basic types of cooling towers: indirect (closed) and direct (open) style. With indirect (closed) cooling towers, the cooling water circulates

Cooling Towers at Faribault Energy Park
Cooling towers at Faribault Energy Park [130].
through tubes in the tower where it is air-cooled through convection from the pipes. In this case, the water being cooled and the air doing the cooling are separated by a pipe and do not mix. In the direct (open) cooling towers, on the other hand, the cooling water mixes with air within the tower which is cooled by a psychrometric process.

There are two types of open cooling towers: crossflow and counterflow. In a crossflow cooling tower, evaporation occurs as air flows perpendicular to water running down over the heat transfer surface. The heat transfer surface is sometimes called the fill, cooling fill, or wet deck; its purpose is to increase the surface area of the water to maximize the amount of cooling by the airflow through the cooling tower. In counterflow open cooling tower, the air flows parallel to and in the opposite direction of the water flowing down. In both cases, as water is evaporated, energy and moisture are added to the air. The resulting air-water vapor mixture (and the energy it holds) are then carried away from the cooling tower by the air being forced through. On cold, wet days cooling towers seem to be emitting smoke, but that is a common misconception. What is actually being seen is the condensed water vapor.

Cooling Tower Wet Deck at MNSU
Cooling tower wet deck at MNSU [129].
There are two main methods of distributing water in cooling towers: gravity and spray distribution. Crossflow cooling towers typically use gravity distribution. This involves a reservoir that can hold hot water to be cooled above the heat transfer surface. The water is forced down through nozzles into the fill by gravity where it is then cooled by the air. Gravity distribution systems can be inspected while in operation and only require minimal pump work since it is not a pressurized system. Spray distributors, which are used on counterflow cooling towers, use spray nozzles which forcibly spray the water to be cooled down onto the fill. Spray distribution systems require additional pump work to supply the pressure needed but must be shut down in order to inspect or perform maintenance on the system.

Drift eliminators are another component implemented into cooling towers which help improve the efficiency. The drift eliminator helps reduce the loss of large amounts of water from the cooling tower. This is done by employing a fill similar to that of cooling fill, but instead the airflow is forced through it before exiting the cooling tower. Large droplets of water in the air impact the drift eliminator getting stopped and returned to the cooling tower while still allowing the rest of the cooling air to pass through.

A heat exchanger can also be used effectively in a cooling tower. Heat exchangers, which contain two chambers separated by a single wall, are an efficient way to transfer heat. One chamber contains the heated material or fluid and the other contains a chilled material or fluid. When two materials come in close contact with each other, the heat transfers from the warmer material to the cooler material until they approach the same temperature.

Minnesota State University, Mankato

Baltimore Air Cooling Towers at MNSU
Baltimore Aircoil Company cooling towers at MNSU [129].
The MNSU utility plant utilizes three Baltimore Aircoil Company dual-cell, crossflow cooling towers. All three are Series 3000, which is a forced draft cooling towers. In a forced draft cooling tower, an axial fan is used to draw air through the cooling tower. A dual-cell cooling tower consists of two attached cooling units, each with its own axial fan. All three of the Baltimore cooling towers have roughly the same specifications: each is roughly 24' x 21.5' x 11', and stands 5' off of the ground to allow for maintenance servicing of the units and the connecting pipes. The cooling towers are constructed of stainless steel to inhibit the formation of rust. MNSU's cooling tower was also designed for a wet-bulb temperature of 76°F and a volumetric flow rate of 3300 gpm.

Maintenance

Cooling towers are often required to be tested every day to ensure the proper chemical composition of the water. The primary reason for this is to avoid bacterial growth. For example, the bacteria that causes Legionnaire's disease can develop in the warm water basins of cooling towers. Once growing in the cooling tower, the bacteria are then released and become airborne. Though Legionnaire’s disease is not contagious, it can still cause flu-like symptoms such as chills, fever, cough, headaches, fatigue, loss of appetite, muscle aches, and diarrhea.

In 1995, an outbreak of Legionnaire's disease hit Mankato, home of Minnesota State University, infecting 23 people and killing one. To stop the outbreak, all 21 cooling towers across the City of Mankato were treated and disinfected. Investigations were then held to determine the source of the outbreak [85]. In this case, all of the people who had been infected lived or worked downwind from the cooling tower at the Immanuel St. Joseph’s Hospital, which, ironically, turned out to be the source of the bacteria. The tower was corrected and the personnel in charge of the unit were taught how to prevent the growth of the bacteria in the future.

How Stuff Works → Water Treatment

In order to operate a cooling tower efficiently, the water must be treated to remove dissolved solids. The solids from untreated water can accumulate on the equipment, causing corrosion as well as introducing airborne impurities and biological contaminants into the circulating water. A simple bleed-off system can prevent this from occurring by discharging the water to help keep the concentration of solids in the water below an allowable limit. If the solids are not bled off, they will continue to accumulate. Biocides, a toxic substance that will kill off all living organisms in the recirculating water, can be used to prevent the development of airborne impurities and biological contaminants.

Mechanical vs. Natural-Draft

Cooling Tower Fan at MNSU
Cooling tower fan at MNSU [129].
Mechanical-draft cooling towers consist of one or more mechanically-driven fans. These fans have multiple blades that usually range from 2 feet to 44 feet in length [86]. An electric motor uses reduction gearing to turn the blades at relatively low speeds in order to achieve high volumetric flow rates at relatively low static pressures. The blades are usually made from aluminum alloys, stainless steel, or fiberglass [87]. However, some are known to be manufactured from plastic or laminated wood. Using a mechanical-draft cooling tower has some advantages, such as: the low initial capital and construction costs (because they are smaller size than natural-draft cooling towers), low physical profile, and the capacity to reliably provide the required quantity of air. However, there are disadvantages to using this type of cooling tower as well, such as: higher power consumption, operating costs, maintenance costs, and noise levels.

Natural-draft cooling towers are very tall—often up to a height of several hundred feet—so that the necessary change in pressure can be obtained. Instead of using fans, natural-draft cooling towers depend on natural driving pressure caused by the difference in density between the cool outside air and the hot, humid inside air. The function of natural-draft cooling towers is based on this change in pressure.

Cooling towers were originally made from wood, then evolved to steel, and finally to the reinforced concrete used today. Due to their distinctive shape, the structures are also called hyperbolic towers. Natural-draft cooling towers are usually used in cool, humid climates where there is a low wet-bulb temperature and a high relative humidity.

A hybrid wet/dry cooling tower incorporates the characteristics of both the natural and mechanical-draft cooling towers. The hybrid tower is constructed with a reinforced concrete hyperbolic shell that is similar to, but smaller than that of a natural-draft tower. The smaller size enables the tower to consume small amounts of power and requires half the space. The hybrid tower also makes use of the electrically-driven fans of a mechanical-draft tower. The fans are positioned around the base of the tower to provide better airflow control and consume less power compared to the mechanical-draft tower. The high velocity and height of the exit air from the tower have eliminated the hot-air recirculation problems mechanical-draft cooling towers typically experience.

Check out this animation showing how cooling towers work.
Want to learn more about the different types of cooling towers? Click Here
This diagram shows a hybrid cooling tower.
Want to understand how a cooling tower works? Watch this walkthrough video.
The wet-bulb temperature is the lowest temperature an object may be cooled to solely through the process of evaporation.
Video of mechanical-draft cooling towers at MNSU.
Want to know how using a variety of Cooling Tower designs can save money? Read this ASHRAE article.