What Are Cooling Towers?

A cooling tower extracts heat from water by evaporation. In an evaporative cooling tower, a small portion of the water being cooled is allowed to evaporate into a moving air stream to provide significant cooling to the rest of that water stream. Evaporative Cooling towers are relatively inexpensive and very dependable means of removing low grade heat from your process.

Cooling Towers are commonly used to provide lower than ambient water temperatures and are more cost effective and energy efficient than most other alternatives. The smallest cooling towers are structured for only a few gallons of water per minute while the largest cooling towers may handle upwards of thousands of gallons per minute. The pipes are obviously much larger to accommodate this much water in the larger towers and can range up to 12 inches in diameter.

Cooling towers can generally be classified by use into either HVAC (air-conditioning) or industrial duty.

HVAC

An HVAC cooling tower is a subcategory rejecting heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at near wet-bulb temperatures. Air-cooled chillers must reject heat to the dry-bulb temperature, and thus have lower average reverse-Carnot cycle effectiveness. Large office buildings, hospitals, schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers.

HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. A ton of air-conditioning is the rejection of 12,000 Btu/hour. The equivalent ton on the cooling tower side actually rejects about 15,000 Btu/hour due to the heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10°F, which amounts to 15,000 Btu/hour, or a chiller coefficient-of-performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 13.65.

Industrial

Industrial cooling towers can be used to reject heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and other industrial facilities. The circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic meters an hour (315,000 U.S. gallons per minute) and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic meters an hour).

If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic meters an hour and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the river or lake to an unacceptable level for the local ecosystem. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water.

Some coal-fired and nuclear power plants located in coastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.

Petroleum refineries also have very large cooling tower systems. A typical large refinery processing 40,000 metric tons of crude oil per day (300,000 barrels per day) circulates about 80,000 cubic meters of water per hour through its cooling tower system.

Heat Transfer Methods

With respect to the heat transfer mechanism employed, the main types are:

• wet cooling towers or simply cooling towers operate on the principle of evaporation.
• dry cooling towers operate by heat transmission through a surface that divides the working fluid from ambient air. They thus rely mainly on convection heat transfer to reject heat from the working fluid, rather than evaporation.
• hybrids are also available.

In a wet cooling tower, the warm water can be cooled to a temperature lower than the ambient air dry-bulb temperature, if the air is relatively dry. As air is drawn past a flow of water, the two flows attempt to equalize. The air, if not saturated, absorbs additional water vapor, leaving less heat in the remaining water flow.

To achieve better performance (more cooling), a media called fill is used to increase the surface area between the air and water flows. Splash fill consists of material placed to interrupt the water flow causing splashing. Film fill is composed of thin sheets of material upon which the water flows. Both methods create increased surface area.

Air Flow Methods

With respect to drawing air through the tower, there are three types of cooling towers:

• Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Counter-intuitively, more moist air is less dense than drier air at the same temperature and pressure. This moist air buoyancy produces a current of air through the tower.
• Mechanical draft, which uses power driven fan motors to force or draw air through the tower.
• Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fill arrangement is also known as draw-through.
• Forced draft: A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through.

Hyperboloid (aka hyperbolic) cooling towers have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperbolic form is popularly associated with nuclear power plants.

Components of a Cooling Tower

• Hot Water Distribution System: An open basin above each fill bank receives the hot water that is piped to each cell in the tower.
• Hot Water Basin: This basin receives the hot water that is piped into each cell in the tower and has removable covers to restrict the influx of debris. Water enters through a removable wave suppressor splash box.
• Fan: Fans are individually adjustable propeller type and driven through v-belts and protected with a belt guard, or with drive shafts and gear boxes.
• Fill, Louvers, and Drift Eliminators: Each fill sheet has louvers and drift eliminators formed by thermoformed PVC. The fills are suspended by hot dip galvanized structural tubing and are elevated above the floor of the cold water basin.
• Cold Water Basin: In a cooling tower, water is supplied from the discharge of the circulating water system to a distribution basin, from which the cooling tower pump takes suction. Accessories include both a side suction connection OR a hole and bolt circle in the basin floor suitable for gravity flow, which have debris screens and anti-cavitation devices.
• Pump: Pumps of a variety of sizes may be used, depending upon the size of cooling tower and the demands of your process. The technical engineers at Cooling Technology, Inc understand that each project is unique and will work diligently to make sure your site’s requirements will be matched to the best cooling tower and pump.

Limitations of Cooling Towers

There are some limitations to using cooling towers. Their ability to cool is based on how much water is lost due to evaporation. The evaporation from a cooling tower is based on the quality of air in the surrounding area. If an area has high humidity, less water will evaporate than in a dry climate. In the winter, cooling towers are usually able to cool more efficiently as the air is drier. If the water is needed to be cooled to higher than 75°F, a cooling tower is recommended, dependent upon wet bulb temperature for the region. If water is needed to be cooler, a chiller may be better suited to your cooling needs than a cooling tower.