Cooling Tower Design

Cooling Tower Design

Cooling tower design is an integral part of providing cost, energy and performance efficiency for a particular cooling tower suited to an application. Factors taken into consideration during the engineering and design process include location, piping, electrical source, tonnage, seismic events, live load, and weather conditions. Various types of cooling towers are available, depending on the specific purpose of the structure. Some of these types include open loop, closed loop, evaporative fluid cooling, counterflow, and crossflow. Several construction materials, such as concrete, fiberglass, steel, and wood, may be used in cooling towers; each presents its own advantages. Pre-engineered cooling tower and custom-designed cooling tower are both available; some cooling tower manufacturers specialize in custom-designed cooling systems, while others focus on ordinary stock cooling towers and equipment.

Cooling Tower From Wikipedia, the free encyclopedia

Natural draft wet cooling hyperboloid towers at Didcot Power Station Didcot Power Station, UK

Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature, or they may rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power plants and building cooling. The towers vary in size from small roof-top units to very large hyperboloid structures (as in Image 1) that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures (as in Image 2) that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site.

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

HVAC

Mechanical draft crossflow cooling tower used in an HVAC application

An HVAC cooling tower rejects heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. Large office buildings, hospitals, and 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 removal of 12,000 Btu/hour (3517 W). The equivalent ton on the cooling tower side actually rejects about 15,000 Btu/hour (4396 W) 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 (5.56 °C), 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 remove heat from various sources such as machinery or heated process material. The primary function 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 metres 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 metres an hour).

If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres 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 receiving 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.

Cooling tower and water discharge of a nuclear power plant

In rare cases, a plant's cooling towers have even been painted to improve public perception as with the Cruas Nuclear Power Plant.

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 tonnes of crude oil per day (300,000 barrels per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system.
The world's tallest cooling tower is the 200 metre tall cooling tower of Niederaussem Power Station.

Cooling tower and water discharge of a nuclear power plant

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. The working fluid and the evaporated fluid (usually H2O) are one and the same.
Dry coolers operate by heat transfer through a surface that separates the working fluid from ambient air, such as in a heat exchanger, utilizing convective heat transfer. They do not use evaporation.
Fluid coolers are hybrids that pass the working fluid through a tube bundle, upon which clean water is sprayed and a fan-induced draft applied. The resulting heat transfer performance is much closer to that of a wet cooling tower, with the advantage provided by a dry cooler of protecting the working fluid from environmental exposure.

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. (see: dew point and psychrometrics). As ambient air is drawn past a flow of water, evaporation occurs. Evaporation results in saturated air conditions, lowering the temperature of the water to the wet bulb air temperature, which is lower than the ambient dry bulb air temperature, the difference determined by the humidity of the ambient air.

To achieve better performance (more cooling), a medium 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 generation methods

With respect to drawing air through the tower, there are four 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. Warm moist air is less dense than drier air at the same 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.
Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow is assisted by a fan.

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. However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants as well. Similarly, not all nuclear power plants have cooling towers.

A forced draft cooling tower

Crossflow

Crossflow is a design in which the air flow is directed perpendicular to the water flow (see diagram below). Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum area. A distribution or hot water basin consisting of a deep pan with holes or nozzles in the bottom is utilized in a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material.

Counterflow

In a counterflow design the air flow is directly opposite to the water flow (see diagram below). Air flow first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow.

Common to both designs:

• The interaction of the air and water flow allow a partial equalization and evaporation of water.
• The air, now saturated with water vapor, is discharged from the cooling tower.
• A collection or cold water basin is used to contain the water after its interaction with the air flow.
• Both crossflow and counterflow designs can be used in natural draft and mechanical draft cooling towers.

Cooling tower as a flue gas stack

At some modern power stations, equipped with flue gas purification like the Power Station Staudinger Grosskrotzenburg and the Power Station Rostock, the cooling tower is also used as a flue gas stack (industrial chimney). At plants without flue gas purification, this causes problems with corrosion.

Wet cooling tower material balance

Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of makeup flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles.

In the above sketch, water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage (W) and some of the water (E) to evaporate. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water.
To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off (D) for disposal. Fresh water makeup (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.

Cooling Tower Operation in Fog and Freezing Weather

Under certain ambient conditions, plumes of water vapor (fog) can be seen rising out of the discharge from a cooling tower (see Image 1), and can be mistaken as smoke from a fire. If the outdoor air is at or near saturation, and the tower adds more water to the air, saturated air with liquid water droplets can be discharged -- what we see as fog. This phenomenon typically occurs on cool, humid days, but is rare in many climates.

• Do not operate the tower unattended.
• Do not operate the tower without a heat load. This can include basin heaters and heat trace. Basin heaters maintain the temperature of the water in the tower pan at an acceptable level. Heat trace is a resistive element that runs along water pipes located in cold climates to prevent freezing.
• Maintain design water flow rate over the fill.
• Manipulate airflow to maintain water temperature above freezing point.