The job of cooling towers is to transfer unwanted heat to the atmosphere. For instance, many industrial processes produce waste heat that is transferred to circulating water via heat exchangers. The circulating water is then cooled by means of evaporation or direct heat transfer to atmospheric air.
Though most people are familiar with the large hyperboloid cooling towers associated with nuclear power plants, most cooling towers are much smaller. They cool water used in many industrial processes such as oil refining, chemical manufacture and power generation. They are commonly used with air conditioning systems for large buildings too.
The first cooling towers were derived from the design of steam engine condensers in the 1800s, which were required to re-condense steam from the pistons and return it to the boiler. For plants without nearby large bodies of cooler water, cooling towers were required that recycled the water after each use. In the early 1900s, two Dutch engineers invented the familiar hyperboloid towers, which were first used at coal-powered electrical plants.
Cooling Methods and Tower Types
There are two general methods used for cooling water in towers:
- Evaporative or wet cooling towers - These types of towers are also referred to as open-circuit coolers. They bring higher-temperature water directly in contact with cooler atmospheric air, which lowers the water temperature through evaporation.
- Indirect or dry cooling towers - Also called closed-circuit coolers, these spread the warmer water inside a metal shell or thin tubing whose outer surface is in contact with cooler air. A car’s engine radiator is an example of this cooling mechanism. The main distinction between evaporative and indirect cooling systems is that the process fluid never comes in contact with the air.
Evaporative towers use one of two methods for passing cooling air through the warm water flow. In a counter-flow design, cooling air flows in the opposite direction of the warm water flow, whereas cross-flow towers move the air horizontally across the warm water flow.
Towers are also distinguished by their air draft method. Natural draft towers utilize the natural buoyancy of the warming air to exhaust it upward. Mechanical draft towers employ fans to assist the flow. Some tower designs mix these methods to compensate for seasonal atmospheric temperature changes.
In the video below you can see and understand the working principle of a cooling tower:
Cooling Tower Components
Some parts of cooling towers have specific terminology:
- Fill refers to the mechanism for spreading the warm water so that it presents a larger surface area to the cooler air. Splash fills break up the water into tiny droplets as it falls over the fill. Film fill, which is more efficient, spreads the water into a thin film. Some fills have a honeycombed pattern to enhance heat transfer further.
- Drift refers to water droplets - not vapor-that approaches the exhaust end of the tower. If drift leaves the tower, it is called blowout. Drift that escapes a tower must be replenished from another source.
- Drift eliminators return the drift to the cold-water basin at the bottom of the tower.
Cooling towers often contain various fans, pumps, nozzles and louvers to move and direct air and water. The most efficient designs use axial fans whose pitch can be changed to adjust airflow or centrifugal fans whose velocity and power consumption are adjusted to suit changing conditions.
Some companies provide turnkey cooling systems that include the tower, temperature controllers, gauges, water treatment and heat exchangers. These package towers require only connections to water and power for installation.
Cooling Tower Performance Factors
When assessing cooling tower performance, there are several factors to consider:
- Range is the difference between the temperature of the process fluid as it exits a heat exchanger and the required cooled temperature of the fluid.
- Approach refers how close a tower is able to cool process fluid to the ambient wet-bulb temperature. Tower manufacturers can usually supply an approach temperature of no less than 2.8 degrees centigrade.
- Capacity is the amount of heat dissipation at a specified flow rate and range that a cooling tower can provide.
Besides these factors, the desired temperature of the returning process fluid must be considered also. Typically, cooler process fluids result in higher efficiency, but that must be weighed against the cost of cooling tower capacity to determine if the higher efficiency pays for itself.