What is a cooling tower? A cooling tower is a heat rejection device that utilizes the evaporative cooling process to transfer waste heat to the atmosphere. Waste heat enters the cooling tower as warm water and is pumped to the water distribution system above the cooling medium. The water is then dispersed down through the tower, while air is simultaneously drawn into the tower. The mixing of the air and water causes a small portion of the water to evaporate, removing heat from the water. This heat is transferred to the air, which is drawn out the top of the tower and expelled into the atmosphere. The resulting cold water then re-circulates back through the heat source in a continuous cycle.
There are two types of cooling towers: crossflow and counterflow.
In a crossflow tower design, the air flow is perpendicular to the water flow. As the water is distributed and flows downward through the fill, air flows horizontally through the fill. Crossflow towers have a smaller physical footprint than counterflow towers of the same capacity, making them advantageous for sites where space is limited.
In a counterflow tower design, the air and water flow parallel to one another in opposite vertical directions. Air enters the tower through air inlets and is drawn up vertically, while water is sprayed through nozzles at the top of the tower and then flows downward through the fill. Counterflow towers are more common than crossflow towers because they have the advantage of lower pumping costs. This is because the water is generally pumped to a lower elevation than in crossflow towers of similar size.
Natural Draft vs. Mechanical Draft
Towers can also be categorized as natural draft or mechanical draft. As the name implies, natural draft towers allow for the circulation of air by natural convection. The air flows through the tower because as it becomes warm and moist, it naturally rises above the cooler, outside air being drawn in. To generate the necessary airflow, natural draft cooling towers require tall stacks to create the required draft. Mechanical draft towers function similarly but utilize fans to create the required airflow. Because of this, mechanical draft towers are often smaller than natural draft towers but increase operational costs. Additionally, there are plume abated towers that combine evaporative cooling with dry heat exchangers to reduce plume (steam) visibility in areas where the plume could create safely issues or be viewed as aesthetically displeasing.
To improve heat transfer and tower efficiency, fill media is installed inside the tower to increase the contact surface area, as well as the contact time, between air and water (allowing more of the waste heat to evaporate). There are two types of fills: film and splash. Film fills cause water to spread into a thin film, exposing a large amount of surface in a very compact volume. Splash fills break the falling water into little droplets, continually exposing more water to the airflow. Both methods allow towers to achieve improved performance; however, the selection and amount of fill vary based on thermal conditions, tower geometry, and water quality.
Drift eliminators remove small droplets of water that are entrained in the exhaust air to minimize the nuisance and health issues that they create if allowed to escape through the top of the tower along with the heated air. Drift eliminators are designed to create a tortuous path for the air stream. When the water droplets move through them, they are forced to change direction and impact the drift eliminator side walls, coalescing and draining back into the wet section of the cooling tower.