From the Springfield nuclear plant to the "double triples" of Didcot Station in England (which have already been demolished), these installations have stood out for the shape of their cooling towers. The formal term is "hyperboloid", but the real question is: why don't they take a straighter, more traditional chimney-like form? The answer lies in the word "cooling", and the latest video from Practical Engineering explains it perfectly.
Thermal power plants can use different types of fuel, but their general goal is usually the same: turn water into steam, which is then injected/transferred to a series of turbines to generate energy. Here we obey the laws of thermodynamics (!), and thermal power plants do too. All that steam must be cooled somehow to repeat the cycle, and spending excessive energy in the process makes no sense.
Some plants take advantage of local features like rivers and lakes (Texas offers several examples), but the rest must build cooling towers. There are many designs with a more "traditional" profile, so to speak, however, Grady from the channel Practical Engineering decided to focus on the concept of the hyperboloid tower, and create a scale model at home. Why does the tower have "that" shape?
Editor's note: Subtitles are in English. Not ideal, but it's watchable.
Natural Draft Hyperboloid Tower: How Does It Work?
One of the first details is that the base of the tower is not closed. Grady's model has several holes, and they are easy to find in real versions. Cold air enters the tower and comes into contact with the water, but that is not enough: a spray system combined with a layer of material that increases the surface area of the liquid optimizes the whole process.
Here is where we move to the most complex section of the video. Grady uses a psychrometric chart to explain something not everyone remembers: hot air rises... and so does moist air. If temperature and pressure remain constant, but we increase air humidity, its density drops. Water molecules (18.01528 g/mol) are lighter than nitrogen and oxygen in the air (28.96 g/mol, includes other gases), which takes us into the fascinating world of convective heat transfer.
The hot water transfers its heat to the air. In turn, it becomes more buoyant, moves upward in the tower, and allows fresh air to enter through the base, but part of the hot water evaporates, removing even more heat from the liquid, and making the air even more buoyant. The most impressive thing is that these towers enable this process with a minimal number of components (pumps, control systems, etc.). In other words, the towers are basically "cloud machines".
As for the reasons for the hyperboloid design, they are several. First, a wide base allows greater air intake, the "closing" in its center accelerates the upward flow, and the wide opening at the top encourages mixing of hot, humid air with cold outside air. However, the main advantage is structural: loosely speaking, the larger the tower, the better it works, but it also needs thin walls, which optimize airflow and reduce costs. A tall cylinder with thin walls is weak (e.g., a soda can without pressure), while the double curve of the hyperboloid reinforces the structure against vertical loads (its own weight) and horizontal ones (wind).
Grady's video ends with other examples of cooling, but he adds that the adoption of the hyperboloid tower in nuclear plants is due to its scale: With an average operation of 50 years, the construction and operating savings in the long term are significant. Furthermore, they are more stable, reducing any possibility of failure in something as delicate as the cooling system of a nuclear plant.
