Cooling towers for nuclear power plants often have the shape of a hyperboloid. At first glance, I thought that an obvious reason for this would be the smaller sectional area, meaning the tower would be less exposed to wind.

However, I did a quick calculation, based on a cooling tower nearby. I modeled the cooling tower as a hyperboloid and calculated its volume and its sectional area. I then calculated the radius needed for a cylindrical tower with the same volume. Its sectional area was just 1.5% larger. By reducing its height from 150 to 145 meters (and increasing the radius, of course), the sectional areas would have been the same.

I know that hyperboloids can be built using only straight pieces of reinforcement. However, those towers are quite big and the circular shape could be very well approximated by some sort of polygon with e.g. 1000 vertices making circularly shaped reinforcement unnecessary.

I can imagine other reasons:

  • The waisted shape could probably lead to a certain compression of the hot vapor and thus help/accelerate the condensation. Is this true? How big could that effect be?

  • Some documents I have read say one needs less material to build a hyperboloid cooling tower. Is there really a big difference in terms of concrete? After a rough calculation, assuming a thickness of 1 meter of concrete all around, I'd estimate that a cylindrical shape would only need approximately 1.5% more concrete.

So, why do almost all cooling towers have this particular shape?

  • $\begingroup$ Just a note: that cooling tower design is seen on other power plants that use the Rankine cycle. I've seen pictures of coal-fired plants that use it. $\endgroup$ – TimWescott Jun 10 '19 at 16:58
  • $\begingroup$ It's probably also worth a note that many nuclear stations don't have towers - none in the UK do that I know of. $\endgroup$ – Will.W Jun 10 '19 at 20:22

The shape is to take advantage of the Bernoulli law.

The narrower throat of the tower creates reduced pressure encouraging faster convection discharge of warm, moist vapors.

Hot water from the turbines is sprayed in at roughly 1/3 height. that compared with cool water on the floor, usually from a near river creates uplift and convective ventilation.

That hyperbolic shape of the tower accelerates this vertical flow.

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  • $\begingroup$ Thanks for the answer. Do you have any information on how big the effect is? Also, what does that mean with respect to all those explanations about "using less material" and "offering maximum stability"? $\endgroup$ – Philipp Imhof Jun 10 '19 at 6:29
  • $\begingroup$ @PhilippImhof, conic-like geometry is highly stable in earthquakes, and any other geometry would have to use more material and reinforcement to resist the same Richter as the hyperboloid, especially the fact that they stay surefooted on the diagonal columns on the lower part. $\endgroup$ – kamran Jun 10 '19 at 6:39
  • $\begingroup$ Thanks again. Do you have any links where I can read more about that topic? $\endgroup$ – Philipp Imhof Jun 10 '19 at 6:53

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