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?