3

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?

Philipp Imhof
  • 133
  • 3

2 Answers2

8

The hyperbolic shape of cooling tower is preferred as this shape is stronger against wind pressure. Consequently, the concrete wall thickness can be reduced and optimised. from 1.5m at the base of the viel, the shell thickness rapidly fall to ~30cm.

The venturi effect resulting in the usage of this shape is, contrary to popular belief, not improving the performance of the cooling tower. Indeed, the Venturi effect increases the speed at the throat which creates a very slight increase in the pressure drop by friction on the walls. This reduces the draft and therefore the air flow available for cooling. This very marginal loss of performance is largely compensated by the gain in resistance of the shell to external winds.

The height of the tower is determined by :

  • Local permit for maximal construction height.
  • A balance between performance / cost / erection time.

If the tower is high, the draft will be stronger, thus the airflow will be higher thus the cooling effect will be better. However, if the tower is high, the cost increase rapidly with the difficulty to bring concrete at such height. But also due to the increased thickness of the shell (due to self weight and the wind load). Sometimes, construction time is a limiting factor, rule of thumb is 1m of shell per day. If the tower is 200m, it might be too long for a project.

Reference : 20 years as a hyperbolic cooling tower design engineer

Vincent
  • 96
  • 1
  • 2
3

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.

Will.W
  • 117
  • 9
kamran
  • 23,517
  • 2
  • 22
  • 42