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I have read that the main reason for the hyperboloid shape of cooling towers you find next to thermal power plants is because it allows more structural strength with less material. According to some sources, it also improves the efficiency of the cooling tower.

Quote from Wikipedia:

Hyperboloid (sometimes incorrectly known as hyperbolic) cooling towers have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperboloid shape also aids in accelerating the upward convective air flow, improving cooling efficiency.

Intuitively, it seems like the venturi effect taking place because of the narrow throat would improve the efficiency as it accelerates the air flow and should thus amplify the natural draft. However, I have found sources claiming this to be a false popular belief.

This answer on a similar post claims:

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.

(This post does not answer my question as it does not say if there are other effects at play)

And the French Wikipedia page for cooling towers reads as following:

Contrary to popular belief, the shape of the tower is not intended to take advantage of the Venturi effect. Although the Venturi effect is present at the tower neck, it reduces cooling performance very slightly because it increases the air velocity. This creates more pressure drop which reduces the draft effect and therefore the overall airflow available for cooling.

(Translated from French to English. This information is not present on the English page)

It therefore seems like the venturi effect is not among the reasons for this design. Does that mean structural strength and material savings are the only reasons for this unique shape?

carl-vbn
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  • Perhaps the benefit is the "natural" draft instead of needing a pump? You seem to discount the natural draft in terms of efficiency but as you don't explain the terms you use for "efficiency" how can anyone be sure? – Solar Mike Apr 17 '22 at 19:16
  • @SolarMike From what I understand the natural draft is caused by the difference of temperature between the hot water and the air inside the tower, but I would like to know if it is amplified by the hyperboloid shape. By "efficiency" I mean the tower's cooling ability, where more cooling is more efficient. – carl-vbn Apr 17 '22 at 19:21
  • Your second link says « as height increases, draft increases, so does velocity and the cooling effect » then goes on to mention time as a factor ie a 290m tower needing 200 days at 1m per day. For an answer written by a cooling tower design engineer why can’t you accept what s/he tells you? – Solar Mike Apr 18 '22 at 03:59
  • @SolarMike I trust them, but I am wondering about the hyperboloid shape (the fact that the middle part is thinner than the top and bottom). They mention the strength factor, which I am aware of and contradict the Venturi effect belief but don't say anything about another physical process related to the shape that directly increases draft, as the English wikipedia page implies such a process exists. If there is none, that's an answer to my question. – carl-vbn Apr 19 '22 at 07:14

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