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Author Topic: Why are cooling towers shaped the way they are?  (Read 25212 times)

Offline chris

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Why are cooling towers shaped the way they are?
« on: 09/03/2007 22:42:21 »
On my way to Durham University this week the train went past the huge cluster of coal fired power stations sitting atop the York, Notts and Derby coalfield. I was looking at the cooling towers and it occurred to me that I haven't got a clue why they are shaped the way they are. Can anyone enlighten me?

Chris


 

Offline Soul Surfer

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Why are cooling towers shaped the way they are?
« Reply #1 on: 09/03/2007 23:18:39 »
An interesting question I remember asking as a child but cannot remember if I got a clear answer and I am producing an answer to this by a bit of theoretical reverse engineering.

Firstly a cooling tower is a big chimney which allows air to be drawn through hot water to cool it partially by evaporation therefore it needs to generate a good updraught  having a shape that is wider at the bottom constricted in the middle ans wider at the top allows this.  However it should minimise the water loss if possible and so the updraught should not be too fast and as much as possible of the evaporation  should be condensed and fall as "rain" inside the cooling tower without being lost  so it needs to be relatively broad compared with chimneys that carry the flue gasses in a power station.

The final detail is the shape which looks to me like a cylindrical hyperbola.  This has properties similar to an arch as used in bridges and is by its nature a good way of putting together a concrete structure that is strong in compression  this is what gives them their particular shape
 

another_someone

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Why are cooling towers shaped the way they are?
« Reply #2 on: 10/03/2007 00:07:02 »
One other aspect that I was wondering about was also about the increased surface area for cooling.

Not sure that the argument about an arch for structural strength, since the whole point about an arch is that it is supported at both ends but unsupported in the middle, and is subject to a gravitational force perpendicular to the points at which the support is given to arch.  The whole point of an arch is to direct the gravitational force to the two end points at which the arch is supported.  A cooling tower is only supported at its base, which is one end of the arch, and the other end of the arch is floating free, and will at most place the upper rim of the tower under tension (and concrete is ofcourse very strong under compression, but week under tension).

The issue of updraught was what occurred to me, as essentially the shape looks very like a typical venturi tubes.  My other thought about an updraught is that it would also maybe push the waste gasses higher up into the atmosphere, but then the same argument would be applicable to most industrial chimneys, and most industrial chimneys are not built like venturi tubes.
« Last Edit: 10/03/2007 00:10:37 by another_someone »
 

Offline eric l

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Why are cooling towers shaped the way they are?
« Reply #3 on: 10/03/2007 12:22:36 »
If my memory does not fail me, it is a shape obtained by rotating a straight line around an axis that is not in the same plane.  This must give a number of advantages for the construction (economy of materials among others). 

There is a page on hyperboloid structures in Wikipedia :
http://en.wikipedia.org/wiki/Hyperboloid_structure
 

another_someone

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Why are cooling towers shaped the way they are?
« Reply #4 on: 10/03/2007 12:51:16 »
If my memory does not fail me, it is a shape obtained by rotating a straight line around an axis that is not in the same plane.  This must give a number of advantages for the construction (economy of materials among others). 

There is a page on hyperboloid structures in Wikipedia :
http://en.wikipedia.org/wiki/Hyperboloid_structure

Interesting, but it would require that the structure be made out of a material capable of sustaining a substantial bending forces - iron bars will do that, concrete will not (are cooling towers actually iron structures?).

Why is this structure commonly used for cooling towers, so much so that it becomes a recognised feature of them, and so rarely for other towers and chimneys?
 

Offline daveshorts

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Why are cooling towers shaped the way they are?
« Reply #5 on: 10/03/2007 14:11:54 »
I would have thought they are reinforced concrete structures.

They are made by taking a cone made up of lots of straight rods, and then rotating the top and the bottom. The reason they want the constriction in the middle makes sense when you think of what speed the air is doing. At the bottom it si going slowly, it accelerates as it goes up the chimney, and then when it reaches the top it slows down again as it meets the stationary air outside.

As the amount of air flowing is a constant all the way up when it speeds up you need less area to carry the same flow of air so the chimney can be narrower.

The conventional chimneys are dealing with much hotter gasses so don't need to bother optimising hardly at all so they just build them as cylinders.
 

Offline eric l

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Why are cooling towers shaped the way they are?
« Reply #6 on: 10/03/2007 15:54:22 »
There is one big difference between cooling towers and chimneys :  in a chimney the gasses cool from bottom to top, while in a cooling tower they (actually only air in this case) get warmer on their way up.  That is why a chimney can be made narrower at the top.

The hyperboloid structure is the structure with the smallest amount of reinforcing steel.

One question :  is air saturated with water heavier (in g/m) than dry air at the same temperature ?  Is there anyone around who has the data ?
 

Offline Bored chemist

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Why are cooling towers shaped the way they are?
« Reply #7 on: 10/03/2007 21:29:05 »
Water vapour is less dense than air so wet air should also be less dense.
 

Offline BillJx

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Why are cooling towers shaped the way they are?
« Reply #8 on: 10/03/2007 22:45:11 »
A cooling tower operates differently than a chimney because as the air rises, it continues to evaporate more water, which obviously occupies more space.  If the cross section didn't increase with height, the flow would be constricted.

In answer to the density of saturated / dry air, density decreases as relative humidity increases.  That's why a falling barometer indicates worsening weather.

Don't think of air as a sponge that "holds" water.  Rather, air is a mixture of N2, O2, H2O, Ar, CO2 and trace molecules.  H2O molecules are lighter than any of the others, so the higher proportion of H2O, the less dense the air.

So, if air doesn't "hold" water, how can it become "saturated"?  Technically, it doesn't, any more than the sun "rises".  But it's a handy expression.
 At one atmosphere pressure, liquid water and steam can co-exist at only one temperature: 100C / 212F.  At higher pressure, the "saturation temperater" (boiling point) increases; at below-atmospheric pressures it decreases.  At a low enough pressure, water will boil at room temperature.
Barring ventilation, water will evaporate, displacing heavier molecules,  until the partial pressure due to H2O is equal to the "saturation pressure" for that temperature.  At that point we say the air is "saturated".  As it it were a sponge.  Which causes much misunderstanding.

 

Offline chris

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Why are cooling towers shaped the way they are?
« Reply #9 on: 11/03/2007 15:00:00 »
But the rising air is losing water, through condensation, as it climbs the tower. Therefore why should there be a need for more space?

Or is the waste steam from the turbines on a closed circuit and a second water feed is used to cool (re-condense) the steam and is itself evaporated in the process?

Chris
« Last Edit: 11/03/2007 15:03:21 by chris »
 

Offline iko

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Why are cooling towers shaped the way they are?
« Reply #10 on: 11/03/2007 17:38:55 »
Hi everyone,
I post some pics trying to understand
what you're discussing here.
I like Image cut&paste!


Did you check this? Here they actually explain
how cooling towers work, quite simply,
good for medical doctors as well!

from AVN Nuclear tech.:     http://www.avn.be/uk/4_nucleaire/tecchp07.asp
« Last Edit: 11/03/2007 17:51:53 by iko »
 

paul.fr

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Why are cooling towers shaped the way they are?
« Reply #11 on: 11/03/2007 17:48:20 »


 
What pretty cooling towers they are.
 

Offline BillJx

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Why are cooling towers shaped the way they are?
« Reply #12 on: 12/03/2007 16:26:17 »
But the rising air is losing water, through condensation, as it climbs the tower. Therefore why should there be a need for more space?

Or is the waste steam from the turbines on a closed circuit and a second water feed is used to cool (re-condense) the steam and is itself evaporated in the process?

Chris



The cooling tower doesn't deal with the steam, if that's what you mean.  The exhaust steam is condensed and the condensate returned to the boiler in a closed cycle.  The condenser is a closed vessel which typically has many cooling-water tubes in it, with the steam space surrounding the tubes.  The condensate is pumped out the bottom and the condensation of the steam produces a low pressure at the turbine outlet.

The cooling water is hot when it comes out of the condenser, and is cooled by evaporation in the cooling tower.  A tremendous amount of energy is contained in the phase difference between steam and water, so a condenser requires a lot of cooling water.  The cooling tower reduces demand on the water source.

Another function of a cooling tower is to cool the plant wastewater.  In our plant we have a cooling tower ahead of the biological treatment ponds.

 

Offline BillJx

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Why are cooling towers shaped the way they are?
« Reply #13 on: 16/03/2007 05:06:32 »
A cooling tower operates differently than a chimney because as the air rises, it continues to evaporate more water, which obviously occupies more space.  If the cross section didn't increase with height, the flow would be constricted.

This part of my post was rather stupid.  In the first place, if the question refers to hyperbolic towers, they are not wider at the top.  They constrict and flare, like a nozzle.  In the second place, for any reasonable temperatures the water vapor can't be much of a percentage of the air, or the humidity will prevent further evaporation.

There are several general cooling tower designs, some drafted by fans and some by natural convection "chimney effect".  I'm quite sure the hyperbolic towers are natural draft.

post edited after some internet research:  The answer is here: http://www.goupstate.com/apps/pbcs.dll/section?category=NEWS&template=wiki&text=cooling_tower

 The pertinent part is under the heading "Cooling Towers of Nuclear Reactors" although the same type of tower is sometimes used in fuel-fired plants, especially in Europe.
« Last Edit: 16/03/2007 18:41:11 by BillJx »
 

Offline daveshorts

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Why are cooling towers shaped the way they are?
« Reply #14 on: 18/03/2007 15:25:55 »
Oh ok, they are a shape with a compound curvature - they curve in two directions - so are very rigid, like a sphere, but can also be made out of straight pieces of steel reinforcing. Very clever
 

Offline JimBrownZA

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Why are cooling towers shaped the way they are?
« Reply #15 on: 01/03/2010 05:59:20 »
My understanding is that the air will rise anyway due to its temperature. Then as it passes through the throat of the venturi, the pressure drops due to Bernoulli's principle, and so the water is released and falls.

By amazing coincidence, I was at those two colourful towers just yesterday- they're here in Johannesburg South Africa. I went up one in an outside lift with mesh sides and rapelled down on a rope for the scariest experience of my life. Then I went again, cos I had paid for two...

Check this whole experience at newbielink:http://www.orlandotowers.co.za [nonactive] and purerush.co.za

Cheers,

Jim
 

Offline chris

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Why are cooling towers shaped the way they are?
« Reply #16 on: 01/03/2010 21:04:53 »
Gosh, yes, now you've mentioned it I remember seeing those towers when I was in Jo'burg in November 2007 - we drove past them!

I don't think, for a second, I'd be brave enough to abseil down them though!


Chris
 

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Why are cooling towers shaped the way they are?
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