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Why did they used to make the mill chimneys so tall?

This question was asked in an Engineering Interview at Cambridge University.

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The taller that chimney, the more the pollution is someone else's problem. – Olin Lathrop Jan 1 '14 at 15:48

Two reasons - which matters more will depend on the context.

  1. making the chimney taller increases the flow through it due to the stack effect. This may be useful if you need to get rid of a lot of exhaust gases quickly as it avoids the cost of having to pump the exhaust gases.

  2. if the exhaust is environmentally unpleasant then injecting it into the atmosphere as high as possible will reduce the chances of turbulence carrying it back down to ground level and poisoning people. It will probably also increase the dispersal rate as the wind speed is likely to be higher well above the ground.

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One mantra was, "The solution to pollution is dilution!" – DJohnM Jan 2 '14 at 0:10
I suppose the stack effect only works up to a point - once the gas has cooled down, you just add more resistance to flow by making the chimney taller. Extra height only makes sense if you are interested in dispersion of pollution over a larger range. – Floris Jun 4 '15 at 4:34

I'd start w/ this quote from wikipedia:

The height of a chimney influences its ability to transfer flue gases to the external environment via stack effect. Additionally, the dispersion of pollutants at higher altitudes can reduce their impact on the immediate surroundings. In the case of chemically aggressive output, a sufficiently tall chimney can allow for partial or complete self-neutralization of airborne chemicals before they reach ground level. The dispersion of pollutants over a greater area can reduce their concentrations and facilitate compliance with regulatory limits."

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Then either skim Google Books' entry for this one, or find a copy at a library:

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