Why did they used to make the mill chimneys so tall? Why did they used to make the mill chimneys so tall?
This question was asked in an Engineering Interview at Cambridge University.
 A: Two reasons - which matters more will depend on the context.


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*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.

*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.
A: 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."

(  http://en.wikipedia.org/wiki/Chimney )
Then either skim Google Books' entry for this one, or find a copy at a library:
http://books.google.com/books?id=3qZAAAAAIAAJ&dq=mill+chimney+height+design&source=gbs_navlinks_s
A: The higher the chimney (a containment of hot air) the faster the gases and the more air / oxygen drawn in supplying fire with more heat potential. The massive heat generated can burn / melt almost any thing. Glenfel was a large chimney made by the area of cladding fitted a few inches away from the concrete wall. Same principal as a forge using blown air.
A: The tall chimneys were first created in the 1800's to catch high winds and spread to a greater and farther area so it is not just in one concentrated area.
A: One point not mentioned is that there are methods to calculate where and if, a chimney plume can cool and sink to the ground. Our safety department calculated this to determine the height required so that a plume would dissipate before reaching the ground.
