The chimney is an essential part of any furnace, boiler, thermal power plant, etc. It is normally used to create as natural draft and provide the air necessary for combustion. But chimneys could also be used as an efficient channel for proper dispersal of gaseous pollutant.
The selection of the height of the chimney used is fairly important especially when it is located in a thickly populated area or is close to the same.
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Keeping the meteorological conditions, especially direction and velocity of wind, in mind, the dispersal of gaseous pollutants could be carried out in such a way that the population around the factory is not affected.
Regarding the stack design, on the basis of observation of past design, several factors that can be used as aids in future stack design or to predict pollutant concentrations to be expected from pollutant stacks have been determined.
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Generally, under the same meteorological conditions, ground-level pollutant concentration at a given point, downwind from a stack, is reduced as the stack height is increased.
This is due to the fact that pollutants emitted from the stack are allowed more time to diffuse in the atmosphere before reaching the ground.
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It is obvious that some of the gases from a chimney will eventually reach the ground by diffusion whatever be the height of discharge or density of the gases. This type of diffusion is known as eddy diffusion, and is far faster than the classical kinetic diffusion.
The visible in flat country by observation of smoke from chimneys which are tall enough for the plume to be undisturbed by down draughts into the lee of buildings or by turbulence generated by the ground contours
It is now possible to calculate with sufficient accuracy the amount of dilution of the gases reaching the ground from a free-flowing plume at a particular height, and, therefore, the maximum time-mean ground-level concentration of any noxious constituent.
This is important to the chemical engineer concern with the specification for the design of a plant for purification including dust and fume removal of waste games. The facts to be considered are:
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1. Wind directions and velocities. (10 year averages can be obtained from the nearest meteorological station or from the meteorological department.) 2 The frequency of radiation type atmospheric temperature inversions likely to extend up to heights above 35 m.
3. Character of the surrounding district in terms of background air pollution.
4. The velocity of discharge from the chimney and the use of a multiple fluid chimney for a plant with an output varying with the season, etc.
5. The aerodynamic effects caused by adjacent buildings, including existing or proposed ‘high-rise’ offices or dwellings within 10 chimney heights of the chimney, and the effects of high ground heights which should be considered in terms of the height of the base level of the chimney.
6. The thermal rise of a hot discharge by virtue of the temperature of the gases.
7. The calculated time-mean ground level concentration of noxious pollutants and dust-fall on ‘sensitive’ areas downwind, taking into accounts their height above chimney base level.
8. The persistence of visibility of the plume.