The Chimney or Stack Effect Explained

You can see the tall flue gas stacks in all the power plants. The function of the stack is to disperse the hot gases, emissions and particulates that leave the boiler to a great height. At these heights the pollutants disperse in a very large area so that ground level concentrations are within permissible levels not harmful for humans or vegetation.

Chimneys where in use from the times of the Roman Empire. Chimneys and fireplaces are a common household item in countries with a cold climate. It does the dual function of removing the hot gases out of the house at the same time bringing in fresh air to the fireplace for combustion.

Flue gas stacks higher than 250 meters are common nowadays for larger power plants. The tallest stack currently is 420 meters in Kazakhstan. Many factors like terrain, dispersion pattern, plume heights, adjacent tall structures, and population density determine the height of the stack.

There is a natural phenomena associated with the chimney or the flue gas stack. This is the natural flow of air up the chimney. This is called the ‘chimney or the stack effect’. This effect is found not only in chimneys but also in tall buildings.

The gas temperature inside the flue gas stack is around 140 ° C. The outside ambient air temperature is around say 30° C. Consider this as two air columns connected at the bottom. The high density and heavier cold air will be always pushing the low density and lighter hot gases up. This causes the natural flow of gases up the flue gas stack. This pressure difference that pushes the hot gas up the flue gas stack or the chimney is the 'chimney or stack effect'.

You can feel the effect if you stand near the doors or openings at the bottom of a stack or at open door of an elevator shaft. Depending on the height it can be gentle draught or heavy suction. This is the chimney or stack effect.

In numerical terms this can be represented as

Chimney effect = 353 x Chimney Height x [1/ Stack gas temperature – 1/ Ambient Temperature]

Where

Chimney effect is in mm of water column.

Chimney height is in mteres.

Temperatures are in ° Kelvin.

For a thermal power plant with a stack height of 250 meters the effect could be around 77 mm of water column. In thermal power plants the stack effect aids the Induced draft fans in removing the hot flue gases from the furnace and dispersing them at the top of the stack.

In tall buildings this effect could create problems for the airconditioning system. In deserts where the outside temperatures are higher than the cool interior of the buildings the effect will be in the reverse.