Flue-gas Stack - Flue-gas Stack Draft

Flue-gas Stack Draft

The combustion flue gases inside the flue gas stacks are much hotter than the ambient outside air and therefore less dense than the ambient air. That causes the bottom of the vertical column of hot flue gas to have a lower pressure than the pressure at the bottom of a corresponding column of outside air. That higher pressure outside the chimney is the driving force that moves the required combustion air into the combustion zone and also moves the flue gas up and out of the chimney. That movement or flow of combustion air and flue gas is called "natural draft", "natural ventilation", "chimney effect", or "stack effect". The taller the stack, the more draft is created.

The equation below provides an approximation of the pressure difference, ΔP, (between the bottom and the top of the flue gas stack) that is created by the draft:

where:
ΔP	= available pressure difference, in Pa
C	= 0.0342
a	= atmospheric pressure, in Pa
h	= height of the flue gas stack, in m
To	= absolute outside air temperature, in K
Ti	= absolute average temperature of the flue gas inside the stack, in K

The above equation is an approximation because it assumes that the molar mass of the flue gas and the outside air are equal and that the pressure drop through the flue gas stack is quite small. Both assumptions are fairly good but not exactly accurate.