The combustion of fuels in industrial furnaces or residential fireplaces produces hot combustion product gases commonly referred to as flue gases. The large temperature difference and hence large density difference between the hot flue gases and the cool ambient air results in a large buoyancy difference between the flue gas and the ambient air. That buoyancy difference then produces a flow of ambient air into the fuel combustion zone and an upward flow of flue gases in the furnace flue gas stacks or the fireplace chimneys. That flow is variously called the flue gas stack effect, the chimney effect, draft, draught, or natural ventilation.

The pressure difference driving force for the stack effect:

The pressure difference between the ambient air and flue gases caused by the difference in their densities is the driving force for the stack effect and it can be calculated with the equation presented below:



SI units:

where:
∆P = available pressure difference, in Pa
C = 0.0342
a = atmospheric pressure, in Pa
h = stack or chimney height, in m
To = absolute outside temperature, in K
Ti = absolute inside temperature, in K

U.S. customary units:

where:
∆P = available pressure difference, in psi
C = 0.0188
a = atmospheric pressure, in psia
h = stack or chimney height, in ft
To= absolute outside temperature, in °R
Ti = absolute inside temperature, in °R

The flow rate induced by the stack effect:

The draft or draught flow rate induced by the stack effect can be calculated with the equation presented below:



SI units:

where:
Q = stack or chimney draft (draught) flow rate, m³/s
A = stack or chimney cross-sectional flow area, m²
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration, 9.807 m/s²
h = stack or chimney height, m
Ti = average inside temperature, K
To = outside air temperature, K

U.S. customary units:

where:

Q = stack or chimney draft (draught) flow rate, ft³/s
A = stack or chimney cross-sectional flow area, ft²
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration, 32.17 ft/s²
h = stack or chimney height, ft
Ti = average inside temperature, °R
Too = outside air temperature, °R

Note:

Both of the above equations assume that the ambient air and the hot flue gases have the same molecular weight which is only approximately true. Hence, both equations are only approximations. However, in most cases, the equations yield quite good approximations.

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Milton Beychok
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