Chimney Sizing
Calculating chimney draft and chimney area
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The capacity of a chimney depends on the potential chimney draft - the pressure difference created by the inside and outside air temperature difference - and the chimney area and height.
The pressure difference will force air and fuel gas from the boiler or fireplace through the chimney and out of the building.
Chimney Draft
The difference pressure draft in a chimney can be expressed as:
dpch = h (ρo - ρi) g (1)
where
dpch = pressure draft in the chimney (Pa)
h = height of chimney (m)
ρo = density of outside air (kg/m3)
ρi = density of inside air (kg/m3)
g = 9.81 - gravitation (m/s2)
(1) can alternatively be expressed as:
dpch = 0.0465 dt h (2)
where
dt = temperature difference between inside and outside air (K, oC)
Chimney Area
The design velocity of air and flue gases in a small furnace should be below 2 m/s. The design velocity of a large furnace should be below 10 m/s.
The chimney area can be calculated as
A = Q / v (3)
where
A = cross-sectional area of chimney (m2)
Q = volume of flue gases at chimney temperature (m3/s)
v = velocity (m/s)
It is a general rule to provide 1100 mm2 chimney area per 1 kW boiler rating.
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