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# Chimney Sizing

## Calculate chimney draft and required chimney area - online calculator.

The capacity of a chimney depends on the potential chimney draft - the pressure difference created by the inside and outside air temperature difference and chimney height - and the chimney area.

The pressure difference in the chimney pulls air and fuel gas from the boiler or fireplace through the chimney and out of the building.

### Chimney Draft

The pressure difference or chimney draft can be expressed as:

dpch = h (ρo - ρi) ag         (1)

where

dpch = pressure draft in the chimney (Pa)

h = height of chimney (m)

ρo = ambient outside density of air (kg/m3)

ρi = inside chimney density of air (kg/m3)

ag = acceleration of gravity (9.81 m/s2)

Since the Ideal Gas Law states that

ρo To = ρi Ti

or

ρi = ρo To / Ti                  (2)

where

T = t + 273 = absolute temperature (K)

where

t = temperature (°C)

- (1) can alternatively be expressed as:

dpchh (ρo - ρo To / Ti) ag

or

dpchh ρo ( 1 - To / Ti) ag               (3)

#### Example - Chimney Draft

A 20 m high chimney operates with flue gas temperature 200 °C (473 K). The outside temperature is 0 °C (273 K). The density of the outside air is 1.292 kg/m3.

The draft in the chimney can be calculated with (3) as

dpch = (20 m) (1.292 kg/m3) ( 1 - (273 K)(473 K)) (9.81 m/s2)

= 107 Pa

#### Chimney Draft Calculator

h - height of chimney (m)

ρodensity outside air (kg/m3)

ti - chimney inside temperature (oC)

to - outside air temperature (oC)

### Chimney Area

The velocity of air and flue gas in a smaller furnace should not exceed 2 m/s. Design velocities of larger furnaces should not exceed 10 m/s.

Required 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)

In general 1 kW boiler heat requires 1100 mm2 of chimney area.

## Related Topics

• Combustion - Boiler house topics, fuels like oil, gas, coal, wood - chimneys, safety valves, tanks - combustion efficiency.

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Temperature

oC
oF

Length

m
km
in
ft
yards
miles
naut miles

Area

m2
km2
in2
ft2
miles2
acres

Volume

m3
liters
in3
ft3
us gal

Weight

kgf
N
lbf

Velocity

m/s
km/h
ft/min
ft/s
mph
knots

Pressure

Pa (N/m2)
bar
mm H2O
kg/cm2
psi
inches H2O

Flow

m3/s
m3/h
US gpm
cfm

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