# Chimney Sizing

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:

dp_{ ch }= h (ρ_{ o }- ρ_{ i }) a_{ g }(1)

where

dp_{ ch }= pressure draft in the chimney (Pa)

h= height of chimney (m)

ρ_{ o }= ambient outside density of air (kg/m^{ 3 })

ρ_{ i }= inside chimney density of air (kg/m^{ 3 })

a_{ g }= acceleration of gravity (9.81 m/s^{ 2 })

Since the Ideal Gas Law states that

* ρ _{ o } T _{ o } = ρ _{ i } T _{ i } *

* or *

* ρ _{ i } = ρ _{ o } T _{ o } / T _{ i } (2) *

* where *

* T = t + 273 = absolute temperature (K) *

* where *

* t = temperature (°C) *

- (1) can alternatively be expressed as:

dp_{ ch }=h ( ρ_{ o }< - ρ_{ o }T_{ o }/ T_{ i }) a_{ g }

or

dp_{ ch }=h ρ(3)_{ o }( 1 - T_{ o }/ T_{ i }) a_{ g }

#### 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/m ^{ 3 } .

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

* dp _{ ch } = (20 m) (1.292 kg/m ^{ 3 } ) ( 1 - (273 K) / (473 K) ) (9.81 m/s ^{ 2 } ) *

<

* = 107 Pa *

#### Chimney Draft Calculator

#### Chimney Draft Chart

### 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 (m^{ 2 })

Q= volume of flue gases at chimney temperature (m^{ 3 }/s)

v= velocity (m/s)

In general * 1 kW * boiler heat requires * 1100 mm ^{ 2 } * of chimney area.

#### Chimney Vent Capacity Chart - Btu/h and kW

## Related Topics

### • Combustion

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

## Related Documents

### Chimney's Explosion Doors

Recommended size of explosion doors or draught stabilizers in oil fired installations.

### Chimney's Size vs. Boiler Power

The size of chimney vs. boiler's heat load.

### Chimneys - Roof Termination

Chimneys and single wall vents roof termination.

### Chimneys and Fireplace Sizing

Chimneys and fireplaces for stoves burning wood or coal as fuel.

### Natural Draft - Air Flow Volume and Velocity

Air flow - volume and velocity - due to stack or flue effect caused by indoor hot and outdoor cold temperature difference.