Νatural draught flow rate (draft flow)

Description

The structure which provides ventilation for hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere causes a flow rate. The height that structure (e.g. chimney) influences its ability to transfer flue gases to the external environment via stack effect.
The 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 draught/draft”, “natural ventilation”, “chimney effect”, or “stack effect”.

Related formulas

Variables

QChimney draught/draft flow rate (m3/s)
Cdischarge coefficient (usually taken to be from 0.65 to 0.70) (dimensionless)
ACross-sectional area of chimney (m2)
gStandard gravity
HHeight of chimney (m)
TiAverage temperature inside the chimney (K)
TeExternal air temperature (K)