Stack Effect - The Flow Induced By The Stack Effect

The Flow Induced By The Stack Effect

The draft (draught in British English) flow rate induced by the stack effect can be calculated with the equation presented below. The equation applies only to buildings where air is both inside and outside the buildings. For buildings with one or two floors, h is the height of the building and A is the flow area of the openings. For multi-floor, high-rise buildings, A is the flow area of the openings and h is the distance from the openings at the neutral pressure level (NPL) of the building to either the topmost openings or the lowest openings. Reference explains how the NPL affects the stack effect in high-rise buildings.

For flue gas stacks or chimneys, where air is on the outside and combustion flue gases are on the inside, the equation will only provide an approximation. Also, A is the cross-sectional flow area and h is the height of the flue gas stack or chimney.

SI units:

where:
Q	= stack effect draft (draught in British English) flow rate, m³/s
A	= flow area, m²
C	= discharge coefficient (usually taken to be from 0.65 to 0.70)
g	= gravitational acceleration, 9.81 m/s²
h	= height or distance, m
Ti	= average inside temperature, K
To	= outside air temperature, K

U.S. customary units:

where:
Q	= stack effect draft flow rate, ft³/s
A	= area, ft²
C	= discharge coefficient (usually taken to be from 0.65 to 0.70)
g	= gravitational acceleration, 32.17 ft/s²
h	= height or distance, ft
Ti	= average inside temperature, °R
To	= outside air temperature, °R

This equation assumes that the resistance to the draft flow is similar to the resistance of flow through an orifice characterized by a discharge coefficient C.