What is stack effect

The phenomenon induced by the density difference between a hot and cold air column that creates a natural flow through a chimney is called stack effect or chimney effect.

The function of the stack is to disperse the hot gases, emissions and particles that leave the furnace. The gas temperature inside the stack is greater than the outside ambient air temperature. If these two air columns are connected at the bottom, dense air will push the light gases up the chimney. Higher is the temperature difference, greater will be the buoyancy force causing the lighter gases to move up.

Flow of gases in furnaces, stacks and other equipments operating at atmospheric pressures involve small difference in pressures  that should be used in mechanical energy balance equation (lecture 21, equation 7). This small pressure difference can be conveniently handled in terms of “draft”.