Definitions: Equivalent area, Extract ventilation, Free area

Equivalent area – a measure of the aerodynamic performance of a ventilator. It is the area of a sharp-edged orifice which air would pass at the same volume flow rate, under an identical applied pressure difference, as the opening under consideration. Extract ventilation – the removal of air directly from a space or spaces to outside. Extract ventilation may be by natural means (e.g. by passive stack ventilation) or by mechanical means (e.g. by an extract fan or central system). Free area – the geometric open area of a ventilator.