Free area characteristics

Free Area is the area within an aperture unobstructed by the components of a damper or air transfer grille product when in the normal or open position through which air is free to flow.

Its size can be given in square metres, square centimetres, square millimetres, square feet or square inches.

The sketch in Fig. 1 shows a typical air transfer grille where the aperture size is indicated by the heavy line i.e. W x H. The shaded area represents the components of the grille and the non-shaded area within the aperture is the free area. As can be seen the free area equals the aperture area minus the shaded area and is often expressed as a percentage of the area of the aperture.