Geometric RWA: Calculation cross section
SHEVS in stairwells usually have to provide 1m² of geometric smoke extraction area. The question often arises as to how this is actually calculated. We will look into this topic and shed some light on the subject. In this article, we assume that a facade window exists and is to be retrofitted with a smoke and heat ventilation system. The window is a bottom-hung window whose tilt fitting is not limited to 15cm as in a "normal" side-hung/tilt window, but can be opened further.
Geometric smoke extraction area - basic requirement
Imagine that you unhinge the window sash and look at the result. Then you see an opening whose clear dimension is the inside width and inside height of the window frame. Suppose that the window outer dimension (before installation) is 110cm x 110cm. Then the clear opening dimension should be about 100cm x 100cm. This gives a geometric opening area of 1m².
So, the first requirement is that the window is large enough to even reach 1m² opening area.
Calculation of the smoke extraction area or stroke length
When a window is tilted, three areas result, the sum of which must correspond to the required geometric cross-section. The three areas (left and right triangle, as well as upper rectangle) can be easily calculated if the stroke length is known:
If you are looking for the stroke length of the drive for a given smoke outlet area, the following formula applies:
Corrections
Depending on how deep the window is installed in the wall (reveal depth), losses are to be expected. You should therefore definitely plan for reserves - i.e. open the window further.
The calculation of the geometric smoke extraction area is explained in detail in one of our YouTube videos starting at the 25th minute - take a look: