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Geometric RWA: Calculation of cross-section

SHE systems in stairwells usually have to offer 1m² of geometric smoke extraction area. The question often arises as to how these are actually calculated. We are investigating this topic and shedding light on it. In this article we assume that a facade window is available and should be retrofitted with an RWA system. The window is a tilt window, the tilt fitting is not limited to 15cm as with a "normal" tilt / turn window but can be opened wider.

Geometric smoke extraction area - basic requirement

Imagine hanging out the window sash and looking at the result. Then you will see an opening, the clear dimension of which is the inside width and inside height of the window frame. Assuming the outside window dimension (before installation) is 110cm x 110cm. Then the clear opening size should be around 100cm x 100cm. This results in a geometric opening area of ​​1m².

The first requirement is that the window is large enough to even reach 1m² of opening area.

Calculation of the smoke extraction area or the stroke length

If a window is tilted, there are three areas, the sum of which must correspond to the required geometric cross-section. The three areas (left and right triangles and the 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 specific smoke extraction area, the following formula applies:


Depending on how deep the window is built into the wall (reveal depth), losses can be expected. So you should definitely plan for reserves - so keep the window open.