Natural ventilation is equivalent to a fan with a pressure equal to :
H = ρ * ∆z
H(AF) = H(AB) + H(BC) + H(CD) + H(DE) + H(EF)
To explain how natural ventilation works, we can use the example of a mine according to its exposure to the sun.
In case N°1, the sun is on the air inlet side. The air inlet is heated by the sun. Since the return air is cooler than the air inlet, the air flows from the return air to the air inlet.
In case N°2, when the sun is on the return air side, the opposite effect occurs and the air will circulate from the air inlet to the return air side.
When the air inlet and return air are at the same temperature there is equilibrium between the two air columns and there is no more air circulating in the galleries.
The air circulation in a mine not equipped with a fan is governed by natural ventilation and depends on the weather conditions outside.
Under these conditions, natural ventilation is not a stable ventilation because the ventilation regime changes over the course of a day according to the exposure of the sun and during the different seasons (depending on weather conditions).