Well, that my friend is the natural convection fan also known as roof ventilator.
Natural convection is the phenomenon of heat transfer by which a movement of fluid (convection) is caused entirely by fluid itself without forcing it to do so by external means. So let us try to understand what happens in a factory without proper roof ventilation.
Industrial activity generates a lot of heat and as the hot air being less dense will rise up due to buoyancy. It loses its heat to the roof and then falls down. This cycle continues and the process is called as Rayleigh Benard convection. You can observe this in the before part of the image that follows. Even though the hot air loses its heat to the roof, there will be no effective ventilation provided as the heat loss is partial which causes the temperature of the room to increase. Another characteristic feature of Natural convection is that the velocity of flow is very less.
The hot air which is near the roof will find itself a way to escape to the atmosphere by rotating the rotor which is located in the ventilator. The breeze outside also has an effect in rotation of the rotor. This causes a reduction in pressure in that region and causes more air to escape through the ventilator leading to an increase in ventilation of the building.
Credit : Quora
Picture Credit : Google
