Question

what do you think what is the reason behind placing the ventilators on the top

Answer 1

Hot air is lighter than cold air, so hot air rises up. Therefore to let the hot air escape for the room the ventilators are placed in the top. 

Answer 2

Hot air has less weight and rises up. Cool air has more weight than compared to hot air . so, in the room , hot air rises up and flows out through the ventilators and cool air remains in the room.