Operation Of A Ventilator

A wind driven ventilator will exhaust air by two different processes.

1. Stack Effect: Regardless of the spinning action, the area of the neck of the vent allows warm air to rise and pass through the vanes. This action proceeds due to the density of air decreasing as temperature increases. Warm air rises and exerts a pressure, called "stack effect." The ability of a vent to hinder this rising air is measured by its discharge coefficient. The higher the discharge coefficient of a vent of equivalent throat size, the higher is the exhaust rate due to stack effect. The discharge coefficient is influenced by internal blockages in the throat of the vent plus gaps between the vanes.
   
   The stack pressure is determined by the difference in temperature between that existing in the throat of the vent and that applying to fresh air entering the space plus the height of the 'stack'. The greater the temperature differential, the higher the stack pressure. Similarly, the greater the height, the higher the pressure.
2. Siphoning Effect: As a vent spins in the wind, a low pressure region is created on the leeward side (back side of the vent). This phenomenon is known as the the Bernoulli effect (see the top picture). This area of low pressure helps create a siphoning effect where the hot internal air of high pressure is sucked out and flows to the low pressure area. The efficiency of a vent to create this siphoning effect is measured by its 'flow coefficient'. Edmonds vents have flow coefficients much higher than equivalent size spherical vane vents. The siphoning effect is influenced by vent design, weight of head and friction of bearing system.