Roof Ventilators
The function of a roof ventilator is to provide a storm- and weatherproof air outlet. For maximum flow by induction, the ventilator should be located on that part of the roof where it will receive the full wind without interference.
One must exercise great care when installing ventilators. If the ventilators are installed within the vacuum region created by the wind passing over the building or in a light court, or on a low building between two buildings, their performance will be seriously influenced. Their normal ejector action, if any, may be completely lost.
The base of a ventilator should be a tapering cone shape. This design provides the effect of a bell mouth nozzle, which gives considerably higher flow than that of a square entrance orifice.
Air inlet openings located at low levels in the building should be at least equal to, and preferably larger than, the combined throat areas of all roof ventilators.
The advantages of natural ventilation units are that they may be used to supplement power-driven supply fans, and under favorable conditions it may be possible to shut down the power-driven units.
Types of Roof Ventilators
Roof ventilators are manufactured in a variety of shapes and designs. Because of this variety, it is possible to classify roof ventilators under a number of broad categories, including the following:
1. Stationary head
2. Revolving
3. Turbine
4. Ridge
5. Siphonage
Some of the examples illustrated in the paragraphs that follow are no longer manufactured, but they will be encountered on older buildings. For this reason, they are included in this chapter.