Wall and Roof Ventilators

Ventilators can be combined with a window or a door frame. 

Wall Ventilators 

These are provided on the walls, and the top of the ventilator should be close to the ceiling so that the vitiated air is removed through the ventilator. This consists of a frame and a shutter which is horizontally pivoted to the frame. The shutter can be opened or closed by means of two cords, one attached to its top rail and the other to the bottom rail. 

Roof Ventilators 

These are normally provided in industrial buildings making use of the stack effect. In this, ventilation rate is affected by the convection effects arising from temperature or vapour pressure difference (between inside and outside of the room), and the difference of height between the outlet and inlet openings. If the inside air temperature is higher than that of outside, the warmer air tries to rise and pass through the opening in the upper part of the building. At the same time, the incoming cooler air from outside through the opening at lower elevation replaces it. The rate of air flow, in addition to the temperature or pressure difference and height difference, also depends upon the ratio between the areas of two openings. 

There are also other types  of roof ventilators. Some are stationary and others change their position with the wind. The stationary type is constructed with a covering for protection against rain, snow, etc., and provides an easy escape of air despite wind direction. Other types have a vane which keeps the wind at the back of the openings and provides an increased flow of air due to an aspirating effect. and are commonly used in barns or warehouses. 

Where ventilation or movement of air is brought about by mechanical means, such as exhaust fans or blowers the results are positive and can be depended upon regardless of outdoor wind and temperature conditions. In addition the desired effects can be had without special architectural planning of windows, doors and other openings. To determine the size and number of ventilators required to handle a specific job in an industrial building, one must keep in mind a few simple rules. There are four primary factors affecting the capacity of a roof ventilator which may be summed up as follows : 

• The difference between the indoor and outdoor temperature is perhaps the greatest factor affecting the air flow. It is a common knowledge that hot air is lighter than cold air and will rise to seek its level. The greater the temperature difference, the faster the rise. Consequently, an initial velocity is given to the air, which aids the ventilator in its performance. 
• The velocity of wind (air in motion against any surface) will produce an area of lower pressure on the leeward of the surface and cause a partial vacuum in the ventilator head, accelerating the discharge of the air in the ventilator stack-a fact utilized advantageously in the design of all efficient ventilators. 
• Height of ventilator above air inlets, or increased elevation, results in greater draft, so it follows that the higher the ventilator head above the air inlets to the room, the greater the exhaust. 
• Exhaust capacity, theoretically, is directly proportional to the cross sectional area of the stacks, assuming of course, that the ventilator heads are equally efficient. In as much as circular areas vary directly as the square of their diameters, it follows that doubling the size of a ventilator will increase its area and exhaust capacity four times.

The above mentioned points must be kept in mind when considering ventilator requirements. One has to be certain that provision is ma& for supply of air to be drawn into rooms to take the place of air which is exhausted by the ventilators. Intake may be in the form of windows, doors, or other openings. If the room is closed most of the time, it will be necessary to install grills or louvers, the free area of which should be twice the combined areas of the ventilator throats. 

Properly designed ventilators do not down draft unless a tremendous amount of air is being removed from the same room by mechanical means. Be sure there are no large  fans or blowers, exhausting great amounts of air from a point near the ventilator. One cannot expect natural ventilation to compete with powerful mechanical exhaust apparatus unless the latter is satisfied by ample provision of air. A large boiler will create a starved air-condition in a room which is not supplied with proper air intakes. 

In instances where ventilators are required to remove large quantities of hot gases or steam, it is best to place a hood over the immediate source. This hood should be as low as possible and should project at least 600 mm around the vat or pol. If a hoist or conveyor makes it necessary for the hood to be placed a considerable distance above the source, this projection should be greater depending upon the height. The hot gases rising into the hood must be drawn away quickly; therefore, a large ventilator area is necessary to cope with the requirements.