Mechanical ventilation is used for applications where natural ventilation is not appropriate. The most basic of mechanical ventilation systems is the extractor fan often found in domestic bathrooms and kitchens where the aim is to remove smoke/odours from the room. These fans are directly connected to a duct that exhausts through an external wall or a roof stack and are either controlled directly via a switch or wired into the lighting circuit so that they operate when the light is switched on.
Commercial and industrial systems are inevitably more complex but operate on the same principle. Commercial kitchens usually have a series of ducts and hoods that are all connected to an extract fan. The air entering the system is usually filtered to keep grease out of the fan mechanism in order to keep it working and avoid blockages.
Some buildings do not have a mechanical air extraction system but are supplied with mechanically drawn air. This is often used for large internal spaces such as offices and workshops and often combined with heating, cooling and humidification systems, so that air entering the space is the right temperature and humidity. The location of air inlets is crucial – they should not be located close to air outlets or sources of smoke or other pollution otherwise polluted air will be drawn into the system.
Air is typically distributed around the building in metal ducting, traditionally of a rectangular cross section but increasingly nowadays of a circular cross section. This design improves air flow. Ductwork should be insulated to reduce heat loss but often historically has not been. The longer and bendier the route of the ductwork is, the less effective it will be. Long runs are typically discouraged. It is important to get the pressure in the system right as if it is too high, it would cause excessive draughts and noise.
Outlets from ventilation ducts can take many forms. Ducts are often located behind suspended ceilings with downward facing outlets fitted with diffusers. Diffusers can be circular or square and direct the air flow so it covers a wide area. Older buildings sometimes have outlets in walls or under windows while occasionally vents can be found in suspended floors, although this is usually used for specialist applications such as computer server rooms. The location of outlets should be carefully planned to ensure maximum benefit from the ventilation system.
In the larger ventilation systems, air is normally drawn into the building via an Air Handling Unit (AHU). This is a device that can contain fans, filters, heating and cooling coils and humidification equipment. Please see the chapter on Air Conditioning for more information on how these work.
It is important to filter air before it enters large central systems. This removes particulates and dust from the air. Commercial cooker hoods typically use carbon-activated filters as these are excellent for trapping grease. General ventilation systems typically use disposable fabric filters that consist of pleated fabric on a wire frame with a cardboard outer frame. These need to be replaced every six months or so (depending on the application). Higher specification filtering systems for specific industrial applications sometimes use oil sprays or electrostatic plates.