General

Air conditioning systems as we know them today were first introduced in America in the 1950's and were originally mostly confined to the larger muIti-storey buildings in the warmer regions of the country. In the early years it is probably true to say that many of the systems were added more for their prestige value rather than any functional purpose.

In more recent times however, factors such as legislation on indoor air quality and the increased usage of heat generating equipment has meant that most types of commercial or public building will now incorporate some form of mechanical ventilation or air conditioning system to provide control over the indoor environment.

Air terminal devices, commonly known as grilles and diffusers are used to supply, exhaust, or transfer air to and from rooms to control the indoor environment. Most commonly, this is to satisfy human comfort requirements as in the case of offices and other commercial buildings, but can also be to offset heat loads generated by factory manufacturing processes or plant.

In the case of the former, the primary requirement of the grille or diffuser is that it should be capable of introducing the supply air without causing draughts, stratification, excessive differences in air temperature or high noise levels.

To those who might view the air terminal as simply a convenient means of disguising the duct opening in the wall or ceiling, such factors would probably be ignored, more often than not with disastrous consequences.

However, the more cautious and experienced ‘ventilation engineer’ will be aware of the pitfalls and will take the trouble to select and size the appropriate terminal device for the application. In short, an otherwise perfectly designed air system which on paper meets the application and building specification requirements, can often be negated simply by virtue of the fact that poor terminal selection renders the area ‘uninhabitable’.

Where a system is primarily designed to offset heat loads produced by a manufacturing process in a factory, the performance requirements of the air terminal are generally not so demanding. In such cases the heat source is usually localised and intense and requires what would normally be considered an excessive air velocity to break it up.

Fortunately, the human body is not so susceptible to draughts at elevated temperatures so man and machine can easily be satisfied under such conditions.