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AIR DISTRIBUTION

Air is supplied into rooms via air diffusers.

These are used to direct the air in one, two, three or four directions if square or rectangular units are installed.

Return or Extract air is removed via a grille. These are not required to direct the air and are simpler in construction.

For continuity and aesthetic reasons diffusers may be used for both supply and extract.

It is difficult to supply or extract air at low level in rooms because draughts may result from low-level supplies and vertical ductwork in rooms is not generally feasible.

It is therefore usual to supply or extract air from the ceiling as shown below, but this means that air velocities at diffusers must be sufficient to ensure adequate mixing within the space otherwise short-circuiting of grilles may occur.

Good mixing

Circular diffusers may be used in some areas especially if they blend into the ceiling layout or room shape.

Another option is to use linear slot diffusers which can give a continuous long system of air distribution. This may be an advantage in some types of room.

In rooms with a very high ceiling it may be necessary to use a nozzle which acts like a jet to force the air down to the occupied space. One difficulty associated with high velocity jets is noise production. This is one of the aspects of air distribution which must be addressed by the engineer.

VENTILATION SYSTEM LAYOUT

When designing ventilation systems it must be remembered that since most of the ductwork is installed within ceiling spaces, it is a good idea to liaise closely with the Architect at the early stages of design so that space requirements are met.

Figure 1 below shows a typical balanced ventilation system layout. One method, which can be adopted, is to run main supply and return ductwork in the ceiling space above corridors and the branches into adjoining rooms.

Ceiling heights in corridors may be lowered to accommodate larger ductwork.

It is common practice to use flexible ductwork to grilles and diffusers. These have several advantages; sharp bends are eliminated, flexible ductwork has better sound adsorption qualities compared to sheet metal, it is easier to install especially in a congested area and it allows more freedom in positioning the grille or diffuser.

A plenum box can be used to connect the ductwork system to the grille or diffuser. This has a larger cross sectional area than the connecting duct and reduces the air velocity before it enters the diffuser thus giving better air distribution over total diffuser area. A less expensive method is to use diffusers with factory-fitted square or round necks, which can be fitted directly to the flexible duct connection.

Each grille or diffuser should have a damper to regulate flow of air. This damper can be an opposed blade type incorporated within the diffuser or a butterfly volume control damper (VCD) positioned in the branch duct. All dampers require access.

Figure 2 below shows a typical ventilation system using linear or slot diffusers. These have the advantage that air can be distributed over a wider area and the ‘coanda’ effect can be utilised. This is where cool air ‘sticks’ to the ceiling before falling gradually into the space below.