Internal air flow and ventilation

Air flow in the interior of buildings may be created by allowing natural ventilation or by the use of artificial mechanical ventilation or air conditioning. The production of buildings using more than one of these options is becoming more frequent. Such buildings are said to be 'mixed-mode'. The overriding principle should be to minimise the need for artificial climate systems and one way to achieve this is to make maximum use of natural ventilation in conjunction with climate sensitive design techniques for the building fabric.

Natural ventilation is possible due to the fact that warm air is lighter than cold air and therefore will tend to rise in relation to cold air. As it rises, colder air is drawn in to compensate: the buoyancy principle. If air flow is to be encouraged to help provide natural ventilation and cooling the following are desirable design features:

• Plan form should be shallow to allow for the possibility of cross-ventilation.

• The most straightforward system of cross flow ventilation is where fresh air is provided with routes through a building from the windward to leeward side. In most office situations this can be considered as a supplement to the main ventilation strategy. Openings on opposite walls to allow cross-ventilation are better than on one or more adjacent walls.

• Building depth should not be more than about five times the floor to ceiling height if cross-ventilation is to be successful.

• For single sided ventilation, depth should be limited to about two and a half times the floor to ceiling height.

• Minimum opening areas should be about 5 per cent of floor area to provide sufficient flow.

• Continuous, secure background ventilation should be available using trickle vents and other devices.

• Windows should be openable, but able to provide controlled air flow. This is particularly difficult in high rise buildings but its problems have been addressed in the 40 storey Swiss Re building in the City of London (see pp. 157-159).

• Atria and vertical towers can be incorporated into the design to allow the stack effect to draw air through the building, though care in meeting fire and smoke movement restrictions may determine the limits of what is possible.

• The effectiveness of natural ventilation and cooling can be improved by the use of low energy controlled lighting and low energy office equipment, thus reducing internal heat gain.

The ventilation system most obviously borrowed from the past is the use of the thermal chimney exploiting the buoyancy principle. A thermal chimney which is warmed by the sun accelerates the process, causing cooler air to be drawn into the building at ground level. If the chimney has a matt black finish it will absorb heat and increase the rate of buoyancy. Portcullis House, admirably demonstrates this technology (Figures 12.1 and 12.11). In fact this building is one of the most overt demonstrations of the dynamics of natural ventilation, with external rising ducts carrying the warmed air from the offices to a thermal wheel on the roof before being expelled. Fresh air, in this case, is drawn in at high level assisted by the thermal wheel (Figures 12.11, 12.12 and 12.13).

Portcullis House

Figure 12.1

Portcullis House, Westminster, London

Figure 12.1

Portcullis House, Westminster, London

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