Evaporative cooling

Another case of 'nothing new under the sun' is evaporative cooling. One of the earliest cases of this being incorporated in a building is the Emperor Nero's megalomanic 'Golden House' which covered most of the centre of Rome. At its centre was the domed octagon room and in one of its sides a waterfall was inset, supplied by a mountain stream. No doubt it performed the dual role of architectural feature and cooling device.

Evaporative cooling works on the principle that molecules in a vapour state contain much more energy than the same molecules in a liquid state. The amount of heat required to change water into vapour is the latent heat of evaporation. This heat is removed from the water, hence 'evaporative cooling', and transferred to the vapour. So, evaporation causes surfaces to cool (Thomas, R. (ed.) (1996) Environmental Design, E & FN Spon).

Evaporative techniques include:

• air that does not already have a high moisture content can be cooled by allowing water to evaporate into it;

• as stated, direct evaporation occurs when air passes through tree foliage, fountains and across pools;

• evaporative cooling is produced if incoming air to a building passes over a dampened surface, or through a spray or damp material across windows;

• direct evaporative cooling is best in dry climates where average relative humidity at noon in summer does not exceed 40 per cent;

• in the case of indirect evaporation, the air does not come into direct contact with the moisture, but can be allowed to pass through tubes or pipes which have their outer surfaces moistened.

An example of a design which incorporates evaporative cooling is the Jubilee Campus at Nottingham University. Sloping glazing directs air which has previously passed across an extensive open air pool into an atrium between teaching and office units. Orientation ensures that the prevailing wind is in the right direction (Figure 12.15).

Figure 12.15

Directed evaporative cooling, Jubilee Campus

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