Book - School of Science and Technology

Methods of ventilation 349indiscriminate use of coal and heavy oil, are no longer the principal pollutant. Their placehas largely been taken by the effluents produced by motor vehicles, in contrast to thosecomplained of by Florence Nightingale in 1860, when she referred toDirty air coming in from without, soiled by sewer emanations, the evaporation fromdirty streets, bits of unburnt fuel, bits of strawand bits of horse dung.When air is introduced into a building by ventilation, it brings with it a proportion ofwhatever pollutants exist outside. If inlet is to be by natural means, i.e. through openwindows and the like, then there is very little to be done to prevent this incursion. Wherethe supply of air is to be by mechanical means then a whole variety of types of filters isavailable. Some of these are discussed later in Chapter 17.The use of activated carbon filters will revive vitiated air by adsorption of manyimpurities such as body odours, sulphur dioxide, petrol fumes, and other noxious productsof civilisation. Filters of this type are, however, high in first cost and have a relatively shortservice life before return to the manufacturers for re-activation becomes necessary. Nevertheless,use of this equipment is growing for certain specialised installations and, with thegrowth, costs may fall and usage spread to more mundane applications.Methods of ventilationWhere air movement is induced either by wind or by the effect of temperature difference,ventilation is termed natural. On the other hand, where air movement results from powerdrive applied to a fan or fans, the arrangement is described as being mechanical. Sinceinlet and extract have to be considered separately, there are four possible combinations ascovered under the sub-headings which follow.Natural inlet and natural extractArrangements of this sort are used in buildings which seem to fall into three categorieswhich, for want of better descriptions, might be called traditional, commercial andindustrial.TraditionalThis category includes most school and university buildings, hospitals, shops, officebuildings constructed between the wars, almost all domestic premises and other roomsand buildings with low levels of occupancy. Conditions internally depend upon cleanoutside air and upon other external features which permit windows to be opened.Where an open fireplace exists, the flue provides a route for exhausting air from theroom. Further, in instances where a fireplace is in use for burning solid fuel, the flue servesthe dual role of not only carrying away products of combustion but also of inducing aflowof room air much greater in quantity than is normally required, as illustrated inFigure 13.1: an adjustable restriction at the throat of the flue is advised in order toameliorate this problem. It must be added that a gas fire connected to a flue will havethe same effect but at a much reduced level. Various methods of providing naturalventilation to rooms have been devised as shown in Figure 13.2:(a) The Dr Arnott ventilator of the 1850s which, built into a chimney breast at high levelin the room, acted to remove `used' air as induced by the flowof products ofcombustion in the flue.