Book - School of Science and Technology

484 Fans and air treatment equipmentPosition of outside air intakeIt might be thought that the purest air supply would be obtainable at the roof of abuilding, but experience shows that in many instances this is not the case. At roof-level,air is certainly free from street dust, but chimneys or contaminated exhausts fromkitchens, toilets or fume cupboards of the same or neighbouring buildings may, withcertain states of the wind, deliver fumes into the intake. Particular care must be taken toensure that drift from cooling towers, or other possible sources of Legionnaires' disease,cannot be carried over into air intakes.An intake at low level, such as near a busy street, would be liable to draw in much roaddust and exhaust fumes from motor vehicles. If a point half way up the elevation of abuilding can be found, this is probably the best. It must, however, be clear of windowswhere fire or smoke might occur, and particularly of lavatory windows.There is no general solution to the problem of the outside air intake position, asobviously every case requires examination of orientation, possible sources of contamination,position of air-conditioning plant, and so on.The selection of the weather louvre is obviously important but quite often left to thearchitect to be provided through the cladding package. In order to avoid problems withrain or snow penetration air velocities should not exceed 2.5 m/s. The free area of thelouvre will depend on the particular arrangement but typically will be 40±50% of the totalarea.Air filtrationAir contaminantsAtmospheric air is contaminated by a variety of particles, such as soot, ash, pollens,mould spores, fibrous materials, dust, grit and disintegrated rubber from roads, metallicdust and bacteria. The heavier particles may be such that under calm conditions theywill settle out of their own volition. These are termed `temporary'. The smokes, fumes andlighter particulate matter remain in suspension and are termed `permanent'. Nonparticulatecontaminants, such as vapours and gases, also exist in the air. Sulphur dioxideis the most damaging, affecting building fabric, vegetation and artifacts; carbon monoxideand other oxides of nitrogen are also present but normally in small concentrations.The unit of measurement for dust particles is the micron (one millimetre ˆ 1000microns). The human hair has a diameter of about 100 microns, and the smallest particlevisible to the naked eye is about 15 microns. The smallest range of particles we needconsider here is of the order of 0.01 to 0:1 mm, which is represented by smokes of variouskinds, such as tobacco smoke. The upper range of particle size we need consider is about15 mm.Pollution in all its forms, and especially atmospheric pollution, continues to be a subjectof increasing public concern, though apart from smoke, fumes and soot it appearsdoubtful whether much of the other airborne dusts and dirt are susceptible of reduction.Tests are regularly carried out and records kept of suspended pollution material and ofsulphur dioxide concentrations.*Tables 17.1 and 17.2, extracted from the Guide Section B2, give typical values for solidsin the air for different localities and typical analyses of dust contamination with respect to* Warren Spring Laboratory, on behalf of the Department of Trade and Industry.