Book - School of Science and Technology

Application 17Table 1.9 Proposed ventilation rates (for sedentary occupants)Level of smokingPercentage ofsmokers (%)Outdoor airsupply rate (litre/s/person) aNo smoking 0 8Some smoking 25 16Heavy smoking 45 24Very heavy smoking 75 36NotesFor accommodation in hospitals refer to the DHSS Building Notes.a Assumes the outdoor air is fully mixed with the room air.and energy use it is desirable to allow the internal temperature to vary from around 20 to22 C in winter to 22 to 24 C in summer.Outside air supply rates in spaces where smoking is not permitted would normally be 8to 12 litre/s per occupant, except where other factors apply. For example, in factories therate of ventilation may be determined by the process, and in bathrooms, toilets andchanging rooms rates would be determined by odour or humidity control. For dwellingspaces the Building Regulations should be consulted.The use of resultant temperature as a design criterion takes account of radiation in thegeneral sense but not of particular local effects which may arise from asymmetricalexposure of the body to:. cold radiation in winter to a single-glazed window. excessive insolation (short-wave radiation) from unshaded glazing. exposure to some internal source of high radiant intensity.A simplification of the data given in the Guide Section A1 suggests that, in the first ofthese situations for a typical window height of 2 m, an occupant seated within a distancerepresented by the square root of the window width (in m) will experience discomfortwhen the outside temperature is at about freezing level. In the case of the discomfortarising from the second situation, there are so many variables that a similar simplificationis not possible; in an extreme case, however, a rise in radiant temperature (t ri ) of 10±15 Kmight be anticipated and the provision of some form of shading to the glazing, preferablyexternal, is the correct solution.As to humidity, too high a level will reduce the ability of the body to lose heat byevaporation, with resultant lassitude; too low a level will produce a sensation of coolnesson exposed flesh, a parched throat and dry eyes. Where temperatures are high, theseextremes are of great importance but in the maritime climate of the British Isles, variationsbetween 40 and 60% saturation are usually acceptable, although 50% saturation isthe usual design target for human comfort. Industrial processes, museums and galleriesand associated storage often require a better standard of humidity control to within aslittle as 5% and even smaller tolerances for some applications.There are two quite extraneous aspects which relate to humidity in any building, thefirst being the likely incidence of condensation on single glazed windows when theirsurface temperature falls, in very cold weather, to below the dewpoint temperature ofthe building air content. The second is the problem of a build up in static electricity,leading to electrostatic shock when occupants touch earthed building components. This isa function of humidities below about 40% coupled with the material and backing of thefloor coverings.