Book - School of Science and Technology

Methods of ventilation 357For summer use only in a small building, infiltration via the type of window ventilator(`trickle' or `night') referred to on p. 351 may suffice. In some circumstances, replacementair may be drawn from another part of the building to serve a dual purpose; a goodexample being extract ventilation from a small kitchen which, by drawing air throughserving hatches, serves to dilute the spread of cooking smells to the associated canteen.For industrial applications, the natural roof ventilator shown earlier might be replacedby some form of fan-powered extract unit as Figure 13.8. Units of this type are made in avariety of patterns with mounting arrangements and weathering covers to suit most typesof roof construction, pitched and flat. The air extracted by such units is discharged at arelatively lowvelocity and has a tendency to hug the roof profile, thus, where fumes andother pollutants are carried in that air, it is better practice to use a vertical jet-dischargeunit as Figure 13.9. Axial-flowfans, which are commonly more noisy, are often fitted tosuch units, together with hinged vanes or dampers, for rain protection when the fan is idle.These units may be connected to either horizontal or vertical ductwork.With the evolution of well detailed weathering methods for roof extract units, theprevious practice of mounting fans to factory gable walls has lost favour. It is necessarythat fans so arranged be fitted with baffles or discharge bends in order to avoid the effectof wind blowing in opposition to the discharge. Self-closing dampers are required, inaddition, to prevent unwanted inlet or discharge when the fan is not in operation. Fordomestic use and indeed for application to small rooms generally, the familiar window orwall mounted extract units manufactured in moulded plastic, Figure 13.10, may wellprovide a solution to a local ventilation problem which is adequate for most of the time.Such units may be provided with speed control and, in some instances, are reversible sothat they may, alternately, be used for either an inlet or an extract duty.A specific area where local authority requirements bear upon the provision of mechanicalextract with natural inlet relates to internal toilets and bathrooms, particularlyin multi-storey housing. An investigation which was reported 30 years ago remainsvalid,* the principal conclusion being that a ventilation volume of 5.5 litre/s should beprovided for each fixture, i.e. per WC pan or bath, and thus 11 litre/s for a bathroom withMounted in windowMounted in wallFigure 13.10 Domestic window or wall mounted fans* Wise, A. F. E. and Curtis, M., `Ventilation of internal bathrooms and water closets in multi-storey flats,'JIHVE, 1964, 32, 180.