Book - School of Science and Technology

366 Ventilationbe noted that, as a result of the direction of a large proportion of the air supply to theventuri slots within the ceiling troughs, the air change rate in the occupied area is reducedpro rata. This reduced level will lead to a reduction in air velocity through servery hatchesand, in the majority of cases, provide a more comfortable working environment for thekitchen staff.Ductwork connectionsDuctwork generally is dealt with in Chapter 16 (p. 436) but that for kitchens, whethersupply or exhaust, is a special case. In many instances, with a view to cleanliness, modernkitchens with canopies are provided with a false ceiling to conceal down-standing beams,ducting, pipework and electrical conduits, etc. It is often argued that the resultant voidcould, if adequate in depth, be used as a plenum for supply air, with ducting reduced toshort lengths and bell mouths, or as a suction chamber at negative pressure for exhaustair. Poor site workmanship by building contractors and the natural permeability of moststructures and materials should be taken into account and also the inevitability of a longtermcleaning problem.As far as supply air is concerned, use of such a ceiling void would be practicable only ifthere were absolutely no possibility of fouling by a noxious leak from, say, a waste pipe ora gas service or, furthermore, that there would be no possibility that a requirement foraccess, perhaps for maintenance, would ever occur.It goes without saying that canopies should always be connected directly by sheet metalducting to the suction side of the exhaust fan. Similarly, in the case of exhaust from akitchen through a ventilated ceiling, sheet metal suction chambers above the ceiling andsheet metal ducting to the fan suction therefrom is much to be preferred. In the interests ofcleanliness and as a precaution against fire, facilities for cleaning, by way of access holesand covers, should be provided at regular intervals along all exhaust ductwork. As acounsel of perfection, similar access on supply ductwork is desirable.Special applicationsIt is not possible within the scope of this present chapter to deal with the whole range ofdemands for ventilation which are imposed by specialist applications. The notes includedin the following paragraphs are therefore no more than an introduction to the widevariety of those demands.Smoke control in escape routesWith the advent of tall buildings and, in their wake, a greater necessity for mechanical airmovement in the occupied areas and the greater use of internal stair and lift cores, somefire authorities realised that the traditional approach to smoke control throughout escaperoutes and firefighting shafts required re-examination. As a result, the concept of airpressurisation to those routes has received much attention. In viewof the importance ofthe subject, it is necessary to refer the reader to BS 5588: Part 4: 1998, Code of Practice forSmoke Control using Pressure Differentials, which provides a detailed approach to theproblem, sets minimum requirements of pressure and air velocities to be achieved,calculation methods together with details of equipment specification.