Book - School of Science and Technology

168 Indirect heating systemsHigh velocitynozzlesHighvelocityductingVenturidiffuserCentral 100%fresh air plantFigure 6.20 High velocity plenum system (Casaire)a quantity not much more than half the maximum permitted by the Health and SafetyExecutive. A similar plant arrangement may be used with the products of combustionflued to outside the building, thereby reducing the carbon dioxide and moisture levelswithin space, but with a resultant reduction in thermal efficiency.The heated air, which represents only about half an air change within the space served,is distributed from the central plant through insulated sheet-metal ducts at a velocity ofabout 35 m/s, to an array of terminal diffusers. Each diffuser is equipped with one or morenozzles which induce a supply of air from the space into circulation via a venturiarrangement. The quantity of air induced, which may be drawn from an area within thebuilding where temperature gradient has created a potential wastage, is about four timesthat of the supply from the central plant and the total is thus enough to allow control overthe final distribution. The output discharge temperature is of the order of 50 Catavelocity of about 5 m/s. Figure 6.20 provides a diagram of the system arrangement.Pipework heat emissionWhatever the heating medium may be; water, steam or air, the piping distributionarrangements have certain features in common. They will expand when heated, asdescribed later in Chapter 8, and they will give out heat throughout their length whetherthis be wanted or not. The magnitude and effect of this second matter must be evaluatedand dealt with by application of thermal insulation when necessary.Pipework heat emissionExposed pipework, as a form of heating surface, is rarely used in current practice but inany system of distribution, main and branch piping running through the spaces to beheated may offer a contribution. The theoretical emission from bare horizontal pipeworkmay be calculated from expressions given in the Guide Section C3 and extracts appropriateto a variety of water temperatures and steam pressures are listed in Tables 6.10,6.12 and 6.14. Emission from pipes fixed vertically varies from the listed values and isabout 25% less for small and 5% less for large pipes.