Book - School of Science and Technology

Indirect systems ± gaseous media 115available already on the site to be served or must be produced by a projected boiler plantto satisfy some process need. The upper limit of pressure at which steam is available will,in consequence of this practice, be determined by other than requirements for spaceheating. A supply at a reduced pressure may be provided but this is unlikely to be atthe atmospheric or even sub-atmospheric levels which were popular in the past. Thissubject is considered further in a later chapter.Terminal equipmentWhen steam is used this may be either as the primary medium in a calorifier (or heatexchanger) to produce hot water, or in heat emitting apparatus of one or more of thefollowing types:. exposed pipes. metal radiant panels or strips. natural or forced (fan) convectors. unit heaters. steam/air heat exchangers for ventilation systems.Indirect systems ± gaseous mediaDistribution of heatHistorically, the culture of the Romans developed a type of heating system which wasprimarily radiant, an art which was lost for some 1600 years. This system was centredabout a furnace room below ground from which hot flue gases were conveyed, via ductsunder the floor and flues in the walls, to emerge at various points about the building.Sometimes proper ducts were formed but in others the whole space under the floor (thehypocaust) was used. The wall flues took the form of hollow tiles, very similar to theirmodern counterparts. A similar but rather more sophisticated system was applied to thetwentieth-century Anglican Cathedral in Liverpool using heated air, fan circulatedthrough a pattern of structural ducts immediately below the floor finish.Air, the only gas now in use as a medium for distribution, has a very low specific heatcapacity per unit volume with the result that the distance over which heat can betransported is limited since duct sizes inevitably are large and the heat loss from them isthus disproportionately large also. This difficulty may be overcome in part by increasingthe velocity within the duct but, by doing so, the power required to drive the fan mayincrease considerably.The warm air equipment used in direct systems, whether oil, gas or electrically fired,could be considered as a source for an indirect air distribution system if any outlet ductingfitted to it were arranged to serve more than one room. In addition, for industrial or commercialapplications, if the air circulated were to be drawn, in some part at least, fromoutside the building then the basic definition of what is called a plenum system would be met.It is perhaps worth emphasis that there is a fundamental difference between a ventilationsystem and a plenum system. Although the components of each are similar, the airsupply provided by the former to the rooms served will be at near-to-room temperatureonly, the building being heated by other means, whereas that provided by the latter will beat a much higher temperature in order to heat as well as to ventilate.