Book - School of Science and Technology

Air systems ± characteristics 165FallRelay pointSee fig 8.20 alsoRelay pointFigure 6.19 Steam main relay pointsCurrent practiceFacilities for the removal of condensate so that the steam supply remains dry are amongthe more important features and supply pipework must be provided with relay points, asin Figure 6.19, to assist in this respect (see Figure 8.20 also, p. 222). Otherwise, pipeworkfollows a two-pipe pattern very much as might be anticipated, incorporating specialisedequipment as described in Chapter 8.Condensate will, in most normal circumstances, be collected and returned to a tank orhot well near to the boiler. Where distance or site levels make direct return impossible,intermediate collection and pumping units may be required. At the hot well, to make upfor any losses which may have occurred, a treated water supply will be provided and thismay alternatively be directly `on line' or via an intermediate store. The optimum size of ahot well will depend upon a number of factors such as the order of fluctuation in thesteam demand, the corresponding ± but not necessarily equivalent ± rate of condensatereturn and the operational routine of the water treatment plant. It is good practice,nevertheless, to provide a minimum size related to the evaporation capacity of the boilerplant over 2 hours.The temperature of the returned condensate is likely to be not much below 100 C butthe hot well should nevertheless be provided with a steam coil to maintain that level. Toavoid cavitation at the suction connections of the pumping equipment provided to returncondensate to the boilers, the hot well should be elevated where this is practicable, toabout 5 m above that level. Where packaged boilers are used, each is likely to be provided,at works, with a small cylindrical `condense receiver' in association with the prewired feedpump and associated controls. The size of such receivers is usually quite inadequate toprovide the necessary storage capacity but their volume may be deducted from thecapacity of the hot well as proposed above.Air systems ± characteristicsPassing reference was made in Chapter 4 to the use of hot air as a heat transfer mediumwithin floor ducts but such an application is a rarity and no purpose would be achieved bypursuing it further here. Table 6.9 lists the principal characteristics of dry air in contextwith its use for space heating and it is of interest to compare these with the equivalentvalues for hot water as listed in Table 6.1. The specific heat capacity of water, in terms ofmass, is four times that of air at the same temperature and in terms of volume thedifference is immensely greater.Ventilation systems are dealt with later (Chapter 13) and it is necessary to draw a quiteclear distinction between the function which they fulfil and that of the so-called plenumsystem, using air as a heating medium, which falls under this present heading. The essenceof the difference lies in the fact that an inlet ventilation system, properly designed, will dono more than introduce a supply of air into the space served at a temperature which is