Book - School of Science and Technology

Water systems ± piping arrangements 1512Drum4Drum258Boilers19Boilers117 63103Figure 6.4 Steam pressurisation (water tube boilers):1, system circulating pumps; 2, flows, boilers todrums; 3, returns, boilers to drums; 4, steam balance pipe; 5, water balance pipe; 6, mixing valve; 7,manual by-pass; 8, pump suction header; 9, flow header; 10, return header; 11, external circuitsfrom a cistern containing treated water. The boiler water level is monitored and themakeup pump is usually controlled by hand.The principal disadvantage of steam pressurisation is that the system is inherentlyunstable since the steam pressure applied is so closely associated with the temperature ofthe water circulated. Any minor variation in the applied load will be relatively critical toperformance:experienced operatives and skilled maintenance personnel are necessary ifsatisfactory service is to be assured. As a result, most current high temperature installationsuse other methods to apply pressure, the equipment necessary being described in Chapter 8.As in the case of medium temperature systems, it is the surface temperature of the heatemitting equipment which restricts selection of type. However, the higher temperaturesavailable permit the use of most forms of radiant surface and these will show to advantagewhere they can be fitted at levels beyond reach. For a building or group of buildings whereheating from an available high temperature source is unsuitable, one or other of thearrangements shown in Figure 6.5 may be used. The second of these, in (b), it should benoted, operates at the same pressure as the high temperature system.Water systems ± piping arrangementsAlthough the materials used and the techniques of construction span a wide range fromthe smallest domestic premises to the largest industrial building, piping arrangements forsystems using water as a heat distributing medium differ only in detail. A line diagram of