Book - School of Science and Technology

150 Indirect heating systemsHGIDEACFFBKJFigure 6.3 Steam pressurisation (shell boiler):A, feed cistern; B, feed pump; C, feed check valve; D, waterline (upper and lower); E, steam space; F, dip pipes; G, cooling water by-pass; H, return; I, flow; J, blowdown;K, system circulating pumproute pipework largely irrespective of site levels, were among the considerations which ledto its increasing popularity. In terms of operation and maintenance, virtually all equipmentneeding attention is, for the water system, concentrated in plant rooms. It has nowcome to be accepted that, for space heating, steam is not the preferred medium except inthe particular circumstances which will be noted later.In order to achieve the elevated temperatures associated with these systems, the practiceadopted initially was as shown in Figure 6.3, and this arrangement still has its advocates.Steam is generated in a more-or-less conventional steam boiler and the water to becirculated is drawn from below the steam/water separation line, to be returned there alsoafter passing through the system. The connections are made in the form of dip-pipes fromthe top of the boiler in order to avoid draining the boiler, with consequent danger, shoulda serious leak occur in the system.The temperature of both the steam and water are the same, at saturation level, and werepressure to fall in the water pipework without any parallel loss of temperature, the waterwould flash into steam and create an unstable condition. In order to prevent this happening, asmall proportion of the water returning from the system is injected into the flow outlet as itleaves the boiler, so reducing the temperature there to below the point of ebullition. AlthoughFigure 6.3 is no more than a diagram, it does illustrate a desirable arrangement whereby flowpipework is routed to below the steam/water interface in order to increase static pressurebefore connection to the circulating pump. Where the size of the installation requires thatmore than one boiler be installed, their respective water levels are kept uniform by use ofbalance pipes, one such joining the water spaces and another the steam spaces.For much larger installations, much use has been made of high velocity water tubeboilers with connections to a steam drum:where a number of boilers of this type arerequired, a common drum or drums are used, as Figure 6.4. The water of expansion whichhas to be dealt with as a result of diurnal temperature fluctuations may well be containedby variations in level within the drum but when the contents of a large system are heatedup from cold, discharge either to a cistern or to drain is necessary.Make-up water for installations of this type, to replace the inevitable losses at valve andpump glands, is provided via a conventional boiler feed pump which draws its supply