Book - School of Science and Technology

164 Indirect heating systemssuccinct guidance in relation to the design of distribution pipework, these few wordsgather together all the salient criteria.Traditional arrangementsDisposal of condensate and elimination of air were the twin problems which dominatedthe design of steam pipework in early years, where system pressures adopted were in therange of 120 to 170 kPa absolute. Single-pipe arrangements were not uncommon, Figure6.18(a), and these were featured in early North American textbooks. Horizontal pipework,where this occurred, was laid to a generous `pitch' to encourage condensate returnand vertical risers were oversized to allow steam and condensate to flow in oppositedirections in the same pipe. Air vents were fitted at the end of each horizontal pipe runand at mid-height on each radiator. Water hammer was endemic.Two-pipe gravity systems were classified as having `wet' or `dry' returns, dependentupon whether the condensate pipework was above or below the water level in the boiler,as in Figure 6.18(b). As in the case of the single-pipe system, radiators were either on, attemperatures between 105 C and 115 C, or off and cold; there was no intermediate level.Water make-up to both single and two-pipe systems was made from a cistern, fitted at aheight sufficient to overcome the system pressure, to a `boiler feeder' which consisted of aball valve within a closed casing connected top and bottom to the steam and watercontents of the boiler.The subsequent vacuum systems where, as the name suggests, use was made of a vacuumpump to assist in removal of air and to return condensate to the boiler, were a substantialadvance and permitted system size to be increased considerably. The later introduction ofsub-atmospheric systems was perhaps the ultimate development since these permittedsteam pressures to be both lowered and controlled such that temperatures could be variedbetween about 55 C and 105 C at source, according to weather conditions. Such precisionwas not unaccompanied by disadvantages, one of the more significant being thenecessity to use specialised components such as glandless valves in an attempt to produce apiping system which remained absolutely `airtight' both in service and after shut-down.FeedRadiatorsAir ventsFeedRadiatorsAir vents Air ventAir ventSteamSteamDry returnDrainWater lineBoilerBoilerWater lineWet returnCheck valve(a)(b)Figure 6.18 Low pressure steam systems:(a) single-pipe gravity return; (b) two-pipe gravity return