Book - School of Science and Technology

148 Indirect heating systemsin continental practice but these relate to circumstances which have little relevance togood practice in the British Isles.The types of heat emitting equipment (see Chapter 7) which are most suited to lowtemperature hot water are floor-panel coils embedded within sheaths (Panelite); heatedacoustic ceilings; metal radiant panels; radiators of all types and natural or fan convectors.Medium temperature hot waterIt could be argued that systems of this type originated from those which enjoyed a voguein about 1920, when fitted with a device known as a heat generator. This was an item ofstatic equipment, containing a mercury column and reservoir, which was interposedbetween the feed and expansion cistern and the boiler. The effect was to multiply thepressure available by density difference and thus permit the working temperature to beelevated accordingly.As now understood, however, systems of this type were so identified in the early 1950sand, as such, were advocated as being a compromise between low and high temperaturepractice. As will be noted later, high temperature systems at that time used steam boilertechniques to apply pressure, split-casing circulating pumps with water cooled glands andeither high quality welding or flanged joints for pipework. These, needless to say, arecorrespondingly costly.It was suggested that medium temperature systems could be constructed using whatwere, effectively, the best of low temperature equipment and methods. The use of screwedjoints was proposed and adopted for valves and other fittings but, for most pipeworkjoints, welding of an adequate commercial standard was becoming the more economicapproach. A further concurrent development was the introduction of the factory madegas pressurisation sets which will be referred to later.Taking mid-point temperatures from the ranges in Table 6.2, the differential, water toair, in a medium temperature system would be 105 20 ˆ 85 K whereas that in a hightemperature system would be 132 20 ˆ 112 K. To achieve the same output, therefore,the medium temperature system would need to have over 40% more radiant heatingsurface. Nevertheless, there are many compensating advantages in terms of capital andrunning costs for smaller installations and, for these reasons, it has come to be acceptedthat medium temperature systems are best applied where the overall load is not greaterthan about 2.5±3.0 MW.Most designers prefer to equip medium temperature systems with one or other of thepackaged pressurising sets which are described in more detail later. However, it is possiblein some instances to provide adequate pressure from a cistern elevated above the highestpoint of the system. A central boiler plant providing primary water to heat exchangers atbasement or ground level in each of a group of buildings, might be so arranged. As in thecase of pressure so applied to thermal storage vessels, Figure 5.11, it is necessary to takesteps to ensure that heated water from the system does not circulate within the expansionpipe and rise to the cistern.As an example of pressure so applied, consider a cistern mounted 17 m above anypipework carrying water at system flow temperature. A gauge pressure would be producedat that point of 17 9:81 ˆ 167 kPa corresponding to 167 ‡ 101 ˆ 268 kPa absolute.From Table 6.1 it will be seen that an absolute vapour pressure of 270 kPa relates toa temperature of 130 C and thus, allowing for a 10 K safety margin, the system could bedesigned to operate with a flow temperature of 120 C.