Book - School of Science and Technology

Chapter 10Boilers and firing equipmentIn the two subsequent chapters, the properties of available fuels or energy sources and thechemistry of combustion will be examined. As an introduction to this subject, however,we now turn to the plant items employed in practice to burn those fuels and to transfer theenergy so liberated to a heating medium. Features which are particular to boilers designedto serve domestic hot water systems are described elsewhere (Chapter 20, p. 573).Basic considerationsBoiler powerThe load imposed upon a boiler plant is derived from knowledge of not only the sum ofthe heat losses calculated for the individual spaces under basic design-temperature conditions,but also of any interaction or diversity between the components thereof. As hasbeen mentioned earlier, the total of the heat losses may well exceed the actual peakdemand due to the fact that infiltration air entering rooms on one side of the buildingmay leave via rooms on the other side. As a result, an adjustment is necessary in order toavoid making allowance twice for the same air-change. In addition, where heating iscontinuous, it is known that further diversities exist and use of the following correctionfactors has been proposed:Single space 1.0Single building or zone, controlled centrally 0.9Single building or zone, controlled per room 0.8Group of buildings with similar pattern of use 0.8The appropriate corrections having been made and the peak load thus estimated, anarbitrary allowance or a calculated estimate must be added to cover unwanted heat lossesfrom distribution pipework and fittings. It is then necessary to consider by how much theboiler rating should exceed this corrected total in order to make sure that the designtemperatures within the various heated spaces may be reached in a reasonable time. Whenheating is continuous, of course, the excess will be a minimum; the more intermittent theusage, the greater the excess capacity required.There is a further case for some margin of capacity, where a boiler plant is thermostaticallycontrolled, in order to provide what might be called acceleration ± the ability ofthe plant to surmount the load under peak conditions while remaining still under control.In addition, it is wise to remember that boiler plants are generally not well maintained.