Book - School of Science and Technology

Criteria for boiler selection 263output which is advantageous in terms of efficiency and avoidance of corrosion. Thesecond boiler is then brought into use during cold weather and can act as a standby for alarge part of the year against breakdown or outage for maintenance. In the past it wasroutine to select each of two boilers to have two-thirds of the total required capacity butmore modern practice, with which the authors do not wholly agree, suggests that eachshould meet no more than half of the total requirement. For larger installations still,three, four or more boilers may be used, giving greater flexibility still since several boilersthat each have a small margin may then give almost a complete one-boiler standby.Selection of the size of individual units for a multi-boiler plant, nonetheless, is a matterfor compromise and cannot be determined wholly by a statistical approach. On the onehand, limitation of the number of spare parts to be stocked suggests the use of equallysized units, e.g. three boilers at 167 kW to meet a gross total of 500 kW. Conversely, tohold maximum combustion efficiency while meeting a load which varies with the externaltemperature, a case might be made for selecting units of unequal size, e.g. for the samegross total of 500 kW, one boiler at 100 and two at 200 kW which would provide five stepsin output instead of three.The first of these two loading arrangements would require no more than a simplesequence controller, manually reset at intervals to change the order of firing (1±2±3, 2±3±1, 3±1±2) in order to avoid undue use of any one boiler. The second example, however,would need a very much more complex sensing system with automatic though limitedpotential (and this beyond the ability of an operative to monitor) for change of sequence.The end result might be that the costs associated with an elaborate sequence arrangementwould outweigh any saving resulting from better annual combustion efficiency!Packaged modulesThe modular approach to the provision of boiler power differs from the conventional inthat it represents the provision of an array of small boiler-burner units, each of which mayhave a rating in the range of 40±100 kW dependent upon the maker. The units areassembled with their water ways connected in parallel, as Figure 10.1, so that, forinstance, an array of six 60 kW units would substitute for a single large boiler rated at360 kW. The principal gain arising from such an arrangement is in flexibility to meetvariable loads at high efficiency. For the example quoted, if only one of the units were tobe fired then the turn-down ratio would be 6:1 at approximately the same efficiency as if allunits were in use.Boiler sequence controllerFlowBoilermodulesSensorFigure 10.1 Typical pipework arrangements for modular boilersReversed return