Book - School of Science and Technology

Types of boiler 273Third passFlueTransferbox andaccessGritcollectorSecond passFurnace tubeBlow downInspectiondoorCombustionchamberAsh disposal(solid fuel)Figure 10.10 Shell boiler of super-economic typeFor applications where a substantial, large capacity boiler is required, this type willprovide a combustion efficiency of the order of 80%. A further advantage is that the typeis not only capable of burning either solid, liquid or gaseous fuel but also of site conversiononce or more during its working life to a fuel other than that for which it was initiallyequipped.PackagedAs boiler design grew in sophistication and the associated firing equipment, controls, etc.,became increasingly complex the market became established for a range of large, factorybuilt,boiler-burner units, incorporating all necessary working parts and pre-commissionedat the manufacturers' works. The packaged boiler may be no more than anassembly of production line components but with the very important difference that allthose components are, as it were, hand picked for compatibility, Figure 10.11.The firing equipment will have been chosen to give the optimum flame shape for thecombustion chamber; the forced or induced draught arrangements will be matched toprovide the most suitable gas flow pattern and the control system will be fitted, wired andproven. Site tasks after delivery, and time for erection and commissioning should bereduced in consequence.In some respects it is unfortunate that the introduction of fire-tube shell boilers inpackaged form coincided with a trend towards smaller furnace volumes, reduced diameterfire tubes and a general move to squeeze the last gram of heat transfer from each kilogramof metal. The consequent need for greater care in water treatment and greater skill inmaintenance has coincided with a period when both care and skill are in short supply. Thepackage concept has acquired some quite undeserved disrepute in consequence.Water tubeBoilers of this type, while available in sizes down to as little as 500 kW, are commonlyused only for much larger duties of up to about 10 MW. The most typical application has