Book - School of Science and Technology

272 Boilers and firing equipmentFlowFlueReturnFigure 10.9 Steel reverse flow boiler (Allen Ygnis)double skin and may be water cooled. A range of ratings from 100 kW to 3.5 MW isavailable for operation at absolute pressures up to 1 MPa (10 bar).Steel shell and fire tube typeIt is convenient to separate boilers falling into this category from those others which aremanufactured from the same material, namely mild steel, and to make brief mention ofearlier patterns from which the present range has evolved. Single-flue Cornish and twinflueLancashire boilers, both types brick-set, are now rarely seen. Nevertheless, in theirday, they had the merits of sturdiness, extreme simplicity and vast thermal storagecapacity. All these were useful attributes, when hand fired using coal of indifferent qualityto meet fluctuating loads, but none of them seem relevant today when viewed in conjunctionwith a combustion efficiency, on test, of only about 60%.For both water and steam service, with ratings between 1 MW and 10 MW, shell andfire tube boilers of both economic and super-economic type are available, one being asillustrated in Figure 10.10. Without any brick setting, either type may have one or twofurnace tubes within the pressure shell, from which the flue gases pass to a combustionchamber at the rear of the boiler. In the two-pass economic design, the gases returnthrough a secondary array of fire tubes to a smoke hood at the boiler front from whichthey are discharged. In the three-pass super-economic design, a transfer box takes theplace of the front smoke hood and a further array of fire tubes conveys the gases backagain to the rear of the boiler for collection prior to dispersal.The manner in which the combustion chamber is constructed, for either type, as arefractory lined external box at the back of the boiler or as a water-immersed pressurevessel within the main shell, types the design as being either dry back or wet back.Apart from the different patterns of mountings fitted, as described later, the principaldifference between the steam and hot water variants of this type is that for steam storagewhereas the latter are commonly drowned, i.e. are completely water filled. Mention hasbeen made in Chapter 6 of a method adopted in some high temperature hot water systemswhereby pressurisation by steam was employed: with this technique, the steam storagespace was retained even though the boiler served a hot water system.