Book - School of Science and Technology

286 Boilers and firing equipmentof open honeycomb construction should be arranged to provide additional coolingthrough circulation of air. To encourage this, it may be possible to arrange that a forceddraught fan draws its air supply via passages formed within the honeycomb, so serving adual purpose by preheating this supply by conduction. It is desirable that a concrete slabbelow a boiler should not reach a temperature of over about 70 C.Regulations, etc.Since oil storage and oil firing both impose some elements of fire hazard, various bodieshave drawn up regulations relating to these installations. A British Standard Code ofPractice (BS 5410: Parts 1 and 2) outlines many of the hazards and proposes ways inwhich they may be reduced. In addition, insurance companies and the fire servicedepartments of some of the larger local authorities have their own regulations and theseshould be ascertained when any new installation is under consideration. Local authoritieshave certain responsibilities under the Clean Air Act which may affect the grade of oilused and the dimensions of the chimney, as referred to in Chapter 12.Burner controlsThe few notes which follow are not intended to be a full and complete dissertationcovering the whole subject of automatic control to oil burners, particularly since manyof the primary devices may be applied with equal validity to the control of automaticfiring equipment for other fuels. Primary functional control is certainly common to all,but unique requirements as to safety and function, particular to individual fuels, aresubjects which cannot be dealt with satisfactorily here.Fully automatic types of oil burner have been referred to and it will be obvious thatthese, by definition, incorporate means for self-igniting and self-extinguishing. Primarycontrol of these functions is normally by either two separate devices, be they thermostatsor pressure sensors, or one single dual-service device. One such item will be the basiccontroller and the other, normally a hand-reset pattern, will be a high-limit safety override.Whether the firing rate is capacity controlled between the extremes of on and offdepends upon the method of operation and, very largely, the size of the fired boiler: smallunits normally operate in the `on/off ' mode whereas larger units incorporate some meansof varying or modulating output. Apart from the nuisance which would arise resultantupon the cycling of a large burner to meet a small load, `on/off ' operation in thosecircumstances would create thermal shock to the boiler.Obviously, both fuel and combustion air must be controlled in parallel, which tends toinvolve complexity if adjustment is envisaged right across the spectrum of capacity. Thisdifficulty is commonly overcome in one of two ways, the simpler being to arrange for twopreset levels of operation, low and high, i.e. perhaps a sequence of 40 and 100% capacity.More complex, but still simpler than the ideal, is an `on/off ' operation up to one thirdload and full modulation to match the load thereafter.With a self-contained burner in which one motor drives both fan and fuel pump, thereis little likelihood of fuel being delivered into the boiler without a parallel supply of air. Intypes where separate drives are necessary, suitable interlocks are available. Residualproblems arise however when a flame is not established due to failure of ignition or,having been established, is later extinguished for some reason such as an intermittentstoppage in the fuel supply.