Book - School of Science and Technology

Boiler firing ± gaseous fuel 287A variety of flame failure devices has been used in the past but it is now almostuniversal practice to incorporate a photo-electric cell either within the burner housingor at some other site where the flame may be viewed. Arrangements are incorporated inthe control sequence such that this cell is out of circuit for a predetermined few seconds onstart-up but that, thereafter, the burner will cut out if no flame is sensed. In someinstances the control circuits provide a facility which will allow a second attempt atignition to be made after a suitable time delay but will ensure that if that second attemptfails, the burner is deactivated until reset by hand.With larger and more sophisticated burners, a commonplace addition to the controlsystem is a purging sequence which arranges for the air supply fan to run for a periodbefore fuel is made available to the burner head and also to run for a similar period aftersupply has been shut off. This sequence is, of course, interlocked with the flame failurecircuit and will be initiated whether or not a flame is established.Boiler firing ± gaseous fuelA gaseous fuel discharged at a low velocity from a simple nozzle will burn with a soft lazyflame at the interface between the gas envelope and the surrounding air. Such a flame isunstable with a low heat flux and if the velocity be increased with intent to produce a moreuseful combustion characteristic, then there will be a tendency for the flame to lift fromthe nozzle and extinguish. A variety of complex jets has been produced to substitute forthe simple nozzle and other methods, principally involving some process of aeration, havebeen evolved to address the problem. Those most commonly used in boiler firing are:Gas supplyPilot supplyBurnerMain cockGovernorFigure 10.22 Neat flame gas burnerSafety shut off