Book - School of Science and Technology

Boiler firing ± gaseous fuel 289Figure 10.7. They are extremely quiet in operation and, as multiple units, may be obtainedin ratings up to about 1 MW. A gas pressure of about 1.5±2 kPa is required at the burner,down stream from the governor.Gas is supplied to a calibrated nozzle fitted to a manifold and flows through a venturito induce a supply of primary air. The resultant mixture is delivered by the manifold to aseries of outlet ports where the flames entrain secondary air from the surrounding space atthe boiler base.Combustion gases rise by convection through channels between the heat exchangesurfaces, boiler height providing an adequate motive force. As illustrated in Figure10.23, it is necessary that the flue terminal of the boiler incorporates a draught diverterand gas dilution device in order to prevent any interaction between that small naturaldraught and the effect of a connected chimney. An atmospheric burner must not be usedin any situation where draught is not so stabilised.Packaged burnersKnown also as forced draught burners, units of this type are built up around an integralfan which is arranged to provide, under pressure, all the air required for combustion. Inmost models, such fans operate with direct coupled drives at relatively high speed and are,as a result, objectionably noisy.As may be seen from Figure 10.24 which illustrates one such burner, both gas and airunder pressure are delivered to the combustion chamber via a complex arrangement ofFanAir boxNozzlesSafetyshutoffPilotsupplyGovernorBurnerheadMaincockGas supplyFigure 10.24 Packaged forced draught gas burner