Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSMost double-furnace boilers are designed tocarry a superheater outlet temperature of 850°F; this is about 100° F higher than the superheateroutlet temperature in a comparablesingle-furnace boiler, given the same quality ofmaterials for boilers, piping, and turbines. Thereason why a higher superheater outlet temperaturecan be used in a double-furnace boilerthan in a comparable single-furnace boiler is thatallowance must be made, in the single-furnaceboiler, for the maximum superheater temperatureswhich might occur under adverse conditionsof load.In spite of the advantages resulting from thecontrol of superheat, double-furnace boilers areno longer being installed in naval combatantships. Experience with these boilers has revealedcertain disadvantages which at the presenttime appear to outweigh the advantages of controlledsuperheat. Some of the disadvantages are:1. In practice, it is not possible to maintainmaximum designed superheat at low steamingrates. Only the steam for the main turbines andthe turbogenerators goes through the superheater;at low firing rates, therefore, the steamflow through the superheater is generally not sufficientto permit a high firing rate on the superheaterside. Thus under some conditions thesteam supplied to the propulsion turbines and tothe turbogenerators may be saturated or onlyvery slightly superheated. As a consequence,therefore, the double-furnace boiler is actuallyless efficient than the single-furnace boiler atlow firing rates.2. The double-furnace boiler is more difficultto operate than the single-furnace boiler,and requires more personnel for its operation.Once there is any appreciable load on the boiler,the high air pressure in the double casings and inthe furnace make it difficult and even dangerousto light burners on the superheater side. In orderto avoid this difficulty, operating personnelwould have to be able to predict the need forsuperheat and light off the burners on the superheaterside before the air pressure had becomeso high. Obviously, such prediction is not alwayspossible.3. The double-furnace boiler is heavier,larger, and generally more complex than asingle-furnace boiler of equal capacity.Single- Furnace BoilersThe older single-furnace boilers that wereinstalled on many World War II ships differ inseveral important respects from the newersingle-furnace boilers that have been installedon ships built since World War II.A single-furnace boiler of the older typeis shown schematically in figure 10-20 and incutaway view in figure 10-21. This boiler producesabout 60,000 pounds of steam per hour atfull power. At full power the steam drum pressureis about 460 psig, the superheater outletpressure is about 435 psig, and the superheateroutlet temperature is about 750° F.SUPERHEATERPROTECTIONEXHAUST VALVEECONOMIZER\Figure 10-20.— General arrangementsingle-furnace boiler.SUPERHEATERPROTECTIONSTEAM VALVEMAIN STEAMof38.39olderThis boiler does not have controlled superheat.When the boiler is lighted off, both thegenerating tubes and the superheater tubes areheated. In order to protect the superheatertubes from overheating, all steam generated inthe boiler must be led through the superheater.The saturated steam goes from the dry pipe inthe steam drum to the superheater inlet; it goesthrough the superheater tubes, out the superheateroutlet, and into the main steam line.Auxiliary steam must go through the superheater(in order to provide a sufficient steamflow to protect the superheater) but must thenbe desuperheated. Desuperheating is accomplishedby passing some of the superheatedsteam through a desuperheater, which is basicallya coil of piping submerged in the water in251