Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSbeen used for some time in stationary powerplants, in locomotives, and in some merchantships. Only a few controlled circulation boilershave been installed in the propulsion plants ofnaval ships, and of this few the majority weresubsequently removed and replaced by conventionalsingle-furnace boilers with acceleratednatural circulation. In theory, however, controlledcirculation has some very marked advantagesover natural circulation, and it isentirely possible that improved designs ofcontrolled circulation boilers may be developedfor future use in naval propulsion plants.In natural circulation boilers, circulationoccurs because the ascending mixture of waterand steam is lighter (less dense) than the descendingbody of relatively cool and steam-freewater. As boiler pressure increases, however,there is less difference between the density ofsteam and the density of water. At pressuresover 1000 psi, the density of steam differs solittle from the density of water that natural circulationis harder to achieve than it is at lowerpressures. At high pressures, controlled circulationboilers have a distinct advantage becausetheir circulation is controlled by pumps and isindependent of differences in density. Becausecontrolled circulation boilers can be designedwithout regard for differences in density, theycan be arranged ip practically any way that isrequired for a particular type of installation.Thus a greater flexibility of arrangement ispossible and the boilers may be designed forcompactness, savings in space and weight requirements,and maximum heat absorption.There are two main kinds of controlledcirculation boilers. One type is known as aonce-through or forced flow boiler; the othertype is usually called a controlled circulationor a forced recirculation boiler. In both types,external pumps are used to force the waterthrough the boiler circuits; the essential differencebetween the two kinds lies in the amountof water supplied to the boiler.In a once-through forced circulation boiler,all (or very nearly all) of the water pumped tothe boiler is converted to steam the first timethrough, without any recirculation. This type ofboiler has no steam drum, but has instead asmall separating chamber. Water is pumpedinto the economizer circuit and from there tothe generating circuit, the amount of flow beingcontrolled so as to allow practically all of thewater to "he converted into steam in the generatingcircuit. The very small amount of waterthat is not converted to steam in the generatingcircuit is separated from the steam in theseparating chamber. The water is dischargedfrom the separating chamber to the feed pumpsuction, if it is suitable for use; if it containssolid matter, it is discharged through the blowdownpipe. Meanwhile, the steam from theseparating chamber flows on through the superheatercircuit, where it is superheated beforeit enters the main steam line.Figure 10-24 shows the boiler circuits of acontrolled circulation (or forced recirculation)boiler. In this boiler, more water is pumpedthrough the circuits than is converted intosteam. The excess water is taken from thesteam drum and is pumped through the boileragain by means of a circulating pump.circuitsThis type of boiler has a conventional steamdrum which contains a feed pipe, steam separatorsand dryers, a desuperheater, and otherfittings. The boiler has an economizer, threegenerating circuits, and a superheater. Circulatingpumps, fitted as integral parts of theboiler, provide positive circulation to all steamgenerating surfaces.Both types of controlled circulation boilershave far smaller water capacity than do naturalcirculation boilers, and therefore have muchmore rapid response to changes in load. For thisreason, automatic controls are required onthese boilers to ensure rapid and sensitiveresponse to fuel and feed water requirements.PRESSURIZED-FURNACE BOILERS.-Aboiler recently developed for use in naval propulsionplants is variously known as a pressurized-furnaceboiler, a pressure-fired boiler ,a supercharged boiler , or a supercharged steamgenerating system .A pressurized-furnace boiler is shown schematicallyin figure 10-25 and in cutaway view infigure 10-26. As may be seen, the boiler isquite unlike other operational boiler types ingeneral configuration. The pressurized furnaceis more or less cylindrical in shape, with thelong axis of the cylinder running vertically. Theboiler drum is mounted horizontally, some distanceabove the pressurized furnace. The drumis connected to the steam and water elementsin the furnace by risers and downcomers, all ofwhich are external to the casing. Some boilersof this type are side-fired. Others (including theone shown) are top-fired; as may be seen in figures10-25 and 10-26, the burners are at the top257