Chapter 10- PROPULSION BOILERS 



been used for some time in stationary power 

 plants, in locomotives, and in some merchant 

 ships. Only a few controlled circulation boilers 

 have been installed in the propulsion plants of 

 naval ships, and of this few the majority were 

 subsequently removed and replaced by conven- 

 tional single-furnace boilers with accelerated 

 natural circulation. In theory, however, con- 

 trolled circulation has some very marked ad- 

 vantages over natural circulation, and it is 

 entirely possible that improved designs of 

 controlled circulation boilers may be developed 

 for future use in naval propulsion plants. 



In natural circulation boilers, circulation 

 occurs because the ascending mixture of water 

 and steam is lighter (less dense) than the de- 

 scending body of relatively cool and steam-free 

 water. As boiler pressure increases, however, 

 there is less difference between the density of 

 steam and the density of water. At pressures 

 over 1000 psi, the density of steam differs so 

 little from the density of water that natural cir- 

 culation is harder to achieve than it is at lower 

 pressures. At high pressures, controlled circu- 

 lation boilers have a distinct advantage because 

 their circulation is controlled by pumps and is 

 independent of differences in density. Because 

 controlled circulation boilers can be designed 

 without regard for differences in density, they 

 can be arranged ip practically any way that is 

 required for a particular type of installation. 

 Thus a greater flexibility of arrangement is 

 possible and the boilers may be designed for 

 compactness, savings in space and weight re- 

 quirements, and maximum heat absorption. 



There are two main kinds of controlled 

 circulation boilers. One type is known as a 

 once-through or f orced flow boiler; the other 

 type is usually called a controlled circulation 

 or a forced recirculation boiler. In both types, 

 external pumps are used to force the water 

 through the boiler circuits; the essential dif- 

 ference between the two kinds lies in the amount 

 of water supplied to the boiler. 



In a once-through forced circulation boiler, 

 all (or very nearly all) of the water pumped to 

 the boiler is converted to steam the first time 

 through, without any recirculation. This type of 

 boiler has no steam drum, but has instead a 

 small separating chamber. Water is pumped 

 into the economizer circuit and from there to 

 the generating circuit, the amount of flow being 

 controlled so as to allow practically all of the 

 water to "he converted into steam in the gen- 

 erating circuit. The very small amount of water 



that is not converted to steam in the generating 

 circuit is separated from the steam in the 

 separating chamber. The water is discharged 

 from the separating chamber to the feed pump 

 suction, if it is suitable for use; if it contains 

 solid matter, it is discharged through the blow- 

 down pipe. Meanwhile, the steam from the 

 separating chamber flows on through the super- 

 heater circuit, where it is superheated before 

 it enters the main steam line. 



Figure 10-24 shows the boiler circuits of a 

 controlled circulation (or forced recirculation) 

 boiler. In this boiler, more water is pumped 

 through the circuits than is converted into 

 steam. The excess water is taken from the 

 steam drum and is pumped through the boiler 

 circuits again by means of a circulating pump. 

 This type of boiler has a conventional steam 

 drum which contains a feed pipe, steam sepa- 

 rators and dryers, a desuperheater, and other 

 fittings. The boiler has an economizer, three 

 generating circuits, and a superheater. Circu- 

 lating pumps, fitted as integral parts of the 

 boiler, provide positive circulation to all steam 

 generating surfaces. 



Both types of controlled circulation boilers 

 have far smaller water capacity than do natural 

 circulation boilers, and therefore have much 

 more rapid response to changes in load. For this 

 reason, automatic controls are required on 

 these boilers to ensure rapid and sensitive 

 response to fuel and feed water requirements. 



PRESSURIZED-FURNACE BOILERS.-A 

 boiler recently developed for use in naval pro- 

 pulsion plants is variously known as a pres- 

 surized-furnace boiler, a pressure-fired boiler , 

 a supercharged boiler , or a supercharged steam 

 generating system . 



A pressurized-furnace boiler is shown sche- 

 matically in figure 10-25 and in cutaway view in 

 figure 10-26. As may be seen, the boiler is 

 quite unlike other operational boiler types in 

 general configuration. The pressurized furnace 

 is more or less cylindrical in shape, with the 

 long axis of the cylinder running vertically. The 

 boiler drum is mounted horizontally, some dis- 

 tance above the pressurized furnace. The drum 

 is connected to the steam and water elements 

 in the furnace by risers and downcomers, all of 

 which are external to the casing. Some boilers 

 of this type are side-fired. Others (including the 

 one shown) are top-fired; as may be seen in fig- 

 ures 10-25 and 10-26, the burners are at the top 



257 



