CHAPTER 10 



PROPULSION BOILERS 



In the conventional steam turbine propulsion 

 plant, the boiler is the source or high tempera- 

 ture region of the thermodynamic cycle. The 

 steam that is generated in the boiler is led to 

 the propulsion turbines, where its thermal 

 energy is converted into mechanical energy 

 which drives the ship and provides power for 

 vital services. 



In essence, a boiler is merely a container 

 in which water can be boiled and steam gen- 

 erated. A teakettle on a stove is basically a 

 boiler, although a rather inefficient one. In 

 designing a boiler to produce a large amount 

 of steam, it is obviously necessary to find some 

 means of providing a larger heat transfer sur- 

 face than is provided by a vessel shaped like a 

 teakettle. In most modern boilers, the steam 

 generating surface consists of between one and 

 two thousand tubes which provide a maximum 

 amount of heat transfer surface in a relatively 

 small space. As a rule, the tubes communicate 

 with a steam drum at the top of the boiler and 

 with water drums and headers at the bottom of 

 the boiler. The tubes and part of the drums are 

 enclosed in an insulated casing which has space 

 inside it for a furnace. As we will see presently, 

 a boiler appears to be a fairly complicated piece 

 of equipment when it is considered with all its 

 fittings, piping, and accessories. It may be help- 

 ful, therefore, to remember that the basic com- 

 ponents of a saturated-steam boiler are merely 

 the tubes in which steam is generated, the drums 

 and headers in which water is contained and 

 steam is collected, and the furnace in which 

 combustion takes place. 



Practically all boilers used in the propulsion 

 plants of naval ships are designed to produce 

 both saturated steam and superheated steam. 

 To our basic boiler, therefore, we must now add 

 another component: the superheater. The super- 

 heater on most boilers consists of headers, 

 usually located at the back or at the bottom of 



the boiler, and a number of superheater tubes 

 which communicate with the headers. Saturated 

 steam from the steam drum is led through the 

 superheater; since the steam is now no longer 

 in contact with the water from which it was gen- 

 erated, the steam becomes superheated without 

 any appreciable increase in pressure as addi- 

 tional heat is supplied. In some boilers, there is 

 a separate superheater furnace; in others, the 

 superheater tubes project into the same furnace 

 that is used for the generation of saturated 

 steam. 



Some question may arise concerning the need 

 for both saturated steam and superheated steam. 

 Many steam-driven auxiliaries— particularly if 

 they have reciprocating engines— require satu- 

 rated steam for the lubrication of the moving 

 parts of the driving machine. The propulsion 

 turbines, on the other hand, and many auxiliaries 

 as well, perform much more efficiently when 

 superheated steam is used. There is more avail- 

 able energy in superheated steam than in satu- 

 rated steam at the same pressure, and the use of 

 higher temperatures vastly increases the ther- 

 modynamic efficiency of the propulsion cycle 

 since the efficiency of a heat engine depends upon 

 the absolute temperature at the source (boiler) 

 and at the receiver (condenser). In some in- 

 stances, the gain in efficiency resulting from the 

 use of superheated steam may be as much as 15 

 percent for 200 degrees of superheat. This in- 

 crease in efficiency is particularly important for 

 naval ships because it allows substantial savings 

 in fuel consumption and in space and weight re- 

 quirements. A further advantage in using super- 

 heated steam for propulsion turbines is that it 

 causes relatively little erosion or corrosion 

 since it is free of moisture. 



BOILER DEFINITIONS 



In order to ensure accuracy and uniformity 

 in the use of boiler terms, the Naval Ship Systems 



230 



