Chapter 10- PROPULSION BOILERS 



Command has established a number of standard 

 definitions relating to boilers. Since these terms 

 are quite widely used, the student will find it 

 helpful to understand the following terms and 

 to use them correctly. 



BOILER FULL-POWER CAPACITY. -The 

 total quantity of steam required to develop con- 

 tract shaft horsepower of the ship, divided by 

 the number of boilers installed in the ship, gives 

 boiler full-power capacity. Boiler full-power 

 capacity is expressed as the number of pounds 

 of steam generated per hour at a specified pres- 

 sure and temperature. Boiler full-power capac- 

 ity is listed in the design data section of the 

 manufacturer's technical manual for the boilers 

 on each ship; it may be listed as capacity at full 

 power or as designed rate of actual evaporation 

 per boiler at full power. 



BOILER OVERLOAD CAPACITY.-Boiler 

 overload capacity is usually 120 percent of 

 boiler full-power capacity. Boiler overload 

 capacity is listed in the design data section of 

 the manufacturer's technical manual for the 

 boilers; it may be listed as boiler overload 

 capacity or as full power plus 20 percent. 



SUPERHEATER OUTLET PRESSURE. -Su- 

 perheater outlet pressure is the actual steam 

 pressure carried at the superheater outlet. 



STEAM DRUM PRESSURE.-Steam drum 

 pressure is the pressure actually carried in the 

 boiler steam drum. 



OPERATING PRESSURE. -Operating pres- 

 sure is the constant pressure at which the boiler 

 is operated in service. Depending upon various 

 factors, chiefly design features of the boiler, the 

 constant pressure may be carried at the steam 

 drum or at the superheater outlet. Operating 

 pressure is specified in the design of the boiler 

 and is given in the manufacturer's technical 

 manual. Operating pressure is the same as 

 superheater outlet pressure or steam drum 

 pressure (depending upon which is used as the 

 controlling pressure) only when the boiler is 

 operating at full-power capacity, for combatant 

 ships, or some other specified rate, for other 

 ships. When the boiler is operating at less than 

 full-power capacity (or other specified rate), 

 the actual pressure at the steam drum or at the 

 superheater outlet will vary from the designated 

 operating pressure. 



DESIGN PRESSURE. -Design pressure is the 

 pressure specified by the boiler manufacturer 

 as a criterion for boiler design. It is often 

 approximately 103 percent of steam drum pres- 

 sure. Operating personnel seldom have occasion 

 to be concerned with design pressure; the term 

 is noted here because there is a good deal of 

 confusion between design pressure and operating 

 pressure. The two terms do not mean the same 

 thing. 



DESIGN TEMPERATURE. -Design tempera- 

 ture is the intended maximum operating tem- 

 perature at the superheater outlet, at some 

 specified rate of operation. The specified rate 

 of operation is normally full-power capacity for 

 combatant ships. 



OPERATING TEMPERATURE. -Operating 

 temperature is the actual temperature at the 

 superheater outlet. As a rule, operating tem- 

 perature is the same as design temperature 

 only when the boiler is operating at the rate 

 specified in the definition of design temperature. 



BOILER EFFICIENCY. -The efficiency of a 

 boiler is the ratio of the Btu per pound of fuel 

 absorbed by the water and steam to the Btu per 

 pound of fuel fired. In other words, boiler effi- 

 ciency is output divided by input, or heat uti- 

 lized divided by heat available. Boiler efficiency 

 is expressed as a percentage. 



FIREROOM EFFICIENCY.-Boiler Efficien- 

 cy corrected for blower and pump steam con- 

 sumption is called fireroom efficiency. Note: 

 Fireroom efficiency is NOT boiler plant effi- 

 ciency or propulsion plant efficiency. 



STEAMING HOURS.-The term steaming 

 hours is used to include all time during which 

 the boiler has fires lighted for raising steam 

 and all time during which steam is being gen- 

 erated. Time during which fires are not lighted 

 is not included in steaming hours. 



HEATING SURFACES.-The total heating 

 surface of a boiler includes all parts of the 

 boiler which are exposed on one side to the gases 

 of combustion and on the other side to the water 

 and steam being heated. Thus the total heating 

 surface equals the sum of the generating surface, 

 the superheater surface, and the economizer 

 surface. All heating surfaces are measured on 

 the combustion gas side. 



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