Principles of naval engineering - Historic Naval Ships Association

Chapter 10- PROPULSION BOILERSCommand has established a number of standarddefinitions relating to boilers. Since these termsare quite widely used, the student will find ithelpful to understand the following terms andto use them correctly.BOILER FULL-POWER CAPACITY. -Thetotal quantity of steam required to develop contractshaft horsepower of the ship, divided bythe number of boilers installed in the ship, givesboiler full-power capacity. Boiler full-powercapacity is expressed as the number of poundsof steam generated per hour at a specified pressureand temperature. Boiler full-power capacityis listed in the design data section of themanufacturer's technical manual for the boilerson each ship; it may be listed as capacity at fullpower or as designed rate of actual evaporationper boiler at full power.BOILER OVERLOAD CAPACITY.-Boileroverload capacity is usually 120 percent ofboiler full-power capacity. Boiler overloadcapacity is listed in the design data section ofthe manufacturer's technical manual for theboilers; it may be listed as boiler overloadcapacity or as full power plus 20 percent.SUPERHEATER OUTLET PRESSURE. -Superheateroutlet pressure is the actual steampressure carried at the superheater outlet.STEAM DRUM PRESSURE.-Steam drumpressure is the pressure actually carried in theboiler steam drum.OPERATING PRESSURE. -Operating pressureis the constant pressure at which the boileris operated in service. Depending upon variousfactors, chiefly design features of the boiler, theconstant pressure may be carried at the steamdrum or at the superheater outlet. Operatingpressure is specified in the design of the boilerand is given in the manufacturer's technicalmanual. Operating pressure is the same assuperheater outlet pressure or steam drumpressure (depending upon which is used as thecontrolling pressure) only when the boiler isoperating at full-power capacity, for combatantships, or some other specified rate, for otherships. When the boiler is operating at less thanfull-power capacity (or other specified rate),the actual pressure at the steam drum or at thesuperheater outlet will vary from the designatedoperating pressure.DESIGN PRESSURE. -Design pressure is thepressure specified by the boiler manufactureras a criterion for boiler design. It is oftenapproximately 103 percent of steam drum pressure.Operating personnel seldom have occasionto be concerned with design pressure; the termis noted here because there is a good deal ofconfusion between design pressure and operatingpressure. The two terms do not mean the samething.DESIGN TEMPERATURE. -Design temperatureis the intended maximum operating temperatureat the superheater outlet, at somespecified rate of operation. The specified rateof operation is normally full-power capacity forcombatant ships.OPERATING TEMPERATURE. -Operatingtemperature is the actual temperature at thesuperheater outlet. As a rule, operating temperatureis the same as design temperatureonly when the boiler is operating at the ratespecified in the definition of design temperature.BOILER EFFICIENCY. -The efficiency of aboiler is the ratio of the Btu per pound of fuelabsorbed by the water and steam to the Btu perpound of fuel fired. In other words, boiler efficiencyis output divided by input, or heat utilizeddivided by heat available. Boiler efficiencyis expressed as a percentage.FIREROOM EFFICIENCY.-Boiler Efficiencycorrected for blower and pump steam consumptionis called fireroom efficiency. Note:Fireroom efficiency is NOT boiler plant efficiencyor propulsion plant efficiency.STEAMING HOURS.-The term steaminghours is used to include all time during whichthe boiler has fires lighted for raising steamand all time during which steam is being generated.Time during which fires are not lightedis not included in steaming hours.HEATING SURFACES.-The total heatingsurface of a boiler includes all parts of theboiler which are exposed on one side to the gasesof combustion and on the other side to the waterand steam being heated. Thus the total heatingsurface equals the sum of the generating surface,the superheater surface, and the economizersurface. All heating surfaces are measured onthe combustion gas side.231