Principles of naval engineering - Historic Naval Ships Association

CHAPTER 10PROPULSION BOILERSIn the conventional steam turbine propulsionplant, the boiler is the source or high temperatureregion of the thermodynamic cycle. Thesteam that is generated in the boiler is led tothe propulsion turbines, where its thermalenergy is converted into mechanical energywhich drives the ship and provides power forvital services.In essence, a boiler is merely a containerin which water can be boiled and steam generated.A teakettle on a stove is basically aboiler, although a rather inefficient one. Indesigning a boiler to produce a large amountof steam, it is obviously necessary to find somemeans of providing a larger heat transfer surfacethan is provided by a vessel shaped like ateakettle. In most modern boilers, the steamgenerating surface consists of between one andtwo thousand tubes which provide a maximumamount of heat transfer surface in a relativelysmall space. As a rule, the tubes communicatewith a steam drum at the top of the boiler andwith water drums and headers at the bottom ofthe boiler. The tubes and part of the drums areenclosed in an insulated casing which has spaceinside it for a furnace. As we will see presently,a boiler appears to be a fairly complicated pieceof equipment when it is considered with all itsfittings, piping, and accessories. It may be helpful,therefore, to remember that the basic componentsof a saturated-steam boiler are merelythe tubes in which steam is generated, the drumsand headers in which water is contained andsteam is collected, and the furnace in whichcombustion takes place.Practically all boilers used in the propulsionplants of naval ships are designed to produceboth saturated steam and superheated steam.To our basic boiler, therefore, we must now addanother component: the superheater. The superheateron most boilers consists of headers,usually located at the back or at the bottom ofthe boiler, and a number of superheater tubeswhich communicate with the headers. Saturatedsteam from the steam drum is led through thesuperheater; since the steam is now no longerin contact with the water from which it was generated,the steam becomes superheated withoutany appreciable increase in pressure as additionalheat is supplied. In some boilers, there isa separate superheater furnace; in others, thesuperheater tubes project into the same furnacethat is used for the generation of saturatedsteam.Some question may arise concerning the needfor both saturated steam and superheated steam.Many steam-driven auxiliaries—particularly ifthey have reciprocating engines—require saturatedsteam for the lubrication of the movingparts of the driving machine. The propulsionturbines, on the other hand, and many auxiliariesas well, perform much more efficiently whensuperheated steam is used. There is more availableenergy in superheated steam than in saturatedsteam at the same pressure, and the use ofhigher temperatures vastly increases the thermodynamicefficiency of the propulsion cyclesince the efficiency of a heat engine depends uponthe absolute temperature at the source (boiler)and at the receiver (condenser). In some instances,the gain in efficiency resulting from theuse of superheated steam may be as much as 15percent for 200 degrees of superheat. This increasein efficiency is particularly important fornaval ships because it allows substantial savingsin fuel consumption and in space and weight requirements.A further advantage in using superheatedsteam for propulsion turbines is that itcauses relatively little erosion or corrosionsince it is free of moisture.BOILER DEFINITIONSIn order to ensure accuracy and uniformityin the use of boiler terms, the Naval Ship Systems230