Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGCOOL WATERFLOWS DOWNHOTGASESMIXTURE OFHOT WATERAND STEAMFLOWS UPFigure 10-2.— Natural circulation(accelerated type).139.17air pressure used in the furnace. Most boilersnow in use in naval propulsion plants operatewith a slight air pressure (seldom over lOpsig)in the boiler furnace. This slight pressure, whichresults from the use of forced draft blowers tosupply combustion air to the boilers, is not sufficientto warrant calling these boilers "pressurized-furnaceboilers." However, a new typeof boiler has recently appeared on the scene andis being installed in some ships. This new boileris truly a pressurized-furnace boiler, since thefurnace is maintained under a positive air pressureof approximately 65 psia (about 50 psig)when the boiler is operating at full power. Theair pressure in the furnace is maintained by aspecial air compressor. Hence we must nowmake a distinction between this new pressurizedfurnaceboiler, on the one hand, and all othernaval propulsion boilers, on the other hand, withrespect to the pressure maintained in the furnace.Type of SuperheaterOn almost all boilers currently used in thepropulsion plants of naval ships, the superheatertubes are protected from radiant heat by waterscreen tubes. The water screen tubes absorbthe intense radiant heat of the furnace, and thesuperheater tubes are heated by convectioncurrents rather than by radiation. Hence, thesuperheaters are referred to as convection-typesuperheaters.On a few older ships, the superheater tubesare not screened by water screen tubes but areexposed directly to the radiant heat of the furnace.Superheaters of this kind are calledradiant-type superheaters. Although radianttypesuperheaters are rarely used at present, itis possible that they may come into use again infuture boiler designs.Control of SuperheatA boiler which provides some means of controllingthe degree of superheat independently ofthe rate of steam generation is said to havecontrolled superheat . A boiler in which suchseparate control is not possible is said to haveuncontrolled superheat .Until recently, the term superheat controlboiler was used to identify a double-furnaceboiler and the term uncontrolled superheatboiler (or no control superheat boiler ) was usedto identify a single-furnace boiler. Most doublefurnaceboilers now in use do, in fact, havecontrolled superheat, and most single-furnaceboilers do not have controlled superheat. However,recent developments in boiler design makesuperheat control independent of the number offurnaces in the boiler. Single-furnace boilersWITH controlled superheat and double-furnaceboilers WITHOUT controlled superheat are bothpossible. The time has come, therefore, to stoprelating the number of furnaces in a boiler tothe control (or lack of control) of superheat.Operating PressureFor some purposes it is convenient to classifyboilers according to operating pressure.Most classifications of this type are approximaterather than exact. Header-type boilers and someolder drum-type boilers are often called "400-psi boilers" even though the operating pressuresmay range from 300 psi (or even lower) to about450 psi. The term "600-psi boiler" is oftenapplied to various double-furnace and singlefurnaceboilers with operating pressures rangingfrom about 435 psi to about 700 psi.The term "high pressure boiler" is at presentused rather loosely to identify any boilerthat operates at substantially higher pressurethan the so-called "600-psi boilers." In general,we will consider any boiler that operatesat 751 psi or above as a high pressure boiler.A good many boilers recently installed on navalships operate at approximately 1200 psi; forsome purposes, it is convenient to group theseboilers together and refer to them as "1200-psiboilers."234