Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGAIR OFFTAKE CROSSOVER PIPES(IN EXHAUST OPENING STRUTS)STEAM LANETUBE SUPPORT PLATECONDENSER TUBESCONDENSER SHELLAIR OFFTAKETUBE PLATESTAYSCONDENSERSIDEWALLSTIFFENERSAIR BAFFLESUPPORTINGFLANGEAIR COOLINGSECTIONSUPPORTINGFLANGEAIR COOLINGSECTIONHOT WELLCAGE GLASSCONDENSATE OUTLETFigure 13-3. — Cross-sectional view of main condenser.98.33sometimes used to support part of the weight ofthe condenser so that it will not have to be entirelysupported by the turbine.Condenser performance may be evaluated bya simple energy balance which takes account ofall energy entering and leaving the condenser.In theory, the entering side of the balance shouldinclude (1) the mechanical kinetic energy of theentering steam, (2) the thermalenergy of the enteringsteam, (3) the mechanical kinetic energyof the entering sea water, and (4) the thermalenergy of the entering sea water. In theory, again,the leaving side of the balance should include (1)the mechanical kinetic energy of the leaving condensate,(2) the thermal energy of the leavingcondensate, (3) the mechanical kinetic energy ofthe leaving sea water, and (4) the thermal energyof the leaving sea water. In considering real condensers,however, the entering and leaving mechanicalkinetic energies of the sea water tendto be small and tend to cancel each other out, themechanical kinetic energy of the entering steamis so small as to be negligible, and the mechanicalkinetic energy of the leaving condensate issmall enough to disregard. With all of these relativelyinsignificant quantities omitted, the enteringside of the balance includes only the thermalenergy of the entering steam and the thermalenergy of the entering sea water, and the leavingside includes only the thermal energy of theleaving condensate and the thermalenergy of theleaving sea water.AIREJECTOR ASSEMBLIESThe function of air ejectors is to remove airand other noncondensable gases from the condenser.An air ejector is a type of jet pump,having no moving parts. The flow through the airejector is maintained by a jet of high velocitysteam passing through a nozzle. The steam istaken from the 150-psi auxiliary steam systemon most ships.The air ejector assembly (fig. 13-4) used toremove air from the main condenser usually consistsof a first-stage air ejector, an inter condenser,a second-stage air ejector, and an aftercondenser. The two air ejectors operate in series.The first-stageair ejector raises the pressurefrom about 1.5 inches of mercury absolute(condenser pressure) to about 7 inches of mercuryabsolute; the second-stage air ejectorraises the pressure from 7 inches of mercuryabsolute to about 32 inches of mercury absolute(about 1 psig).The first-stage air ejector takes suction onthe main condenser and discharges the steam-airmixture to the inter condenser, where the steamcontent of the mixture is condensed. The result-condensate drops to the bottom of the intering352