Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGcondenser. To avoid this difficulty, provision ismade for returning some of the condensate to themain condenser when the condensate reaches acertain temperature. As a rule, the recirculatingline branches off the condensate line just afterthe gland exhaust condenser. In most installations,the recirculating valve in this recirculatingline is thermostatically operated.VENTCONDENSERThe vent condenser is actually a part of thedeaerating feed tank, being installed in the tanknear the top. It is described separately herebecause it is functionally quite separate from thedeaerating feed tank.In the vent condenser, as in the air ejectorcondensers and the gland exhaust condenser,condensate on its way from the main condenserto the deaerating feed tank is used to cool andcondense the steam from a steam-air mixture.The vent condenser receives steam and air fromthe deaerating feed tank. The steam condensesinto water, which falls toward the bottom of thetank. The air goes to the gland exhaust condenserand is vented to atmosphere. The condensatewhich is used as the cooling medium in the ventcondenser is sprayed out into the deaerating feedtank and is deaerated before beingused as boilerfeed.DEAERATING FEED TANKThe deaerating feed tank serves to heat, deaerate,and store feed water. The water is heatedby direct contact with auxiliary exhaust steamwhich enters the tank at a pressure just slightlygreater than the pressure in the tank. Thedeaerating feed tank is usually designed to operateat a pressure of about ISpsigand to heat thewater to between 240° and 250° F.One type of deaerating feed tank is shown infigure 13-5. Condensate enters the tank throughthe tubes of the vent condenser and is forced outthrough a number of spray valves in a spray head.The spray valves discharge the condensate in afine spray throughout the steam-filled upper sectionof the deaerating feed tank. The very smalldroplets of water are heated, scrubbed, and partiallydeaerated by the relatively air-free steam.As the steam gives up its heat to the water, muchof the steam is condensed into water. Thedroplets of water (including both the enteringcondensate sprayed out from the vent condenserand the steam condensed in the deaerating feedtank) are collected in a cone-shaped baffle whichleads them through a central port, to thedeaerating unit.Steam enters the deaerating unit, picks up thepartially deaerated water, and throws ittangentiallyoutward through the curving baffles of thedeaerating unit. In this process, the water is evenmore finely divided and is throughly scrubbed bythe incoming steam. Thus the last traces of dissolvedoxygen areremovedfromthe water. Sincethe water enters the deaerating unit at saturationtemperature, having already been heated by thesteam in the upper part of the deaerating feedtank, the incoming steam does not condense toany marked degree in the deaerating unit.Therefore all (or practically all) of the incomingsteam is available for breaking up, scrubbing,and deaerating the water.The thoroughly deaerated water falls into thestorage space at the bottom of the tank, where itremains under a blanket of air-free steam untilit is pumped to the boilers. Meanwhile, the mixtureof steam plus air and other noncondensablegases travels over the spray head (where muchof the steam is condensed as it heats the incomingcondensate) and over the tubes of the ventcondenser (where more steam is condensed intowater which then goes into the deaerating unit).The air and other noncondensable gases, togetherwith a little remaining steam, goto the gland exhaustcondenser.As shown in figure 13-5, the deaerating feedtank has a recirculating connection that allowswater to be sent back to the condenser from thedeaerating feed tank. The recirculating line isused to provide a high enough condensate levelin the condenser so that the condensate pump cantake suction. The recirculating line is also usedat slow speeds and when the plant is first startedup to ensure a sufficient supply of cooling condensateto the air ejector condensers and to thegland exhaust condenser and to keep the deaeratingfeed tank at the prescribed temperature.The deaerated feed water from the deaeratingfeed tank is pumped to the boiler by the feedbooster pump and the main feed pump. The feedbooster pump takes suction from the bottom ofthe deaerating feed tank and discharges to thesuction side of the main feed pump. Thefeed booster pump provides a positive suctionpressure for the main feed pump and thus preventsthe hot water from flashing into steam atthe main feed pump suction. The main feedpump operates at variable speed in order tomaintain a constant discharge pressure under all354