Principles of naval engineering - Historic Naval Ships Association

CHAPTER13CONDENSERS AND OTHER HEAT EXCHANGERSThis chapter deals with the major pieces ofheat transfer apparatus found in the condensateand feed system of the conventional steam turbinepropulsion plant. Heat exchangers discussedhere include the main condenser, the air ejectorcondenser, the gland exhaust condenser, the ventcondenser, the deaerating feedtank, and the auxiliarycondenser. The arrangement of piping thatconnects these units is discussed in chapter 9 ofthis text.MAIN CONDENSERThe main condenser is the heat exchanger inwhich exhaust steam from the propulsion turbinesis condensed as it comes in contact withtubes through which cool sea water is flowing.The main condenser is the heat receiver of thethermodynamic cycle— that is, it is the low temperatureheat sink to which some heat must berejected. The main condenser is also the meansby which feed water is recovered and returnedto the feed system. If we imagine a shipboardpropulsion plant in which there is no main condenserand the turbines exhaust to atmosphere,and if we consider the vast quantities of freshwater that would be required to support even oneboiler generating 150,000 pounds of steam perhour, it is immediately apparent that the maincondenser serves a vital function in recoveringfeed water.The main condenser is maintained under avacuum of approximately 25 to 28.5 inches ofmercury. The designed vacuum varies accordingto the design of the turbine installation andaccording to such operational factors as the loadon the condenser, the temperature of the outsidesea water, and the tightness of the condenser. Thedesigned full-power vacuum for any particularturbine installation may be obtained from themachinery specifications for the plant. Someturbines are designed for a full-power exhaustvacuum of 27.5 inches of mercury when the circulatingwater injection temperature is 75° F;others are designed for a full-power exhaustvacuum of 25 inches of mercury with a circulatingwater injection temperature of 75° F.It is often said that an engine can do a greateramount of useful work if it exhausts to a lowpressure space than if it exhausts against a highpressure. This statement is undeniably true, butfor the condensing steam power plant it may besomewhat misleading because of its emphasis onpressure. The pressure is important because itdetermines the temperature at which the steamcondenses. As noted in chapter 8 of this text, anincrease in the temperature difference betweenthe source (boiler) and the receiver (condenser)increases the thermodynamic efficiency of thecycle. By maintaining the condenser under vacuum,we lower the condensing temperature, increasethe temperature difference betweensource and receiver, and increase the thermodynamicefficiency of the cycle.Given a tight condenser and an adequate supplyof cooling water, the basic cause of the vacuumin the condenser is the condensation of thesteam. This is true because the specific volumeof steam is enormously greater than the specificvolume of water. Since the condenser is filledwith air when the plant is cold, and since someair finds its way into the condenser during thecourse of plant operation, the condensation ofsteam is not sufficient to establish the initial vacuumnor to maintain the required vacuum underall conditions. In modern shipboard steam plants,air ejectors are used to remove air and othernoncondensable gases from the condenser. Thecondensation of steam is thus the major causeof the vacuum, but the air ejectors are requiredto help establish the initial vacuum and then toassist in maintaining vacuum while the plant isoperating.348