Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGsuperheat decreases if the feed temperature isincreased, more saturated steam is generatedfrom the burning of the same amount of fuel.The increased quantity of saturated steam causesan increase in the rate of flow through the superheater.Since there is no increase in the amountof heat available for transfer to the superheater,the degree of superheat drops slightly.Under conditions of constant load and a constantrate of combustion, what happens to thesuperheat if the amount of excess air'* is increased?To see why an increase in excess airresults in an increase in temperature at thesuperheater outlet, we must take it step by step:1. An increase in excess air decreases theaverage temperature in the furnace.2. With the furnace temperature lowered,there is less temperature difference between thegases of combustion and the water in the boilertubes.3. Because of the smaller temperature difference,the rate of heat transfer is reduced.4. Because of the decreased rate of heattransfer, the evaporation rate is reduced.5. The lower evaporation rate causes areduction in the rate of steam flow through thesuperheater, with a consequent rise in the superheateroutlet temperature.In addition to this series of events, anotherfactor also tends to increase the superheateroutlet temperature when the amount of excessair is increased. Large amounts of excess airtend to cause combustion to occur in the tubebank rather than in the furnace itself; as a result,the temperature in the area around the superheatertubes is higher than usual and the superheateroutlet temperature is higher.Any appreciable amount of moisture in thesteam entering the superheater causes a verynoticeable drop in superheat. This occurs becausesteam cannot be superheated as long asit is in contact with the water from which it isbeing generated. If moisture enters the superheater,therefore, a good deal of heat must beused to dry the steam before the temperatureof the steam can rise.The condition of the superheater tube surfaceshas an important effect on superheaterThe term "excess air" is used to indicate any quantityof combustion air in excess of that which istheoretically required for the complete combustion ofthe fuel. Some excess air is necessary for efficientcombustion, but too much excess air is wasteful, asdiscussed in a later section of this chapter.254outlet temperature, K the tubes have soot on theoutside or scale on the inside, heat transfer willbe retarded and the degree of superheat will bedecreased.If the water screen tubes have soot on theoutside or scale on the inside, heat transfer tothe water in these tubes will be retarded. Thereforethere will be more heat available fortransfer to the superheater as the gases ofcombustion flow through the tube bank. Consequently,the superheater outlet temperature willrise.The single-furnace boiler is lighter andsmaller, for any given output of steam, than thedouble-furnace boiler. Because the singlefurnaceboiler supplies superheated steam atlow steaming rates, the overall plant efficiencyis better with this type of boiler than with thedouble-furnace boiler. The single-furnace boilerhas the further advantage of simplicity of operationand maintenance. Although the singlefurnaceboiler considered here does not havecontrolled superheat, this lack is less importantthan might have been supposed, since some ofthe theoretical advantages of controlled superheathave not been entirely realized in practice.The basic design of the single-furnace boilerhas been used increasingly. Except for experimentalboilers, no double-furnace boilers havebeen installed on combatant ships since WorldWar II. The newer single-furnace boilers operateat approximately 600 psi or at approximately1200 psi. Operating temperature at the superheateroutlet is quite commonly 950° F for the1200-psi boilers; this is 100° F higher than theoperating temperature of most double-furnaceboilers, and 200° F higher than the operatingtemperature of the older single-furnace boilers.One of the most noticeable differences betweenthe older and the newer single-furnaceboilers is the change in furnace design. Higherheat release rates are possible in the newerboilers. Although these newer single-furnaceboilers are not the type that we refer to as"pressurized-furnace" boilers, they do oftenuse a slightly higher combustion air pressurethan the older single-furnace boilers. The use ofhigher air pressure causes an increase in thevelocity of the combustion gases, and the increasedvelocity results in a higher rate of heattransfer to the generating tubes. Because of theincreased heat release rates, a newer singlefurnaceboiler is likely to have a water-cooledroof and water-cooled rear walls as well aswater-cooled side walls.