PRINCIPLES OF NAVAL ENGINEERING 



superheat decreases if the feed temperature is 

 increased, more saturated steam is generated 

 from the burning of the same amount of fuel. 

 The increased quantity of saturated steam causes 

 an increase in the rate of flow through the super- 

 heater. Since there is no increase in the amount 

 of heat available for transfer to the superheater, 

 the degree of superheat drops slightly. 



Under conditions of constant load and a con- 

 stant rate of combustion, what happens to the 

 superheat if the amount of excess air'* is in- 

 creased? To see why an increase in excess air 

 results in an increase in temperature at the 

 superheater outlet, we must take it step by step: 



1. An increase in excess air decreases the 

 average temperature in the furnace. 



2. With the furnace temperature lowered, 

 there is less temperature difference between the 

 gases of combustion and the water in the boiler 

 tubes. 



3. Because of the smaller temperature dif- 

 ference, the rate of heat transfer is reduced. 



4. Because of the decreased rate of heat 

 transfer, the evaporation rate is reduced. 



5. The lower evaporation rate causes a 

 reduction in the rate of steam flow through the 

 superheater, with a consequent rise in the super- 

 heater outlet temperature. 



In addition to this series of events, another 

 factor also tends to increase the superheater 

 outlet temperature when the amount of excess 

 air is increased. Large amounts of excess air 

 tend to cause combustion to occur in the tube 

 bank rather than in the furnace itself; as a result, 

 the temperature in the area around the super- 

 heater tubes is higher than usual and the super- 

 heater outlet temperature is higher. 



Any appreciable amount of moisture in the 

 steam entering the superheater causes a very 

 noticeable drop in superheat. This occurs be- 

 cause steam cannot be superheated as long as 

 it is in contact with the water from which it is 

 being generated. If moisture enters the super- 

 heater, therefore, a good deal of heat must be 

 used to dry the steam before the temperature 

 of the steam can rise. 



The condition of the superheater tube sur- 

 faces has an important effect on superheater 



The term "excess air" is used to indicate any quan- 

 tity of combustion air in excess of that which is 

 theoretically required for the complete combustion of 

 the fuel. Some excess air is necessary for efficient 

 combustion, but too much excess air is wasteful, as 

 discussed in a later section of this chapter. 



outlet temperature, K the tubes have soot on the 

 outside or scale on the inside, heat transfer will 

 be retarded and the degree of superheat will be 

 decreased. 



If the water screen tubes have soot on the 

 outside or scale on the inside, heat transfer to 

 the water in these tubes will be retarded. There- 

 fore there will be more heat available for 

 transfer to the superheater as the gases of 

 combustion flow through the tube bank. Conse- 

 quently, the superheater outlet temperature will 

 rise. 



The single-furnace boiler is lighter and 

 smaller, for any given output of steam, than the 

 double-furnace boiler. Because the single- 

 furnace boiler supplies superheated steam at 

 low steaming rates, the overall plant efficiency 

 is better with this type of boiler than with the 

 double-furnace boiler. The single-furnace boiler 

 has the further advantage of simplicity of oper- 

 ation and maintenance. Although the single- 

 furnace boiler considered here does not have 

 controlled superheat, this lack is less important 

 than might have been supposed, since some of 

 the theoretical advantages of controlled super- 

 heat have not been entirely realized in practice. 



The basic design of the single-furnace boiler 

 has been used increasingly. Except for experi- 

 mental boilers, no double-furnace boilers have 

 been installed on combatant ships since World 

 War II. The newer single-furnace boilers operate 

 at approximately 600 psi or at approximately 

 1200 psi. Operating temperature at the super- 

 heater outlet is quite commonly 950° F for the 

 1200-psi boilers; this is 100° F higher than the 

 operating temperature of most double-furnace 

 boilers, and 200° F higher than the operating 

 temperature of the older single-furnace boilers. 



One of the most noticeable differences be- 

 tween the older and the newer single-furnace 

 boilers is the change in furnace design. Higher 

 heat release rates are possible in the newer 

 boilers. Although these newer single-furnace 

 boilers are not the type that we refer to as 

 "pressurized-furnace" boilers, they do often 

 use a slightly higher combustion air pressure 

 than the older single-furnace boilers. The use of 

 higher air pressure causes an increase in the 

 velocity of the combustion gases, and the in- 

 creased velocity results in a higher rate of heat 

 transfer to the generating tubes. Because of the 

 increased heat release rates, a newer single- 

 furnace boiler is likely to have a water-cooled 

 roof and water-cooled rear walls as well as 

 water-cooled side walls. 



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