Chapter 10- PROPULSION BOILERS 



Most double-furnace boilers are designed to 

 carry a superheater outlet temperature of 850° 

 F; this is about 100° F higher than the super- 

 heater outlet temperature in a comparable 

 single-furnace boiler, given the same quality of 

 materials for boilers, piping, and turbines. The 

 reason why a higher superheater outlet tem- 

 perature can be used in a double-furnace boiler 

 than in a comparable single-furnace boiler is that 

 allowance must be made, in the single-furnace 

 boiler, for the maximum superheater tempera- 

 tures which might occur under adverse condi- 

 tions of load. 



In spite of the advantages resulting from the 

 control of superheat, double-furnace boilers are 

 no longer being installed in naval combatant 

 ships. Experience with these boilers has re- 

 vealed certain disadvantages which at the present 

 time appear to outweigh the advantages of con- 

 trolled superheat. Some of the disadvantages are: 



1. In practice, it is not possible to maintain 

 maximum designed superheat at low steaming 

 rates. Only the steam for the main turbines and 

 the turbogenerators goes through the super- 

 heater; at low firing rates, therefore, the steam 

 flow through the superheater is generally not suf- 

 ficient to permit a high firing rate on the super- 

 heater side. Thus under some conditions the 

 steam supplied to the propulsion turbines and to 

 the turbogenerators may be saturated or only 

 very slightly superheated. As a consequence, 

 therefore, the double-furnace boiler is actually 

 less efficient than the single-furnace boiler at 

 low firing rates. 



2. The double-furnace boiler is more diffi- 

 cult to operate than the single-furnace boiler, 

 and requires more personnel for its operation. 

 Once there is any appreciable load on the boiler, 

 the high air pressure in the double casings and in 

 the furnace make it difficult and even dangerous 

 to light burners on the superheater side. In order 

 to avoid this difficulty, operating personnel 

 would have to be able to predict the need for 

 superheat and light off the burners on the super- 

 heater side before the air pressure had become 

 so high. Obviously, such prediction is not al- 

 ways possible. 



3. The double-furnace boiler is heavier, 

 larger, and generally more complex than a 

 single-furnace boiler of equal capacity. 



Single- Furnace Boilers 



The older single-furnace boilers that were 

 installed on many World War II ships differ in 



several important respects from the newer 

 single-furnace boilers that have been installed 

 on ships built since World War II. 



A single-furnace boiler of the older type 

 is shown schematically in figure 10-20 and in 

 cutaway view in figure 10-21. This boiler pro- 

 duces about 60,000 pounds of steam per hour at 

 full power. At full power the steam drum pres- 

 sure is about 460 psig, the superheater outlet 

 pressure is about 435 psig, and the superheater 

 outlet temperature is about 750° F. 



ECONOMIZER 

 \ 



SUPERHEATER 

 PROTECTION 

 EXHAUST VALVE 



SUPERHEATER 



PROTECTION 



STEAM VALVE 



MAIN STEAM 



Figure 10-20.— General arrangement 

 single-furnace boiler. 



of 



38.39 

 older 



This boiler does not have controlled super- 

 heat. When the boiler is lighted off, both the 

 generating tubes and the superheater tubes are 

 heated. In order to protect the superheater 

 tubes from overheating, all steam generated in 

 the boiler must be led through the superheater. 

 The saturated steam goes from the dry pipe in 

 the steam drum to the superheater inlet; it goes 

 through the superheater tubes, out the super- 

 heater outlet, and into the main steam line. 



Auxiliary steam must go through the super- 

 heater (in order to provide a sufficient steam 

 flow to protect the superheater) but must then 

 be desuperheated. Desuperheating is accom- 

 plished by passing some of the superheated 

 steam through a desuperheater, which is basi- 

 cally a coil of piping submerged in the water in 



251 



