Chapter 10- PROPULSION BOILERS 



the steam drum. Heat transfer takes place from 

 the steam in the desuperheater to the water in 

 the steam drum. The desuperheated steam which 

 passes out of the desuperheater and into the 

 auxiliary steam line is once again at (or very 

 close to) saturation temperature. 



Thus far we have considered the flow of 

 steam as it occurs after the boiler has been cut 

 in on the steam line. But what happens when a 

 cold boiler is lighted off? How can the super- 

 heater tubes be protected from the heat of the 

 furnace after fires are lighted but before suffi- 

 cient steam has been generated to ensure a safe 

 flow through the superheater? 



Various methods are used to protect the 

 superheater during this critical period imme- 

 diately after lighting off. Very low firing rates 

 are used, and the boiler is warmed up slowly 

 until an adequate flow of steam has been estab- 

 lished. Many— but not all— boQers of this type 

 have connections through which protective steam 

 can be supplied from another boiler on the same 

 ship or from some outside source such as a 

 naval shipyard or a tender. As shown in figure 

 10-20, this steam comes in (under pressure) 

 through the superheater protection steam valve. 

 It enters the superheater inlet, passes through 

 the superheater tubes, goes out the superheater 

 outlet, passes through the desuperheater, and 

 then goes into the auxiliary exhaust line by way 

 of the superheater protection exhaust valve. 



On single-furnace boilers which do not have 

 a protective steam system for use during the 

 lighting off period, even greater care must be 

 taken to establish a steam flow through the 

 superheater. In general, the steam flow is es- 

 tablished by venting the superheater drains to 

 the bilges while warming up the boiler very 

 slowly. 



On the basis of the classification methods 

 given earlier in this chapter, we may consider 

 this older single-furnace boiler as one which 

 has the following characteristics: It is a water- 

 tube boiler with natural circulation of the ac- 

 celerated type. It is a drum-type (rather than a 

 header-type) boiler. It has tubes which are ar- 

 ranged roughly in the shape of the letter D— 

 hence it is often called a D-type boiler. It has 

 only one furnace. It does not have controlled 

 superheat. It is often classified as a "600-psi 

 boiler," although it actually operates at about 

 435 psig. 



As previously noted, the degree of superheat 

 obtained in a single-furnace boiler of the type 

 being considered is primarily dependent upon 



the firing rate. However, a number of design 

 features and operational considerations also 

 affect the temperature of the steam at the super- 

 heater outlet. 



Design features that affect the degree of 

 superheat include (1) the type of superheater 

 installed— that is, whether heated by convection, 

 by radiation, or by both; (2) the location of the 

 superheater with respect to the burners; (3) the 

 extent to which the superheater is protected by 

 water screen tubes; (4) the area of superheater 

 heat-transfer surface; (5) the number of passes 

 made by the steam in going through the super- 

 heater; (6) the location of gas baffles; and (7) the 

 volume and shape of the furnace. 



Operational factors that affect the degree of 

 superheat include (1) the rate of combustion; 

 (2) the temperature of the feed water; (3) the 

 amount of excess air passing through the furnace; 

 (4) the amount of moisture contained in the steam 

 entering the superheater; (5) the condition of the 

 superheater tube surfaces; and (6) the condition 

 of the water screen tube surfaces. Since these 

 factors may affect the degree of superheat in 

 ways which are not immediately apparent, let us 

 examine them in more detail. 



How does the rate of combustion affect the 

 degree of superheat? To begin with, we might 

 imagine a simple relationship in which the degree 

 of superheat goes up directly as the rate of com- 

 bustion is increased. Such a simple relationship 

 does, in fact, exist— but only up to a certain 

 point. Throughout most of the operating range 

 of this boiler, the degree of superheat goes up 

 quite steadily and regularly as the rate of com- 

 bustion goes up. Near full power, however, the 

 degree of superheat drops slightly even though 

 the rate of combustion is still going up. Why 

 does this happen? Primarily because the in- 

 creased firing rate results in an increased 

 generating rate, which in turn results in an 

 increased steam flow through the superheater. 

 The rate of heat absorption increases more 

 rapidly than the rate of steam flow until the 

 boiler is operating at very nearly full power; 

 at this point the rate of steam flow increases 

 more rapidly than the rate of heat absorption. 

 Therefore the superheater outlet temperature 

 drops slightly. 



Suppose that the boiler is being fired at a 

 constant rate and that the steam is being used 

 at a constant rate. K we increase the tempera- 

 ture of the incoming feed water, what happens to 

 the superheat? Does it increase, decrease, or 

 remain the same? Surprisingly, the degree of 



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