PRINCIPLES OF NAVAL ENGINEERING 



the floor, front wall, side walls, and rear wall 

 with refractory material. The refractory lining 

 serves to protect the furnace casing and to pre- 

 vent loss of heat from the furnace. Refractories 

 retain heat for a relatively long time and thus 

 help to maintain the high furnace temperatures 

 required for complete and efficient combustion 

 of the fuel. Refractories are also used to form 

 baffles which direct the flow of combustion 

 gases and protect drums, headers, and tubes 

 from excessive heat. 



There are many different kinds of refractory 

 materials. The particular use of each type is 

 determined by the chemical and physical char- 

 acteristics of the material in relation to the 

 required conditions of service. Refractories 

 commonly used in the furnaces of naval pro- 

 pulsion boilers include firebrick, insulating 

 brick, insulating block, plastic fireclay, plastic 

 chrome ore, chrome castable refractory, high 

 temperature castable refractory, air-setting 

 mortar, and burner refractory tile. 



Casings, Uptakes, and Smokepipes 



In modern boiler installations, each boiler 

 is enclosed in two steel casings. The inner cas- 

 ing is lined with refractory materials, and the 

 enclosed space constitutes the furnace. The 

 outer casing extends around most of the inner 

 casing, with an air space in between. Air from 

 the forced draft blowers is forced into the space 

 between the inner and the outer casings, and 

 from there it flows through the air registers 

 and into the furnace. 



The inner casing encloses most of the boiler 

 up to the uptakes. The uptakes join the boiler to 

 the smokepipe. As a rule, the uptakes from two 

 or more boilers connect with one smokepipe. 



Both the inner and the outer casings of boil- 

 ers are made of steel panels. The panels may 

 be flanged and bolted together, with gaskets 

 being used at the joints to make an airtight 

 seal, or they may be welded together. The cas- 

 ings are made in small sections so they can be 

 removed for the inspection and repair of boiler 

 parts. 



Saddles and Supports 



Each water drum and water header rests 

 upon two saddles, one at the front of the drum 

 or header and one at the rear. The upper flanges 

 of the saddle are curved to fit the curvature of 

 the drum or header, and are welded to the drum 



or header. The bottom flanges, which are flat, 

 rest on huge beams built up from the ship's 

 structure. The bottom flange of one saddle is 

 bolted rigidly to its support. The bottom flange 

 of the other saddle is also bolted to its support, 

 but the bolt holes are elongated in a fore-and- 

 aft direction. As the drum expands or contracts 

 because of temperature changes, the saddle 

 which is not rigidly fastened to the support 

 accommodates to the changing length of the 

 drum by sliding backward or forward over the 

 support. The flanges which are not rigidly fas- 

 tened are known as boiler sliding feet . 



Airheaters 



Some boilers of recent design have steam- 

 coil airheaters to preheat the combustion air 

 before it enters the furnace. A typical steam- 

 coil airheater consists of two coil blocks, each 

 coil block having three sections of heating coils 

 in a single casing. Each individual section has 

 rows of copper-nickel alloy tubes, helically 

 wound with copper fins. Airheaters used in the 

 past on some older naval ships were installed 

 in the uptakes and the combustion air was pre- 

 heated by the combustion gases; these airheaters 

 thus utilized heat which would otherwise have 

 been wasted. The use of these older airheaters 

 was discontinued in naval ships because the sav- 

 ing of heat was not considered sufficient to jus- 

 tify the added space and weight requirements. 

 The new steam-coil airheaters use auxiliary 

 exhaust steam as the heating agent; they are in- 

 stalled near the point where the combustion air 

 enters the double casing. 



Fittings, Instruments, and Controls 



The major boiler components just described 

 could not function without a number of fittings, 

 instruments, and control devices. These addi- 

 tional boiler parts are merely mentioned here 

 for the sake of completeness; they are taken up 

 in detail in chapter 11 of this text. 



Internal fittings installed in the steam drum 

 may include equipment for distributing the in- 

 coming feed water, for separating and drying the 

 steam, for giving surface blows to remove solid 

 matter from the water, for directing the flow of 

 steam and water within the steam drum, and for 

 injecting chemicals for boiler water treatment. 

 In addition, many boilers have desuperheaters 

 for desuperheating the steam needed for auxil- 

 iary purposes. 



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