PRINCIPLES OF NAVAL ENGINEERING 



COOL WATER 

 FLOWS DOWN 



MIXTURE OF 

 HOT WATER 

 AND STEAM 

 FLOWS UP 



HOT 

 GASES 



139.17 

 Figure 10-2.— Natural circulation 

 (accelerated type). 



air pressure used in the furnace. Most boilers 

 now in use in naval propulsion plants operate 

 with a slight air pressure (seldom over lOpsig) 

 in the boiler furnace. This slight pressure, which 

 results from the use of forced draft blowers to 

 supply combustion air to the boilers, is not suf- 

 ficient to warrant calling these boilers "pres- 

 surized-furnace boilers." However, a new type 

 of boiler has recently appeared on the scene and 

 is being installed in some ships. This new boiler 

 is truly a pressurized-furnace boiler, since the 

 furnace is maintained under a positive air pres- 

 sure of approximately 65 psia (about 50 psig) 

 when the boiler is operating at full power. The 

 air pressure in the furnace is maintained by a 

 special air compressor. Hence we must now 

 make a distinction between this new pressurized- 

 furnace boiler, on the one hand, and all other 

 naval propulsion boilers, on the other hand, with 

 respect to the pressure maintained in the fur- 

 nace. 



Type of Superheater 



On almost all boilers currently used in the 

 propulsion plants of naval ships, the superheater 

 tubes are protected from radiant heat by water 

 screen tubes. The water screen tubes absorb 

 the intense radiant heat of the furnace, and the 

 superheater tubes are heated by convection 

 currents rather than by radiation. Hence, the 

 superheaters are referred to as convection-type 

 superheaters. 



On a few older ships, the superheater tubes 

 are not screened by water screen tubes but are 



exposed directly to the radiant heat of the fur- 

 nace. Superheaters of this kind are called 

 radiant-type superheaters. Although radiant- 

 type superheaters are rarely used at present, it 

 is possible that they may come into use again in 

 future boiler designs. 



Control of Superheat 



A boiler which provides some means of con- 

 trolling the degree of superheat independently of 

 the rate of steam generation is said to have 

 controlled superheat . A boiler in which such 

 separate control is not possible is said to have 

 uncontrolled superheat . 



Until recently, the term superheat control 

 boiler was used to identify a double-furnace 

 boiler and the term uncontrolled superheat 

 boiler (or no control superheat boiler ) was used 

 to identify a single-furnace boiler. Most double- 

 furnace boilers now in use do, in fact, have 

 controlled superheat, and most single-furnace 

 boilers do not have controlled superheat. How- 

 ever, recent developments in boiler design make 

 superheat control independent of the number of 

 furnaces in the boiler. Single-furnace boilers 

 WITH controlled superheat and double-furnace 

 boilers WITHOUT controlled superheat are both 

 possible. The time has come, therefore, to stop 

 relating the number of furnaces in a boiler to 

 the control (or lack of control) of superheat. 



Operating Pressure 



For some purposes it is convenient to clas- 

 sify boilers according to operating pressure. 

 Most classifications of this type are approximate 

 rather than exact. Header-type boilers and some 

 older drum-type boilers are often called "400- 

 psi boilers" even though the operating pressures 

 may range from 300 psi (or even lower) to about 

 450 psi. The term "600-psi boiler" is often 

 applied to various double-furnace and single- 

 furnace boilers with operating pressures ranging 

 from about 435 psi to about 700 psi. 



The term "high pressure boiler" is at pres- 

 ent used rather loosely to identify any boiler 

 that operates at substantially higher pressure 

 than the so-called "600-psi boilers." In gen- 

 eral, we will consider any boiler that operates 

 at 751 psi or above as a high pressure boiler. 

 A good many boilers recently installed on naval 

 ships operate at approximately 1200 psi; for 

 some purposes, it is convenient to group these 

 boilers together and refer to them as "1200-psi 

 boilers." 



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