PRINCIPLES OF NAVAL ENGINEERING 



3o Do not attempt to operate a pump while 

 either the speed limiting governor or the constant 

 pressure governor is inoperable. Be sure that the 

 speed limiting governor and the constant pres- 

 sure governor are properly set. 



4. Do not use any boiler feed system pump 

 for any service other than boiler or feed water 

 service, except in an emergency. 



FORCED DRAFT BLOWERS 



On most steam-driven surface ships, forced 

 draft blowers are used to furnish the large 

 amount of combustion air required for the burn- 

 ing of the fuel oil. A forced draft blower is es- 

 sentially a very large fan, fastened to a shaft and 

 housed in a metal casing. As a rule, two blowers 

 are furnished for each boiler; they are synchro- 

 nized for equal distribution of load. 



Most forced draft blowers are driven by 

 steam turbines. However, some blowers for in- 

 port use and some main blowers on auxiliary 

 ships are driven by electric motors. Most tur- 

 bine-driven blowers are direct drive, rather than 

 geared; but some geared turbine drives are used. 



On most ships, the forced draft blowers take 

 suction from the space between the inner and 

 outer stack casings and discharge slightly pre- 

 heated air into a duct that leads to the space be- 

 tween the inner and outer casings of the boiler. 



Types of Forced 

 Draft Blowers 



Two main types of forced draft blowers are 

 used in naval ships: centrifugal blowers and pro- 

 peller blowers. The main difference between the 

 two types is in the direction of airflow. The cen- 

 trifugal blower takes air in axially at the center 

 of the fan and discharges it tangentially off the 

 outer edge of the blades. The propeller blower 

 moves air axially— that is, it propels the air 

 straight ahead in a direction parallel to the axis 

 of the shaft. Most forced draft blowers now in 

 naval use are of the propeller type. However, 

 some older ships and some recent auxiliaries 

 have centrifugal blowers. 



Centrifugal blowers may be either vertical 

 or horizontal. In either case, the unit consists of 

 the driving turbine (or other driving unit) atone 

 end of the shaft and the centrifugal fanwheel at 

 the other end of the shaft. Inlet trunks and dif- 

 fusers are fitted around the blower fanwheel to 

 direct air into the fanwheel and to receive and 

 discharge air from the fan. Centrifugal blowers 



are fitted with flaps in the suction ducts. In the 

 event of a casualty to one centrifugal blower, air 

 from another blower blows back toward the 

 damaged blower and closes the flaps. 



Both horizontal and vertical propeller 

 blowers are used in naval combatant ships. In 

 general, single-stage horizontal blowers are 

 used on older ships and two- stage or three- stage 

 vertical blowers on ships built since 'VorldWar 

 II. 



Balanced automatic shutters are installed in 

 the discharge ducts between each propeller 

 blower and the boiler casings. These shutters are 

 locked in the closed position whenever the blower 

 is taken out of service so that the blower will not 

 be rotated in reverse. 



Figure 15-32 and 15-33 show two views of a 

 single-stage horizontal propeller-type blower. 

 As may be seen, the blower is a complete unit 

 consisting of a driving turbine and a propeller- 

 type fan. The entire unit is mounted on a single 

 bed plate. 



The air intake is screened to prevent the en- 

 trance of foreign objects. The blower casing 



38.111X 

 Figure 15-32.— Horizontal propeller-type blower 

 (view 1). 



414 



