Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERING3o Do not attempt to operate a pump whileeither the speed limiting governor or the constantpressure governor is inoperable. Be sure that thespeed limiting governor and the constant pressuregovernor are properly set.4. Do not use any boiler feed system pumpfor any service other than boiler or feed waterservice, except in an emergency.FORCED DRAFT BLOWERSOn most steam-driven surface ships, forceddraft blowers are used to furnish the largeamount of combustion air required for the burningof the fuel oil. A forced draft blower is essentiallya very large fan, fastened to a shaft andhoused in a metal casing. As a rule, two blowersare furnished for each boiler; they are synchronizedfor equal distribution of load.Most forced draft blowers are driven bysteam turbines. However, some blowers for inportuse and some main blowers on auxiliaryships are driven by electric motors. Most turbine-drivenblowers are direct drive, rather thangeared; but some geared turbine drives are used.On most ships, the forced draft blowers takesuction from the space between the inner andouter stack casings and discharge slightly preheatedair into a duct that leads to the space betweenthe inner and outer casings of the boiler.are fitted with flaps in the suction ducts. In theevent of a casualty to one centrifugal blower, airfrom another blower blows back toward thedamaged blower and closes the flaps.Both horizontal and vertical propellerblowers are used in naval combatant ships. Ingeneral, single-stage horizontal blowers areused on older ships and two- stage or three- stagevertical blowers on ships built since 'VorldWarII.Balanced automatic shutters are installed inthe discharge ducts between each propellerblower and the boiler casings. These shutters arelocked in the closed position whenever the bloweris taken out of service so that the blower will notbe rotated in reverse.Figure 15-32 and 15-33 show two views of asingle-stage horizontal propeller-type blower.As may be seen, the blower is a complete unitconsisting of a driving turbine and a propellertypefan. The entire unit is mounted on a singlebed plate.The air intake is screened to prevent the entranceof foreign objects. The blower casingTypes of ForcedDraft BlowersTwo main types of forced draft blowers areused in naval ships: centrifugal blowers and propellerblowers. The main difference between thetwo types is in the direction of airflow. The centrifugalblower takes air in axially at the centerof the fan and discharges it tangentially off theouter edge of the blades. The propeller blowermoves air axially— that is, it propels the airstraight ahead in a direction parallel to the axisof the shaft. Most forced draft blowers now innaval use are of the propeller type. However,some older ships and some recent auxiliarieshave centrifugal blowers.Centrifugal blowers may be either verticalor horizontal. In either case, the unit consists ofthe driving turbine (or other driving unit) atoneend of the shaft and the centrifugal fanwheel atthe other end of the shaft. Inlet trunks and diffusersare fitted around the blower fanwheel todirect air into the fanwheel and to receive anddischarge air from the fan. Centrifugal blowers38.111XFigure 15-32.— Horizontal propeller-type blower(view 1).414