INFORMATION SHEET

COMBUSTION AIR SYSTEM
Information Sheet Number 62B-204-I
 
 
 INTRODUCTION
 
 The combustion air system provides the proper amount of air which combines with the fuel to achieve optimum combustion.  Combustion air systems are similar from steam ship to steam ship except for prime movers of the blower fans (some are electric motor driven and some are steam turbine driven).
 
 REFERENCES
 
 a. Principles of Naval Engineering  NAVPERS 10788 series
 b. Forced-Draft Blowers  NSTM Chapter 554
 c. Boilers  NSTM Chapter 221
 
 INFORMATION
 
A. The function of the combustion air system is to supply the large amounts of air necessary for the complete combustion of the fuel oil.  Most ships have two high-capacity, turbine driven, axial flow, forced-draft blowers that supply each boiler during normal operations.  Most ships also have one electric motor driven light-off blower for each boiler to provide air for light-off when steam is not available (Refer to figure 1).
 
 
 COMBUSTION AIR FLOW
 

 
B. Depending on the ship's design the main forced-draft blowers (FDBs) may be steam turbine driven or electric motor driven.  The steam turbine driven FDB may be horizontally or vertically mounted.  Air is drawn in from the uptake space and discharged through ducting to the boiler air casing.  FDB air output is controlled by the automatic combustion control (ACC) system.
 
1. To control the amount of air from an FDB, we can either vary the speed of the prime mover or install moveable radial vanes or dampers at the blower's inlet or outlet.
 
2. Steam turbine and variable speed electric motor FDBs change blower speed to control air output.
 
3. Single and multiple constant speed FDBs are motor driven and use a combination of radial vanes or dampers at either the inlet or the outlet to control air output.  The multiple constant speed FDB has better efficiency at low speeds and low power.
 
C. Some ships are designed with one electric light-off FDB per boiler.  The electric FDB takes its suction from the uptakes and discharges through ducting to the boiler air casing at approximately 4.5" H2O with all air registers closed.  Boiler air casing pressure is measured in inches of water which is the amount of air pressure required to support a column of water of so many inches high.  A single, manual butterfly-type shutter on the discharge side isolates the light-off FDB from the combustion air system when the blower is secured.  When shifting from a motor driven FDB to a turbine driven FDB, close and lock the butterfly valve (shutter) prior to stopping the electric motor, or windmilling and eventual disintegration of the motor driven blower may result.
 
D. Windmilling is the backward rotation of the FDB fan caused by air from another FDB back-flowing through the idle blower from the discharge side.  Windmilling is catastrophic to idle turbine driven FDBs as the turbine bearings have no oil flow.
 
E. Windmilling the electric light-off blower would cause the motor rotor to overspeed and lead to disintegration of the unit.  A phenomena similar to windmilling is stalling.  If one FDB is delivering more air than the other, the air imbalance from the blower delivering the higher output will tend to block the air discharge from the other.  If the imbalance is severe enough, the lower output blower can stall due to lack of air flow.  For this reason, FDB speed control elements are set to maintain FDBs within 300 rpm to prevent stalling.
 
F. A turbine-driven FDB uses auxiliary steam to power a single-stage steam turbine. A combined exhaust relief valve protects the turbine casing against over-pressurization.  Two labyrinth glands prevent the escape of steam where the turbine rotor shaft penetrates the casing.  Turbine gland leak-off is removed by the auxiliary gland exhaust system.
 
1. Turbine speed is regulated by a governor which positions the turbine steam admission valve to control the amount of steam entering the turbine.  Blower speed is controlled:
 
2. By the automatic combustion control system in the automatic mode, or
 
3. By the console operator at the boiler control console in the remote manual mode, or
 
4. By the operator directly manipulating the governor speed adjusting knob or steam throttle valve in the local manual mode.
 
G. The vane axial fan, or compressor, is a two-stage assembly with dynamically balanced stainless steel propeller blades attached to the FDB main shaft.  Two stationary blade rings or guide vanes are installed, one between the two propellers and one after the second row of propellers.  These guide vanes direct the air flow and minimize rotation and turbulence of the air between the propellers and the discharge air system ducting (refer to figure 2).
 
 FORCED DRAFT BLOWER

 
 
 
H. Counter-balanced, automatic return shutters are located in the air discharge ducting between the FDB and the boiler air casing.  These shutters close automatically when a FDB is secured to prevent the FDB from windmilling when  idle.
 
I. The area between the boiler's inner air casing and outer air casing forms the boiler air casing and serves as the ducting to direct the combustion air to the air registers.  Air registers admit and direct the combustion air from the air casing to the furnace in such a way that it properly mixes with the fuel for proper combustion.  This design allows incoming combustion air to be preheated prior to entering the furnace.  The combustion air also carries away heat from the inner casing which would otherwise be transferred to the outer casing and contribute to high heat in the fireroom and a burn hazard to personnel.  Boiler air casing leaks, no matter how slight, decrease the efficiency of the boiler and add to a high heat environment.  Air casing pressure is measured with a diaphragm-type gauge (boiler draft gage) which is calibrated in inches of water pressure for increased accuracy (27.7"H2O = 1 psig).
 
J. Before lighting fires in any propulsion boiler, the boiler must be purged.  Purging removes any potentially combustible vapors in the furnace before the light-off torch is inserted.  A proper purge must change the furnace air five times (five volumetric changes).  Obviously, the time required to complete the purge depends on the amount of air provided.  Every boiler has a purge table which converts windbox pressure, in inches of water, into purge time.  figure 3 is a sample purge table.
 
 BOILER PURGE TABLE FOR LHA 1-5
 
 BEFORE LIGHTING FIRES PURGE THE BOILER
 WITH ALL BURNER AIR REGISTERS WIDE OPEN
 IN ACCORDANCE WITH ONE OF THE FOLLOWING
 

 
1. NOTES:
 
2.  If an air pressure between listed values is used, select the time required for next lower air pressure listed.
 
3.  The guidance provided in the boiler air purge table assumes the presence of an explosive mixture of hydrocarbon vapors in the boiler, air casing, uptake, and stack at the beginning of the purge.  No allowance is made for fuel accumulation or pools of fuel in the furnace. In every instance, if inspection of the furnace indicates oil accumulation, DO NOT LIGHT FIRES until the fuel has been wiped up and all traces removed. Use the purge times prior to initial light-off and prior to relighting fires after flame failure and after securing all burners.
 
4.  If doubt exists, inspect the furnace for unburned fuel when ignition is not obtained within three seconds during light-off or after any uncontrolled loss of fires.

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