A. Air system components and function (See Figure 1 )
1. The smoke stack as viewed from outside the ship, is actually an indirect contact, counter flow heat exchanger in disguise. It is comprised of two parts, i.e. the inner and outer stacks.
a. The outer stack lends structural support to the inner stack and provides for the passage of cooler outside air to the forced draft blowers. In addition to this, it also protects the topside personnel from the hot surfaces of the inner stack as well as helping to reduce the ship's Infra Red (IR) signature. The louvers on this structure allow the air to enter and help keep moisture and debris out.
b. The uptake space is the area between the inner and outer stack where the FDB's take their suction. Access to this space on most ships is commonly made from the main deck. Uptake spaces also receive hot air that is discharged from the firerooms exhaust ventilation. This additional source of air adds to the economical operation of the boiler.
c. The inner stack provides a path for the hot combustion gases to exhaust to the atmosphere. Heat from the inner stack and uptake space preheats the cooler incoming air, thus reducing the temperature on the inner stack and providing a small amount of preheat to the combustion air. Because the stacks are subjected to harsh conditions such as corrosion and thermal expansion, they are thoroughly inspected as part of the routine 18 month boiler inspection.
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Air and Combustion Gas Path
Figure 1
2. Forced draft blowers are designed to supply combustion air at the required volume to each boiler, drawing air in from louvers in the outer stack and discharging air through ducts that are connected to the airtight casing around and under the boiler.
a. The electric light-off fan (blower) on ships so equipped, shall be used in all normal (non-emergency) light-off's until steam from the boiler being lighted off is available to drive the steam driven blowers"( IAW REF. C, para. 4-4.2). On most ships (with certain exceptions), each boiler is equipped with its own light-off blower. This machine is preferred for light-off as the air supplied is of a lower volume than that of the main FDBs. The lower the pressure, the lower the volume (CFM) will be at the air casing windbox. With lower pressure, the velocity will be lower as the air flows through the air register assembly.
This is preferred as it ensures against excess air that would inhibit stable combustion and the potential for explosive conditions to occur such as possible white smoke and/or loss of boiler fires.
3. The electric light-off blower has a manually operated butterfly valve (FLAP) which must be locked closed when the main FDB is placed in operation. If the flap is not closed and the light-off blower secured after the main blower (particularly steam driven) is placed in operation, the electric blower may abruptly reverse direction and over-speed causing the fan assembly to disintegrate and the motor windings to burn. Aside from the material damage which can be incurred, personnel safety is paramount. Personnel in the vicinity during this occurrence could be struck with the resulting shrapnel.
a. Some ships such as CV-67 have no light-off blowers. In this type of configuration, the first boiler is lit off by setting Zebra on the space. Supply ventilation is secured and exhaust ventilation placed on high. This causes pressure in the uptakes (14.7 psia) to be higher than that in the main machinery room (MMR). The automatic shutters for all three FDBs are then locked open. When the boiler air registers are opened, the higher pressure from the uptakes will flow into the boiler. Draft will be enhanced if the boiler is under a steam blanket lay-up due to the differences in density caused by the increased temperature. Not only is the boiler purged using this method but fires are lit and the boiler continues to be fired until the steam drum pressure is above 200 psi. At this time the main and auxiliary steam stops are opened and the steam forced draft blowers are started. Blower flaps are then unlocked from the open position and the secured blowers shutters are pinned shut. Normal space ventilation can then be restored and zebra secured.
B. Main forced draft blowers can be either steam or electric driven. The type installed depends upon the ship type and function it will perform in war time. Ships that must have a high degree of maneuverability require blowers that have a wide performance range and rapid automatic response capability. On the other hand, a ship that usually operates port-to-port at relatively constant power will do very well with electric constant speed blowers.
1. Steam turbine-driven FDBs are classified as vane axial machines, a propeller type pump similar to that of the main circulating pump in that it delivers a high volume at a low pressure. Steam FDBs are either of horizontal or vertical design (Figure 2). The volume of air delivered to the boiler is determined by its speed. Steam driven blowers are used in approximately 90 percent of all Navy ships having conventional boilers. Ships with these blowers installed will have two or more per boiler. Examples of this are CVs which have three steam driven blowers per boiler or LHAs/LHDs with two steam blowers per boiler.
Horizontal Forced Draft Blower
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Figure 2
2. Electric FDBs (Figure 3) are classified as centrifugal by design and have either a single or double inlet. Their discharge is regulated by either varying the inlet vane position, variable speed motors, or a combination of the two. An example of this would be the AE-26 class which has one electric FDB per boiler.
Centrifugal Forced Draft Blower
Figure 3
3. Counter balanced automatic non-return
shutters are airflow shutoff devices which are located within the ducting
between the blower discharge and the boiler outer casing. The shutter/s function as a check valve that
automatically prevents reverse air flow through the blower. It incorporates a locking feature with which
the shutter can be locked opened or closed so that it no longer functions
automatically. Reverse air flow through
a secured blower will cause a short circuiting of the air by allowing it to
take the path of least resistance back to the uptake space. This in turn will produce windmilling of the
secured blower as well as cause the on-line blower/s to speed up due to the
loss of air flow. The extent of this
casualty is covered in depth in the following automatic boiler controls
lesson.
4. The entire boiler is double cased, the area between them is termed the air casing. Here, the air picks up heat that is given off by the inner casing and preheats it, resulting in improved combustion and increased thermal efficiency. Another advantage of this design is the significant decrease in the machinery space ambient temperature which would otherwise drastically reduce the watchstanders safe stay time. The area within the air casing where the air registers are located is called the windbox.
5. Boiler air casing pressure is measured by the windbox pressure gauge (Figure 4). This is a diaphragm type gage calibrated in inches of water ( 27.7" H20 = 1 PSIG).
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Windbox Pressure Gauge
Figure 4
6. Air registers (Figure 5) when open, admit air from the windbox and direct it in a swirling motion to the furnace in such a way that it mixes with the fog-like oil particles for proper combustion. Air passing through the register assembly is separated into two distinct streams. The first, called primary air, is that which passes through the diffuser (or impeller) and mixes with the fuel as it is emitted from the atomizer. In addition, it also prevents the flame from being blown out. The other, secondary air, is the air that passes around the diffuser. Due to the restriction of flow caused by the diffuser, the greater quantity of air is in the secondary air stream. This air mixes with the fuel particles after they leave the diffuser, encompassing and giving shape to the flame. Not only does this enhance efficiency, but helps to impede against flame impingement to surrounding tubes and/or refractories.
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Radial Vane Air Register Sliding Sleeve Air Register
Figure 5
C. Forced draft blower operation
1. Since a turbine is more efficient at about 60% to 80% of its rated speed, it is more efficient to operate one FDB at higher speeds than it is to operate two or more at low speeds. This is normally the preferred auxiliary steaming configuration for ships having boilers with more than one main forced draft blower. Single blower operation is effective up to 60% of boiler load.
2. Multiple blowers are used for routine underway and maneuvering operations. When two or three blowers operate as a cluster and discharge to a common boiler, they are said to operate in parallel. When in parallel operation, speed differences up to 300 rpm can be tolerated without resulting in unstable blower operation. Speed differences in excess of 300 rpm may cause unstable operation (surging and stalling) to one or all blowers in that cluster.
a. Surging occurs when one of the blowers is running significantly out of parallel with the other/s. When this happens the blowers are said to "fight" with one another. This causes an oscillation in the air flow to the air casing and the boiler is caused to "pant". If extreme, the casing and burner flame patterns can actually be seen to heave in and out!
b. Stalling is when one blower completely overtakes the other/s causing the counter weighted shutters of the slower machine/s to close. When this occurs, the effected blower can no longer discharge its air. Like any other pump, the medium which is being moved provides its cooling. This is known as windage and if left to operate under this condition for a length of time will cause overheating and thermal stress to the propeller blading.
3. Purging of the boiler furnace (See Figure 1) is done prior to light-off to clear the furnace, boiler gas circuit, and stack of possible explosive vapors. To accomplish this, a minimum of five volumetric changes of fresh air must be blown through the boiler. A "volumetric change" is the complete exchange of air from the firebox to the exit of the stack. Because of the differences in boiler size, uptake and smoke pipe systems, fan size and output, each boiler requires a different purge time. These times have been calculated and together with wind box pressure as an index of airflow, have been issued to all ships. These tables have been printed on metal plates and shall be mounted in clear view of the operator. This information is also contained in Ref. C, Appendix H. The minimum purge time is listed adjacent to the pressure. The pressure is measured with all air registers closed. If the pressure should fall between two listed values, use the purge time corresponding to the next lower pressure. Minimum times are based on the assumption that there is no unburned fuel in the furnace. If unburned fuel is present, it shall be removed before starting the purge. A boiler purge is conducted with all air registers opened.
D. Combustion air flow path (Figure 1 Combustion Air and Gas Flow Path)
1. Combustion air enters through the louvers in the outer stack and flows down between the inner and outer stack to and through the uptake space where the forced draft blowers take a suction at the plenum chamber.
2. The forced draft blowers discharge the combustion air through the automatic non-return shutters.
3. The combustion air then flows between the inner and outer boiler casings (air casing).
4. At the windbox, the air enters the air registers where it mixes with the fuel and is combusted.
5. The combustion gases then pass through the screen wall tubes, superheater, main generating bank, and economizer. At the breaching (the base of the stack), the gasses enter the smoke pipe (inner stack) and exit outside the ship.
E. Safety precautions
1. Before admitting steam to a blower, ensure it is not seized in position and the fan is free of foreign material. This can be done by turning it over by hand or with the assistance of a strap wrench.
2. Keep the oil sump full of clean oil, and sample IAW PMS.
3. Never run a blower without a visible flow of oil in the return sight glass, or an indication of low or abnormally high temperature or pressure.
4. Test the speed limiting governor by hand for freedom of movement before starting. It should also be tested monthly with steam IAW PMS.
5. In case of vibration or unusual noise, secure the blower immediately!
6. When a blower is secured, ensure the automatic shutters are locked in the closed position.
7. Never start a blower without first unlocking the automatic shutters.
8. Never work on any machinery without ensuring the valves are shut and safety wired, and danger tags properly attached.
9. When a steam driven FDB is brought on line, ensure the motor driven FDB butterfly valve is shut with locking pin installed before securing.
10. Ensure any installed blower suction covers are removed prior to starting the blower.
11. Never open the automatic shutters on an idle blower when the boiler is in operation since this may cause a boiler explosion.
12. Ensure pressure side of lube oil system flanges are covered by flange shields.