Various Equipments - New Age International

5
Various Equipments
In previous chapters we studied fuel and its burning. We also studied water required for the boilers. In
this chapter we will study the various equipments used for boilers. The equipments include mountings,
fittings and axillaries. After going through this chapter it will be easier for engineers to understand
various types of boilers which are described in the next chapter.
5.1 FEED WATER TANK AND DEAERATOR ASSEMBLY
Purpose
Soft water/DM water contains dissolved gases like oxygen, carbon dioxide. These gases corrode the
boiler-drum, drum-internals and boiler tubes. Therefore, it is necessary to remove these gases to the
maximum extent before storing it in feed tank. These gases being non-condensable accumulate in the
top portion of the boiler drum resulting in creation of hot spots.
For small boilers, provision of deaerator is uneconomical and therefore water is directly stored in
the feed tank and heated by steam. Due to temperature the solubility of gases reduces and evolves out.
Remaining gases are scavenged by dosing chemicals like sodium sulfite to the feed tank.
For large boilers precise control of feed water quality is of utmost importance and therefore, effective
removal of dissolved gases is necessary. For this purpose equipment known as deaerator is employed.
The functioning of a deaerator is to remove the dissolved gases present in the feed water. This can
be achieved by heating the water to the saturation temperature so that entrapped gases are released.
The gases then go through a vent condenser where steam accompanying the gases is condensed by
the feed water.
Normally deaerator is placed above the feed tank and treated as one unit. The position of a deaerator
and feed water tank is an important point to be considered. It should be placed at a height to give
positive suction head to the feed pumps so as to avoid cavitation. Minimum prescribed height is 12 ft.
and can go beyond 100 ft. depending upon the suction created by the feed pump.
The volume of deaerator depends upon the extent to which the oxygen content from the feed water
is to be removed. The feed water tank size depends upon the storage required and it acts as a balance
wheel of the system.
The deaerator can be of truncated cone cylinder type with completely guided steam path ensuring
water distribution and regular scrapping of gases at all loads.

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The deaeration system is comprised of the following equipment:
1. Deaerator
2. Storage tank/feed tank
3. Vapor cooler
4. Heater
5. Overflow controller
Description
Deaerator: It consists of a cylindrical vessel. The lower part of the vessel is connected to the feed
tank and upper part is connected to the feed water supply and condenser. The water is fed through
pipe connections situated in the boxes of mixing device in the upper part of the deaerator. From
mixing device the water then flows successively across perforated trays as shown in the Fig. 5.1. The
first few trays are fastened to the deaerator head and last few trays are fastened to the lower portion
of the deaerator. Rings are provided to the inner part of the shell, which acts as a hydraulic seal
between trays and shell wall.
Hot condensate is fed after few trays having its own spraying arrangement. The water fed to the
upper part of the deaerator gets heated up to boiling temperature by the steam, which is supplied to the
feed tank. The separated gases rise to the upper part of the deaerator from where they are removed.
The deaerated water flows down to feed tank on which deaerator is mounted.
Feed Tank/Storage Tank
As stated earlier storage tank helps to store and supply feed water to the boiler. The reservoir is having
the capacity so that in case of emergency it is capable of supplying feed water for a reasonable period.
For power boilers this capacity is in the range of 10 to 15 minutes.
To Vapor Cooler
Feed Water In
Deaerator
Trays
Manhole
Steam
FW Storage Tank
Overflow
Fig. 5.1
Deaerator

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EXPOSURE TO BOILERS
Feed tank is provided with outlet, overflow, vent, gauge glass, inspection ladder, manhole, etc.
Two overflow funnels are provided and connected to the overflow container. In case of increase in
level water flows out and maintains the level in the feed tank.
Vapor Cooler
The purpose of vapor cooler is to cool the vapors and condense so that non-condensable gases can be
separated. It is a vertical vessel consisting of a shell and tubes. Gases enter at the bottom of the shell
and exit from the top of the shell. In the process vapors get condensed and collect at the bottom and
gases with small quantity of moisture exit from the top. Cooling water enters in the tube having
multiple passes cools the vapor, and comes out. One of the type of vapor cooler is as shown in the
Fig. 5.2.
Vent
Cooling Water
Cooling Water
Gases
Baffles
Moisture and Gases
Gauge Glass
Drain
To Top of Storage Tank
Fig. 5.2
Vapor Cooler
CBD Flash Tank
If the boiler is provided with continuous blowdown system, the blowdown pipe is connected to a
flash tank and the flash steam is used for the deaerator. This arrangement enables to recover heat from
the blowdown water.
Over Flow Controller
The feed tank is maintained above atmospheric pressure to increase boiling point of the feed water.
Therefore, overflow line of the deaerator cannot be kept open to the atmosphere. It is therefore,
necessary to provide overflow controller so that when the level in feed tank increases beyond limit the
controller opens the overflow valve and closes when the level comes to preset value.

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The construction is as shown in the Fig. 5.3.
From Top of Storage Tank
Float
Control Valve
From Side of Storage Tank
Overflow
Fig. 5.3 Overflow Controller
5.2 BURNERS
Burner is an equipment, which supplies fuel and air in a scientific way to the furnace for efficient
combustion. The designs of the burners differ depending upon the type of fuel to be handled. They are
classified as under:
1. Coal fired burner
2. Oil fired burner
3. Gas fired burner.
4. Gas and Oil combination burner.
1. Coal Fired Burners
They are classified as:
(a) Fix burners
(b) Tilting burners.
Coal is pulverized for coal fired burners. The brief description of coal fired burners is as under:
(a) Fix burners
In this type of burner the direction of coal and air fed to the furnace cannot be changed and hence
flame position remains unchanged. Therefore, these burners are called fix burners.
Coal is pulverized in a coal mill and conveyed to the burner through a pipe. Coal conveying is done
by means of an air. This air is known as primary air. The pulverized coal along with primary air then
enters in a furnace. At the throat of a burner additional air is supplied such that the total air is sufficient
for complete combustion. This additional air is called secondary air. Secondary air therefore, serves
two purposes:
(i) Adequate air for complete combustion
(ii) Turbulence for effective combustion.
When the boiler stops or trips furnace remains hot and can damage burner throat. To overcome
this problem a small amount of air is continuously supplied at the burner throat even if the boiler trips.
This air is known as tertiary air.

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EXPOSURE TO BOILERS
One may think that the primary air quantity itself can be increased which is sufficient for complete
combustion and secondary air system can be totally avoided. If the secondary air system is avoided it
will be at the cost of following disadvantages.
(i) Velocity of air in coal mill will increase resulting in carry-over of big coal particles along with
fine coal particles.
(ii) Air frictional losses will increase resulting in requirement of fan having higher discharge head.
(iii) Velocity of coal and air will increase in coal carrying pipes resulting in high wear and tear, which
will reduce life of the pipes.
(iv) High velocity coal and air mixer will result in increased flame length. It will be necessary to
increase the furnace width so as to avoid flame impingement on the water wall.
(v) Turbulence will be inadequate resulting in more excess air requirement and in turn inefficient
performance of a boiler.
(b) Tilting burners
Tilting burners are normally provided in the corner fired furnaces or tangentially fired furnaces.
Principle of pulverized coal conveying is more or less similar to pulverized coal conveying to fix
burners. At the mouth of the burner tiltable fins are provided. These fins if tilted in upward or
downward direction coal and air supplied to burner also change the direction accordingly.
Power generation depends upon the quantity of steam supplied. However, superheat steam
temperature is critical for turbine. If the burner is tilted upwards, flame moves upwards and water
wall gets less heat energy where as superheater gets more heat energy.
Exactly the reverse phenomenon occurs if the burner is tilted downwards. This means these burners
can control superheat steam temperature at the cost of steam generation. Therefore, it will be necessary
to adjust the fuel supply as per demand which can be achieved by cutting in or cutting out the burners
or by increasing or decreasing fuel flow.
Coal quantity, coal quality, steam generation and superheat steam temperature are interrelated.
Tilting burners are capable of handling changes in coal quality. The burner position is kept in such a
way that the steam generation and superheated steam temperature is as per requirement.
2. Oil Fired Burners
Fuel oil contains a range of petroleum products. It is a clean fuel having a very low percentage of ash
content. Fuel oil requires comparatively less excess air than coal. Normally 15 to 20% of excess air is
sufficient in oil firing system.
Oil is fired by means of oil fired burners, which are classified as:
(a) Air atomized burners
(b) Steam atomized burners
(c) Pressure atomized burners
(d) Rotary burners.
(a) Air atomized burners
These burners can be further subdivided in two categories:
(i) Low air pressure burners
(ii) High air pressure burners.

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Description
Fuel oil is preheated, filtered and supplied to the burner. In a burner as shown in the Fig. 5.4 there is
an air spinning arrangement. Air is supplied by means of a blower. Part of the air goes for atomization
and balance to diffuser. Spinning arrangement in the burner on one side mixes the air in fuel and on the
other side delivers air and oil mixture in the form of spray. This spray is in a conical form.
Nozzle
Air
Fig. 5.4
Inside Air Mixing
Oil
Air Atomized Burners
Air atomized burners are useful when only one burner is installed. However, one air blower can
operate more than one burner. These burners are not very much popular in boilers, as they require
very high power for fuel atomization. Furnaces other than boiler furnaces low-pressure air burners
are in use in many industries.
In low-pressure air atomized burner oil is supplied by gravity and air is supplied by blower. The fuel
is atomized by air. Fine atomization is not necessary as air and fuel molecules are being mixed properly
before entering in the furnace. Low-pressure burners require about 20 to 30% of combustion air for
atomization where in medium-pressure oil burners require about 3 to 5% of the combustion air for
atomization. Balance air is fed to the furnace through the diffusers, which is the part of the burner assembly.
Feature
Air atomized burners have following features:
(i) Medium-pressure air atomized burners have 5:1 turned down ratio, wherein low air pressure oil
fired burners have 2:1 turned down ratio.
(ii) Maintenance is very simple.
(iii) With the help of one blower number of burners can be operated.
(iv) Flame control is comparatively easy.
(v) Fuel supply at correct temperature is essential.
(b) Steam atomized burners
Out of various methods of fuel atomization steam atomization is one of them. Brief description of the
burner is as under:
Description
These burners can be further classified as under:
(i) Inside mixing type
(ii) Outside mixing type
In inside mixing type burners steam is mixed in the mixing chamber with fuel oil for fuel atomization.
In outside mixing burners steam and fuel oil are mixed before it enters the burner.

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EXPOSURE TO BOILERS
Preheated fuel oil is pumped in a central barrel of the burner. Central barrel is surrounded with a
steam, which flows through a coaxial tube, concentric to barrel. Preheated oil further gets heated in
the barrel due to steam causing reduction in fuel viscosity. The oil and steam will get mixed in the
mixing chamber. In a mixing chamber arrangement is made to create a whirl. The mixture of fuel and
steam comes out in the furnace in the form of mist whirl and with high velocity. The whirl and high velocity
create a conical sheet of oil mist. An air is supplied through a diffuser to achieve complete combustion.
Normally 4 to 5% of total steam generated is required for oil atomization. For proper atomization it
is essential to have dry and saturated steam. Therefore, insulation to the steam pipe of the burner is
provided. In addition to this a steam trap installation just before burner is inevitable. Fuel oil pressure
is normally 5 to 6 kg/cm 2 . Therefore, for pumping the oil to the burner screw pump or gear pump is
required. Normally steam pressure is maintained more by one kg/cm 2 than the oil pressure.
(c) Pressure atomized burners
Quite often, these burners are called as pressure jet burners. These burners are most popular and used
widely for boilers in industries.
Description: Clean, filtered and preheated fuel oil is pumped in a barrel of a burner. At the end of
the barrel a swirl chamber is provided. The fuel oil from swirl chamber passes through a nozzle,
which is atomized.
As shown in Fig. 5.5 the burner gun consists of a fuel supply pipe, swirl chamber and a nozzle. The
fuel oil flows from burner pipe to the swirl chamber under pressure. The swirl chamber is designed
for the fuel oil to take a tangential pass. Due to pressure fuel oil enters in the nozzle orifice. The orifice
hole being small the fuel oil velocity increases. Due to high velocity and whirl action fuel oil comes out
of burner nozzle in a mist form. This mist forms a cone.
The burner gun is housed in a wind box, which is a part of oil burner assembly. Wind box consists
of number of vanes, which supplies air in a whirl form.
The whirling air then passes through a diffuser. The diffuser is nothing but a perforated plate or
cone mounted on the burner gun. The distance of diffuser from burner tip is very critical for flame
stability and homogeneous mixing of fuel and air.
The air fuel mixture enters in a furnace. To guide the mixture flow a conical throat is provided to a
furnace. A conical throat is generally made out of castable refractory.
Combustion is efficient if atomization is proper for which correct viscosity, temperature and
pressure of fuel oil is necessary. It is also necessary to maintain steady fuel oil pressure. Fluctuation
in fuel oil pressure is mainly due to surges. Provision of fuel oil bypass line minimizes the surges. In
these types of burners part of the fuel oil is continuously bypassed and returned to the fuel oil tank.
Good designed burners operate with very good combustion efficiency. Burner nozzle orifice wears
out at a faster rate affecting the atomization and ultimately combustion. The nozzle replacement from
time to time is very essential depending on the nozzle condition. Nozzle orifice being very small it is
susceptible for choking. Therefore, clean and filtered fuel oil supply has utmost importance.
Fuel Supply Pipe
Tangential Swirl Chamber
Nozzle
Turbulance Chamber
Fig. 5.5
Pressure Jet Burner

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(d) Rotary burners
Rotary cup burners operate on low fuel oil pressure.
Description: As shown in the Fig. 5.6 furnace oil enters in a rotary cup through a burner pipe at
low pressure. Rotary cup is normally rotated by an electric motor or by an air impeller. The speed of
the rotary cup is maintained very high i.e., in the region of 4500 to 8000 rpm. The furnace oil falls on
the cup at a point where the linear velocity of the oil is minimal. Due to the centrifugal action fuel oil
spreads on a conical shape of rotary cup and accelerates. Fuel oil finally reaches to the tip of the
rotary cup and thrown to the furnace at high velocity. Effective spray is achieved by adjusting the
primary airflow. The primary airflow governs the air pressure and velocity. For complete combustion
secondary air is supplied as shown in the figure.
Oil Film
Oil
Rotary Cup
Primary Air
Secondary Air
Fig. 5.6
Rotary Burners
Rotary cup burners have high turned down ratio. They operate at low fuel oil pressure. These
burners are not very sensitive for fuel oil viscosity. These burners offer an easy maintenance. However,
the short falls of the burner are rotary cup damage and once in a shift cleaning of cup is necessary.
The Fig. 5.7 indicates the droplet size and droplets distribution for rotary cup burner, pressureatomized
burner and conventional burner.
1.4
Droplet Distribution
% Micron
1.0
0.6
0.2
100 200 300 400
Droplet Size in Microns
Dots – Improved Rotary Burner Lines – Conventional Burner
Dash – Pressure Atomized Burner
Fig. 5.7 Comparison of Droplet Distribution of Various Burners

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EXPOSURE TO BOILERS
3. Gas Fired Burners
Burner constitutes of auto ignition-fuel control-flame sensing systems. It comprises of a burner,
combustion blower, gas train and an electrical control panel. These burners are suitable for Natural
gas, LPG, Biogas and allied gaseous fuels.
Requirements of gas burners
1. Fully Automatic Operation - having built-in safety feature like pre-purging facility.
2. Auto ignition, flame supervision and fuel control system.
3. Safe and quiet working.
4. Highly reliable and efficient.
5. Strong and robust construction.
4. Gas & Oil Combination Burner
This burner has a ring with holes. The gas flows through the ring and comes out from the holes.
At the center of the ring a burner gun is provided for furnace oil. The combination of two burners
gives flexibility to the operator to switch over from one fuel to other. Air for combustion is supplied
through the wind box. The entire unit can be mounted on a furnace wall with the help of castable
refractory.
Refractory work
Gas Inlet
Furnace
Gas Burner
Mechanical
Oil Burner
Movable Blade Register
Register Box
Fig. 5.8 Gas & Oil Combination Burner
5.3 COAL MILLS
Purpose: If the coal is pulverized the exposed surface area increases. Combustion becomes faster
and need of excess air reduces. It leads to increased thermal efficiency. Coal pulverization has many
advantages in addition to increased thermal efficiency such as flexibility for using wide range of coal
variety, fast response, ease of handling large quantity of fuel, low banking losses, etc.

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63
Therefore, for large capacity boilers pulverized coal firing is always preferred. Pulverization of coal
takes place by mainly three actions such as impact, attrition and crushing. For pulverizing coal the
equipment used is a mill called as coal mill. Most of the coal mills use all the three actions.
1. Ball Mill or Tube Mill
The ball and tube mill (Fig. 5.9) has a slowly revolving cylinder whose length is somewhat greater
than its diameter, turning at about 20 rpm. The interior of this cylinder is fitted with cast-lined rods
and is filled about half its volume with steel balls whose diameters vary from 25 mm to 50 mm. While
in rotation these balls are carried up about two-third of its way up the periphery i.e., they ride up the
rising side of the cylinder. Then the mixture above the balls pushes them down as it rises too high. The
coal intermingles with the balls. This continuous climbing of the balls make them seam against the
coal pieces, reducing it to powder form. Pulverization is due to impact of the falling balls, attrition
through their sliding over each other as well as over the lines, and crushing as they roll over the liners
and also over each other. The larger pieces are broken principally by impact, and the fine grinding is
done by the rolling and sliding action of the balls.
Motor
To Burner
Fan
Coal Hoppers
Fan
To Burner
Motor
Classifier
Classifier
Preheated Air
Fig. 5.9 Ball Mill
Hot air enters the mill through the hollow tubes at each end and the swirling air picks up the coal
fines. The mixture enters in a classifier, taking a cyclonic flow. It throws the oversized particles
outwards, which return to the tube for grinding. The oversized particles, which have been dried in the
classifying process, are returned with the raw coal. The mixture of coal has reduced average moisture
content, which enters in the mill. This recirculation of dried coal reduces the tendency of wet coal to
choke the feed and passages.

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EXPOSURE TO BOILERS
Advantages
(a) Low maintenance cost
(b) Reliability in operation
(c) Quick response to change in fuel demand
(d) Considerable quantity of fuel in the drum, which acts as a storage reservoir from which
sudden increase in fuel demand may be met with. Approximately 8 min. reserve is available if
there is interruption in fuel supply.
Disadvantages
(a) The power consumption of ball mill per ton of coal pulverized, is high at partial loads.
(b) Ball mill is relatively large for small capacities and therefore requires large floor area.
(c) With high moisture content ball mill capacity greatly falls
(d) Ball mills are not suitable for intermittent operation as the large amount of heat, stored in the
coal and balls may cause overheating and result in fires when the mill is idle.
(e) Ball mill produces 70 to 75% dust having 200-mesh fineness.
2. Ball and Race Mill
The ball and race mill crushes coal between two moving surfaces, balls and races. The diameter of
ball is from 20 cms to 30 cms for large mills and somewhat less for smaller once. If the grinding
member is a roller; its diameter usually varies from 1/4 th to 1/3 rd of the ring diameter, varying from 25
to 60 cms. The width of the roll face varies from 15 cms to 30 cms depending up on the mill size.
Normally if the balls are used as grinding element they are confined between two races. Pressure to
crush the coal is obtained by forcing these together with heavy springs or hydraulic pressure. One of
the designs is shown in the Fig. 5.10. The balls may be driven either rotating the upper or lower race.
Some additional grinding pressure is obtained by the centrifugal force of the rotating balls. Coal enters
the mill through coal feeder, which falls on the inner side of the races. The moving balls and races
catch coal between them to crush coal into powder. Air supplied by FD fan enters the mill through the
annular space surrounding the races. As it flows between the balls and races it picks up the coal dust
and enters in a classifier, which is located above in the mill. The classifier can be adjusted for the
fineness of the coal dust.
Rotating races turn at about 225 to 280 rpm. The mill can handle coal having as high as 20%
moisture. The airflow rate fixes the amount of coal dust carried from the mill. Based on coal dust
removed the coal feeder responds through automatic controls. Mill feeder and fan need up to 14 kW
hr/ton coal pulverized.
Advantages
(a) Lower power consumption than any other type of mill
(b) Reliability in operation
(c) Quick response to change in fuel demand
(d) It is compact in design
(e) Occupies less floor space
(f) Maintenance is much less than hammer mill.

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Disadvantages
(a) Maintenance is slightly higher than the ball mill
(b) It is not suited from cost point of view as manufacturing in sizes below 800 tons/hr is costlier.
Hydraulic Jack
Coal Air Mixture
Classifier
Coal from Coal Feeder
Spring
Ball
Race
Stationary Disk
Rotating Disk
Hot Air
Gear
Fig. 5.10
Ball and Race Mill
3. Bawl Mill
(Refer Fig. 5.11)
The bawl mill has a grinding ring carried on the slopping side of the rotating bowl. The bawl rotation
gives the bawl a rim speed of about 360 M/min. The spring-loaded stationary rollers just clear the
grinding ring. The feeder discharges coal into the rotor bawl. Centrifugal force throws the coal to the
bawl rim and up the slopping sides to be crushed under the rollers. Hot air is fed into the mill for
drying and conveying of the dust particles. A classifier creates a cyclonic flow, where the hot air fuel
mixture enters. Coarser particles are returned back for further grinding.
An exhauster takes the dust-laden air from classifier and passes it on to the burner.

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EXPOSURE TO BOILERS
Coal Hopper
Fine Coal
Coal
Coal Dust
Spring
To Burner
Gear
Roller
Bowl
Motor
ID Fan
Hot Air from Air Preheater
Fig. 5.11 Bawl Mill
The general arrangement of the bawl mill consists of the following:
(a) Dust guard seal assembly
(b) Skirt assembly
(c) Scrapper assembly
(d) Bawl ring assembly
(e) Bawl extension ring
(f) Separator body and linear assembly
(g) Grinding roller assembly
(h) Classifier cone
(i) Classifier vane
(j) Outlet venturi
(k) Mill discharge valve assembly.
However for simplicity all the part are not shown in the Fig. 5.11. Names of the parts and assemblies
are self-explanatory. Therefore, the reader can visualize the mill construction comfortably.
4. Hammer Mill or Impact Mill
(Refer Fig. 5.12)
The impact mill or hammer mill consists of series of hammers attached to hammer arm fitted to the
rotor. Grinding is accomplished by a combination of impact on the larger particles and attrition on the
smaller ones. An air system is provided to supply flow of air to the mill. Built-in or an external
classifier is used.

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Advantages
(a) Mill is compact.
(b) Simple in construction.
(c) Can be manufactured in various sizes.
(d) Excellent drier and can be used for extremely wet coal.
(e) Drying of wet coal is fast and positive.
Disadvantages
(a) Maintenance cost is very high.
(b) Dust fineness cannot be maintained throughout the life of the wearing parts.
(c) The maximum capacity has the limitation.
Stationary Pegs
Primary Air Fan
From Coal Feeder
Air In
Motor
Rejector Arms
Hammer
Rotating Pegs
Fig. 5.12
Hammer Mill
Wear Out of Mill Parts
· Balls in ball mills—125 to 375 g/t of coal ground.
· Hammers in hammer mill—200 g/t of coal ground.
· Balls and rings in ball and race mills—30 g/t of coal ground.
Types of Classifiers Used in Coal Mills
The powder leaving the mill also contains a certain quantity of excessively large particles approximately
0.3 to 5 mm. This fraction (so called ‘oversize particles or returning residues’) is separated from
remaining powder in classifier and returns back to coal mill. The classifiers are roughly divided into
stream or gravity type, according to application of centrifugal force or gravity to separate the coarse
particles.

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EXPOSURE TO BOILERS
1. Stream Type Classifier: Centrifugal classifier is one of the most applicable types of classifiers.
It operates so that the stream of milled fuel and carrying gas, leaving the mill, strikes against the plate
in the top of inside cone, then it enters the vane rim by means of which it sets in turbulence and
discharges through upper throat provided with telescopic cup. Centrifuged oversized particles of coal
are carried away from inside cone or pipes with flap enclosure.
Fineness of powdered coal leaving the classifier is regulated partly by displacing of vanes and
partly by position of the cup. It is used for all types of mills.
Spiral classifier is not so wide spread and is mostly used in combination with multiple row heater
mills. Pulverized coal fineness is regulated by dislocating the flaps.
Bending classifiers also work on the same principle.
2. Gravity Type Classifier: They are used only at multiple raw beater/hammer mills. Coarse
particles are separated first by reduction in velocity of mixture flow, but often the pits are additionally
created by various boiler internals (impacts, partitions, louvers, etc.), which support classifying effect.
5.4 COAL BURNING EQUIPMENT
For efficient combustion of fuel particle size, air to fuel ratio and conducive environment are necessary.
For fuel oil burning we have seen the different types of burners used. In case of solid fuel like coal,
mainly two methods are used. First is pulverized fuel firing, and second is stoker fuel firing. However,
for small boilers hand firing is preferred to reduce capital cost.
Coal burning can be classified as under:
1. Hand firing
2. Stoker firing
(a) Over feed stoker firing
(i) Traveling grate stoker firing
· Chain grate stoker firing
· Bar grate stoker firing
(ii) Vibrating grate firing
(iii) Spreader stoker firing
(b) Under feed stoker firing
(i) Single retort
(ii) Multiple retort
3. Pulverized firing
(i) Unit system
(ii) Central system
1. Hand Firing
This method is useful only for small boilers due to coast considerations. For large boilers it is
impracticable to feed the coal manually.
2. Stoker Firing
The types of stokers used are mainly traveling grate or spreader type.
(a) Over Feed Supply of Coal
In over feed stoker firing system crushed coal is fed to the furnace above the point of entry of air. Air
is supplied from the bottom of the furnace by means of FD fan. The coal burns on the bed and