1. 1.STEAM POWER PLANT
Different types of fuels used for steam generation.:- Generally there are three types of fuels can be burnt in
any type of steam power plant. They are 1) Solid fuels 2) Liquid fuels 3)Gaseous fuels.
Gaseous fuels:
The gaseous fuels widely used in steam power plants are natural gas, Blast furnace gas. Gaseous fuels may
be either natural gases or manufactured gases. Since the cost of manufactured gases is high only natural
gases are used for power generation. Natural gas is colorless , odorless and is non poisonous its calorific
value lies between 25000KJto 50000 KJ/ m3 .
The various manufactured gases are coal gas, Coke oven gas , Blast furnace gas and producer gas. These
manufactured gases play an less important role in the steamgeneration.
Advantages:
1) Excess air required is less 2) Uniform mixing of fuels and air is possible 3) Handling is much more
easier compared to the coal. 4) The load changes can be met easily. 5) There is no problem of ash disposal.
6) Operational labourer required is less.
Disadvantages:
1) Storage of gaseous fuels is not easy compared to liquid fuels due to the risk of explosions.
2) The plant must be located near the natural gas field other wise transportation cost increases.
Liquid fuels:
The liquid fuels used in the thermal plant to generate the steam instead of coal as it offers the following
advantages over the coal.
1) The calorific value of liquid fuels is about 40% higher than that of coal or solid fuels.
2) The storage space required for liquid fuels is less.
3) Instantaneous ignition and extinction of fire is possible.
4) Stand by losses are minimum. 5) Efficiency of the boiler is high. 6) Ash handling system can be
eliminated. 7) Oil can be easily metered. The rate of fuel supply to furnace is easy to control.
Disadvantages
1) The overall combustion efficiency of the liquid fuel fired power plant is less compared to the coal fired
power plants
2) The availability of the liquid fuel resources are very limited as compared to the coal resources
Ex. Heavy oils, Bunker C oil , Viscous residue oil, Petroleum and its by products .
Solid Fuels:
Example for solid fuels is Coal. The term coal refers to the rocks in the earth’s crust , produced by the
decaying of the plant materials accumulated overt the milli ns of years ago. Different types of coals that are
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used for steam generation are 1) Lignite 2) Sub Bituminous 3) Bituminous coal 4) Semi Anthracite coal 5)
Anthracite coal.
1)Lignite:
It is lowest grade of coal . Its having 30% of moisture calorific value is about 14650 to 19300 KJ/ Kg. Due
to the high moisture content and low calorific value lignite is not easy to transport over long distances. It is
usually burnt by the utilities at the mine sites.
2) Sub Bituminous:
Its calorific value is slightly less than that of the Bituminous coal. Its calorific value is in between 193000
to 26750 KJ / Kg. The moisture content is about 15 to 30%. It is brownish black in colour. These coals
usually burned in the pulverized form.
3) Bituminous coal:
Bituminous coal is widely used in all purposes. It is used in steam generation and in the production of the
coal gas and producer gas. Its moisture content may vary from 6 to 12 %. Its calorific value ranges from
25600 KJ/Kg to 32600 KJ /Kg. Bituminous coal burn easily especially in pulverized form.
4) Semi Anthracite
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2. It is an intermediate coal between Bituminous and Anthracite coal. It ignites more easily than anthracite to
give a short flame changing from Yellow to Blue . It is having the following properties’
Moisture content = 1 to 2%
Volatile matter = 10 to 15 %
Calorific value= 36000 to 36960 KJ / Kg
5) Anthracite:
It is the most mature and hard form of solid fossil fuel. It is having an fixed Carbon content ranging from
92 to 98%. Anthracite is good domestic fuel for heating and is some times used for steam generation.
Selection of coal for steam generation.
Selecting an suitable coal for steam generation is an very difficult task. The firing qualities of coal are every
important when we are considering an combustion equipments. Slower burning coal generates high fuel bed
temperature s and therefore requires forced draught fan. The fast burning coals require large combustion
chamber. Such coals are suitable for meeting an sudden demand for steam. The most important factors
which are to be considered in the selection of coal are sizing and caking, Swelling properties and ash fusion
temperature . Some times the selection of coal depends on the ash content also.
The following properties of the coal are to be considered for the selection of the coal for steam generation
in steam power plants.1) Burning rate of coal 2) Sizing, Caking , Swelling properties of coals. 3) Finess of
coal.
Selection of site
The following factors should be considered while selecting the site for a steam power station
1) Availability of fuel.
2) Nature of load
3) Cost of land.
4) Availability of water.
5) Transport facilities.
6) Ash disposal facilities.
7) Availability of labour.
8) Size of the plant.
9) Load center.
10) Public problems.
11) Future extensions.
Combustion equipments for steam generators.
The combustion equipment is one of the important component of the steam generator. The combustion
equipment used must have the ability to meet the following requirements.
1) Through mixing of fuel and air.
2) Maintaining of optimum air fuel ratio leading to complete combustion over the full load range.
3) ready and accurate response to the load demand
4) continuous and reliable ignition of the fuel.
Coal handling :
The coal handling plant needs extra attention , while designing a thermal power station, as almost 50% to
60% of the total operating costs consists of fuel purchasing and handling . Fuel system is designed in
accordance with the type and nature off fuel. Plants may use coal oil or gas as the fuel. The different stages
in coal handling are shown below.
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3. Coal delivery
The method of transporting coal to a power station depends on the location of the plant, but may be one or
more of the following : rail, road , river or sea. Plants situated near river or sea may make use of the
navigation facilities. Stations which cannot make use of these facilities may be supplied coal either by
trucks or by rail. Transportation of trucks is usually used in case the mines are not available. In case rail
transport is to be adopted , the necessary siding for receiving the coal should be brought as near the station
is possible.
Unloading:
Just what kind of equipment will do the best job for unloading depends first of all on how the coal is
received. If the coal is delivered in dump trucks and if the plant site is favourable we may not need
additional unloading equipment. When coal transported by using by sea or rivers unloading bridge or tower
and portable conveyors are used. In case the coal received by rail in hoppers cars, again the coal may be
unloaded quickly by using any of the facilities such as car shakers , Car throwing equipments , Car
dumpers(Rotary), coal accelerators .
Preparation:
If the coal is brought to the site un sized and sizing is desirable for storage or firing purposes. The coal
preparation plant may be located either near the coal receiving point or at the point of actual use. The coal
preparation plant may include the following equipments a) Crushers b) Sizers c) Dryers and d) Magnetic
separators.
Coal preparation plant is as shown below.
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4. The raw coal is crushed in to required size using crushers. The crushed coal is passed over the sizer which
removes unsized coal and feeds back to the crusher. The crushed coal is further passed to the drier to
remove the moisture from the supplied coal . Before supplying the coal to the storage hopper , the iron
scrap and particles are removed with the help of magnetic separators.
Transfer:
Transfer means the handling of the coal between the unloading point and the final storage point from where
it is discharged to the firing equipment. The equipments used for the transfer of coal may be any one of the
following or a suitable combination there of:a) Belt conveyors b) Screw conveyors c) Bucket Elevators d)
Grab bucket Elevators e) Skip hoists and f) Flight conveyors.
Belt conveyor
Cross section of belt drive
The belt conveyors are suitable for transporting large quantities of coal over large distances. It consists of
endless belt made up of rubber, canvas or balata running over a pair of end drums or pulleys and supported
by series of rollers provided at regular intervals. The return idlers which support the empty belt are plain
rollers and are spaced wide apart. Belt conveyors can be used successfully up to 20 degree inclination to the
horizontal . The load carrying capacity of the belt may vary from 50 to 100 tons /h and it can easily be
transported through 400 meters.
Advantages:
1) It is most economical method of coal transfer.
2) The rate of coal can be regulated by varying the speed of the belt.
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5. 3) The repair and maintenance charges are minimum.
4) The coal can be protected.
5) The power consumption is minimum.
Disadvantages;
1) It is not suitable for short distances and greater heights.
Screw conveyor
It consists of an helicoids screw fitted to a shaft as shown in the figure. The driving mechanism is
connected to one end of the shaft and the other end of the shaft is supported in an enclosed ball bearing.
The screw while rotating in a trough transfers coal from one end to the other end as shown in figure. The
diameter of screw is 15 cm to 50 cm and its speed varies from 70 to 120 rpm and the maximum capacity is
125 tones per hour.
Advantages:
1) It requires minimum space and is cheap in cost
2) It is most simple and compact
3) It can be made dust tight.
Disadvantages:
1) The power consumption is high.
2) The maximum length limited to 30 meters.
3) The wear and tear is very high therefore life of the equipment is less.
Bucket elevators:
These are used extensively for vertical lifts, through their for horizontal runs is not ruled out. These
elevators consists of relatively small size buckets closely spaced on an endless chain. The coal is carried
by the buckets from the bottom and discharged at the top. Centrifugal type and continuous type bucket
elevators are most commonly used. The maximum height of the elevator is limited to 30.5 m and
maximum inclination to the horizontal is limited to 60 degree. The speed of the chain required in first case
is 75 m/min and continuous type is 35 m/min for 60 tonnes capacity per hour.
Advantages:
1) Less power is required.
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6. 2) Coal can be discharged at elevated places.
3) Less floor area is required.
Disadvantages:
Its capacity is limited to 60 tons per hour and hence not suitable for large capacity stations.
Grab bucket
Grab bucket conveyor is form of hoist which lifts and transfers the load on a single rail or track form one
point to another. This can be used with crane or tower as shown in figure A 2-3 cu-m bucket operating over
a distance of 60 m transfer nearly 100 tons of coal per hour. Its initial cost is high but operation cost is
less.
Flight conveyor
This type of conveyor is generally used for transfer of coal when filling of number of storage bins situated
under the conveyor is required. It consists of one or two strands of chain to which steel scrappers are
attached The scraper scraps the coal through a trough and the coal is discharged in the bottom of the trough
as shown in figure.
Advantages:
1) It requires small head room
2) The speed can be regulated .
3) It can be used for as well as coal transfer.
4) It requires less attention.
Disadvantages:
1) There is excessive wear and tear and hence the life of the conveyor is less.
2) The repair and maintenance charges are high.
3) The restricting the operating speed to 300m/min is required to reduce the abrasive action.
4) Power consumption is high per unit of coal or ash handled.
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7. Skip hoist:
It is used in high lifts and handling is not continuous. It consists of vertical or inclined hoist way, a bucket
or a car guided by the frame, and a cable for hoisting the bucket.
Advantages:
1) It requires very low maintenance.
2) Power requirement is low.
3) It can handle larger size clinkers.
4) It can be used for handling ash as well as coal
5) It needs minimum floor area.
Disadvantages:
1) The initial cost is high
2) This is not suitable for continuous supply of coal.
3) There is excessive wear of skips and ropes which need frequent replacements.
Out door storage: Whether the storage is large or small , it needs protection against losses by weathering
and by spontaneous combustion. With proper methods adopted even largeroutdoor storage can remain safe.
In order to avoid the oxidation of coal the compact layers are formed. To avoid spontaneous combustion
air is allowed move evenly through the layers.
Indoor storage or Live storage:
This is usually a covered storage provided in plants, sufficient to meet day’s requirement of the boiler.
Storage is usually done in bunkers made of steel or reinforced concrete having enough capacity to store
the requisite of coal. From the coal bunkers coal is transferred to the boiler grates.
Weighing: A frequent part of in plant handling is keeping tabs on quantity and quality of coal fired. For
weighing weigh bridge is used. Coalis weighed in transit also by using belt scale.
Fuel Firing methods
Selection of firing method adopted for a particular power plant depends on the follow factors.
ing
1) Characteristics of fuel available.
2) Capacity of the plant.
3) Load factor of the power plant.
4) Nature of load fluctuations .
5) Reliability and efficiency of the various combustion equipments.
Depending upon the combustion equipments used boilers can be classified as
1) Solid fuel fired.
2) Liquid fuel fired.
3) Gaseous fuel fired
Solid fuel firing
The classification of combustion system used for coal burning given below.
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8. Hand firing system is the simplest method for solid fuel firing but it can not be used in modern power plant.
The most commonly used methods forfiring the coal are 1) Stoker fire 2) Pulverized fire.
Stoker fire:
Stoker is fuel burning mechanism used for burning fuel on grate. This type of burning mechanism is
suitable where the coal is burned. Stokers are classified as
1) Over feed stokers
2) Under feed stoker.
In over feed stoker the direction of air and coal are opposite to one another. The coal is supplied on to the
grates above the point of air admission. In under feed stoker coal is fed from underneath the grate between
the two tuyers. The direction of fuel and air is same.
Over feed stoker.
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9. Typical overfeed stoker is as shown in the figure. Coal is fed on to the grate above the point of air
admission.
The pressurized air coming from the FD fan enters under the bottom of the grate. The air passing through
the grate opening is heated by absorbing the heat from the ash and grate itself , where as the grate and ash
get cooled . As hot air passes through the incandescent coke layer O2 reacts with Carbon to form Carbon
dioxide. This is an exothermic reaction and releases heat required for continuation of combustion process.
It continues till all the oxygen is consumed. If the incandescent layer thick, CO2 may be partly reduced to
CO(CO2 + C – 2CO) The gasses leaving the incandescent layer are N2 , CO, CO2, H2 . A slight water
reaction may take place with the moisture in air (H2O + C – H2 + CO) . This is an endothermic reaction
and may bring down the temperatureof the bed and gas. Stream of gases then passes through the distillation
zone where volatile matter is added from raw coal and then moisture is picked up in the drying zone and
finally emerges above the fuel bed. The gases leaving the upper surface of the fuel bed contain combustible
volatile matter, N2, CO2, CO, H2 and H2O , If the combustion of Carbon , Hydrogen and volatile matter is to
be completed following have to provide .
a) Sufficient fresh air or secondary air is supplied .
b) Ignition point should be in the range 10000C – 13000 C.
c) Creating turbulence by supplying secondary air at right angles to the up flowing gas stream from fuel
bed.
It does not help supplying if the secondary air supplied along with primary air, since more primary air
produces only more carbon monoxide. The presence of the Carbon monoxide in the exhaust gases indicates
the in complete combustion leads to decrease in the efficiency of combustion equipments.
Types of Over feed stoker
1) Traveling grate stoker
2) 2) Spreader stoker
1) Traveling grate stoker
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10. The traveling grate stoker is as shown in the figure. This type of stoker has the grate which is moving from
one end of the furnace to the other end. This grate may be chain grate type or bar grate type chain grate
stoker is made up of series of Cast Iron chain links connected by pins to form an endless chain. The bar
grate stoker is made up of a series of Cast Iron sections mounted on a carrier bars. The carrier bars are
mounted and ride on two endless drive chains. he traveling grate stoker consist of an endless chain which
forma support for the fuel bed . The chain travels over the two sprocket wheels which are at the front and
rear end of the furnace. The front end sprocket wheel is connected to variable speed drive mechanism. The
grate can be raised or lowered as needed. Simultaneous adjustment of grate speed, fuel bed thickness, and
air flow control, the burning rate so that nothing but ash remains on the grate by the time it reaches furnace
rear. The ash falls on to the ash pit, as the grate turns to make the return trip. A coal gate at the rear of the
coal hopper regulates coal. As the raw coal or green coal on the grate enters the furnace, surface coal gets
ignited from heat of furnace flame and radiant heat rays reflected by ignition arch. The fuel bed becomes
thinner towards the rear of furnace as combustible matter burns off. The secondary air supplied helps in
mixing the gases and supplies oxygen to complete combustion. The coal should have minimum ash content
which will form an layer on the grate . It helps in protecting grate from over heating.
Advantages:
1) Simple and initial cost is low.
2) Its maintenance costs are low
3) It is self cleaning stoker.
4) Heat releases rates can be easily controlled.
5) It gives high heat release rates per unit volume of furnace .
Disadvantages
This ca not be use for high capacity boilers 200t / hr or more.
The temperature of preheated air is limited to 1800c.
The clicker troubles are common.
The ignition arches are required.
The loss of fuel in ash can not be avoided.
Spreader Stoker
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11. The coal from coal hopper is fed by a rotating feeder, a drum fitted with short blades on its surfaces, to the
spreader or distributor below. Which projects the coal particles on to the grate holding an ignited fuel bed.
The finer particles burn in suspension and the coarse Particles are consumed on the grate. The speed of the
feeder directly proportional to the steam out put of the boiler. The secondary air helps in creating
turbulence and completing combustion. In high capacity boilers may have traveling grate stoker in addition
to spreader stoker. The grate consists of Cast Iron links underneath the grate connect all the bars to a lever.
Moving lever makes the ash fall through to the ash pit below. Spreader stokers capable of burning any type
of coal.
Advantages:
1) Almost any type of the coal can be burnt.
2) Clinkering problem is less.
3) It is having quick response to varying load.
4) The quantity of excess air required is less.
5) The operation cost is low.
Disadvantages:
1) The problem of fly ash is high. It requires an dust collector to prevent the environment pollution.
2) Coal particles trapping mechanism is necessary to prevent their escape with excess air.
3) Its operating efficiency decreases with varying sizes of coal.
Vibrating stoker
Its operation of similar to that of the chain grate stoker except that fuel feed and fuel bed movement are
accomplished by vibration. The vibration and the inclination of the grate cause the fuel to move through
the furnace towards the ash pit. The vibrating conditions of the fuel bed permits the use of wider range of
fuels. Vibrating grate stokers are suitable for medium volatile Bituminous coal and Lignite but at reduced
burn rates.
Under feed stoker
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12. In this type of stokers the fuel and air move in the same direction. In this case coal is fed from underneath
the grate by screw conveyor or by a ram. Primary air after passing through the holes in the grate meets the
raw coal . As the air diffuse through the bed of raw coal picks up moisture and then pass through the
distillation zone where volatile matter is added. When gas stream next passes through the incandescent
coke region, volatile matter burns readily with the secondary air fed at the top. The gases in this type stoker
are at higher temperature than over feed stoker. The under feed method is best suited for burning semi
Bituminous and Bituminous coals high in volatile matter.
Types of Under feed stokers.
1) Single retort Stoker 2) Multi retort stoker.
1) Single retort stoker
The arrangement of single retort stoker is shown in figure in the form of two views. The fuel is placed in
large hopper on the front of the furnace and then further fed by reciprocating ram or screw conveyor in to
the bottom of the horizontal trough. Air is supplied through the tuyers provided along upper edge of the
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13. grate. The ash and clicker are collected on the ash plate provided with dumping arrangement. The coal
feeding capacity of a single retort stoker varies from 100 to 2000 Kg / Hr.
2) Multi retort stoker
Tuyers
Incandescent zone
Distilat ion zone
Green coal
Stoker ram
Extension grat e
D per
am W box
ind
inlet
D per
am Pushers
D arge
sh
isch
A
D gauge
aft
connection
Multi retort stoker is as shown in figure . It consists of series of alternate retorts and tuyers boxes for
supply of air . Each retort is fitted with reciprocating ram for feeding and pusher plates for uniform
distribution of coal. Coal falling from hopper is pushed forward during inward stroke of the stoker ram.
Then distributing ram pushes the entire coal down length of the stoker. The ash formed is collected at the
end as shown in the figure. The number of retorts may be vary from 2 to 20 with burning capacity varying
from 300 kg to 2000 Kg /hr/retort.
Advantages
1) High thermal efficiency compared to chain grate stoker.
2) Combustion rate is high 3) Combustion is continuous 4) Grate is self cleaning 5) Smoke less operation
6) Stokers are suitable for non clinker high volatile and low ash content coal.
Disadvantages
1) It requires large building space.
2) Clicker problems are high.
3) Low grade coals with high ash content can not be burn economically.
4) Initial cost of the unit is high.
Pulverized fuel firing system.
In pulverized fuel firing system the coal is grinded in to a fine powder form with the help of grinding mill
and then projected in to the combustion chamber with the help of hot air current. This hot air is known as
the primary air. The amount of the air required for complete combustion is supplied separately in the
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14. combustion chamber. It helps in creating turbulence, so that uniform and intimate mixing of coal particles
and air can take place inside combustion chamber. The efficiency of the pulverized fuel firing system
mostly depends upon the size of the particles of the coal in the coal powder . The finess of the coal
particles should be such that 70% of it would pass through 200 mesh sieve and 98% through a 50 mesh
sieve.
Coal handling for pulverized fuel plant is shown in figure
Advantages:
1) Any grade of coal can be used.
2) Stand by losses are reduced and banking losses are eliminated.
3) Efficiency of combustion is high compared to other methods of solid fuel firing methods.
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15. 4) Boiler unit can be started up from cold rapidly and efficiently.
5) Practically free from slagging and clinker troubles.
6) Furnace has no moving parts subjected high temperatures.
7) The furnace volume required is less.
8) This system works successfully with or in combination with gas and oil.
9) Greater capacity to meet the peak loads.
10) Practically no ash handling problems.
11) The structural arrangements and flooring are simple.
12) The external heating surfaces are free from corrosion.
Disadvantages:
1) Coal preparation plant is necessary.
2) High capital cost.
3) Handling of fly ash makes the system uneconomical.
4) Special equipment is needed to start this system.
5) Larger building space is needed especially with central system..
6) Skilled operators are required.
7) Refractory material surfaces are affected by high furnace temperatures.
8) Atmospheric pollution created by the fly ash is can not be completely eliminated.
9) The possibility of explosion is more as coal burns like gas.
10) The maintenance of furnace brick work is costly.
There are two methods of pulverized fuel firing. They are unit system and central or Bin system. In unit
system each burner of the plant has its own pulveriser and handling units. In central or Bin system fuel is
pulverized in the central plant and then distributed to each burner with the help of high pressure air current.
Unit system
In unit system each burner of the plant has its own pulverizer and handling units. The pulveriser an
together with feeder , separator and fans may be arranged to form an complete unit or mill. The number
of units required depends on the capacity of the boiler. Raw coal from coal hopper fed to the
pulverizing mill through feeder . Hot air from or flue gases passed through the feeder to dry the coal
before feeding to the pulveriser. The pulverized coal is carried from the mill with the help of induced
draught fan as shown in figure. This further carries the coal through the pipes to the burner. Secondary
air supplied to the burner before fuel entry in to the combustion chamber is as shown in figure helps in
creating the turbulence as well as supplying additional air required for completing the combustion of
the coal particles in the furnace.
Advantages:
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16. 1) It is simple in layout and cheaper than central system.
2) It allows direct control of combustio rate from the pulveriser.
n
3) Maintenance charges are less.
4) The coal transportation is simple.
Disadvantages.
1) The performance of pulverizing mill is poor.
2) Degree of flexibility is less than central system.
3) The fault in the preparation unit may put entire steam generator out of use.
4) There is excessive wear and tear of the blades of fan as it handles air and coal particles.
5) Strict maintenance of the mill is required because the entire plant operation depends on it
Central or Bin system
The central system or Bin system fuel is pulverized in the central plant and then distributed to each
burner with the help of high pressure air current. Crushed and sized coal is fed to the drier from coal
bunker by gravity as shown in figure. The dried coal fed to the pulverizing mill with the help of air, as
shown in figure ,separated in the cyclone separator. The separated pulverized coal is transferred to the
central bunker using conveyor as shown in figure. Oversized coal particles are fed back to the
pulverizing mill for further processing. The storage bin may contain 12 to 24 hours of supply of
pulverized coal. The energy consumption is 15 to 25 KW-Hr / Ton of coal pulverized.
Advantages:
1) The reliability of plant is high.
2) The central system is flexible . Supply of the coal can be maintained to the burners without any
interruption.
3) Burner operation is independent of coal preparation.
4) The pulverising mill may work at the part load because of storage capacity available in the storage
bin.
5) Power consumption per ton of coal handled is less .
6) As the fans handle only air there is no problem of excessive wear and tear.
7) The labourers required is less .
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17. Disadvantages.
1) Initial cost is high and occupies a larger space.
2) The overall power consumption per ton of coal handled is higher than unit system due to high
power consumption by auxiliaries.
3) The operation and maintenance charges are higher than unit system of same capacity.
4) There is possibility of fire hazard due to the stored pulverized coal.
Equipments or components of the pulverized coal fired plant.
The main equipments used in the pulverized coal fires plant are
1) Primary crushers .
2) Magnetic separators.
3) Coal driers .
4) Pulverizing mill
5) Burners.
Primary crushers
Crushing of coal is required when we are handling un sized coal. Plant using pulverized coal generally
specifies the top size, larger than what can not be handled by the pulveriser, making crushing necessary
to prepare coal for pulverization following types of crushers are used.
1) Ring crushers
2) Hammer mill crushers.
3) Bread ford breaker.
4) Rotary breaker
5) Single roll crushers.
Ring crushers
In this type of crushers coal is fed at top of crushers and is crushed by the action of ring that pivot off
centre on a rotor. Adjustable plate helps in varying the size of discharge coal. It can be used as off or on
plant site.
Hammer coal crusher
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18. In this type of coal crusher also the coal is fed from the top and is crushed by the action of swinging
hammers that are pivoted on a rotor. Swinging hammers are attached to the central drum. As the drum
rotates coal particles coming in between the swinging hammers and adjustable plates crushed . The
crushed sized coal falls out of the crusher through the opening provided at the bottom. The adjustable
plates used to vary the size of discharge coal.
Brad ford crushers.
It is used in large capacity plant. It comprises of large cylinder consisting of perforated steel screen
plates to which lifting shelves are attached inside . The cylinder rotates slowly at about 20 rpm and
receives feed at the one end. The coal is lifted by the shelves , the breaking action is accomplished by
the repeated lifting and dropping of the coal until its size permits it to discharge through the perforation
made . The size of the perforation determine the size of crushed coal. The main advantage is
rejection of the foreign matter and to produce relatively uniform size coal particles.
Rotary breakers.
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19. The crushing of coal takes place between the rotating cylinder and rollers. The crushing action is
combination of both lifting and dropping of the coal and also by the crushing action of coal between
rollers and rotating cylinder.
Single roll crushers
The crushing of coal takes place between adjustable plate and rotating single roller having teeth on the
circumference. The size of the coal particle can be varied by varying the gap between adjustable plate
and rotating roller.
Pulveriser:
Pulverisers are devices that are used to produce coal in the powder form. They are also called as
pulverizing mills. The pulverizing process consists of three stages namely i) Feeding ii) Drying iii)
Grinding . Feeding system controls automatically air required for drying and transporting pulverized
fuel to the burner depending on the boiler demand. For pulverization of coal has to be dry and dusty.
Dryer are an integral part of the pulverizing equipment. For drying coal part of primary air passing
through the air preheater at 3500c is utilized. The third stage of pulverization process is the grinding
and equipment used for this action is known as the grinding mill. Four different types of pulverizing
mills are used .
a) Ball and race mill
b) Bowl mill.
c) Ball mill.
d) Hammer mill.
a) Ball and race mill
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20. This is also known as the contact mill. The coal is crushed between two moving surfaces ball and race.
The upper race is stationary and the lower race is driven by worm and gear, holds the steel balls
between them. The coal is allowed to fall on the inside of the race from feeder or hopper. Moving balls
and race catches coal between them to crush in to a powder. Springs are used to hold down the upper
race and adjust the force needed for crushing. Hot air supplied picks up the coal dust as it flows
between the ball and races and then enters in to the classifier, moving and fixed vanes make the entering
air to form a cyclonic flow which helps to through the oversized particles on to the wall of classifier.
The oversized particles slide down for further grinding in the mill. The coal particles of required size
carried to burners with air from the top of the classifier.
b) Bowl mill
20

21. The bowl mill grinds the coal between a whirling bowl & rollers mounted on pivoted axis. The
pulveriser consists of stationary rollers and power driven balls in which pulverization takes place as the
coal passes between the bowl and rollers. The hot primary air supplied in to the bowl picks up coal
parcels and passes through the classifier. Where oversized coal particles falls back to bowl for further
grinding. The required size coal particles along the primary air supplied to the burner.
c) Ball Mill with double classifier
21

22. The line diagram of the ball mill is as shown in figure . It consist of a large cylinder partly filled with
varying sized steel balls. The coal from coal hopper fed in to the cylinder with the help of crew
conveyor. At the same time required quantity of hot air from air preheater is also enters. As the cylinder
rotates pulverization takes place between the balls and the coal. The stream of hot air picks up the
pulverized coal and pass through the classifier. The oversized coal particles thrown out of the air stream
in the classifier and fine coal particles are passed to the burner through exhaust fan. Ball mill capable of
pulverizing 10 tons of coal / hr containing 4% moisture requires 28 tons of steel balls and consumes 20-
25 KW – Hr energy per ton of coal pulverized.
d) Hammer mill
The hammer mills have swinging hammers connected to an inner ring and placed within the rotating
drum. The coal to be pulverized is fed in to the path of hammers. Grinding is done by the combination
of impact on large particles and attrition on small particles . The hot air is supplied to dry the coal as
well as carrying coal particles to burners. It is compact low in cost and simple in operation. How ever
its maintenance is costly and its capacity is limited. The power consumption is high when fine powder
is required.
22

23. Pulverised fuel burners.
Burners are devices use to burn coal particle by uniform mixing of coal and air and creation of
turbulence within the furnace. The air which carries pulverized coal in to the furnace through the
burner is primary air. The secondary air required for completing combustion is supplied separately
around the burner or else where in the furnace.
The main requirements of pulverized fuel burners are.
1) It should mix thoroughly primary air with coal particles and secondary air.
2) It should create turbulence and maintain stable combustion.
3) It should control the flame shape and it travel in the furnace.
4) The velocity of primary air and coal particles should be same as that of flame velocity to avoid flash
back.
5) The burner should have ability to with stand overheating due internal fires and excessive abrasive
wear.
Types of pulverized burners are
1) Long flame or U- Flame burners. Or streamlined burners.
2) Turbulent burners.
3) Tangential burners.
4) Cyclone burners
1) Long flame burners.
The tertiary air supplied around the burner to provide better mixing of primary air and fuel. The burner
discharges air and fuel mixture vertically down wards with no turbulence to provide long flame. Heated
secondary air supplied at right angles to the flame creates turbulence that required rapid combustion.
This type of burners are suitable for burning low volatile slower burning coal particles.
2) Turbulent Burners.
23

24. These burners are also called as short flame burners. Turbulent burners can project flame horizontally
or at small inclination to the furnace. The fuel – primary air mixture and secondary hot air are arranged
to pass through the burner in such a way tat there is good mixing and the mixture is projected in highly
turbulent form in to the furnace. The mixture burns intensely and combustion is completed in a short
distance. The burning rate of turbulent burners is high compared to other types of burners. Turbulent
burners are preferred for high volatile coal and they are used in modern power plants.
3) Tangential burners.
It consists of four different burners located at 4 corners of the furnace. The discharge of fuel and air
mixture directed tangentially to an imaginary circle in the centre of the furnace. The swirling action
creates necessary turbulence required for completing the combustion in short period. The tips of the
burners can be angled through a small vertical arc. So as to raise or lower the position of turbulent
combustion region in the furnace. It helps in maintaining constant super heat temperature of steam as
load varies. This arrangement can provide 1000c difference in furnace gas exit temperature.
Advantages
1) Parts of burners are well protected.
2) High combustion efficiency and turbulence existing throught the furnace.
3) Liquid, gaseous and pulverized fuel can be readily fired either separately or in combination.
24

25. Cyclone Burner
It consists of horizontal cylinder of water cooled construction , 2 to 3 meters in diameter and 2.5 m in
length . The horizontal axis of the burner is slightly deflected downward towards the boiler. These
burners are externally attached to the furnace. The cyclone burner receives pulverized coal carried by
the primary air tangentially to the cylinder at outer end creates strong and highly turbulent Vortex.
Secondary air enters in to the cylinder tangentially to complete the combustion. These burners can be
rotated by 30 degree up and down it helps in controlling the super heater temperature.The fuel
supplied burns quickly with high heat liberate rates with temperature around 20000c . The ash forms the
molten film over the inner wall surface and molten ash flows to an ash disposal system. The cyclone
burners give best results with low grade fuel.
Advantages
1) Crushed coal can be used instead of costly pulverized coal.
2) It can burn low grades of coal.
3) Percentage of excess of air required is less
4) Combustion efficiency is high
5) Combustion rates ca be easily controlled by varying fuel and air supply
6) High furnace temperature can be obtained.
Ash handling system
25

26. General layout of ash handling system
Large quantity of ash is produced by the power plants. Which are burning coals having high ash
content. The ash should be discharged and dumped at sufficient distance from the power plant because
of the following reasons.
1) The ash content is dusty
2) It is very hot when it comes out of the furnace
3) It produces poisonous gases and corrosive acids whenmixed with water.
The amount of ash produced is as large as 20% of total coal burnt during the day. In order to handle this
large quantity of ash use of mechanical handling equipment becomes necessary. Any ash handling
system consists of the following operations.
1) Removal of ash from the furnace.
2) Carrying of ashes from ash hopper to storage with the help of conveyor.
3) Quenching of hot ash before carrying is desirable and necessary as it offers the following
advantages.
a) Reduces the temperature .
b) Reduces dustiness of ash.
c) Reduces the corrosive action.
d) Disintegrate large clinkers in to smaller one .
e) It act as sealing against the air entering in to boiler.
Ash handling equipment.
The main requirements of good ash handling plants are listed below.
1) It should be capable of handling large volume of ash.
2) It should be capable of handling large clickers with minimumattention.
3) The plant should have high rates of handling.
4) The operation should be noise less as much as possible.
5) It should deal effectively both hot and wet ash.
6) The initial cost, operating and maintenance charges should be minimum as per as possible.
The generally used ash handling systems are classified in to four groups.
1) Mechanical handling system
2) Hydraulic handling system.
26

27. 3) Pneumatic handling system.
4) Steam jet system.
Mechanical ash handling system.
This system of handling ash is used in low capacity power plants. The hot ash coming out of furnace
allowed to fall on to the belt conveyor moving through the water trough . Cooled ash carried
continuously by belt conveyor to the ash bunker . The ash is removed from the ash bunker to the
dumping site with the help of trucks.
Hydraulic handling system.
In this system ash is carried with the flow of water . The hydraulic system, is subdivide in to low
velocity system and high velocity system.
Low velocity system.
In this system water trough is provided just below the boiler and water is made to flow through the
trough. The ash falling directly in to the drain and it is carried by water to the sump. In the sump ash is
separated from water , separated water is used again while the ash collected in the sump is removed to
the dumping yard. The capacity of this system is 50 tons/ hr.
27

28. High velocity or high pressure system.
The ash hoppers below the boilers are fitted with water nozzles at the top and on the sides. The top
nozzle quench ash and side nozzle provide driving force to carry the ash through a trough. The cooled
ash with high velocity water is carried to the sump. The water is re circulated again after separating it
out from the ash. Capacity of the system is 120 tons /hr and distance is 1000 meters.
Advantages
1) The system is clan , dustless, totally enclose and pollution free.
2) The ash can be discharged at a considerable distance.
3) Its handling capacity is large hence it can be used in large capacity powerplants.
4) Working parts do not come in contact with ash.
5) It can also be used to handle molten ash.
Pneumatic handling system.
28

29. In primary and secondary separation working on cyclone principle and then it is collected in the ash hopper
as shown in the figure. The clean air is discharged from the top of the secondary air separator in to the
atmosphere through the exhauster. Exhauster may be mechanical type with filter or washer to ensure that
the exhauster handles clean air or it may use steam jet or water jet for its operation. Mechanical exhausters
are used in large power stations. While steam exhausters are used in small and medium power stations. The
pneumatic system can handle abrasive as well as fine materials such as fly ash as soot.
The capacity of system varies from 15 -25 tons/hr.
Advantages:
1) The system is flexible.
2) There is no spillage and re handling.
3) No chances of ash freezing and sticking of the materials , ash can be discharged freely by gravity.
4) Dustless operation as the system is totally closed.
5) Cost / ton of ash handled is comparatively less.
Disadvantages.
1) Wear and tear of pipes is high and hence the maintenance costs are high.
2) The operation is noisy compared to other systems .
Steam jet system
In this type of ash handling system, a jet of high pressure steam is passed in the direction of ash travel
through a conveying pipe in which ash from the boiler ash hopper is fed. The ash is deposited in the ash
hopper . The velocity is given to the steam by forcing it through the pipe under pressure greater than that of
atmosphere.
Advantages:
1) It does not requires any auxiliary drivers.
2) Capital coat and maintenance costs are low.
3) It requires less space.
4) Equipment can be installed in any position
Disadvantages
1) Noisy operation.
2) Wear and tear of pipes is high.
3) Capacity of this system is limited to 15 tons/hr
29

30. Dust collection
Any gas borne matter larger than 1 micron (0.001mm) in diameter we called it as dust. If the particles are
mainly ash particles then it is called fly ash. If the particles are in turn mixed with some quantity of carbon,
then the matter is known as the cinders. The size of cinders is usually greater than 100 micron. Incomplete
combustion volatile components of fuel produces smoke, consists of particles smaller than 10 micron. The
removal of dust and cinders from flue gas can be achieved by using dust collectors. These are classified as
1) Mechanical dust collectors.
2) Electrical dust collectors.
Mechanical dust collectors.
The basic principle used in the mechanical dust collection is as shown in the figure.
a) Sudden velocity decreasing method: Enlarging cross sectional area off the dust carrying pipe helps
in slow down of the gas so that dust particles will have the chance to settle out are allowed to fall
down.
b) Abrupt change of flow direction.: When gas makes a sharp change in flow direction the heavier
particles tend to keep goinig in original direction and so settle out.
c) Impingement upon small baffles: The larger dust particles may be knocked out of the gas stream by
impingement on baffles. These are used to drop large cinders from the gases.
Mechanical dust collectors can be further classified as wet type and dry type. The wet type dust collectors
are also called as scrubbers . Scrubbers operate with water sprays to wash dust from the air. Large quantity
of wash water is required for central power stations and this system is rarely used. This also produces waste
water that may require chemical neutralization before it may be discharged in to the natural water bodies.
Scrubbers may be 1) Packed type 2)Spray type 3) Impingement type
Dry collectors
Dry collectors are the most commonly used . One example for dry dust collectors is cyclone separator. In
this type of mechanical dust collector, a high velocity gas stream carrying the dust particles enters at high
velocity and tangential to the conical shell. This produces a whirling motion of the gas within the chamber
and throws heavier dust particles to the sides and fall out of the gas stream and are collected at the bottom
30

31. of the collector. The gas from the conical shell is passed through the secondary chamber as shown in figure
for final separation.
Advantages
1) Maintenance cost is low.
2) Efficiency is higher for bigger size particles.
3) Its efficiency increases with increasing the load.
Disadvantages
1) It requires more power than other collectors.
2) It is not flexible.
3) Pressure loss is comparatively high.
4) The collection efficiency decreases as the finess of dust particles increases.
5) It requires large head room.
Electrostatic precipitator.
Electrostatic precipitator are extensively used in removal of fly ash from electric utility boiler emissions.
The dust laden gas is passed between oppositely charged conductors and it becomes ionized. As the dust
laden gas passed through these charged electrodes, both negative and positive ions are formed. The ionized
gas is further passed through the collecting unit which consists of set of vertical plates. Alternates plates
are charged and earthed. As the alternate plates are grounded, high intensity electrostatic field exerts a force
on positively charged dust particles and drives them towards the grounded plate. The deposited dust
31

32. particles are removed from the plates by giving the shaking motion to the plates . Dust removed collected in
the dust hoppers.
Advantages
1) It is more effective in removing small particle.
2) Its efficiency s high.
3) The drought losses are least.
4) It provides ease of operation.
Disadvantages:
1) Use of electrical equipment for converting AC in to DC is necessary.
2) The space required is larger than wet system.
3) Collectors must be protected from sparking .
4) The running costs are high.
High pressure boilers.
For generating steam up to 30 bar pressure with flow rates up to about 30 tones / hr can be achieved with
the use of shell boilers using fire tube principles. These boilers are known as the low pressure boilers. High
pressure boilers are operating with pressures ranging from 30 bar to 300 bars, steam flow rate vary between
30 to 650 tones /hr and maximum temperature is around 6000c having furnace height varying from 32 to 62
m. In high pressure boilers water tube principle is used. The boilers operating at pressure 221 bar . Then
such boilers are known as the sub critical boilers. The boilers operating above 221 bar steam pressure are
called super critical boilers.
32

33. The unique feature of high pressure boilers are
i) Method of water circulation.
Use of natural circulation is limited to sub critical boilers with pressures less than 221 bar. In high pressure
boilers forced circulation of water is used instead of natural circulation. With the increase in pressure in the
boiler, the pressure difference causing the natural flow of water decreases and this becomes zero at critical
pressure of steam 221 bar, because the density of water and steam is same at critical pressure. Therefore the
use of forced circulation becomes necessary. Forced circulation of water is achieved with the help of
pumps these pumps are known as forced circulation pumps.
ii) Arrangement of drums and tubing.
In order to avoid large resistance to flow of water these boilers have a parallel set of tubes arrangement.
They have small steam separating drum or may be entirely free of drum.
iii) Improved method of heating.
The following methods are used to improve the heating.
1) heat added to produce steam can be avoid by eliminating latent heat of evaporation at pressure
above critical (221 Bar).
2) Super heated steam is used to heat water by mixing.
3) Heat transfer coefficients can be improved by increasing gas and water velocities above sonic
velocity.
Advantages:
1) Scale formation is avoided due to the use of high velocity of water.
2) Light weight tubes can be used.
3) Reduction in number of tubes used.
4) Boilers are capable of meeting rapid load changes.
5) Completely eliminates the hig head which is needed for natural circulation.
h
6) Since all parts are heated uniformly which eliminates danger of overheating and setting up thermal
stresses.
7) Construction time required is less.
Lamont boiler
33

34. The schematic arrangement of Lamont boiler is as shown in figure .The feed water from hot well is pumped
in to the steam separating drum with the help of the feed pump. The circulating pump draws water from the
drum and delivers it under pressure to the headers. These headers distribute the water to the steam
generating tubes or evaporator, part of the water evaporated is separated in the steam separator drum. The
steam from the top of the drum is allowed to enter super heater s located in the path of hot gases. As the
steam is drawn from the super heater , an equivalent quantity of feed water is supplied through the
economizer in to the drum. The large quantity of water circulated prevents the tubes from being
overheated. These boilers can be built to generate 45 to 50 tones/hr of super heated stem at pressure of 130
bar and at a temperature of 5000c . The major disadvantage of this boiler is the formation of scale due the
presence of dissolved gases in the water it decreases heat transfer rate and efficiency of the boiler.
Loeffler boiler.
34

35. The major difficulty of salt deposition and sediments experienced in Lamont boiler was solved in Loeffler
boiler by preventing the flow of water to the boiler tubes. In Loeffler boiler, feed water passes through an
economizer before its entry in to the evaporating drum. Superheated steam mixing with drum water and
evaporates it in to saturated steam. The saturated steam flows through radiant and convective super heaters.
About 2/3rd of the steam returns to the demand 1/3rd leaves as the steam generator out put. The steam
coming out of from HP turbine is passed through reheater before supplying to LP turbine. The steam
generating capacity of this boiler is 100 tones/hr at 140 bar pressure. It is best suited for land and sea
transport power generation.
Benson:
One of the difficulty experienced in Lamont boiler is the formation and attachment of bubbles on the inner
surfaces of the heating tubes.
In Benson boiler the difficulty of bubble formation experienced in Lamont boiler is avoided by raising the
boiler pressure to critical pressure (221.6Bar) . The arrangement of the boiler components is as shown if the
figure. The Benson boiler is the drum less once through boiler. This boiler takes the feed water in at one
end and discharges it as superheated steam at the other end. Feed water flows through the radiant tube
section to evaporate partly. Where major part of the water is converted in to steam. The remaining water is
evaporated in the convection evaporator tubes. The saturated high pressure steam is further passes through
super heater before leaving the unit.
Major problem that experienced with this boiler is the salt deposition. To avoid this difficulty the boiler is
normally flashed out after every 4000 working hours to remove the salt. Capacity of this boiler is 150 tones
/ hr of steam generation with pressure 300 bar at 6000c.
Advantages
1) There may be no pressure limitation and it may be as high as super critical.
2) Absence of drum and hence cost is less.
3) Evaporation is quick.
4) Light in weight.
5) Space re4quired is less.
6) Expansion problem is less compared to drum type boiler.
Disadvantage
1) The deposition of salt in evaporator tube is common.
2) Over heating of tubes incase of insufficient water supply.
35

36. 3) It requires close coordination between steam generation and feed water supply.
4) There is a greater chance of corrosion of evaporator tubes.
Velox boiler.
The Velox boiler is a high pressure , forced circulation pressurized or forced combustion boiler with the
limitation of firing with oil or gas. Air is compressed to about 2.5 bar in an compressor run by gas turbine
before being supplied to an furnace. Compressed air helps in generating high velocity gas and also at the
same time release of greater amount of heat . The heat transferred from gases to water while passing
through the annulus to generate the steam. The mixture of water and steam thus formed then passes in to
separator. The separated steam is further passed in to the super heater and then supplied to the prime move.
The water removed from mixture is again passed in to the water tubes with the help of a pump. The gases
coming out of the combustion chamber are used for superheating steam in super heater. The gases coming
out of the gas turbine are used to heat water in economizer. The capacity of this boiler is limited to 100
tones /hr
Advantages:
1) High combustion rates are possible.
2) Low excess air is required.
3) It is very compact.
4) It can be quickly started.
Schmidt Boiler
36

37. It consist of two separate circuits. In primary circuit steam is produced from distilled water. The generated
steam is passed through an heating coil located in evaporator drum. The steam produced in the evaporator
drum from impure water is passed through the super heater and then supplied to the prime mover. The high
pressure condensate formed in the submerged heating coil is circulated through the low pressure feed water
pre heater to raise the fed water temperature to its saturation temperature.
Advantages:
1) Overheating of tubes is completely eliminated.
2) It is capable of taking wide fluctuation of loads.
3) Removal of salts deposited is easy.
Diesel Engine power generation.
37

38. Introduction: Diesel electric engine plants which are in the range of 2 to 50 MW capacity used as central
stations for small capacity and they are universally adopted to supplement hydroelectric or thermal stations
where stand by generating plants are essential for starting from cold conditions.
The demand for diesel engine plants increased in some countries due to the difficulties in establishing the
new hydroelectric steam , steam power plants and enlargement of old power plants.
The diesel engines are most efficient than any other heat engines of comparable sizes . It is cheap in first
cost. It can be started quickly and brought in to the service. Its manufacturing periods areshort.
Applications:
The following are the very important applications of the diesel power plants.
1) Peak load plants: Since the diesel plants can be started quickly and it has no stand by losses it can
be used as peak load plant in combination with thermal and hydel plant.
2) Stand by unit: This can be used as stand by unit to supply part load when required. There are many
situation like when main unit fails or can not cope with demand, due to less rain fall in a particular
year, hydroelectric power plant can not meet the demand. Thus diesel units are installed as stand by
unit to supply power in parallel to generate the short fall of power.
3) Central stations: Due to the ease in installation, starting stopping, diesel electric plants can be used
as central stations where the capacity required is very small.
4) Starting stations: The diesel units are used to run the auxiliaries for starting the large thermal power
plants.
5) Mobile units: These plants are mounted on the trailer are used to supply the power to construction
works that are carried out in remote areas where there is no power.
6) Nursing stations: In remote areas where there is no supply of power from main grid diesel electric
plant can be installed. When the power becomes available by main grid, this plant can be shifted to
other place such as diesel electric plants is generally called nursery uni or stations.
t
7) Emergency plant: Diesel electric plant can be used as the emergency plant to meet the power
requirement when ever there is an power interruption, in the emergency situations like tunnel
lighting, hospitals, Telecommunication and watersupply.
Advantages:
1. Design and installation are very simple
2. Can respond to varying loads without any difficulty.
3. The stand by losses are less.
4. Occupy less space .
5. Can be started and put on load quickly .
6. Require less quantity of water for cooling purposes.
7. Overall capital cost is lesser than that for steam plants .
8. Require less operating supervising staff as compared to that for steam plants.
9. These plants can be located very near to the load centre.
10. Lubrication system is more economical as compared with that of a steam power plant.
Disadvantages:
1.High operating cost
2) High maintenance and lubrication cost.
3) Diesel units capacity is limited . These plants cannot be constructed in large size.
4) In a diesel power plant noise is a serious problem.
5) Diesel plants cannot supply over loads continuously.
6) The life of the diesel plant is quite small.
General layout of diesel engine power plant:
38

39. The essential components of diesel engine power plants are shown in the figure. It consists of the following
components.
1) Engine: This is the main component of the plant which develops required power by burning fuel.
Which is directly coupled to the generator
2) Air filters and super chargers: Air filters remove dust from the air which is taken by the engine.
Super charger is used to increase the pressure of air supplied to the engine to increase the engine
power.
3) Exhaust system: This includes the silencers, connecting ducts. The high temperature of exhaust gas
is utilized for heating oils or air supplied to the engine.
4) Fuel system: It includes the storage tank, fuel pump, fuel transfer pump, strainer and heaters. The
fuel is supplied to the engine according to the engine load on the plant.
5) Cooling system: It includes water circulating pumps, cooling towers, or spray ponds, water filters
on plants. This system helps in maintaining the temperature of engine within the allowable limits.
6) Lubrication system: It includes the oil pumps, oil tank filters, coolers and connecting pipes. It
helps in reducing the wear and tear of the moving parts.
7) Starting system: This includes compressed air tank. The function of this system is to start the
engine from cold condition by supplying compressed air.
8) Governing system: The function of the governing system is to maintain the speed of the engine
constant irrespective of load. This can be done by varying the fuel supply to the engine according to
the load.
Types of diesel engines:
Different types of diesel engines are used 1) Four stroke 2) Two stroke 3) Horizontal engine 4) Vertical
engine5) Single cylinder 6) Multi cylinder 7) Indirect ignition 8) Direct ignition 9) naturally aspirated 10)
Super charged engine 11) Air cooled engine 12) Water cooled engine etc. Four stroke multi cylinder
engines are generally used.
Methods of starting diesel engines:
39

40. It is difficult to start even smallest diesel engine by hand cranking as the compression pressures required
are extremely high. Therefore some mechanical system must be used to start the engine.
There are three methods are used in starting the diesel engines they are:
1) By an auxiliary engine: An auxiliary engine mounted close to the main engine drives the diesel
engine plant through clutch and gear . Once the engine stared auxiliary starting device automatically
disengages.
2) Storage battery and electric motor: By using an electric motor, in which a storage battery of 12 to
36 volts is used to supply power to an electric motor that drives the engine.
3) Compressed air system: In this system compressed air is used to start the diesel engine. The
compressed air about 17 bar. The compressed air from this air tank is admitted to the few engine
cylinder making them to work like reciprocating air motor to run the engine shaft. Fuel is admitted
in to the remaining cylinders and ignited to start the engine. Compressed air system is widely used
to start the diesel engine plants.
Different systems of diesel engine power plants
1) Fuel storage and supply system:
The fuel storage system and supply system depends on the type of fuel, size of plant and type of engine
used and so on. The fuel oil may be delivered at the plant site by many means such as trucks, railway
wagons or Barges and oil tanks with the help of unloading pump the fuel oil is delivered to the main tank
from which oil is pumped to small service storage tank known as engine day tank through the strainers. In
order to reduce the pumping power input oil is heated either by hot water or steam which reduces the
viscosity and so the power input. From storage tanks oil flows under gravity to the engine pumps. This type
of system is as shown in figure and it is generally preferred for medium size or big size plants. The location
of storage tank above ground or below ground depends on the local conditions. The heating requirements
depends on the climate conditions.
Fuel Injection system:
It is an heart of the engine and its failure means stopping of engine. It performs the following functions.
40

41. 1) It filters the fuel ensuring oil free from dirt
2) It measures the correct quantity of the fuel to be injected in each cylinder.]
3) It times the injection process in relation to the crank shaft revolution.
4) It regulates the fuel supply.
5) It atomizes the fuel oil under high pressure for better mixing with hot air leading to efficient
combustion.
6) It distributes the atomized fuel properly in the combustion chamber.
There are two methods used in atomizing the fuel.
1) Air injection system 2) Mechanical injection system
Air injection system is obsolete and mechanical injection is invariably used. In mechanical injection or
solid injection system fuel oil is forced to flow through the spray nozzles at pressure above 100 bar. There
are three types of solid injection system.
1) Common rail injection system
2) Individual pump injection system.
3) Distributor system.
1) Common rail injection system
This system consists of a high pressure pump which distributes fuel to a common rail or header to which all
the fuel injectors are connected. In common rail system, the fuel injectors are operated mechanically. The
metering and timing of fuel injection is accomplished by the spray valve. Then amount of fuel to be
injected in to the cylinder in controlled by the lift of the needle valve in the injector. The quantity of the
fuel injected depends on the duration of valve opening size and number of holes in the nozzle tip, fuel
pressure and air pressure in the cylinder.
2) Individual injection system:
41

42. As the name employs, the system has an independent high pressure pump for each cylinder with meters,
pumps and controls the timing of fuel injection. Each cylinder is provided with one injector and the pump.
The fuel is brought to the individual pump from storage tank or day tank through filters, low pressure
pump. The high pressure pump is equipped with control mechanism of injecting fuel at the proper time , a
rocker arm actuates the plunger and thus injects the fuel in to the cylinder . This is the most popular fuel
injection system used.
3) Distributor fuel injection system.
The above figure shows the arrangement of distributor system. In this system, a metering and high pressure
pump is used to pump the metered quantity of fuel in to the rotating distributor which distributes the fuel to
42

43. the individual cylinders at the correct timing. The number of injection strokes per cycle for the pump is
equal to the number of cylinders. The fuel is fed to high pressure pump from storage tank through course
filter, low pressure pump and the fine filter.
Air supply system:
A large diesel power plant requires considerable amount of air as 4 to 8 m3 / Kw-hr . The air contains
considerable amount of dust and , therefore it is necessary to remove this dust from air before entering in to
the cylinder which would otherwise cause excessive wear in the engine. An air supply system of diesel
power plant begins with intake located out side the building provided with filters. The filters may be oil
impingement or oil bath or drag type depending upon concentration of dust in the air. In cold climate
conditions the air intake system needs heating and necessary heating of air is provided by using the heat
from the exhaust gases.
Exhaust system:
The purpose of exhaust system is to discharge the engine exhaust to the atmosphere with minimum noise.
The following fig shows the exhaust system. The exhaust manifold connects the engine exhaust outlets to
the exhaust pipe which is provided with a muffler or silencer to dampen the fluctuating pressure of exhaust
line which in turn reduces the noise . To isolate the exhaust system from engine vibration flexible exhaust
pipe is used. A provision is made to extract heat from exhaust system if the heating is required for fuel oil
heating or building heating or process heating.
Cooling system:
43

44. The temperature of the gases inside the cylinder vary from 3500c to 27500c . If there is no external cooling,
the cylinder walls and piston tend to attain the temperature of the gases which may be of the order of
10000c to 15000c . The cooling of the engine is necessary forthe following reasons.
1) To prevent the disintegration of the lubricating oil.
2) To avoid the seizure of the piston.
3) Overheating of cylinder may lead to pre ignition of fuel air mixture which affects the performance
of the engine.
4) The strength of the materials used for various engine parts decreases with temperature. It may lead
to the cracking of the different parts of the engine due to uneven heating.
5) At high head temperature , the volumetric efficiency and hence the power out put of the engines are
reduced.
If the engines are not properly cooled temperatures inside the engine causes disintegration of the lubricating
oil on the liners , wrapping of valves and pistons takes place. Proper cooling of engine is necessary to
extend the life of the plant. Therefore exit temperature of the cooling water must be controlled. If it is too
low, lubricating oil will not spread properly and wearing of piston and cylinder takes place . If it is too high,
the lubricating oil burn. Therefore maximum exit temperature of water is limited to 700c. Based on cooling
medium used cooling systems are classified in to
1) Air cooling system.
2) Liquid or indirect cooling system.
1) Air cooling system:
It can be used in very small engines and portable engines by providing fins on the cylinder. The fins are
arranged in such a way that they are at right angles to the cylinder axis and air flow should be such that the
fin surfaces are exposed to maximum air flow.
Advantages:
1) The cooling system is simple as no cooling pipes, radiators are involved.
2) For a given power the weight of air cooled engines is lesser than the liquid cooled engines.
3) The design of air cooled engines is simple as no water jackets are required surrounding the engine.
4) The engine is free from freezing problems as in the case of water cooled engines.
Disadvantages:
1) The noise level is high.
2) Non uniform cooling.
3) The power produced by air cooled engines
2) Liquid or indirect cooling system.
In this method of cooling engines the water jackets are provided in the cylinder wall and cylinder head
through which cooling liquid can be circulated. Heat is transferred from the cylinder barrels and cylinder
44

45. head to the liquid by conduction and convection. The cooling fluid it self is cooled by transferring heat to
the air in a radiator or in cooling towers.
Diesel engines are always water cooled. Various methods used forcirculating water around cylinder are
Big diesel engines are always liquid cooled. Liquid cooling system is further classified as
1) Open cooling system.
2) Natural circulating system.
3) Forced circulation system.
4) Evaporation cooling system.
Open cooling system
This system is suitable only for those plants where plenty of water is available. The hot water coming out
of the engine is not cooled for reuse but it is discharged.
Diesel engine
Hot water discharged
PUMP
in to river system
Natural circulation system (Thermo syphon cooling)
This system is closed one and its design is such that water may circulate naturally due to density difference
in water at different temperatures. The following fig shows the natural circulation system. It consists of
water jacket radiator and a fan. When water is heated its density decreases and tends to rise while the colder
molecules tend to sink. Circulation of water then is obtained as water heated in the water jacket tends to rise
and water cooled in the radiator with the help of air passing over radiator either by ram effect or by fan or
jointly tends to sink.
Thermostatic controlled circulation system:
45

46. It is as shown in the figure. It is also closed one. The system consists of pump, water jacket in the cylinder,
radiator, fan and a thermostat. The coolant is circulated through the water jacket with the help of pump
which is usually a centrifugal type, driven by engine. The thermostat provided in the engine upper nose
regulates the temperatures of cooling water. Stand by diesel power plant up to 200 KVA use this type of
cooling system. In case of bigger plant, the hot water is cooled in a cooling tower and recalculated again
and again. There is a need of small quantity of cooling make up water.
Evaporative cooling system:
In this method water is allowed to evaporate by absorbing the latent heat of evaporation from cylinder
walls. Evaporated water is once again converted in to liquid water by allowing it flow through the radiators.
Where it gives up the latent heat absorbed in the cylin walls to the air flowing across the radiator tubes.
der
General layout of the water cooling system
46

47. A common water cooling system used in diesel plant is as shown in figure. The water which is not pure will
cause deposits at temperature about 500c. Therefore it is necessary to purify the water before entering in to
the system and to prevent the growth of algae. Which may reduce the heat transfer due to fouling. The
cooling water is treated with Calgon to control the scaling in the different parts of the system and it is also
chlorinated once per shift to prevent algae growth which would cause the rapid tube fouling . To prevent
corrosion of tubes Sodium Chromate is added.
Based on the water circuit system cooling system is divided in to single circuit system and double circuit
system.
In the single circuit system may be subjected to corrosion in the cylinder jackets because of the dissolved
gases in the cooling water. The double circulating system completely eliminates internal jacket corrosion
but corrosion may exist in raw water circuit
Lubrication system:
The purpose of lubrication system is to provide sufficient quantity of cool filtered oil to give positive and
adequate lubrication to all moving parts of the engine. The following are the function of lubrication.
1) To reduce the friction which reduces the power required to overcome the same.
47

48. 2) To reduce the wear and tear between the moving parts.
3) To cool the surfaces by carrying away heat generated by friction.
4) To carry away the particles of carbon and metal scrap.
5) To provide sealing between the moving piston and cylinders.
6) To reduce engine noise and to increase the engine life.
7) To avoid corrosion and deposits.
General outline of lubrication system
The forced feed circulation is generally used to lubricate all the parts. The general equipments which are
used in lubrication system are pumps, oil cleaners, oil coolers, storage, sump tanks and safety devices. The
frictional losses of engine will appear as the heating of the lubricating oil during its circulation through the
engine. It is necessary to remove this heat for proper functioning of the lubrication. The lubricating oil is
cooled in an oil cooler before supplying to engine. The cooling is done by using the water from the sump
of the cooling tower.
The oil from the engine is filtered by passing through the metal screen strainers and ultimate cleaning is
accomplished by passing oil through the centrifugal cleaners . The oil is heated to increase the fluidity of
the oil before passing through the cleaning system by using hot water or steam circulating in the heaters.
The lubrication system is classified as
a) Mist lubrication system.
b) Wet lubrication system
1) Splash lubricating system
2) Pressure feed system
3) Splash pressure feed system
c) Dry sump lubrication.
a) Mist lubrication system:
This system is used for two stroke engines. In this type of lubrication system small quantity of
lubricating oil is mixed in the fuel tank. These engines are lubricated by adding 2 to 3 percent
lubricating oil in the tank The oil and fuel mixture is induced through carburetor . The gasoline is
vaporized and the oil in the form of mist , goes via crankcase in to the cylinder. The oil which impinges
on the crankcase walls lubricate the main and connecting rod bearing and rest of the oil which passes on
the cylinder during the charging and scavenging periods, lubricates the piston and , piston rings and the
cylinder
b) Wet lubrication system:
1) Splash lubricating system:
48

49. This system is used on small four stroke stationary engines. A splasher or dipper is provided under each
connecting rod which dips in to the oil in the trough at every revolution of crank shaft and oil is
splashed all over the interior of the crank shaft. Surplus oil eventually flows back to the oil sump. Oil
level in the trough is maintained by means of a oil sump which takes oi from sump through a filter.
l
2) Pressure feed lubrication system
The oil is drawn from sump through strainer which prevents foreign particles and is pumped with help
of gear pump submerged in the oil. An oil hole drilled in the crankshaft from the centre of each
crankpin to the centre of an adjacent main journal , through which oil can pass from main bearing to
the crank pin bearing. The cylinder walls, cam, piston and piston rings are lubricated by oil spray from
around piston pins and main connecting rod bearings. A pressure regulator fitted at the delivery point of
the pump helps in regulating pressure of lubricating oil in the circuit. Excess oil is returned back to the
sump. A pressure regulating valve is also provided on the delivery side of this pump to prevent
49

50. excessive pressure. This system finds favor from most of the engine manufacturers as it allows high
bearing pressure and rubbing speeds.
3) Splash pressure feed lubrication
In this case lubricating oil is supplied under pressure to main and cam shaft bearings . Splash is used to
lubricate crankpin bearings.
c) Dry sump lubrication system
In the dry sump lubrication system the supply of oil is carried in an external tank with the help of
scavenging pump through strainer and filter. The pump is placed out of the sump . The capacity of the
scavenging pump is always greater than oil feed pump. The supply tank is usually placed behind the
radiator. The dry sump is generally used in large stationary marine engines. The oil pressure may vary
from 3 to 8 Kgf / cm2 . Dry sump lubrication system is generally adopted for high capacity engines.
DRAUGHT
Draught is mechanism of creation of small pressure difference that is required maintain the constant
flow of air for combustion of fuel and to discharge the gases through the chimney to atmosphere.
Draught can be produced by using chimney, fans , steam or air jet or combination of these .
The purpose of draught is to
1) Helps in allowing desired volume of air flow in to the furnace.
2) Helps in overcoming the resis tances offered to the flow of air through the furnace.
3) Discharge gases at sufficient height to avoid pollution to atmosphere.
Classification of draught system.
50

51. I) Natural draught.
II) Artificial draught
a) Steam jet draught
i) Induced draught
ii) Balanced draught.
b) Mechanical draught .
i) Induced draught
ii) Forced draught.
iii) Balanced draught.
Natural draught:
If the draught is produced only with the help of chimney it is known as natural draught.
Artificial draught:
If the draught produced by other than chimney it is known as artificial draught. Artificial draught can be
produced either by using fan or using steam jet. Depending on the positions of fans or steam jet they are
further classified as Induced , forced and balanced draught.
Mechanical draught:
This type of draught is produced by using fans in the gas flow path depending on the locations and number
of fans we have ,
a) Forced draught fan system:
The forced draught system fans are installed at the base of the boiler. This draught system is known as
the positive draught system. The fans or blowers installed at the base of the boiler forces the air through
the fuel bed, Economiser, air preheater and to the chimney. The furnace has to be gas tight to prevent
the leakage of gases in the boiler house . Since the FD fans handle cold air, so they consume less power
and less maintenance problems.
b) Induced draught
51

52. In this type of draught system a fan or blower is located at the base of the chimney. The ID fan sucks
the burned gases from furnace and the pressure inside the furnace is reduced below atmosphere and
induces the atmospheric air to flow through the furnace. The draught produced is independent of the
temperature of the gases . This draught is similar to the natural draught system in action but the total
draught produced is the sum of draught produced by the fans and chimney.
Advantages of Forced draught over induced draught :
1) Forced draught fans does not requirewater cooled bearing.
2) The tendency of air leak in to the furnace reduced.
3) The life of the FD fan blades is hight.
4) The power required for FD fans is less compared to Induced draught fans.
c) Balanced draught.
It is the combination of the both forced draught and induced draught system. In this system FD fan over
comes the resistance of fuel bed and air pre heater. The induced draught fan removes the gases from the
furnace maintaining the pressure in the furnace just below atmosphere.
Advantages of mechanical draught.
1) Easy control over the combustion process.
2) The efficiency of thermal power plant improves.
3) It reduces the chimney height.
4) Low grade fuel can be used with high intensity of mechanical draught.
52

53. 5) The fuel burning capacity of grate is enhanced.
Steam jet draught system.
This type of draught is produced by introducing the steam in the flow path of the gases depending on
the location of steam jet we have induced jet draught and forcedjet draught systems.
Induced jet draught:
This is used in locomotive boilers. In this method of draught the steam from the boiler is led to smoke
box through the nozzle to produce draught but when it is running air enters through the damper and
forces it way through the grate besides the induced draught is also produced by utilizing the exhaust
steam from the cylinder through the nozzles placed in the smoke box.
Forced jet draught
In this case steam nozzle is placed in a diffuser pipe. The steam from the boiler after being throttled to 2
bar enters the nozzle and emerging out with a great velocity dragging air with it. The mixture of air and
steam with high kinetic energy passes in to the diffuser pipe where the kinetic energy is converted in to
the pressure energy which forces the air out of chimney.
Natural draught:
The natural draught is produced by chimneyor stack. It is caused by the density difference between the
atmospheric air and the hot gas in the stack. This type of draught is useful for small capacity boiler and
it does not play much important role in the high capacity thermalpower plant.
consider the chimney above grate level is H. The pressure acting on the grate from chimney side
P1 = Pa + Wg H
= Pa + g gH (1)
Where Pa – Atmospheric pressure at chimney top.
g gH – Pressure due to the column of hot gas of height H meters.(Chimney side)
And g = Average mass density of hot gas Kg/m3
Similarly the pressure acting on the grate on open side
P2= Pa + a gH (2)
a gH – Pressure acting on the grate on open side by the column of cold air out side the chimney of
height H meters.
a – Average mass density of cold air out side the chimney.
The net pressure acting on the grate P = P2 – P1
= gH(a -  g ) (3)
53

54. This difference in pressure is responsible for causing flow of air through the combustion chamber and
discharge of gases through the chimney it is known as static draught. This draught can be increased
either by increasing height of chimney or by reducing the density of gases.
If the acting pressure is in terms of mm of water(head).
hw x Ww = H (Wa – Wg)
hw w g = gH(a -  g )
(hw x 1000x g)/1000 = gH(a -  g )
 hw = H(a -  g ) mm of water (4)
Draught in terms of hot column of gas .
Let Hg is the hot column of gas in meters.
Hg x Wg = H (Wa – Wg)
Hg x g g = gH(a -  g)
Hg = H(a/g - 1) Meters of hot column of gas (5)
Calculation of chimney height and cross section:
If m kg be the mass of air supplied per kg of fuel . Then m+1 will be the mass of flue gases. Assuming
volume of air and gas is same at same temperature, at 00c or 2730 K and at atmospheric pressure one kg of
air occupies volume equal to
v = RT/P = 287 x 273 / 1.013 x 105 = 0.7734 m3 (1)
The volume of the gases at higher temperaturecan be calculated as follows.
Let Tg – Mean absolute temperature of flue gases 0K
Ta – Mean absolute temperature of out side air 0K.
Volume of one kg of air at temperature Ta = (0.7734 x Ta) / 273 (2)
Volume of m kg of air at temperature Ta = (0.7734 x Ta x m) / 273 (3)
Volume of m+1 kg of flue gases at temperature Tg = (0.7734 x Tg x m) / 273 (4)
Hence density of air at temperature Ta = Mass / Volume(air)
a = (m x 273) / (0.773 x Ta x m )
= 353 / Ta (5)
Similarly density of gases at temperature Tg = Mass of hot flue gas / Volume of hot flue gas
= (m+1 x 273) / (0.7734 x m x Tg)
g = [353 x / Tg] x [( m+1) / m] kg / m3 (6)
The draught is the difference in pressure between hot gas columnin chimney of height H and cold air
column of same height H and thus
P = P2-P1 = gH(a -  g) (7)
Substituting the value of a and  g in to the equation from equations 5 and 6
P = 353 gH [ 1/Ta – (m+1)/m x (1/Tg)] N/m2 (8)
If hmm be the draught measured in water column then
h = H(a -  g) mm of water.
= 353 H [ 1/Ta – (m+1)/m x (1/Tg)] (9)
If Hg is the height of a column of hot gas expressed in meters which would produce the pressure P in N/
m2 Then, Hg = {P(N/ m2)} / {Density ((Kg/ m3) x g }
From equation 8
Hg = 353 gH [ 1/Ta – (m+1)/m x (1/Tg)] / { [353 x / Tg] x [( m+1) / m] } x g
= H x (m/ m+1) x Tg [ 1/Ta – (m+1)/m x 1/Tg]
= H [Tg /Ta x ( m/m+1)–1] in meters of hot gas (10)
Cross sectional area of chimney:
Velocity of flue gases in ideal chimneyis C = (2gHg)1/2 (1)
54

55. In practical chimney we can not avoid draught losses. Let Hg be the losses in the chimney equivalent to
hot gas column in meter then the velocity of gas in the chimney is
C = (2gHg- Hg)1/2
= 4.4294 x (Hg) 1/2 ( 1- Hg / Hg) 1/2
= K (Hg) 1/2 (2)
Where K= 4.4294 x ( 1- Hg / Hg) 1/2
From experiments it is found that K = 1.1 for steel , K= 0.825 for Brick.
From continuity equation the mass of hot gases flowing through any cross section of the chimney
Mg = A x C x g
 A = (Mg / g ) x (1/C)
= (Mg / g ) x [1/ {K(Hg) 1/2 } ] (3)
From equation 3 cross sectional area of chimney can be calculated and from this area the diameter of base
of chimney can be calculated using formula A= D2/4
 The diameter of the chimney = (A x 4/)1/2
Condition for maximum discharge
Theoretical velocity of flue gases produced by static draught is C = (2gHg)1/2 (1)
Where Hg is the height of a column of flue gases corresponding to draught pressure.
Hg = H [Tg /Ta x ( m/m+1)–1] in meters of hot gas column (2)
Substituting the value of Hg in to the equation (1) C = {2g H [Tg /Ta x ( m/m+1)–1] }1/2 (3)
We know that Pv = RT or g = P / R T g = K / T g (4)
Tg – Temperature of hot flue gas g density of gases K – Constant.
This shows that the density of gasses is inversely proportional to its temperature.
Mass of gases flowing through chimney is given by
Mg = Area x Velocity x Density.
= A {2gH[(m/m+1) x Tg/Ta -1}1/2 x K/Tg
= Ax K/Tg { 2gH[(m/m+1) x Tg/Ta -1}1/2
= Ax K’/Tg { 2gH[(m/m+1) x Tg/Ta -1}1/2 (5)
Where K’ = A x K
We can write the above equation as
Mg = K’x (2g/Tg2)1/2 { 2gH [(m/m+1) Tg/Ta -1] }1/2
Using Kg = K’x(2g)1/2
Mg = Kg { (m/m+1) 1/TgTa – 1/Tg2] 1/2 (6)
In the above equation 6 , Tg and Mg are only two variables all other parameters are constants. To find the
condition for maximum discharge differentiate Mg with respect to Tg and equate to zero. Thus for
maximum discharge.
dMg/dTg = 0
= d/dTg { Kg [ (m/m+1) 1/TgTa – 1/Tg2] ½ }
= Kg x ½ x -(m/m+1) x 1/ Ta x 1/Tg2 + 2/Tg3 / { [(m/m+1) x 1/TgTa-1/Tg2] 1/2 }=0
(m/m+1) x 1/ Ta x 1/Tg2 = 2/Tg3
Tg / Ta = (m+1)/m x 2
Thus we can see that the absolute temperature of the chimney gas bears a certain ratio to the absolute
temperature of the out side air. Using this value of Tg/Ta in to the equation of height of chimney.
H max = H [ (m/m+1) x Tg / Ta -1]
Replacing Tg/ Ta = 2 ( m+1/m) = H [ m/m+1 x 2 ( m+1) /m-1]
Hmax = H (7)
Thus for maximum discharge of flue gases draught produced is equal to the height of the chimney
therefore.
Maximum discharge (Mg)max = A x g (2gHg)1/2
= A . (P / RTg) x (2gHg)1/2
= A P / RTg x Ta /Ta (2gHg)1/2
Substitute the value of Tg / Ta in to the above equation
55

56. = (APm/2RTa) x (2gHg)1/2 / (m+1) (8)
The draught in mm of water column is
h = 353H[1/Ta –( m+1) / m x 1/Tg]
For maximum discharge the condition is T g = 2Ta (m+1)/m
Substitute this value in to the equation 9
h = 353H [1/Ta – ( m+1) / m x (m/m+1)x 1/ (2xTa)]
= 176.5H/Ta mm of water .
Efficiency of the chimney.
The temperature of the flue gases leaving the chimney in case natural draught is higher than that of the flue
gases leaving in case of artificial draught. This leads to the certain minimum temperature needed to produce
a given draught for a given height of chimney. This shows that the draught is created at the cost of thermal
efficiency of the boiler plant. Therefore efficiency of the chimney is defined as the ration of the energy
equivalent of draught produced by artificial draught fan system expressed in meter head to the energy
equivalent expressed in per kg of gases of additional heat carried away by the flue gases in the natural
draught system.
Let Tg be the temperature of the flue gases in the chimney for natural draught at 00C.
Tg’ Temperature of flue gases in chimney for artificial draught 00C.
Hg is the column of the flue gases equivalen of draught produced by artificial draught meter head.
t
o
Cp is specific heat J/Kg K of gas.
chimney = Hg/JCp(Tg-Tg’) = H [ (m/m+1) x Tg/Ta -1 ] / JCp (Tg – Tg’)
Power required to drive the fan ( Artificial draught / Mechanical draught)
V – Volume of the flow gases through the fan m3/min .
H= Draught produced by the fan mm of water.
P = Draught in N/m2 = Efficiency of fan.
We have P = w gh = 1000/1000 x (gh)
The work done on the gas
W = PV / 60 = ghV/60 Watts
Power required to drive the fan = ghV / (60 x 1000 x )
Problems:
1) A 200 m high 4 m dia stack emits 1000kg/s of 1000c gases in to 50c air. The prevailing wind velocity is
50 Km/h. The atmosphere is in a condition of neutral stability. Calculate the height of the gas plume.
Soln : Data- H = 200 m , D = 4 m , Mg = 1000 kg/s , Tg = 1000c Ta = 50c
Vw = 50 x 1000/3600 = 13.89 m/s .
Using correlation of Carson and Moses.
H = 2.62 (Qe)1/2 / Vw – 0.029 VsD/Vw
Where D = Stack dia, m , Vw = Wind velocity , m/s ,
Qe = heat emission from plume , watts = MgCp(Tg-Ta)
= 1000x1.005 (100-5) = 95475 KW
Vs = Stack gas exit velocity , m/s,
= Mg/gA
Where g = Pg/RTg = 101.325 / (0.287 x 373) = 0.9465 Kg/m3 .
A =  x D2 / 4 =  x 42 / 4 = 12.56m2
Vs = 1000/ (0.9465 x 12.56) = 84.12 m/s
56