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Fossil Fuel Steam Generator-II

Hello, I welcome you all in this course of Power Plant Engineering. We will continue

to discuss today, the Fossil Fuel Steam Generators and specifically today, we will discuss the

high pressure boilers. So, high pressure boiler is a topic to be covered today. Now, there

are many benefits of the high pressure boilers and nowadays in the industries high pressure

boilers are used. As, I told you in the previous, in my previous

lecture, the high pressure boilers are mainly water tube boilers. They are mainly water

tube boilers. They have a very high efficiency, the high pressure boilers have efficiency

more than 90 percent and nowadays green boilers are also there. The green boilers have efficiency

more than 98 percent.

So, the green boilers, they have efficiency more than 98 percent and very low level of

NOx solution to the surroundings. Regarding the pressure and temperature as I said earlier

high pressure boilers have pressure more than 80 bar. So, the pressure varies from 80 to

300 bars and a temperature of steam 450 to 585 even it exceeds 600 degree centigrade.

So, high pressure boilers have you can have the temperature of steam up to 600 degree

centigrade, pressure up to 300 bars. So, it can go up to 300 bars. Normally it

is between 200 250 bars. They have a small diameter tubes. The smaller diameter tubes

are taken for this boiler for a purpose because when the tube diameter is small, then surface

to volume ratio is high. When surface to volume ratio is high, more heat interaction will

take place between flue gases and the water which is flowing inside the tubes.

But at the same time if you take a smaller diameter tube, more pumping power will be

required; more power will be required to pump the fluid through the tubes. But at the same

time the more heat transfer will be there. There is force circulation of water inside

the tubes in high pressure boilers. Heating is high, there are several arrangements. So,

that there is an improved heating in the boilers. They are compact, some of the high pressure

boilers do not have any drum. So, drum constitutes a considerable amount

of mass in the boilers. So, when there is no drum, at least 15 to 20 percent of the

mass of the boiler is reduced. So, some of the boilers do not have any drums; so, they

are compact. And, some of the boilers are once through construction, once through construction

means in the boiler house steam is a water is a feed water is entering from one side

and steam superheated steam is leaving from another side. So, these type of boilers are

known as once through boilers. We will start, we will take some classical

high pressure boilers.

So, the most popular boiler is La Mont boiler which was introduced in 1925. The steam generation

capacity of La Mont boiler is 50 tons per hour, pressure is 170 bar and temperature

it can it take up to 500 degree centigrade. In a La Mont boiler, there is a boiler shell

right and it narrows down from the top and this is the place for flue gas to leave the

boiler. And, fuel is burned at the bottom of the boiler in fire grate.

So, there is a fuel burning place here, these are the fuel burns. There is a drum in the

boiler, any drum there is a stratification, bottom side is occupied by water, top is occupied

by a saturated steam. From drum, this water this is known as feed water. Feed water is

sent to the boiler with the help of a pump. So, there is a pump here and there is a header.

I am showing only one tube, but there are several tubes.

So, there is a header here, distribution header here. After distribution header, the water

enters the boiler right and the wall of the boiler is covered with the firebrick. So,

there is no heat transmission to the surroundings right. The water when enters so, from header

then it is going to the drum. So, movement of the fluid is like this right. When the

water enters the boiler, the heat transmission to the fluid is by radiation heat transfer.

In high pressure boilers, the 60 to 70 percent heat transmission by radiation heat transfer

right. After radiation, heat transfer there is convective heat transfer here. Convective

transfer between the flue gases and because this because the when the fuel is burning

here, the entire area is filled with a flue gases hot glue gases. So, first the water

entry into the boiler it is. It is actually tubes are embedded in the wall of the boiler

this tubes. I am just showing the schematic otherwise when the water is entering the boiler,

the tubes are embedded in the wall of the boiler and heat transmission is by radiation

not by a convection right. When the tubes, when the flues when the water

reaches here and then in this zone, the convective heat transfer takes place. In this zone, the

convective heat transfer takes place right then, again I will have to erase. Now after

taking convective heat, the steam enters the drum where separation of water and steam takes

place and it is a closed cycle right. Now here, now we want superheated steam from

the boiler. So, again the steam is taken from here and there is a super heater and from

here, we get there is steam supply port and from this side superheated steam is supplied.

So, again the steam is passed through the flue hot flue gases. So, that it can trap

heat for the flue gases and get converted into the superheated steam and still after

superheating there is a lot of heat is going with the flue gases, it has to be trapped.

So, there is an economizer, economizer is an accessory, but now in high pressure boiler,

it is almost essential part of the boiler. Now what economizer does? Now, the hot gases

which are going with the hot sorry the heat which is going with the flue gases can be

trapped by feed water. So, this heat which is going to with the flue gases, there is

a suppose there is a pump feed water pump and this feed water pump feeds the water in

this area. Now, when it is feeding water in this area,

the feed water gets heated. So, that is an advantage. Suppose to the boiler, we are supplying

feed water right 25 degrees centigrade. Now, with this economizer the temperature can be

increased from 25 degrees centigrade to 70 degrees centigrade. So, that is the amount

of equivalent amount of heat will be saving while converting water into the steam.

So, the water instead of supplying water to the boiler here at 25 degrees centigrade,

we are supplying water at 70 degree centigrade. So, again there is a closed loop here and

water is coming and the feed water is coming from this side. So, this is economizers. So,

after connective heating, there is a coil which is economizer coil and the purpose of

economizer is to heat the feed water with the help of flue gases.

After the economizer still there is a lot of heat which is going out of flue gases because

purpose efficient design of the boiler is more and more, how much amount of heat we

can try from the boiler. Now, here air preheater is required provided. Now, the purpose of

air preheater is to preheat the air. So, air is circulated in the preheater and this hot

air is emerging from this side. Suppose there is a fan or the blower in this side and the

hot air which is emerging from this side is supplied to the combustion chamber right for

burning the fuel. Reason being this heat which is coming with

the hot air; suppose, we are using normal atmospheric air at 25 degrees centigrade for

burning the fuel that is case 1 and the case 2. When this hot air is also which is hot

air maybe at let us say 100 degree centigrade or 120 degree centigrade, this hot air is

also; this hot air is used in the second case this hot air is used for a burning the fuel.

So, definitely in the second case trapping of heat is more.

So, basically the purpose of this arrangement is to trap the heat. First the water is entering

in the shell, radiation heat transfer after radiation heat also there is a convection

heat transfer between the tubes and the flue gases then economizer to heat the preheat

to preheat the feed water. And then there is air pre heater to heat the air which is

being used for combustion.

So, this is the arrangement of a La Mont boiler. This is the schematic arrangement which I

have explained you here, they have put the air pump here. Again it is a u type of I have

put air pump here. It does not make any difference and air is used for preheating ok. After the

La Mont boiler, we will take up Benson boiler.

Now, Benson boiler is a drum less boiler it does not have any drum. So, it is a once through

boiler. So, I will make the same type of schematic here also, there is a shell, there is a neck

of the shell and then exhaust of the gasses right. It is a once through boiler. So, feed

water feed water will be entering from the top because economizer has to be at the top

economizer below that the convective heating, below that radiation heating.

So, water is entering here in economizer and after entering economizer, the water is this

tube is coming out and then there is ash pit there is a grate and the furnace and here

it is again radiation heat transfer is taking place. The tube is embedded in the wall of

the furnace and radiation heat transfer between the tube and the flue gases is taking place.

It is the once through boiler. So, the steam will not come out. So, after digging heat,

there is a convection heat transfer. Flue gases are moving in upward direction.

Now, I will explain you again feedwater is coming from here this is the economizer. After

economizer, the feedwater is going is entering from here. It is coming out to the boiler

this feedwater tube and then again it is entering the boiler. Then it is there is a radiation

heat transfer, then convection heat transfer and after convection heat transfer, the superheating

has to be taken right. After this superheating of the steam if required, it is always required

here. So, super heated superheating of the flue

this steam takes place and after superheating takes place here and after superheating, it

emerges from the steam emerges from the boiler. So, if this boiler has different components,

this is economizer. There is radiation heat transfer, there is convective heat transfer

and at the same time there is a super heater and after superheating, the steam is supplied

from the side. Now, air preheating is also required here.

So, for the purpose of air preheating again, there is a fan or blower which makes the air

flow through the pipe and this pipe is surrounded by the flue gases, hot flue gases and the

air is heated and then this air is again supplied to the combustion chamber.

Now, a schematic of this boiler is shown here. This cold they have shown pump this side.

So, cold air is coming from this side living from this side and this air is used for burning

the fuel here in the grate. This type of arrangement is shown. This is a, this is embedded tubes

in the wall and the radiation heat transfer is taking place between the tubes and the

flue gases. So, this is the arrangement of the Benson boiler. This boiler does not have

any drum to collect the feed water to collect the steam.

So, it is a once through boiler, feed water is entering from one side and superheated

steam is leaving from another side.

The capacity of Benson boiler is 150 tons per hour, pressure is 200 bars quite high

and temperature also we can attain 650 degree centigrade.

Now, after the Benson boiler, we will take Loeffler boiler. So, this boiler has, I mean

higher capacity you can go up to 100 tons per hour steam generation in this boiler.

Pressure is 140 bar pressure and it can provide a steam at 500 degree centigrade right. So,

this boiler has arrangement like this, there is a one U, another U and there is an evaporator

in this boiler. The beauty of this boiler is a steam generated with the help of superheated

steam. So, even salt water can be used here because and there is no problem of shooting

at all because generation of steam it is India, direct generation of a steam and generation

of steam is with the help of superheated steam. So, there is a evaporator here. We are mixing

of superheated steam. Suppose superheated steam is coming from this side and water is

spread from this side right feed water is coming from the side and here mixing of water

and superheated this steam takes place which results in generation of steam right. This

steam is pumped, this is a boiler house. This steam is pumped through the boiler and this

is economizer. So, in this boiler the flue gas there is a

grate the fuel which is burned here. So, this area is filled with the flue gases. So, this

area is filled with the flue gases right. So, this is economizer and for the purpose

of superheating for the purpose of superheating, the water tubes is sent here is passed through

this is super heater. I will I will explain it again. This is evaporator; we are mixing

of superheated steam with feed water takes place which results in generation of a steam.

Now, this steam is pumped through this combustion area this flue gases area and in this area

there is an economizer or convective heating not economizer. Sorry I am sorry, this is

not economizer this is convective heating of the. So, the steam generated in the evaporator

is circulated in this area where convective heat transfer takes place between hot flue

gases and the steam. Then steam is goes steam goes to the super

heater, this is super heater area; this is super heater. After rebirthing from the super

heater, the steam is divided in two parts. One-third part is used for power generation

and two-third part of the steam this is high temperature high pressure steam, this high

temperature high pressure steam goes to the evaporator.

Now, this one-third part goes to the high pressure turbine. So, if you look at the Rankine

cycle temperature entropy diagram. So, expansion of takes place from state 1 to state 2 right.

Now exhaust of this goes to the reheater reheating takes place. So, this is reheater. So, here

the reheating takes place and after reheating, it goes to the low pressure turbine.

So, one-third of the exhaust it goes to the high pressure turbine, two-third of the sorry

not exhaust; one-third of the superheated steam goes to the high pressure turbine to

generate power. Two-third of the steam goes to the a evaporator for generation steam.

This steam again goes to this chamber takes the heat and become superheated steam. Now,

exhaust of the high pressure turbine again goes to the side of the boiler to take heat

and to generate steam in the low pressure turbine. Now this side of the turbine is economizer.

So, there is a water well or it is called a hot well in the hot well water is pumped

and this is economizer. Now, from here the heat is taken because this is a feed water

which is coming here. Feed water, it takes heat from the hot flue gases and the hot flue

gases are moving. So, hot gases are moving like this from here. So, first convective

heat transfer to the tubes, then superheating, then reform the purpose of reheating and for

economizer and then they are leaving the boiler right.

This is a schematic of this Loeffler boiler. Here you can see that this is grate, this

is evaporated drum, this is super heater and here it is reheater for the purpose of reheating

steam and this is economizer and from here, air preheater is also provided because after

economizer air preheater will also be provided to preheat the air to trap the maximum amount

of heat from the boiler right.

So, this is the arrangement of Loeffler boiler and the last one which I am going to discuss

is Velox boiler. In Velox boiler, the efficiency of the Velox boiler is not high. It is between

55 to 60 percent and the capacity of the Velox boiler is 100 times per hour. Velox boiler,

it has again a big huge combustion chamber and it has number of tubes. Actually it is

a circular area of the tube along the wall of the tube and in each array, in annular

space concentric tubes. These are concentric tubes and in these concentric

tubes the annular space is filled with water and inside and outside tube, there is there

are flue gases right. And there is a drum or a steam separator, the steam from the bottom

side. This steam is separator is connected with these so, that the number of tubes, I

am showing only two tubes and from the bottom side, it is connected with this and there

is a close loop a pump is there to circulate the water in the annular space.

So, in a boiler shell, there is a array of suppose this is a boiler shell. There is an

area of circular area of tubes. It is a assembly it is not a single tube, but it is assembly

of tube there are two concentric tubes. So, in the annular space, there is water right

and this water picks heat and the entire space is filled with flue gases. But how this flue

gases here? The generation of flue gases is different.

There is a fuel tank fuel tank and from fuel tank with the help of a pump, fuel is supplied

to the boiler and for burning of these fuel, high pressure air is supplied. The pressure

of air is approximately three bar and that is with the help of a air pump and velocity

of flue gasses is very high. It is supersonic maybe 500 or 600 meters per second. It is

very high the velocity of the flue gases is very high and they leave boiler from this

arm and as in the case of other boilers, there is also a super heater here right.

So, a steam which is being generated is collected here in this drum because there is a close

loop. So, water which is in annular space will get evaporated and it will be collected

in the upper half of this steam separator. From steam separator, the water will go to

the super heater and here the sorry this steam will go to the super heater and from here

the super heated steam will be supplied. But still a lot of energy is remaining with the

flue gases. So, that energy has to be trapped. So, for that purpose what is being done, the

exhaust which is coming from the superheated a still it is a very high velocity. The velocity

of this exhaust is approximately 125 meters per second sorry; yes, the velocity of this

exhaust is 125 meters per second, it contains a lot of energy. So, in order to trap the

heat, it turbine is gas turbine is provided. So, the exhaust of this boiler runs the gas

turbine and gas turbine is coupled with a compressor.

It is coupled with a compressor and this compressor provides the compressed air for burning the

fuel here. I am just repeating the exhaust of the super heater. It is at a very high

velocity the velocity is of the order of 125 meters per second. This exhaust runs the turbine.

It turbine the gas turbine, this gas turbine is coupled with a compressor and this compressor

centrifugal compressor or axial compressor, it compresses air and supplies air to burner

for burning the fuel. Because fuel supply is high and we require sufficient if sufficient

amount of air is required. So, this sufficient amount of require air

at three bar pressure is supplied to the burner and an exhaust of this gas turbine is also

at a quite high temperature and this heat can be trapped in the form of economizer.

So, for the purpose of feed water which is feed water which is supplied here and this

feed water is heated with the help of the exhaust from sorry not from here; exhaust

from the turbine, exhaust from the turbine heats the feed water.

So, exhaust of the turbine is feeds the feed water in a in a heat exchanger. This hot feed

water goes to this separation drum and from separation drum. It is circulated inside the

boiler. This saturated steam is generated; this saturated steam goes to the super heater

right. And, the flue gases after this gases which are leaving this drum through this through

the super heater, they go to the turbine. Turbine the power is generated to run the

compressor, exhaust sort of turbine is used for heating the feed water or used in the

economizer.

Now, this is the arrangement of Velox boiler. Here combustion chamber is shown. This is

super heater, this is turbine compressor which supplies air for burning the fuel, then number

of tubes you can see here and there is the steam separator. This is an economizer and

this is the entire arrangement for the Velox boiler.

So, today we have completed a four classical high pressure boilers. Today, we have discussed

four classical fossil fuel steam generators. In the next class, we will start with the

mountings on boilers. Thank you.

 

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