Tangential-firing furnace: It depends for its operation on the

Downshot-firing furnaces:

The vertical or downshot method

of firing

^is adopted

where coals of low volatile

content

are the main fuel. Low volatile

content

requires a long particle

residence

time for

complete combustion and this result in the U-flame concept.

Tangential-firing furnace: It ^depends for its ^{operation on}

^the

maintenance

of

a

turbulenl

zone

in

the center

of

the furnace. The burner can be

tilted

through a range

of t30'

^about the horizontal

to

maintain steam temperature control.

Cyclone

furnace

-Was developed

in

1940s

-Widely used to burn poorer grade of coal

-Ash should not contain high sulfur content or high

Gfmt

^ratio

Advantage

of

cyclone

furnace

molten slug and 40o/o as

fly

ash.

(Only

40o/o ash leaves

with

the flue ^gases, compared

with

about 80% for PF furnace)

Disadvantage

of

cyclone furnace

Fluidised-bed

combustion (FBC)

-Commercrahzed

in

1920s and 1930s

-Ability

to

utilize

most combustible material including coal, biomass ^and other waste material

-FBC differs from

cyclone furnace

in

that

sulfur

^{is removed during the}

combustion process and also that the combustion occurs at

^lower

temperature resulting

in

lower NOy production.

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Atmospheric FBC-AFBC (non-pressu rized) -Thermal efficiency is

upto 40%

Pressurized FBC (PFBC) -Therrnal efficiency is

over 40yo

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What

is

boiler

Stagging anrl Fouling?

A

small proportion

of

ash may

howevir

be deposited on the heat transfer surfaces

of boiler

(ttrbes, super heater

tubis,

reheater

tubes etc

are

directly

_exposed

to flue gas that

contains

a lot of fly

ash).

Ash

rs

deposited in fwo main ways

Slagging: Where fused or resolidifled molten material forms on surfaces

'exposed to high radiant heat fluxes and gas

temperatures

is

called slugging.

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Fluidized bed combustion _{FBC)

Fouling: Where sintered or

cemented deposits

form on

^convection

surfaces is called fouling.

What

are the ways

to

remove

boiler

slagging and

fouling?

to the

waterside,

for which an

allowance

must be

^made

at

the boiler design stage.

temperatures and low heat

fluxes.

r

slugging.

much larger

than

the

furnace

of

an equivalently rated

oil

^f,red

unit.

Steam generators (SG)

-A

steam generator

is a

complex combination

of

economizet, ^boiler, superheater, reheater, and air preheater.

-A boiler is the

one where saturated

liquid is

converted

into

saturated steam.

-Boilers can be classified as either a utility or an industrial

steam generator.

Utility

^steam generators

are used by electric utilities ^for

generating electricity.

-Modern steam generators can be of the

following

types

.

Subcritical water-tube drum type

.

Supercritical once-through type

. ljltra-supercritical

*Steam capacities range

from

125

to

¹²⁵⁰

^{Kg/s giving}

^{a power output}

between 125 and 1300

MW

Fossil fueled SG can be

broadly

classified as

o

Fire-tube boilers

o

Water-tube boilers

o

Natural-circulationboilers

o

Controlled circulation boilers

.

Once-through boiler

.

Sub-critical pressure boiler

7

I

.

Super-critical pressure boiler

o

Ultra-supercritical pressure boiler

Fire

tube

boilers

-Are not generally used in power

utilities

-Upper

limit

of pressure

is

18 bar

-Upper

limit _of

steam capacity is 6.2 kg/s Advantages of

fire

tube

boiler

are

o Low first

cost

. Reliability

in operation

o

Unskilled workers can run the boiler

r

Less draughts required

o

_Quick response to load changes Two types

of fire

tube boilers are

l)

Externally fired (Ex: Locomotive, Lancashire etc)

2)

Internally fired

(Ex:

scotch-marine boiler, package boiler) Package

type boilers

have

certain

atlvantages

. Highly

compact

o

No

ID

fan is required

o

The entire unit is fabricated in the indushy

.

Occupies _{less space}

.

Easy to operate

Water

tube

boilers

-Used in most power plants

-Modern

high

pressure SG requires more super-heating and re-heating surface and less boiler surface than old units.

-The pressure

is high _that

means

a

smaller

boiler

surface because the latent heat

of

vaporization decreases rapidly

with

pressure.

-In

nafural circulation

boiler,

heat transfer

to

the rvater tubes around the walls is mostly

by

radiation

from

the

fuel

flame and less

by

convection from flue gases.

-Forced

circulation

boilers,

originally

known as

La Mont

boilers, use a sr-:scial pump

for

water circulation.

Both

the above

type

operates

at

subcritical pressures (<221.2

bar)

and rely on drum and recirculation.

What

is

once-through boiler?

-Once though boilers or drum less boilers do not need recirculation. The water enters

the bottom

of,

the

tube and

is

completely transformed to steam

by

the

time it

reaches the

top.

^There

is no facility for

removing dissolved salts by periodic blow down.

Ilolv you will identify

a

boiler whether it is fire tube or water

tube

from

seeing outside?

-Fire tube boilers are generally small in size. The main

^physical

difference between

flre

tube and water tube boiler is that there exists two drums

in water tube boiler

whereas

no drum in fire

tube

boiler.

The upper drum is steam drum and the lower drum is mud drum.

What are the differences

between

fire tube boiler and water

tube

boiler?

Fire

tube

boiler Water

tube

boiler In this boiler the hot flue

gases is

present inside

the

tubes and water surrounds them

Water is

^present

inside the

tubes

and the hot flue

gases surrounds them

They are low pressure boilers. They are high pressure boilers

The

steam generation

rate in

fire tube boiler is low

Steam generation rate in water tube boiler is high

The transportation

and

erection in this type of boiler is

difficult.

The

transportation and erection is easy as its parts can be separated.

The overall efficiency of this boiler is ^{7 5o/o}

The overall

efficiency

is upto 90%

with

the economizer.

Operating cost is low. Operating cost is high.

It

can be operated

with

less skilled person.

A skilled person is required

to operate this boiler.

Low maintenance cost. High maintenance cost

They are

light in'weight

They are heavy in weight

It is

suitable

for

industrial process application.

It

is suitable power plants.

What is boiler mounting? Write the name and function of boiler

mountings

- Boiler

mountings are the devices

which

are

fitted

on the

boiler for

its safe

and

continuous

working. In

^general

there are

seven mountings.

They are

1.

Water Level Indicator

. It is ^fitted in front of the boiler and ^generally

^present

two in number.

. It

is used to indicate the water level inside the boiler.

2. Pressure gauge

. It

is also present in front of the boiler,

. _It is used to

^measure

the

pressure

of the steam inside

the boiler.

.

The pressure gauges generally used are of Bourdon type 3. Safety Valves

.

Safety valves are attached to the steam boiler chest.

. It is used to prevent explosion due to excessive

internal pressure. When the internal pressure inside the

boiler

exceeds its

working

pressures than the safety valves

blow off

the steam and maintains the internal pressure.

.

Generally two safety valves are present on a boiler.

4. Stop

Valve

(steam stop valve)

. It is usually fitted on the highest part of the boiler with

the help

of

a flange.

.

The main function of the,stop valve is

1. To control the flow of steam from the boiler to the

^main

steam pipe.

2.

To completely shut

off

the steam supply when required.

5.

Blow off Cock

. It

is

fitted

at the bottom of the boiler drum.

.

The functions of

blow off

cock is

1.

To empty the boiler whenever required.

2.

To discharge the scale, mud and sediments.

6. Feed Check

Valve

. It

is non-return valve

. It

^regulates

the supply of ^{water which} is ^pumped into

^the

boiler by feed pump.

7. Fusible

Plug

. It

is fitted to the crown plate of the furnace or firebox.

. Its function is to extinguish fire in ^the furnace when

^the water

level in

the

boiler falls to

an unsafe

limit. This

avoids the explosion that

rray

takes place because

of

the overheating

of

the furnace plate.

. During

disaster

or high

current

flow it will automatically

^shut down

What is boiler

Accessories?

Write the

name

and function of boiler

accessories

-The boiler accessories are the integral parts

of

the boiler. They are used in the boiler to improve its efficiency.

1.

Air preheater

. It

is used to recover heat from the exhaust ^gases.

. It

is installed between the economrzer and the chimney.

"

Combustion efficiency

will

be increased 2. Super heater

. It

is placed in the path of hot flue ^gases from the furnace.

" ^A

^super

^{heater is an important}

^accessory

^{used in the}

^boiler.

Its main function is to increase the temperature

of

saturated steam without raising its pressure.

.

Utilizes the exhaust ^gas and increases the thermal efficiency.

3.

Economizer

. It is used to heat the feed water by the utilization of

^heat

from the hot fuel gases before

it

leaves the chimney.

. An

economizer improves the economy of the steam boilers.

'

Is

boiler

feed

pump mounting or

accessories???

v

-Boiler

feed

pump is

neither mounting

nor

accessories rather

it is

^an

integral part

of

a

boiler.

So, feed pump is a part

of

^{a boiler.}

Evaporation factor

Evaporation factor : + _B-C ^(it

^{T is less}

^than

¹⁾

Where,

A

is to and from rating ⁽²²⁵⁷ KJ/Kg) B is enthalpy

of

steam at operating ^pressure

C is enthalpy

of

feed water at operating temperature Super

critical power plant

-Super

critical power plant

operate

at

temperatures

resulting in

^higher

efficiencies upto 46%

-Operating pressure is above critical pressure but less than 300 bar.

-A

super

criticat

power

planl

uses

aboiler

that operates

at

5800C; sub- critical plants operate at 4550C

-Lower emissions than traditional (sub-critical) coal-fired power plants.

Ultra supercritical power plant

-Boilers operate at pressures over 300 bar

-Further increase

in

thermodynamic

efficiency is

possible. Efficiency can reach upto 48% using two stage reheating.

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