BOILERS

Prepared By :

Engr.muhammad ishaqAssistant professor

Department of mechanical engineering Balochistan UET Khuzdar

Steam BoilersHeat it up....!!!!

Brief about Boiler….

What is Boiler ??

Boiler is a closed pressure vessel in which steam is generated with capacity exceeding 25 liters , gauge pressure greater than or equal to 1 kg/cm2 and water is heated at 1000 C or above…

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What is a Boiler?

Introduction

• Vessel that heats water to become

hot water or steam

• At atmospheric pressure water

volume increases 1,600 times

• Hot water or steam used to transfer

heat to a process

Simple Boiler..

According to geometric orientation of

boiler..

If the axis of the boiler is Horizontal, the boiler is known as Horizontal boiler…

If the axis is vertical, it is known as vertical boiler…

If the axis is inclined, it is known as inclined boiler…

types of Boiler

Vertical Boiler Horizontal Boiler

According to relative position of water

and hot gases..

Fire Tube Boilers: If the hot gases of combustion from the furnace pass through the tubes and water is surrounding the tubes is called fire tube boilers.

Water Tube Boilers: If the water passes through the tubes and hot gases are surrounding the tubes is called water tube boiler.

Fire Tube Boiler Water Tube Boiler

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Type of Boilers

2. Water Tube Boiler

(Your Dictionary.com)

• Used for high steam

demand and pressure

requirements

• Capacity range of 4,500

– 120,000 kg/hour

• Combustion efficiency

enhanced by induced

draft provisions

• Lower tolerance for

water quality and needs

water treatment plant

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Type of Boilers

(Light Rail Transit Association)

1. Fire Tube Boiler

• Relatively small steam

capacities (12,000

kg/hour)

• Low to medium steam

pressures (18 kg/cm2)

• Operates with oil, gas

or solid fuels

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According to location of furnace..

Externally Fired Boiler: In this boiler the furnace is placed outside the boiler shell. Generally water tube boilers are externally fired.

Internally Fired Boiler: In this boiler the furnace is placed inside the boiler shell. Generally fire tube boilers are internally fired boiler.

According to method of water

circulation..Natural circulation boilers: In this boiler, water flow

take place naturally, by density difference of water. The flow of water and steam are set up due to density difference resulting from difference in temperature. The water from steam drum falls under gravity to header where its temperature rises and density reduces, steam moves upward to steam drum due to low density. Normally, the low capacity boilers are natural circulation type boiler..Forced circulation type boilers: In this boiler, water

flow takes place by a pump. High pressure boilers are forced circulation boilers..

Natural Circulation Boilers Forced Circulation Boilers

Further there are other type bases..

According to working pressure : High, Medium and Low pressure Boilers.

According to mobility of boiler : Stationary Boiler, Mobile Boiler.

According to number of tubes in boiler : Single Tube Boiler, Multi Tube Boiler.

We are going to study here three boilers …

Cochran Boiler

Lancashire Boiler

Babcock and Wilcox Boiler

Cochran boiler is modification of simple vertical boiler where heating surface has been increased by increasing numbers of fire tubes. It is used for steam generation at low rate.

Cochran Boiler..

Characteristics of Boiler Specification of Boiler

Vertical

Multi Tubes

Fire Tube

Internally Fired

Natural circulated Boiler

Working Pressure: 6.5 bar (max 15 bar).

Steam capacity 3500 kg/hr (max 4000 kg/hr).

Efficiency 70% to 75%.

Construction..

The boiler consists of a cylindrical shell, hemispherical fire box, fire tubes and chimney.

The hemispherical crown of boiler gives good strength to withstand pressure of steam inside the boiler.

The hemispherical shape of furnace can withstand high heat and is also useful to increase radiant heat transfer from the furnace to hemispherical furnace wall.

The grate is placed at the bottom of furnace and ash pit is located below the grate.

The furnace and the combustion chamber are connected by short flue pipe.

Working..

The water is supplied to the boiler through the feed check valve.

Because of the burning coal at the furnace produced hot gases enters combustion chamber through flue pipe.

This hot gases enters into horizontal fire tubes. Convection heat transfer take place from flue gases passing

inside the tubes to water surrounded the tubes. The flue gases coming from of fire tubes enter into smoke

box. Finally they discharge to atmosphere through a chimney. The ash formed is collected in ash pit and the steam is

collected through antipriming pipe on top of the shell.

Mountings..

Steam pressure gauge, water level indicator, dead safety valve, feed check valve, blow-off cock, steam stop valve.

This boiler is also fitted with fusible plug, antipriming pipe and dampers.

Advantages..

1. It is compact and portable boiler therefore minimum floor area is required.

2. Initial cost is less.

3. It’s a mobile boiler.

4. Quick and easy installation.

5. Any type of fuel can be used.

Disadvantages..

1. Steam raising capacity is less due to vertical design.

2. Water along with steam may enter the steam pipe under heavy loads due to small steam space.

3. Efficiency is poor in smaller sizes.

Lancashire boiler is simple in design, easy to operate and less operating and maintenance cost. It is one of the most commonly used stationary boilers. It is normally used in sugar mills, textile industries where power generation as well as process heating is required..

characteristics Specification

Horizontal

Multi Tubes

Fire Tube

Internally Fired

Natural circulated Boiler

Working Pressure: 16 bar maximum.

Steam capacity : 9000 kg/hr.

Efficiency 50% to 70%.

Construction..

It consists of a cylindrical shell and two fire tubes, cylindrical shell is placed over the brick structure.The fuel grates are provided at the front end and

inside of two main fire tubes, a fire bridge is provided at the end of the grate to prevent coal and ash particles entering into the interior of the furnace tubes.Superheater is provided at the end of the main flue

tubes in passage of flue gases, while an economiseris at the end of the side flues, before exhausting the gases to chimney.

Working..

The coal is introduced to the grate through fire holes.

The combustion of coal takes place in presence of air which is regulated by damper.

The combustion will produce hot gases.

Path of flue gases..

G rate Fire tube(MF) BF SF Chimney Atm

Advantages..

1. Due to three phases of flue gases, the heating surface area per unit volume of boiler is large.

2. The fluctuation in load can be easily met by this boiler due to large reservoir.

3. Easy operation, low maintenance costs, easy to clean and inspect.

4. By use of economiser and superheater, maximum heat of flue gases is utilized, so efficiency of boiler can be increased.

Disadvantages..

1. Maximum working pressure is limited to 16 bars.

2. Due to brick work, more floor area is required.

3. Response of pressure build up is less.

4. The furnace is inside the tubes therfore the grate area is restricted.

This is an example of water tube boiler and is used in stationary and marine engine. The efficiency of this boiler is much greater than that of the fire tube boiler. This boiler is exclusively used when pressure is above 10 bar and steam generating capacity is required higher than 7000 kg/hr.

Characteristics of Boiler Specification of Boiler

Horizontal

Multi Tubes

Water tube

Externally Fired

Natural circulation of water

Forced circulation of air and hot gases

Solid as well as liquid fuel fired

Working Pressure: 40 bar maximum.

Steam capacity : 40,000 kg/hr.

Efficiency 60% to 80%.

Construction..

It consists of inclined water tubes, a steam and water drum, a mud box and superheater.The drum is connected to up take and down take

header by short riser tubes.These headers are connected to a series of

inclined water tubes.The water tube inclined to horizontal about 150

or above to bring natural circulation of water. The hand hole is provided in header in front of each tube for cleaning and inspections of tubes.

Working..

The water is fed into the drum trough the feed check valve, due to gravity water passes through the short riser tubes, header and fills up the inclined water tubes.Then water collects in drum. Initially one half of

drum is filled up with water. The coal is introduced to furnace grate by help of stroker.Produced hot gases due to fired coal is first passed

upward through passage between tubes.The sine wave of the hot gases is due to baffles

plates, damper controls the flow of air in to the tubes.

Continue..

The water comes to contact with hot gases and it is heated and its density decreases.Low density water moves upward in water tubes.

The water tubes just above the furnace is heated comparatively at a higher temperature than the rest of it. Therefore low density water converted into steam in their path and rises into the drum through the uptake header. So here it is denoted as natural circulation.Then steam enters through antipriming pipe and

flows in the superheater tubes where it is further heated and is finally taken out through the main steam stop valve when engine needed.

Advantages..

1. The steam generation capacity of boiler is too high, about 2,000 to 40,000 kg/hr.

2. Replacement of the defective tubes is easy.3. The draught looses as compare to other boilers

is minimum.4. It is used in power station for generating large

quantity of steam.5. Boiler is required less space area compared to

fire tube boilers and offers greater operational safety.

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Type of Boilers

Tangential firing

6. Pulverized Fuel Boiler

• Pulverized coal powder blown with combustion

air into boiler through burner nozzles

• Combustion

temperature at 1300 -

1700 °C

• Benefits: varying coal

quality coal, quick

response to load

changes and high pre-

heat air temperatures

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Type of Boilers

Agriculture and Agri-Food

Canada, 2001

7. Waste Heat Boiler

• Used when waste heat

available at medium/high

temp

• Auxiliary fuel burners

used if steam demand is

more than the waste heat

can generate

• Used in heat recovery

from exhaust gases from

gas turbines and diesel

engines

BOILER MOUNTING AND ACCESSORIES

BOILER MOUNTING

• These are different fittings and devices which are necessary for the operations and safety of a boiler. Normally these device are mounted over boiler shell.

• According to IBR following mountings should be fitted to the boilers:

1. Two safety valves

2. Two water level indicators

3. A pressure gauge

4. A steam stop valve

5. A feed check valve

6. A blow off cock

7. An attachment for inspector’s test gauge

8. A man hole

9. Mud holes or sight holes

PRESSURE GAUGE

• A pressure gauge is an instrument by means of which thepressure exerted inside a vessel can be measured.

• There are two types of pressure gauges, one is Bourdon tubepressure gauge and the other is diaphragm type gauge.

WATER LEVEL INDICATOR

• The water level indicator isrequired to ascertain the waterlevel of the boiler.

• Two water level indicatorshould be fitted for each boilerin such a place that the waterlevel can constantly be seen.

STEAM STOP VALVE

• The function of steam stop valve is to regulate the flow of steam from boiler to main steam pipe.

• To shut off the steam completely when required.

FEED CHECK IN VALVE

•To allow the feed water to pass into the boiler.

• To prevent the back flow of water from the boiler in the event of the failure of the feed pump.

BLOW-OFF COCK

FUSIBLE PLUG

• To extinguish fire in the event of water level in the boiler shell falling below certain specified limit. It protects fire tubes from burning when the level of the water in the water shell falls abnormally low and the fire tube or crown plate which is normally submerged in the water, gets exposed to steam space which may not be able to keep it cool

SAFETY VALVE• A safety valve is a valve mechanism which automatically releases a

substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits.

• The safety valve may be classified as

1. Dead weight safety valve

2. Lever safety valve

3. Spring loaded safety valve

4. High steam and low water safety valve

Dead weight safety valveLever safety valve

Spring loaded safety valve

High steam low water safety valve

Boilers Accessories• These are auxiliary plants or parts required for steam boilers for

their operation and to increase efficiency of the boiler .

• Following accessories are commonly used with boiler:

1. Feed pumps

2. Injector

3. Economizer

4. Air preheater

5. Superheater

6. Steam separator

7. Steam trap

FEED PUMP• A boiler feed water pump

is a specific type of pump used to pump feed water into a steam boiler. The water may be freshly supplied or returning condensate produced as a result of the condensation of the steam produced by the boiler. These pumps are normally high pressure units that take suction from a condensate return system and can be of the centrifugal pump type or positive displacement type.

ECONOMISER• In boilers, economizers are

heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. Economizers are so named because they can make use of the enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the cold water used to fill it

AIR PREHEATERUsed to recover heat from the exhaust

flue gases• Installed in between the economizer

and chimney.• Temperature of the required air for

combustion is passed through the air preheater where its temperature is raised.

• Then hot air passed to the furnace.ADVANTAGES• Gives higher furnace temperature

which results more heat transfer thus increase evaporative capacity

• Approximately boiler efficiency increases about 2% for each 30-35oC rise in air temperature.

SUPERHEATER• A super heater is a device

used to convert saturated steam or wet steam into dry steam used in steam engines or in processes, such as steam reforming. There are three types of super heaters namely: radiant, convection, and separately fired. A super heater can vary in size from a few tens of feet to several hundred feet (a few meters to some hundred meters).

STEAM SEPRATOR• The steam separator is fitted before

the steam engine or steam turbine to separate water particles from steam.

• A Steam separator, sometimes referred to as a moisture separator, is a device for separating water droplets from steam. The simplest type of steam separator is the steam dome on a steam locomotive. Stationary boilers and nuclear reactors may have more complex devices which impart a "spin" to the steam so that water droplets are thrown outwards by centrifugal force and collected.

STEAM TRAP

• A steam trap is a device used to discharge condensate and non condensable gases with a negligible consumption or loss of live steam. Most steam traps are nothing more than automatic valves. They open, close or modulate automatically. Others, like venturi traps, are based on turbulent 2-phase flows to obstruct the steam flow.

• The three important functions of steam traps are:

1. Discharge condensate as soon as it is formed.

2. Have a negligible steam consumption.

3. Have the capability of discharging air and other non-condensable gases.

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Assessment of a Boiler

1. Boiler performance

• Causes of poor boiler performance-Poor combustion

-Heat transfer surface fouling

-Poor operation and maintenance

-Deteriorating fuel and water quality

• Heat balance: identify heat losses

• Boiler efficiency: determine

deviation from best efficiency

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Assessment of a Boiler

Heat Balance

An energy flow diagram describes geographically

how energy is transformed from fuel into useful

energy, heat and losses

Stochiometric

Excess Air

Un burnt

FUEL INPUT STEAM

OUTPUT

Stack Gas

Ash and Un-burnt parts

of Fuel in Ash

Blow

Down

Convection &

Radiation

Assessment of a Boiler

Heat Balance

Balancing total energy entering a boiler against

the energy that leaves the boiler in different forms

Heat in Steam

BOILER

Heat loss due to dry flue gas

Heat loss due to steam in fuel gas

Heat loss due to moisture in fuel

Heat loss due to unburnts in residue

Heat loss due to moisture in air

Heat loss due to radiation & other

unaccounted loss

12.7 %

8.1 %

1.7 %

0.3 %

2.4 %

1.0 %

73.8 %

100.0 %

Fuel

73.8 %

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Assessment of a Boiler

Heat BalanceGoal: improve energy efficiency by reducing avoidable losses

Avoidable losses include:

- Stack gas losses (excess air, stack gas

temperature)

- Losses by unburnt fuel

- Blow down losses

- Condensate losses

- Convection and radiation

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Assessment of a Boiler

1. Boiler Efficiency

Thermal efficiency: % of (heat) energy input that is

effectively useful in the generated steam

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