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Denubo, Michael Anthony B. Group No. 2 / Seat No. 08

ME144L –  A1 October 14, 2014

Engr. Sublime O. Carvajal

Learning Task #1

Boiler

I.  Overview

A boiler  is a closed vessel in which water or other fluid is heated. The heated or

vaporized fluid exits the boiler for use in various processes or heating applications, including

 boiler-based power generation, cooking, and sanitation.

The pressure vessel in a boiler is usually made of steel (or alloy steel), or historically

of wrought iron. Stainless steel is virtually prohibited (by the ASME Boiler Code) for use in

wetted parts of modern boilers, but is used often in superheater sections that will not be exposedto liquid boiler water. However electrically-heated stainless steel shell boilers are allowed under

the European "Pressure Equipment Directive" for production of steam for sterilizers and

disinfectors.

In live steam models, copper or brass is often used because it is more easily fabricated

in smaller size boilers. Historically, copper was often used for fireboxes (particularly for steam

Figure 1. Boiler  

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locomotives), because of its better formability and higher thermal conductivity; however, in

more recent times, the high price of copper often makes this an uneconomic choice and cheaper

substitutes (such as steel) are used instead.

For much of the Victorian "age of steam", the only material used for boilermaking was

the highest grade of wrought iron, with assembly by rivetting. This iron was often obtained from

specialist ironworks, such as at Cleator Moor (UK), noted for the high quality of their rolled plate and its suitability for high-reliability use in critical applications, such as high-pressure

 boilers. In the 20th century, design practice instead moved towards the use of steel, which is

stronger and cheaper, with welded construction, which is quicker and requires less labour.

Cast iron may be used for the heating vessel of domestic water heaters. Although such

heaters are usually termed "boilers" in some countries, their purpose is usually to produce hot

water, not steam, and so they run at low pressure and try to avoid actual boiling. The brittleness

of cast iron makes it impractical for high pressure steam boilers.

II.  Types of Boiler

A.  Fire Tube Boiler is one of the most basic

types of boiler and the design is also very

old. It was popular in 18thcentury. It was

mainly used for steam locomotive engines.

Operation of fire tube boiler is as simple as

its construction. In fire tube boiler, the fuel

is burnt inside a furnace. The hot gases produced in the furnace then passes

through the fire tubes. The fire tubes are

immersed in water inside the main vessel

of the boiler. As the hot gases are passed

through these tubes, the heat energy of the

gasses is transferred to the water surrounds

them. As a result steam is generated in the

water and naturally comes up and is stored

upon the water in the same vessel of fire

tube boiler. This steam is then taken out

from the steam outlet for utilizing for required purpose. The water is fed into the boiler

through the feed water inlet. As the steam and water is stored is the same vessel, it is

quite difficult to produce very high pressure steam from. General maximum capacity of

this type of boiler is 17.5 kg/cm2 and with a capacity of 9 Metric Ton of steam per hour.

In a fire tube boiler, the main boiler vessel is under pressure, so if this vessel is burst

there will be a possibility of major accident due to this explosion.

Figure 2. Diagram of a Fire Tube Boiler  

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A.1. Types of Fire Tube Boiler

According to the location of furnace there are two types of fire tube boiler and

these are external furnace and internal furnace type.

There are mainly three types of external furnace fire tube boiler:

(1) Horizontal return tubular fire tube boiler

(2) Short fire box fire tube boiler 

(3) Compact fire tube boiler 

There are also two types of internal furnace fire tube boiler:

(1) Horizontal tubular and

(2) Vertical tubular fire tube boiler. 

Horizontal return fire tube boiler is most suitable for low capacity thermal power

 plant. The main constructional features of this boiler are one big size steam drum which

lies horizontally upon supporting structures. There are numbers of fire tubes come from

furnace and also aligned horizontally inside the drum. When the drum is filled with water

these tubes are submerged in water.

The fuels (normally coal) burnt in the furnace and combustible gasses move into

the fire tubes, travel through these tubes from rear to front of the boiler drum and finally

the gases come into the smoke box. The hot gasses in the tubes under water transfer heat

to the water via the tube walls. Due to this heat energy steam bubbles are created and

come up upon the water surface. As the amount of steam is increased in that closed drum,

Figure 3. Horizontal Return Tubular (HRT) fire-tube boiler

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steam pressure inside the drum increases which increase significantly the boiling

temperature of the water and hence rate of production of steam is reduced. In this way a

fire tube boiler controls its own pressure. In other words this is a self pressure controlled

 boiler.

Advantage of fire tube boilers are:

(1) Compact in construction

(2) Fluctuation of steam demand can be met easily and

(3) Cheaper than water tube boiler.

Disadvantage of fire tube boiler are:

(1) Due to large water the required steam pressure rising time quite high.

(2) Output steam pressure cannot be very high since the water and steam are kept

in same vessel.

(3) The steam received from fire tube boiler is not very dry and

(4) In a fire tube boiler, the steam drum is always under pressure, so there may be

a chance of huge explosion which resulting to severe accident.

B.  Water Tube Boiler

In water tube boiler, boiler feed waterflows through the tubes and enters the boiler

drum. The circulated water is heated by thecombustion gases and converted into steam at

the vapour space in the drum. These boilersare selected when the steam demand as well as

steam pressure requirements are high as in thecase of process cum power boiler / power

 boilers.

Most modern water boiler tube designsare within the capacity range 4,500  –  120,000

kg/hour of steam, at very high pressures. Manywater tube boilers nowadays are of

“packaged” construction if oil and /or gas areto be used as fuel. Solid fuel fired water tube

designs are available but packaged designs are

less common.

The features of water tube boilers are:

Forced, induced and balanced draft provisions

help to improve combustion efficiency. Less

tolerance for water quality calls for water treatment plant. Higher thermal efficiency

shifts are possible.

Figure 4. Diagram of a Water TubeBoiler

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C.  Other Types of Boiler

  Packaged Boiler

  Fluidized Bed Combustion (FBC) Boiler

  Atmospheric Fluidized Bed Combustion (AFBC) Boiler

 

Pressurized Fluidized Bed Combustion (PFBC) Boiler  Atmospheric Circulating Fluidized Bed Combustion Boilers (CFBC)

  Stoker Fired Boilers

  Spreader Stokers

  Chain-grate or Traveling-grate Stoker

  Pulverized Fuel Boiler

  Waste Heat Boiler

  Thermic Fluid Heater

Figure 6. Diagram of a CFBC BoilerFigure 5. Diagram of a Stoker Fired Boiler

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III.  Fuels

The source of heat for a boiler is combustion of any of several fuels,  such

as wood, coal, oil,  or  natural gas. Electric steam boilers use resistance- or  immersion-

type heating elements. Nuclear fission is also used as a heat source for generating steam, either

directly (BWR) or, in most cases, in specialized heat exchangers called "steam generators"(PWR). Heat recovery steam generators (HRSGs) use the heat rejected from other processes such

as gas turbine. 

IV.  Safety

To define and secure boilers safely, some professional specialized organizations such as

the American Society of Mechanical Engineers (ASME) develop standards and regulation codes.

For instance, the ASME Boiler and Pressure Vessel Code is a standard providing a wide range ofrules and directives to ensure compliance of the boilers and other  pressure vessels with safety,

security and design standards.

Historically, boilers were a source of many serious injuries and property destruction

due to poorly understood engineering principles. Thin and brittle metal shells can rupture, while

 poorly welded or riveted seams could open up, leading to a violent eruption of the pressurized

Figure 7. Diagram of Waste Heat Boiler

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steam. When water is converted to steam it expands to over 1,000 times its original volume and

travels down steam pipes at over 100 kilometres per hour. Because of this, steam is a great way

of moving energy and heat around a site from a central boiler house to where it is needed, but

without the right boiler feed water treatment, a steam-raising plant will suffer from scale

formation and corrosion. At best, this increases energy costs and can lead to poor quality steam,

reduced efficiency, shorter plant life and unreliable operation. At worst, it can lead to

catastrophic failure and loss of life. Collapsed or dislodged boiler tubes can also spray scalding-

hot steam and smoke out of the air intake and firing chute, injuring the firemen who load the coal

into the fire chamber. Extremely large boilers providing hundreds of horsepower to operate

factories can potentially demolish entire buildings. The Locomotive, by Hartford Steam Boiler

Inspection and Insurance Company, Published by Hartford Steam Boiler Inspection and

Insurance Co., 1911, Item notes: n.s.:v.28 (1910 – 11), Original from Harvard University,

Digitized December 11, 2007 by Google Books, Link to digitized document: an article on a

massive Pabst Brewing Company boiler explosion in 1909 that destroyed a building, and blew

 parts onto the roof of nearby buildings. This documents also contains a list of day-by-day boiler

accidents and accident summaries by year, and discussions of boiler damage claims.

A boiler that has a loss of feed water and is permitted to boil dry can be extremely

dangerous. If feed water is then sent into the empty boiler, the small cascade of incoming water

instantly boils on contact with the superheated metal shell and leads to a violent explosion that

cannot be controlled even by safety steam valves. Draining of the boiler can also happen if a leak

occurs in the steam supply lines that is larger than the make-up water supply could replace.

The  Hartford Loop was invented in 1919 by the Hartford Steam Boiler and Insurance Company

as a method to help prevent this condition from occurring, and thereby reduce their insurance

claims.