INTRODUCTION
Boiler, device for heating water or generating steam above atmospheric pressure.
All boilers consist of a separate compartment where the fuel is burned and a compartment
where water can be evaporated into steam.
FIRE TUBE BOILERS
Savery, Watt, and Newcomen engines all operated at pressures only slightly above
atmospheric pressure. In 1800 the American inventor Oliver Evans built a high-pressure
steam engine utilizing a forerunner of the fire-tube boiler. Evans's boiler consisted of two
cylindrical shells, one inside the other; water occupied the region between them. The fire
grate and flue were housed inside the inner cylinder, permitting a rapid increase in steam
pressure. Simultaneously but independently, the British engineer Richard Trevithick
developed a similar “Cornish” boiler. The first major improvement over Evans's and
Trevithick's boilers was the fire-tube “Lancashire Boiler,” patented in 1845 by the British
engineer Sir William Fairbairn, in which hot combustion gases were passed through tubes
inserted into the water container, increasing the surface area through which heat could be
transferred. Fire-tube boilers were limited in capacity and pressure and were also,
sometimes, dangerously explosive.
EXAMPLES OF FIRE TUBE BOILERS
1. Simple Vertical Boiler:-
A simple vertical boiler produces steam at a low pressure and in small quantities.
It is used for low power generation or at places where the space is limited. The
construction of this type is shown in fig below.
It consists of a cylindrical shell surrounding a nearly cylindrical fire box. The fire
box is slightly tapered towards the top. At the bottom of the fire box, there is a grate. The
fire box is fitted with two or more inclined cross tubes F, F.
An uptake tube passes from top of the fire box to the chimney. The hand holes are
provided opposite to the end of each water tube for cleaning deposits. A man hole is
provided at the top for a man to enter and clean the boiler and a mud hole is also provided
at the bottom of the shell.
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2. Cochran Boiler or Vertical Multitubular Boiler:-
There are various designs of vertical multitubular boilers. It is most efficient type
of such type of boilers.
This boiler consists of an external cylindrical shell and a fire box as shown in the
fig below. The hemispherical crown of the boiler shell gives maximum space and strength
to withstand the pressure of steam inside the boiler. The fire box and the combustion
chamber are connected through a short pipe. The fuel gases from the combustion
chamber flow to the smoke box through a number of smoke tubes. These tubes have 62.5
mm external diameter and are165 in number. These gases, then, pass to atmosphere
through chimney. A man hole near the top of the crown on the shell is provided for
cleaning.
At the bottom of the fire box, there is a grate. If the boiler is used for oil firing, no
grate is provided. The oil burner is fitted at the fire hole.
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3. Lancashire Boiler:-
It is a stationary fire tube boiler, internally fired, horizontal and natural circulation
boiler. It is used where working pressure and power required are moderate. These boilers
have cylindrical shell of 1.75m to 2.75m diameter. Its length varies from 7.25m to 9m. It
has two internal flue tubes having diameter 0.4 times that of shell.
A Lancashire boiler with brick setting is shown in the fig below.
4. Cornish Boiler:-
it is similar to Lancashire boiler in all respects , except there is only one flue tube
in Cornish boiler instead of two as shown in the fig below.
The diameter of Cornish boiler is generally, 1m to 2m and its length varies from
5m to 7.5m. The diameter of flue tube may be about 0.6 times that of shell. The capacity
and working pressure of a Cornish boiler is low as compared to Lancashire boiler.
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WATER TUBE BOILERS
Boiler pressures, however, remained limited until the first successful design of a
water-tube boiler, patented in 1867 by the American inventors George Herman Babcock
and Stephen Wilcox. In the water-tube boiler, water flowed through tubes heated
externally by combustion gases, and steam was collected above in a drum. This
arrangement used both the convection heat of the gases and the radiant heat from the fire
and the boiler walls. Wide application of the water-tube boiler became possible in the
20th century with such developments as high-temperature steel alloys and modern
welding techniques, which made the water-tube boiler the standard type for all large
boilers.
Modern water-tube boilers can operate at pressure in excess of 5000 psig (lb/sq in
gauge) and generate more than 9 million lb of steam per hour. Because combustion
temperatures may exceed 1650° C (3000° F), the water flow is controlled by natural or
forced circulation. By using so-called super heaters, modern boilers can achieve almost
90 percent fuel efficiency. Air preheaters heat the incoming air with combustion gases
that are discharged to the stack; water preheaters use the flue gases to heat the feedwater
before it enters the boiler. Draft control and chemical treatment of the water to avoid
scale deposits and corrosion also contribute to efficient operation.
EXAMPLES OF WATER TUBE BOILERS
1. Babcock and Wilcox boiler:-
It is straight tube and stationary type water tube boiler as shown in the fig below.
It is consisted of a steam and water drum. It is connected by a short tube with uptake or
riser at the back end. The water tubes are inclined to the horizontal and connect the
uptake header to the down take header. The headers are curved when viewed in the
direction of tubes so that one tube is not in the space of other, and are provided with hand
holes in the front of the tubes and are covered with caps. A mud box is also provided with
each down take header. A fire bricks baffle causes hot gases to move upwards and
downwards and again upwards before leaving the chimney. The dampers are provided by
a chain.
The boiler is suspended on steel girders and surrounded on all the four sides by
fire brick walls. The doors are also provided. A steam super heater consists of a large
numbers of steel tubes and contains two boxes. The boiler is fitted with usual mountings
such as safety valve, feed valve, water level indicator and pressure gauge.
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2. La-Monte Boiler:-
This is a modern high pressure water tube boiler working on a forced circulation.
The circulation is maintained by a steam turbine using steam from the boiler. A
diagrammatic sketch of La-Monte steam boiler is shown in the fig below. The feed water
passes through the economizer to an evaporating drum. Then, it is drawn to the
circulating pump through the tube. The pump delivers the feed to the headers. The header
distributes water through nozzles in to these tubes passes into the drum. The steam in the
drum is then drawn through the super heater.
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3. Loeffler Boiler:-
This is a water tube boiler using a forced circulation. Its main principle of
working is to evaporate the feed water by means of super heated steam from the super
heater. A diagrammatic sketch of a Loeffler boiler is shown in the fig below.
The feed water from the economizer tubes is forced to mix with the superheated
steam in the evaporating drum. The saturated steam thus formed is drawn from the
evaporating drum by a steam circulating pump. This steam passes through the tubes of
the combustion chamber walls and then enters the super heater. From the super heater
about one-third of the superheated steam passes to the turbine and the remaining two-
third is used to evaporate its feed water in the evaporating drum.
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