1. MODULE 7

2. BOILERS
• A boiler is a device in which steam is
generated.
• The boilers are divided into two types:
1. Fire Tube Boiler and
2. Water Tube Boiler

3. • Superheated steam is required, the wet steam is
removed from the steam space and is piped to a
super heater. This consists of a long tube or
series of tubes which are suspended across the
path of the hot gases from the furnace.
• If a control of degree of superheat is required, as
in some of the larger boilers, then an
attemperator is fitted. The control of the degree
of superheat is obtained by the injection of water
or steam into the superheated steam. If an
attemperator is fitted, the super heater is
generally divided into two parts .
• The first part is called the primary super heater.
Then comes the attemperator followed by the
second part of the super heater called secondary
super heater.

5. • The economizer is really a heat exchanger in
which the feed water being pumped into the
boiler at a higher temperature. Hence , less
energy is required to raise the steam, or, if the
same energy is supplied, then the more steam
is raised. The result is higher thermal
efficiency.

7. • Auxiliary equipment fitted to boilers:
a. A pressure gauge – this will record the gauge
pressure of the saturated steam formed in the
steam space.
b. A water gauge glass – this will record the water
level in the boiler. Often, two are fitted in case
one breaks.
c. A pressure relief valve- this is fitted as a safety
precaution and is set to blow- off at a particular
pressure

8. FIRE TUBE BOILERS
• Example of fire tube type boiler is Lancashire
Boiler. Heat from the hot flue gases passes
through the tubes and water surrounds the flue
• The basic elements of the boiler are a large steel
shell through which pass two large-bore
cylinders called flues.
• This boiler is provided with three gas passes. The
first pass is through the boiler. The second pass is
back under the boiler. The third pass is round the
sides of the boiler.

9. Fire tube boiler

10. • These passes are an attempt to extract the
maximum amount of energy from the hot flue
gas before they are released to atmosphere.
• The quality of steam produced by a boiler in
one hour is referred to as its evaporative
capacity.
• The evaporative capacity of a boiler will
depend upon its design, the type of fuel and
furnace and on the quality of fuel.
• The Lancashire , like other shell type boilers,
can be worked up to a pressure of about 1.7
MN/m2 (17 bar).

11. • The general range of sizes of the economic
boiler are from the small size of about 3m
long and 1.6m diameter to the large size
about 6.6m long and 4m diameter. Equivalent
evaporation ranges from about 900 kg
steam/h to about 14000 kg steam/h.

12. WATER TUBE BOILERS
• In water tube boiler Water is going to pass
inside the tubes and steam circulates around
them.
• In most water-tube boilers the water
circulation is by natural convection but there
are a few designs in which forced convection is
employed.

13. Water tube boiler

14. • The headers are connected by a large number
of water tubes. Between the water tubes and
the drum a super heater is fitted. Baffles are
introduced across the water tubes to act as
deflectors to the flue gas stream. The high end
of the tubes is at the hottest end of the boiler
and the low end of the tube is the cool end of
the boiler.

15. • A very large water –tube boiler of modern design
called the radiant heat boiler
• The boiler is fired with pulverished coal.
• One of the difficulties of a pulverized fuel burning
boiler is that the ash in the coal is also pulverized
and hence blown into the furnace and passes up
with the flue gas out of the boiler. This dust ash
must be removed so that it does not pollute the
atmosphere. The dust ash is removed in the
precipitator.
• One type of precipitator gives a vortex motion to
the flue gas. The dust is thus flung out of the gas
and is collected for disposal. This type of
precipitator is called a cyclone precipitator.

16. • Two of the main pollutants appear to be
sulphur oxides(Sox) and the other nitrogen
oxides(Nox ). These can form acids and
compounds which have a corrosive effect on
surroundings and buildings and also have a
contributory effect on the formation of what
is generally referred to as acid rain.
• Problems:
• Boiler calculation.

17. COAL FIRING
• Coal was the fuel most generally used for boiler
firing in the industrial nations of the world.
• The bed consists of a thick layer of fine inert
particles of such natural substances as sand or
limestone's. If such a bed has air blown through
it, the air being evenly distributed through a grate
at this base by means of a device such as a
plenum chamber, then, at a particular velocity
and mass flow of air, the bed will begin to
behave like a fluid , or, in other words, it
becomes a fluidised bed.

19. • If particles of coal are added to the fluidised
bed, they become well mixed throughout the
bed. If the temperature of the bed is high
enough, then the coal will burn.
• A device is required for the removal of the coal
ash at the base of the bed through the bed gate.
A further device, sometimes referred to as an
arrester, is located at the top of the combustor to
intercept any small ash particles.
OIL FIRING: oil burner is fitted into the side of the
boiler furnace. A large boiler, as in a power
station, can employ as many as 32 oil burners.

20. GAS FIRING:
Many of smaller boilers of the shell type are gas
fired. In these boilers burning gas jets are fed
into tubes which pass through the water space
in the boiler much in the same way as
illustrated in the economic boiler. Gas fired
boilers can be either horizontal or vertical.

21. EVAPORATION
• The objective of evaporation is to concentrate
a solution consisting of a non- volatile solute
and a volatile solvent.
• Evaporation is conducted by vaporizing a
portion of the solvent to produce a
concentrated solution of thick liquor.
• Evaporation differs from drying in that the
residue is a liquid – sometimes a highly
viscous one – rather than a solid.

22. • Some of the most important properties of
evaporating liquids are as follows:
1. Concentration – although the thin liquor fed to
an evaporator may be sufficiently dilute to have
many of the physical properties of water, as the
concentration increases, the solution becomes
more and more individualistic. The density and
viscosity increase with solid content until either
the solution becomes saturated of the liquor
becomes too sluggish for adequate heat
transfer.
2. Foaming – some materials, especially organic
substances, foam during vaporization. A stable
foam accompanies the vapor out of the
evaporator, causing heavy entrainment.

23. 3. Temperature Sensitivity- Many fine chemicals,
pharmaceutical products, and foods are damaged
when heated to moderate temperatures for a
relatively short times.
4. Scale- Some solutions deposit scale on the
heating surfaces. The overall coefficient then
steadily diminishes, until the evaporator must be
shut down and the tubes cleaned. When the
scale is hard and insoluble, the cleaning is difficult
and expensive.
5. Materials of Construction- Whenever possible,
evaporators are made of some kind of steel.
Many solutions, however, attach ferrous metals,
or are contaminated by them. Special materials
such as copper, nickel, stainless steel, aluminum,
impervious graphite, and lead are then used.

24. 6. Single and Multiple Effect Operation- when a
single evaporator is used, the vapor from the
boiling liquid is condensed and discarded. This
method utilizes the steam ineffectively. If the
vapor from one evaporator is fed into the
steam chest of the second evaporator and the
vapor from the second is then sent to a
condenser, the evaporation becomes double
effect.

25. TYPES OF EVAPORATION
• The main types of steam heated tubular
evaporators in use today are:
1. Long-tube vertical evaporators
a. Upward flow (climbing-film)
b. Downward flow(falling-film)
c. Forced Circulation
2. Agitated-film evaporators

26. • Types of long tube vertical evaporator:
• 1.falling film evaporator.
• 2. forced circulation evaporator.
• 3.agitaged film evaporator.

27. PERFORMANCE OF TUBULAR
EVAPORATORS
• The principal measures of the performance of
a steam-heated tubular evaporator are the
capacity and the economy. Capacity is defined
as the number of pounds of water vaporized
per hour. Economy is the number of pounds
vaporized per pound to steam fed to the unit.
• Evaporator capacity-
q= UA∆T

28. HEAT PIPE
• A term heat pipe as the name implies, is a device
for transferring heat from a source to sink by
means of evaporation and condensation of a fluid
in a sealed system.
• The heat pipe is a very simple and very efficient
heat transfer device. It can be considered a
super- thermal conductor that transmits heat by
the evaporation and condensation of a working
fluid. It can easily transmit 5000 times heat
transmitted by best known metal conductor and
with a temperature drop of less than 5⁰C per
meter length of heat pipe.
• The heat pipe was first invented in 1942 by
Richard S.Gauler, a general Motors Engineer.

29. • No immediate use were found for it and the
patent expired in 1959. in the early 60s the idea
was revived by the Atomic nergy Commission and
Natural Aeronautics and Space Administration.
• Different Types of Heat Pipes-
• A simple vertical pipe contains a simple vertical
pipe. It consists of a container,(pressure and
vacuum tight) a wick structure and small amount
of working fluid as mentioned earlier. In this
system the wick capillary action brings the
condensate back to an evaporator region and
cycle is completed.
• Another type of heat pipe is a wickless pipe.