1. MET 401 Power Plant Engineering
Chapter 4
Boiler and steam generators
Dr. Taib Iskandar Mohamad

2. Steam generator
To generate steam at desired rate, pressure and
temperature
Fuel and air burn in furnace  Heat water Steam
Used in fossil fuel and nuclear fuel power plants.
Classification:
1. Utility steam generator
2. Industrial steam generator
3. Marine steam generator
2

3. Steam generator components
• Economizer
• Boiler
• Superheater
• Reheater
• Air heater
• Auxiliaries (ash handling, control, fan, burner
etc)
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4. 4

5. 5

6. Boiler
• A boiler is an enclosed vessel where combustion
heat is transferred into water until it becomes
heated water or steam.
• The hot water or steam under pressure is then
usable for transferring the heat to a process.
• When water is boiled into steam, its volume
increases about 1,600 times, producing a force
that is almost as explosive as gunpowder.
6

7.  The boiler system comprises of: feed water system, steam system
and fuel system.
 The feed water system provides water to the boiler and regulates
it automatically to meet the steam demand.
 Various valves provide access for maintenance and repair.
 The steam system collects and controls the steam produced in
the boiler.
 Steam is directed through a piping system to the point of use.
 Throughout the system, steam pressure is regulated using valves
and checked with steam pressure gauges.
 The fuel system includes all equipment used to provide fuel to
generate the necessary heat. The equipment required in the fuel
system depends on the type of fuel used in the system.
 A typical boiler room schematic is shown in Figure 2.1.

8. Figure 2.1: Boiler Room Schematic.

9.  The water supplied to the boiler that is converted into
steam is called feed water.
 The two sources of feed water are:
◦ (1) Condensate or condensed steam returned from the
processes and
◦ (2) Makeup water (treated raw water) which must come from
outside the boiler room and plant processes.
 For higher boiler efficiencies, the feed water is
preheated by economizer, using the waste heat in the
flue gas.

10.  There are virtually infinite numbers of boiler designs
but generally they fit into one of two categories:
◦ 1) Fire Tube Boiler, and
◦ 2) Water Tube Boiler.

11. Fire tube or ―fire in tube‖ boilers; contain long steel
tubes through which the hot gasses from a furnace pass
and around where the water to be converted to steam
circulates. (Refer to Figure 2.2). Fire tube
boilers, typically have a lower initial cost, are more fuel
efficient and easier to operate, but they are limited
generally to capacities of 25 tons/hr and pressures of 17.5
kg/cm2.

12. Figure 2.2: Fire Tube Boiler.

13. Water tube or ―water in tube‖ boilers in which the
conditions are reversed with the water passing through
the tubes and the hot gasses passing outside the tubes
(see figure 2.3). These boilers can be of single- or
multiple-drum type. These boilers can be built to any
steam capacities and pressures, and have higher
efficiencies than fire tube boilers.

14. Figure 2.3: Water Tube Boiler.

15. The packaged boiler is called so because it comes as a
complete package. Once delivered to site, it requires
only the steam, water pipe work, fuel supply and
electrical connections to be made for it to become
operational. Package boilers are generally of shell type
with fire-tube design so as to achieve high heat transfer
rates by both radiation and convection (Refer to Figure
2.4).

16. Figure 2.4: Packaged Boiler Cut-Away View.

17. Fire Tube Boiler
 Advantages:
◦ Relatively inexpensive.
◦ Easy to clean.
◦ Compact in size.
◦ Available in sizes from 176 kW/hr to 14,650 kW/hr.
◦ Easy to replace tubes.
◦ Well suited for space heating and industrial process
applications.

18. Fire Tube Boiler
 Disadvantages:
◦ Not suitable for high pressure applications 1.72 MPa and
above.
◦ Limitation for high capacity steam generation.

19. Water Tube Boiler
 Advantages:
◦ Available in sizes that are far greater than the firetube design.
Up to several million pounds per hour of steam.
◦ Able to handle higher pressures up to 35 MPa.
◦ Recover faster than their firetube cousin.
◦ Have the ability to reach very high temperatures.

20. Water Tube Boiler
 Disadvantages:
◦ High initial capital cost.
◦ Cleaning is more difficult due to the design.
◦ No commonality between tubes.
◦ Physical size may be an issue.

21. Economizer
 Economizer is a mechanical device intended to reduce energy
consumption, or to perform another useful function such as preheating
a fluid.
 The feed water from the high pressure heaters enters the economizer and
picks up heat from the flue gases after the low temperature superheater.
Many types of economizer are designed for picking up heat from the flue
gas. These can be classified as an inline or staggered arrangement based on
the type of tube arrangement. The staggered arrangement is compact and
occupies less volume for the same amount of heat transfer when compared
to the inline arrangement.

22. Economizers are also designed with 1- plain tube and 2- fined tubes.
The fins can be longitudinal or spiral. All these types are suitable for
clean fuels like gas, oil, and low ash coals. For high ash coals, only the
plain tube inline arrangement is used. This is mainly to reduce ash
erosion and thus reduce erosion failures. These economizers pick up
about 50 to 55 degrees centigrade in a large capacity boiler, which will
reduce the flue gas temperature by about 150 to 170 degree centigrade.
The boiler designers always keep the economizer water outlet
temperature to about 25 to 35 degrees below the drum saturation
temperature. This is done to mainly avoid steaming in the economizer. A
steaming economizer generally is less reliable. As a rule of thumb, for
every one degree pick up of economizer water temperature, there will be
a drop of about 3 to 3.5 degrees

23. Type:-

25.  Economizer Advantages :-

1- A lowering of the flue gas temperature by 100 C
 2- will improve the efficient rate by 5%
 3- No falling below the dew point.
 4- It does not cause condensation.
 5- No special requirement on the material for flue gas ducts and
chimney.
 6- Sufficient thermal buoyancy for the flue gases

26. A superheater is a device used to convert saturated
steam or wet steam into dry steam used for power
generation or processes.

32.  An air reheater (APH) is a general term to
describe any device designed to
heat air before another process (for
example, combustion in a boiler) with the
primary objective of increasing the thermal
efficiency of the process. They may be used
alone or to replace a recuperative heat system
or to replace a steam coil.

33.  Tubular type
 Operation:
 Tubular reheaters consist of straight tube bundles which
pass through the outlet ducting of the boiler and open at
each end outside of the ducting. Inside the ducting, the
hot furnace gases pass around the reheater
tubes, transferring heat from the exhaust gas to the air
inside the reheater.

34.  The tubular reheater ductings for cold and hot air
require more space and structural supports than a
rotating reheater design

35. Operation:
In this design the whole air preheater casing is
supported on the boiler supporting structure itself
with necessary expansion joints in the ducting.
The vertical rotor is supported on thrust bearings at
the lower end and has an oil bath lubrication,
cooled by water circulating in coils inside the oil
bath. This arrangement is for cooling the lower end
of the shaft, as this end of the vertical rotor is on
the hot end of the ducting. The top end of the rotor
has a simple roller bearing to hold the shaft in a
vertical position.

36.  The boiler flue gas contains many
dust particles (due to high ash
content) not contributing towards
combustion, such as silica, which
cause abrasive wear of the
baskets, and may also contain
corrosive gases depending on the
composition of the fuel.

40.  1—2: Isentropic Pumping w h2 h1
p
 2 –3 Constant (High)pressure Heat
addition
q in , SG h3 h2
 3 –4 Isentropic expansion in HP turbine.
w hp , out h3 h4
 4—5 Constant (Low) Pressure
Reheating.
q in , RH h5 h4
 5 – 6 Isentropic expansion in LP turbine. w lp , out h5 h6
 6 – 1 Constant pressure condensation.
q out h6 h1