A boiler is a closed vessel that produces steam from water through combustion of fuel. Water is pumped into the boiler and heated by flue gases, vaporizing it into steam. Steam is used for power generation, heating buildings, and industrial processes. There are two main types of boilers - fire-tube boilers where hot gases pass through tubes surrounded by water, and water-tube boilers where water passes through tubes surrounded by hot gases. Water-tube boilers are more efficient but more expensive. Boilers include components like drums, tubes, and mountings to operate safely and control steam production.

1. Steam generating Unit
(A Boiler)
 Boiler:
 A boiler is a closed vessel in which water or other fluid is heated.
 The fuel doesn’t necessarily boil
 Boiler:
 A fuel –burning apparatus or container for heating water, in particular.
 A household device providing a hot –water supply or serving a central heating
system.
 A tank for generating steam under presence in a steam engine.
 Properties of a boiler:
 The heated or vaporized fluid exits the boiler for use in various processes or heating
applications, including water heating, central heating, boiler –based power
generation, cooking & sanitation.
 Steam Generating Unit:
 A boiler is a closed vessel in which steam is produced from water by combustion of
fuel.
 How does a boiler work?
 Water is pumped into the boiler at operating pressure
 Heat of flue gases vaporizes water to form steam.
 Steam formed is passed into steam space above the water space

2.  Application of steam generators:
 Integral components of steam turbines used as prime mover to drive generators
to produce electricity
 Used in various industries including cement production, textiles
 Used to produce distilled water for medicines, pharmaceuticals and others
 Used for heating large buildings, in agriculture
 Principal components of a Boiler:
 Boiler drums, tubes, furnace
 Boiler mountings
 Boiler accessories
Boiler Mountings:
Devices that are required for proper operation, safety and control.
Example: Water level indicator, pressure gauge, steam stop valve, safety valves, fusible plug,
feed check valve etc.
 Boiler accessories:
 Devices used to increase the efficiency
 Example: Pre –heater, Economizer, Super heater, Faced Pump or injector, Baffler
etc.

3.  Classification of boiler:
1) Tube content
 Fire tube boiler
 Water tube boiler
2) Axis of Shell
 Horizontal
 Vertical
 Inclined
3) Location of furnace
 Externally fixed
 Internally fixed
4) Method of Circulation:
 Natural
 Forced
5) Mobility:
 Stationary
 Portable
6) Usage:
 Packaged
 Unpackaged
7) Pressure:
 High
 low
8) Tubes:
 Single –tube
 Multi –tube

4.  Now we briefly discuss about Fire –tube boiler & Water –tube boiler:
 Fire –tube boiler:
In fire –tube boiler there are numbers of tubes through which hot gases are passed and water
surrounds these tubes.
Figure: 1:- Cochran Boiler

5.  Fire –tube boiler
 Working Principle:
 Hot flue gases are fed through the tubes
 Tubes are surrounded by water

6.  Water –tube boiler:
In water tube boiler the water is heated inside tubes and hot gases surround these tubes.
“Water –tube boiler is reverse of the Fire –tube boiler”

7.  Water –tube boiler
 Working Principle:
 Small parallel tubes contain water connected to drum with header
 Gases pass outside the tubes contained water

8.  Fire –tube boiler VS Water –tube boiler:
Fire –tube boiler Water –tube boiler
 Hot flue gases are fed through the tubes
 Tubes are surrounded by water
 Small parallel tubes contain water connected
to drum with header
 Gases pass outside the tubes contained water
 Low initial cost  High initial cost
 Efficiency 70-75% (up to 80%)  Higher efficiency (up to 92%)
 Capacity 20kg/hr – 3000kg/hr
(up to 10000kg/hr)
 High pressures and capacity (50000 kg/hr)
 Furnace location: internal  Furnace location: external
 Floor area: small  Floor are: large
 Evaporation: slow  Evaporation: fast
 Water treatment: No  Water treatment: yes
 Suitable for fluctuating loads  Suitable for power plant
 Boiler Efficiency:
The ratio of the quantity of the heat utilized actually by water and steam to the quantity of
the heat supplied.
 Boiler Capacity:
The amount of steam of a boiler can supply per hour.
 Boiler Pressure:
The designated operating pressure of a boiler at which it can safely deliver the steam.
 Function of boiler Mountings:
 Water level indicator: indicate the safety level of water
 Pressure gauge: indicate the steam pressure
 Steam stop valve: to regulate the supply of steam to turbine
 Fusible plug: to stop the boiler if it gets try to prevent damage
 Safety valve: to release the internal boiler pressure whenever necessary
 Function of boiler accessories:
 Air pre –heater: utilize the waste of chimney to preheat the furnace air
 Economizer: utilize the waste heat of chimney to preheat the boiler feed water
 Super –heater: Utilize the waste heat to dry and /or super heat the steam leaving
the boiler (to increase steam quality and boiler efficiency)
 Baffles: help feed water in extracting heat from the flue gases
@shohel, MEE’16, SUST