Steam Boilers

Steam Boilers

Steam boilers are also called steam generators, are closed vessels, generally made of steel, and are used for the generation of steam at a desired pressure and temperature. 

A steam generator consists of furnace boiler or evaporator, superheater, economizer, air preheated etc

The function of a boiler is to evaporate water into steam at a pressure higher than the atmospheric pressure.

Water-free from impurities such as dissolved salts, gases and non-soluble solids should be supplied to boilers. This is done by suitable water treatment.

Steam is useful for running steam turbines in electrical power stations, ships and steam engines in railways locomotives. It is also useful for many other industrial applications. Boiler furnace can use either solid, liquid or gaseous fuel.

Boilers mainly classified as fire-tube boiler and water-tube boilers. In fire tube boilers, hot gases from the furnace pass through the tubes which are surrounded by water. In the water tube boilers, the water circulates inside the tubes which are heated from outside by hot gases from the furnace.

Figure:- General arrangement of a steam power plant

Classifications of Steam Boilers

Steam boilers may be classified in many ways. The important classification follows:-

1. According to the contents in the tube

a) Fire Tube or Smoke Tube steam boilers

In fire tube steam boilers, the flames and hot gases, produced by combustion of fuel, pass through the tubes (called multi-tubes)

Example:- simple vertical boilers, locomotive boiler, and falcon boiler.

b) Water Tube steam boilers

In water tubes boilers, the water is contained inside the tubes (called water tubes) which are surrounded by flames and hot gases from outside. Example:- Babcock and Wilcox boilers, Stirling boiler, la-mount boiler, Benson Boiler and Loeffler Boiler.

2. According to the position of the furnace

a) Internally fired boilers-

In internally fired steam boilers, the furnace is located inside the boiler shell. Most of the fire tube steam boilers are internally fired.

b) Externally fired boiler

In externally fired steam boilers, the furnace is located inside the boiler shell. Most of the fire tube steam boilers are internally fired.

In internally fired steam boilers, the furnace is arranged underneath in a brick-work setting. Water tube steam boilers are always externally fired.

3. According to the axis of the boiler shell

a) Vertical-axis boiler

In vertical steam boilers, the axis of the shell is vertical that is why it is known as verical boiler

b) Horizontal-axis boiler

whereas in horizontal  boilers, the axis of the shell is horizontal, that is why is know as horizontal boiler.

4. According to the number of tubes

a) Single Tube boiler-

In single tubes steam boilers there is only one fire tube or water tube simple vertical boilers and Cornish boilers are single tube boilers.

b) Multi Tubes boiler

In multi-tubular steam boilers, there are two or more fire tubes or water tubes.

5. According to the method of circulation of water and steam

a) Natural circulation boiler

In natural circulation steam boilers, the circulation of water is by natural convection currents which are set up during the heating of water. In most of the steam boilers, there is a natural circulation of water.

b) Forced circulation boiler

In forced circulation steam boilers, there is a forced circulation of water by a centrifugal pump driven by some external power. Use of forced circulation is made in high problem such la-mount, Benson boilers etc

6. According to the movement of boiler

a) Stationary Boiler

The stationary steam boilers are used in power plants, and in industrial process work. These are talked stationary because they do not move from one place to another.

b) Mobile Boiler

The mobile steam boilers are those which move from one place to another. These boilers are locomotive and marine boilers.

7. According to the pressure

  1. Boilers operate up to 30 bar:- pressure may be classified as low-pressure boilers.
  2. Pressure 30 to 60 bar:- medium pressure boilers
  3. Pressure above 60 to 70 bar:- high-pressure boilers.

Boilers having pressure above 221.2 bar are known as supercritical pressure boilers.

Formation of Steam in Steam boiler:-

let us take 1 kg of water and heat at standard pressure

Figure:- Formation of steam.

Boiling Point of Water:-

At standard atmospheric pressure, the temperature is 100° C, It increases with pressure. The value can be obtained from steam tables. At 10 bar, the boiling point is 179.9°.

Evaporation of water:-

The temperature remains constant. There will be a mixture of water and steam till the point C. At the midpoint m, only 0.5 kg of water would


If water is heated above its saturation temperature corresponding to saturation pressure, then it becomes the superheated steam. It is called superheating.

Types of Steam Boilers:-

  1. Cochran boiler
  2. Locomotive boiler
  3. La-mount boiler
  4. Benson boiler
  5. Cornish boiler
  6. Wilcox & Babcock boiler

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