Various types of boilers used in thermal(steam) power plant-efficiency

Introduction to Boiler

A boiler or steam generator is a closed vessel which generates steam by heating water. According to American Society for Mechanical Engineers a boiler or steam generating unit is defined “As a combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the eat so made available to the fluid being heated and vaporized.” The heat energy supplied to water to convert it into steam comes from the external source such as from burning coal in thermal plants, heat energy generated through nuclear fission reactions in nuclear plants e.t.c. The steam generated can be used for various purposes such as for generating power in power plants by driving turbines through the steam generated, in textile and chemical industries e.t.c. Boiler is the most important and crucial component in a power plant. Accordingly utmost care is taken in selection of materials used for its construction, design, erection and maintenance of boiler in power plants.

Working principle of steam boiler

Basically boiler is a shell and tube heat exchanger. The boiler is a closed vessel in which water and hot gases simultaneously flow in shell and tube of boiler or vice versa. The hot gases are produced due to heat energy generated by continuous burning of fuel. These hot gases come in contact with the intermediate metal separating the hot gases and water and heat it up. The heat energy flows from hot gases and metal due to convection. This metal in turns transfers the heat to water flowing on the other side and converts it into to steam.

Types of Boilers

The boilers can be classified broadly based on following parameters

a) The boilers are divided in to three types based on direction of principle axis of the shell as Horizontal, Vertical and Inclined Boiler.
If the principal axis of the boiler is horizontal, the boiler is called horizontal boiler. If the axis is vertical, it is called vertical boiler and if the axis is inclined it is called as inclined boiler. Due to its horizontal structure in the event of any malfunctioning of boiler the parts of horizontal boiler is can be inspected and repaired easily compared to vertical boiler. The horizontal boiler occupies more space whereas the vertical boiler occupies less floor area.
b) Based on the relative positions of water and hot gases containing heat energy
as Fire Tube and Water Tube boilers.

Fire tube boilers:

In fire-tube boilers, combustion gases pass through the inside of the tubes with water surrounding the outside of the tubes. Examples: Cochran, Lancashire and Locomotive boilers. The advantages of a fire-tube boiler are its simple construction and less rigid water treatment requirements. The disadvantages are the excessive weight per pound of steam generated, excessive time required to raise steam pressure because of the relatively large volume of water, and inability to respond quickly to load changes due to the large water volume.

Water Tube Boiler

In the water tube boilers, the water is inside the tubes and hot gases surround them.
Examples: Babcock and Wilcox, Stirling, Yarrow boiler etc. In a water-tube boiler, the water is inside the tubes and hot gases pass around the outside of the tubes. The advantages of a water-tube boiler are a lower unit weight per pound of steam generated, less time required to raise steam pressure, a greater flexibility for responding to load changes, and a greater ability to operate at high rates of steam generation.

c) Based on location of furnace as externally fired and internally Fired

Externally Fired Boilers:
The boiler is known as externally fired if the fire is outside the shell.
Examples: Babcock and Wilcox boiler, Stirling boiler etc.

Internally Fired Boilers:
In case of internally fired boilers, the furnace is located inside the shell.
Examples: Cochran, Lancashire boiler etc.

4.Based on method of water circulation as Forced circulation and Natural Circulation

Forced Circulation Boiler:
In forced circulation type of boilers, the circulation of water is done by a forced pump. In once through generators water is forced in to by feed water pumps.
Examples: Velox, Lamomt, Benson Boiler etc.

Natural Circulation Boiler:
In natural circulation type of boilers, circulation of water in the boiler takes place due to natural convention currents produced by the application of heat. This natural circulation occurs due to density gradients along the network.
Examples: Lancashire, Babcock and Wilcox boiler etc.

5. According to the pressure of steam produced Higher Pressure and Low Pressure Boilers.

High pressure boilers:

The boilers which produce steam at pressures of 80 bar and above are called high pressure boilers.
Examples: Babcock and Wilcox, Velox, Lamomt, Benson Boiler etc.

Low Pressure Boilers
The boilers which produce steam at pressure below 80 bars are called low pressure boilers.
Examples: Cochran, Cornish, Lancashire and Locomotive boiler e.t.c.

6. Stationary and Portable
The boilers are classified based on mobility as either stationary or mobile.

  • Stationary boilers are used for power plant steam, for central station utility power plants, for plant process steam etc. They will be stationary once erected in the site.
  • Mobile boilers or portable boilers include locomotive type, and other small units for temporary use at sites.

7. Single Tube and Multi Tube Boiler
The fire tube boilers are classified as single tube and multi-tube boilers, depending upon whether the fire tube is one or more than one.

Examples: Cornish, simple vertical boiler are the single tube boiler and rest
of the boilers are multi-tube boiler.

Total Efficiency of boiler

The efficiency of boiler depends on so many factors such as the design of the boiler, heat cycle employed, built in losses, heat lost to flue gases, properties of water and hot gases, convective and radiative loss, losses due to incomplete burning of fuel e.t.c.

The boiler efficiency is given as
n= heat content of outgoing steam separated/heat energy supplied by the fuel

    = ma*(hf-hf1)/C

Where

ma*(h – hf1) is the heat gained by the steam from the boiler per unit time
ma  = mass of steam in kg
C is the calorific value of steam in KJ/kg
h=enthalpy of steam at predefined pressure and temperature
h = hf (specific enthalpy) for hot water at pressure p

h = hf +hfg for dry saturated steam at pressure p
h = hf +x*hfg for steam with dryness fraction x at pressure p
X is known as steam quality or equivalent evaporation
h= hf + hfg + cp*(Tsup-Ts) for superheated steam at pressure p and temperature

Tsup and Ts is the saturation temperature corresponding to the pressure at the outlet of boiler.
The values of hf, hfg, cp, Ts can be obtained from steam tables. In the steam tables specific enthalpies, specific heat, heat capacity, steam saturation temperature will be given for specific intervals of pressure. The intermediate values of hf, hfg, cp, Ts which are not available in the steam table can be interpolated linearly. Typical boiler efficiencies are in the range of 75-90%.