Types of Fire Tube Boilers Design-Operation

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Fire tube boiler

Fire tube boiler is the one of the simplest and oldest boiler designs. The term Fire tube boiler is coined because of the fact that the combustion products pass through the tubes surrounded by water in a shell. Fire tube boilers are simple in construction and are mostly used for low pressure applications. There are many variants of fire tube boilers such as vertical boiler, Locomotive boiler, Cornish boiler, Lancashire boiler, Cochran boiler e.t.c. with different construction features.

Fire tube boiler operation

In the fire tube boiler fuel such as coal, oil, natural gas is burnt in the combustion chamber typically located at the bottom of the fire tube boiler(it can be external to the boiler). The burning of fossil fuels generates hot flue gases which carry the heat energy produced in the burning of fuel. The hot gases are allowed to flow through the tubes of fire tube boiler. These tubes are immersed in the water filled inside the cylindrical shell of boiler. As the gases flow through the tubes they exchange they heat the water surrounding the tubes and as a result of this steam is produced. The steam is also stored in the boiler or delivered continuously from the boiler to the purpose for which it will be used. The water level inside the boiler is maintained by providing the continuously supply of feed water by using the feed water pumps.

Types of Fire tube boilers

The furnace volume or combustion chamber can either be inside or external to the shell that contains the water.

Based on the location of combustion chamber or furnace volume Fire tube boilers are subdivided into three groups. They are External furnace type and internal furnace type Fire tube boilers.

Depending on the design of the Fire tube boiler:

These are subdivided as
Cornish boiler: Cornish boiler was first adopted by Trevithick a Cornish engineer. It consists of a flat cylindrical shell and a small flue tube containing the furnace passes through it.
Lancashire boiler: Lancashire boiler is similar to Cornish boiler in construction but instead of one flue tube, two flue tubes are used.
Locomotive boiler: It is a stationary boiler and is used in locomotives. It is efficient in steam production and is compact. It is horizontal multi tubular design. The main advantages of Locomotive boiler are low cost of construction, high steam capacity and low installation cost.
Vertical boiler: It is the simplest fire tube boiler which consists of a cylindrical shell containing greater portion of water and remaining portion is occupied by steam. It contains cross tubes and a furnace at the bottom of the boiler. The combustion gases after heating the water are allowed to escape into atmosphere
Cochran boiler: It is a vertical multi tubular boiler and has a number of horizontal fire tubes. The furnace is one piece construction and is flawless.
Scotch marine boiler: The Scotch marine is one of the most popular Fire tube boilers for higher steam capacities at high pressures. It uses a large number of small-diameter tubes. Large number of tubes gives the advantage of higher heating surface area. The furnaces are internally fired and leave the boiler though chimney to atmosphere.
Immersion type boiler: The Immersion Fired boiler is a once through fire-tube boiler. The fire tubes themselves act as furnace with multiple burner valves injecting gas under pressure.

Fire tube boiler design

The pressure vessel, tubes, combustion chamber are the main components in a fire tube boiler. Fire tube boiler consists of a cylindrical shell containing mostly water space (volume of shell occupied by water) and little steam space (volume of shell occupied by water). The fuel will be burnt at the bottom of the pressure vessel in combustion chamber. After the fuel is burnt hot gases are produced and are allowed through cylindrical steel tubes inside the shell. In some of the fire tube boiler designs tubes are horizontal with respect to shell axis such as in Cochran boiler, Cornish boiler and Locomotive boiler and in some designs they are inclined at an angle as in vertical fire tube boiler. The number of tubes, diameter of tubes through which the flue gases flows depends on the water and steam operating conditions based on which the flue gas operating temperatures required to achieve those conditions are derived. In Cornish and Lancashire design the number of tubes is one independent of operating conditions. Pressure gauges and water level indicators are provided inside the fire tube boilers to monitor the pressure inside the boiler and the level of water. Safety valves are provided inside the fire tube boiler in the event of occurrence of some fault in the boiler due to which the pressure inside the boiler increases. The safety valves are opened automatically when the pressure inside the boiler reaches a threshold. The safety valves relieve the pressure inside the boiler by dumping the steam inside the fire tube boiler to the atmosphere.

Advantages of fire tube boiler

  • Fire tube boilers are appropriate for high pressures as gas is contained inside tubes
  • Fire tube boilers are cheaper than water tube boilers
  • In fire tube boilers the deposits will be formed only inside the tube hence cleaning the tubes is easier
  • Chances of explosion are less even if it explodes it Involves lesser risk due to lower operating pressures
  • Fire tube boilers are more fuel efficient and easier to operate
  • Fire tube boiler requires less skill for efficient and economic working.

Disadvantages of fire tube boiler

  • The steam capacity (rate of steam production) and pressure of steam in fire tube boilers are limited
  • Fire tube boilers are difficult in construction and transportation
  • Fire tube boilers occupies more floor area
  • A separate steam drum with accessories is required to achieve good steam quality.
    Due to large volume of water the time required to raise the steam pressure is high.
  • The steam quality of Fire tube boiler is poor.