Boilers are enclosed vessels used to produce steam, primarily for power generation. The steam produced can also be used for domestic purposes such as cooking, heating and sanitation. In power plants, boilers produce steam which are used to drive a turbine which generate electricity. Boilers require a source of heat which heats up the water contained in the boiler to produce steam. For power plants that use fossil fuels the source of heat involves the burning of coal,oil or natural gas. The combustion process releases huge amounts of energy which enter the boiler and heat up the water. For nuclear power plants heat is produced by nuclear fission. In ships heat recovery boilers use the exhaust gases from the main engine to heat the water with little or no fuel consumption. This process is also used in power plants where a gas turbine and steam boiler are used.

Boiler Types

  • Fire tube boiler
  • Water tube boiler

Fire Tube Boiler

These boilers are constructed such that water surrounds the tubes enclosed in a cylindrical shaped iron casing. The furnace or firebox is located inside the cylinder and permanently surrounded by water. The fire tubes through which the heat passes are connected to the furnace. As the heat passes through the tubes, the water surrounding the tubes are heated and steam is produced. The steam then passes through an outlet to where it will be used for various purposes. There is a safety valve installed on the boiler that opens when required to relieve the boiler of excess pressure which may have built up inside the cylinder. There is a chimney attached to the boiler to exhaust waste steam into the atmosphere. There is also a drain valve which is opened periodically to drain the boiler of waste water and clean it of all dirt to ensure the efficiency of the boiler. These types of boilers are commonly used in locomotives that use steam to drive the trains. Fire tube boilers comparatively have low rate of steam production but have a high steam storage capacity. They mostly burn solid fuels, but can also be adapted to use liquid and gas fuels.

Water Tube Boilers

Water tube boilers are constructed such that the tubes are connected to drums which hold steam and water respectively. The lower drums contain water while the upper drums contain steam and water. The heat source is a furnace enclosed in the boiler. In water tube boiler steam flows inside the tubes which receive heat from the furnace by radiation. The water in the tubes receive the heat by conduction and convection. As the heating process continues, the temperature of the water increases until steam is produced. The steam then passes to the steam drum where it is stored and distributed for various uses. For power generation the steam flows from the steam drum to a superheater where it is further heated and converted to superheated steam. This superheated steam is used to drive the turbines used for power generation. Water tube boilers generally give high steam generation rates but have less steam storage capacity compared to fire tube boilers and are generally preferred for high pressure applications such as propulsion of warships. Water tube boilers are also installed with safety valves to relieve excess pressure built up inside the boiler. The safety valves prevent the boiler from exploding when the pressure inside the boiler exceeds its maximum working pressure.

Boiler Materials

The boiler pressure vessel is usually made of steel or a steel alloy or previously wrought iron. Stainless steel is not used on parts exposed to moisture due to the danger of corrosion, but ferrite stainless steel is often used in superheater sections that will not be exposed to boiling water. In other models especially boilers used in steam locomotives, copper was used to fabricate the body because of the high thermal conductivity of copper, but due to the high cost of copper, steel was adopted as the preferred material for the construction of boilers.

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