Steam Boiler

Evaporating the water at appropriate temperature and pressure does the generation of steam. It might look that to perform this function is an easy task. But if large quantities of steam at high pressure are to be produced rapidly and with economy then a considerable amount of skill and specialized equipments are required. The production of steam at high pressure in large quantities would not be possible and economical if the water is heated in ordinary way in a vessel with furnace at its bottom. The heat will go to waste in large quantities by radiation. There will not be an efficient heat transfer to water and evaporation of water may not be controlled.

In order to overcome these defects water is evaporated in a closed vessel, in which heat transfer and the evaporation process is controlled. This equipment is known as steam boiler.Boiler is thus, a vessel in which steam is generated. Heat is produced in the boiler by burning of fuel. This heat is transferred to the water contained in the boiler and the water evaporates to form steam. As has been discussed earlier that steam is generated at a desired saturation pressure, the boiler thus also maintains the required pressure in it. A boiler is therefore also known as a pressure vessel. The construction and appearance of a boiler depends upon the arrangements made for burning the fuel and the mode of transfer of this heat to water.

i. Components of a Steam Boiler
Every boiler consists of three distinct regions, such as, the space for burning of fuel and flue gases, the space for water and the space for steam .The Fuel Space: It consists of the furnace chamber and the passage of hot gases as they flow through the boiler. The fuel is burned in the furnace chamber or the fire box. If the fuel is a solid fuel, such as, coal it is burnt by placing it over a grate.The grate consists of cast iron bars with spacing between the bars for the flow of air. If the fuel is oil or gas it is burned with help of a specially designed oil or gas burner. The combustion of fuel is maintained by a steady supply of air to the furnace. The waste furnace gases escape the boiler through a high chimney. A high chimney helps in safe discharge of waste gases in the atmosphere and also creates a necessary pressure differential for flow of air and gases in the boiler.

The Water Space and Steam Space: The water and steam in the boiler is contained in metallic water drums and tubes. The water space is the volume of the drum or shell that is occupied by water, and steam space is the volume of the entire shell not occupied by water. The level at which the water stands in the boiler shell is known as water level and it is indicated by a water level indicator. The level of water fluctuates in the boiler at times but for the best operation the variation should be small. The heating surface is the surface of shell exposed to the fire or the hot gases from the fire.

The general constructional features of a boiler are illustrated in fig. 9.1. In its simplest form, the boiler shell is kept vertical. It consists of a large cylindrical shell.A firebox is provided in the shell by suitable positioning it with metal plates. The firebox carries a grate at the bottom. The water space in the shell surrounds the firebox from all sides.

The Uptake and Chimney: A tube called the uptake starting from the top of firebox passes through the shell and connects the base of the chimney. The chimney is placed at the top of the uptake. One or more cross tubes are is fitted across the firebox to increase the heating surface area and to ensure better circulation of water. A fire-hole is provided in front of the boiler slightly above the grate level.A manhole and a hand hole are provided in the shell to have an access for cleaning.To drain the mud that settles down a mud hole is provided at the bottom. All the openings are closed with suitable covers.

ii. Functioning of a Boiler 

The coal burns in the firebox over the grate. Heat produced by burning fuel in the firebox is radiated to the water in the boiler shell. The hot gases while moving upward come in contact with cross tubes and then pass through the uptake tube.

During this contact, they give part of their heat to the metal of the tubes and this heat is transferred to water. The water gets heated up and evaporates. The steam produced by evaporation of water gets accumulated in the boiler shell, in the space above the surface of water. The steam generated in the boiler is then tapped off through a suitable valve fitted in the steam portion of the shell.

A boiler also has component units which are primarily intended for the safety of boiler. These components are mounted on the body of the boiler and are known as Boiler mountings. Various boiler mountings include: pressure gauge, safety valve,water-level indicator, fusible plug, the safety and satisfactory working of the boiler largely depends upon the reliability of these mountings. We shall discuss various essential boiler mountings in detail in Unit-10.

In the smaller boilers the firebox forms an integral part of the boiler shell. But in large boilers it is separately constructed of brickwork and is known as settings. The brickwork forms the walls of furnace and combustion chamber. It confines the heat to the boiler and makes passage through which the hot gases pass. It also provides support to the boiler shell.

Steam is generated in the boiler under the set of conditions of inlet water and exit steam, while a certain rate of fuel is being consumed. It is neither practical nor expected that whole of the heat produced by the combustion of fuel in a boiler will be transferred to water for generation of steam. Certain amount of heat will go to waste through radiation and through flue gases, etc. Nevertheless, a good boiler is one which gives the economy of steam generation and provides high performance efficiency.

iii. Requisites of a Good Boiler

We would now specify and lay down the necessary requisites of a good boiler.

Requisites of a good boiler could be listed as follows;

a) It should be capable of producing maximum amount of steam with minimum cost.

b) It should be capable of quick starting and should be able to meet the variations in steam requirements.

c) All parts and components should be easily accessible for inspection and repair.

d) In order to make the best use of heat supplied, the boiler should have proper arrangement of circulation of water and hot gases.

e) It should be safe in working.

There are many factors for selection of a boiler for a given situation. The most important ones are: the required steam capacity, working pressure in the boiler and the availability of fuel. Based on these the size and the type of boiler are selected.We therefore now need to know about various types of boilers and how these are classified.

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