14-12-2012, 12:33 PM
INTRODUCTION TO BOILERS
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INTRODUCTION
Boiler is a metal container in which a liquid is heated and changed into a vapor. Most boilers change water into the vapor steam. Steam is used to heat buildings and processes. It changes from vapor to liquid form as it delivers heat into a room or building, giving off even more heat as a result. Some heating systems, called hydronic systems, circulate hot water rather than steam. However, the heat source in these systems is still referred to as a boiler. Steam produced in boilers is also used in steam turbines and for refining oil or drying paper.The process of heating a liquid until it reaches it's gaseous state is called evaporation.
Heat is transferred from one body to another by means of
(1) radiation, which is the transfer of heat from a hot body to a cold body through a conveying medium without physical contact,
(2) convection, the transfer of heat by a conveying medium, such as air or water and
(3) conduction, transfer of heat by actual physical contact, molecule to molecule.
The heating surface is any part of the boiler metal that has hot gases of combustion on one side and water on the other. Any part of the boiler metal that actually contributes to making steam is heating surface. The amount of heating surface a boiler has is expressed in square feet. The larger the amount of heating surface a boiler has the more efficient it becomes.
BOILER TYPES:
There are virtually infinite numbers of boiler designs but generally they fit into one of two categories:
Firetube or as an easy way to remember "fire in tube" boilers, contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be changed to steam circulates.
In the fire-tube boiler, gases flow through tubes surrounded by water. This type of boiler is used in most steam locomotives, in small factories, and sometimes in heating homes.
In a firetube boiler the heat (gasses) from the combustion of the fuel passes through tubes and is transferred to the water which is in a large cylindrical storage area.
STEAM BOILER SYSTEMS:
The feedwater system provides water to the boiler and regulates it automatically to meet the demand for steam. Valves provide access for maintenance and repair. The steam system collects and controls the steam produced in the boiler. Steam is directed through piping to the point of use. Throughout the system steam pressure is regulated using valves and checked with steam pressure gauges. The steam and feedwater systems share some components. The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system. All fuels are combustible and dangerous if necessary safety standards are not followed.
COMBUSTION:
Is the method of combining the fuel and air systems in a source of heat at sufficient temperature to produce steam. Combustion may be defined as the rapid chemical combination of oxygen with the combustible elements of a fuel. Only three combustible, chemical elements are of any significance: carbon, hydrogen and sulfur. The boiler combustion furnace in which the fuel burns provides a chamber in which the combustion reaction can be isolated and confined so that it can be controlled. In a scotch marine boiler it is referred to as a Morrison tube or in other boilers the firebox area. The convection surfaces are the areas to which the heat travels that is not transferred in the combustion furnace. Here additional heat is removed. The burner is the principal device for the firing of oil and/or gas. Burners are normally located in the vertical walls of the furnace. Burners along with the furnaces in which they are installed, are designed to burn the fuel properly.
STEAM TO WATER CYCLE:
In a steam heating system steam leaves the main steam line and enters the main steam header. From the main header piping directs the steam to branch lines. Branch lines feed steam through a riser to the steam heating equipment. At the heating equipment heat is transferred to the building space. As the steam releases heat to the building space and is cools it turns back to water or condensate. The condensate is separated from the steam by a steam trap. The steam trap allows condensate to pass but not the steam. The condensate passes through the condensate return line and is collected and directed back to the boiler to repeat the steam to water process.
Referring back to the teapot example, after repeated use it began to acquire a "buildup" of solids from the water. The same separation of solids in the water occurs in the boiler but since it is operating continuously and at higher temperatures this "buildup" can occur very rapidly. When this occurs the heat transfer can not be achieved as readily which requires more fuel to produce the steam. If continued unchecked damage to the metals in the boiler shell and tubes will result.