Boiler is an enclosed vessel in which water is heated and circulated until the water is turned in to steam at the required pressure. Coal is burned inside the combustion chamber of boiler. The products of combustion are nothing but gases. These gases which are at high temperature vaporize the water inside the boiler to steam. Some times this steam is further heated in a super heater as higher the steam pressure and temperature the greater efficiency the engine will have in converting the heat in steam in to mechanical work. This steam at high pressure and temperature is used directly as a heating medium, or as the working fluid in a prime mover to convert thermal energy to mechanical work, which in turn may be converted to electrical energy. Although other fluids are sometimes used for these purposes, water is by far the most common because of its economy and suitable thermodynamic characteristics.
Fire tube Boilers : In fire tube boilers hot gases are passed through the tubes and water surrounds these tubes. These are simple, compact and rugged in construction. Depending on whether the tubes are vertical or horizontal these are further classified as vertical and horizontal tube boilers. In this since the water volume is more, circulation will be poor. So they can't meet quickly the changes in steam demand. High pressures of steam are not possible, maximum pressure that can be attained is about 17.5kg/sq. cm. Due to large quantity of water in the drain it requires more time for steam raising. The steam attained is generally wet, economical for low pressures.
Water tube Boilers : In these boilers water is inside the tubes and hot gases are outside the tubes. They consist of drums and tubes. They may contain any number of drums. Feed water enters the boiler to one drum. This water circulates through the tubes connected external to drums. Hot gases which surround these tubes will convert the water in tubes in to steam. This steam is passed through tubes and collected at the top of the drum since it is of light weight. So the drums store steam and water. The entire steam is collected in one drum and it is taken out from there. As the movement of water in the water tubes is high, so rate of heat transfer also becomes high resulting in greater efficiency. They produce high pressure, easily accessible and can respond quickly to changes in steam demand. These are also classified as vertical, horizontal and inclined tube depending on the arrangement of the tubes. These are of less weight and less liable to explosion. Large heating surfaces can be obtained by use of large number of tubes.
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