Steam boilers are used to produce saturated or superheated steam, which is then put to work in a variety of ways. Steam is used for its heat energy in heating, cooking and reboiling operations. It also is used for its pressure energy in reciprocating steam engines and steam turbines. Steam boilers create the steam by converting heat energy from fuel combustion, nuclear reactors, concentrated sunlight or waste heat from other processes..
Two Basic Types
While early boilers were as simple as iron tanks over a wood fire, modern boilers use either a fire tube or water tube design. Either type incorporates a fire box in which fuel and air are continuously introduced and burnt. The hot combustion gases are then used to heat up either the inside or outside of a tube. Water on the opposite side of the tube is thus boiled into steam.
Fire Tube Boilers
Fire tube boilers consist of a boiler shell containing the water to be boiled. Long horizontal tubes carry the hot combustion gases through the shell. The heat passes from the inside of the tube to the water on the outside of the tube. There may be several passes of parallel tubes as the gases wind their way from the firebox back and forth to the stack.
Water Tube Boilers
Water tube boilers hold water in vertical tubes, called risers, which extend from the water drum on the bottom to the steam drum on the top of the boiler, where the steam header exits. These typically surround the firebox in many layers like a dense forest. As steam bubbles form, they rise to the steam drum, where the steam exits through the steam header.
Superheaters are added to the outlets of standard boilers to add more energy, increasing the temperature and pressure of the steam -- and thus its capacity to do work. Superheated steam is generally used to drive engines and turbines with its pressure.
The key to boiler design is the containment of potentially dangerous high pressure steam within the boiler at extremely high temperatures. This calls for special attention to materials, construction techniques, and procedures, such as uniform heat-up and cool-down.
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