Heat is the energy source that converts water into steam. The fuel source to provide the necessary heat can come in many different forms. From wood, coal, oil, natural gas, municipal waste or biomass, nuclear fission reactors and the sun. Each type of fuel provides the heat source to boil water. They just do it in different ways. Some are eco-friendly, while others are quite dirty.
Fire Tube Boilers
The initial steam generators, also called boilers, required a fire box for the fuel. These started out being wood-burning and quickly changed to coal-burning. The fire box has tubes running through the water chamber, heating the water to steam and then releasing the fuel fume gasses out through a smoke stack. Railroad train engines and boats were the first to use this type of steam generation for power(see Reference 1).
Water Tube Boilers
Water tube boilers came about so that the steam generated could be produced at a higher pressure. The water flowed through tubes at an angle while the heat flowed up and around the tubes. The higher steam pressure gave greater force to push a piston or turn a turbine wheel utilizing less heating fuel.
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Combustion Heat Generator
Combustion heat generators follow a similar heat exchange concept as the tube boilers, but can product even higher pressures for power. These are used mainly in power plants producing electricity. Their steam pressures can almost meet or, in some super-critical steam-designed plants, exceed the critical water pressure of 221 bar. The temperature of the steam at these highly compressed rates can reach over 500 degrees Celsius.
Heat Recovery Steam Generator
The heat recovery steam generator, or heat exchanger, recovers the high pressure hot gas steam and utilizes that steam after running it through a chain of heat exchanges to run other lower-powered machines. This recovered steam can even be used at these lower pressures to supply steam heat for other industrial buildings or even homes (see Reference 2).
Nuclear Power Plant Steam Generators
There are two major types of nuclear steam generators; the (BWR), Boiling Water Reactor and the (PWR), Pressurized Water Reactor. The BWR-cooled water is turned into steam within the nuclear reactor itself and run to the turbine outside the containment area. The PWR-cooled water is pressurized in excess of 100 bar and there is no water boiling process inside the reactor. It is then run to the turbine and through a cooling process for recirculation (see Reference 3).
Solar Power Steam Generators
Solar power steam generators are the cleanest source of boiling water. The water is run through tubes inside a solar panel. The sun heats the water and then the water runs through a steam turbine, creating electricity. There are no waste products and no pollution (see Reference 4).