Information AboutBoiler |
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OVERVIEW Construction of boilers is mainly limited to Copper , Steel , Stainless Steel , and Cast Iron . In Live Steam toys, Brass is often used. The source of heat for a boiler is Combustion of any of several Fuel s, such as Wood , Coal , Oil , or Natural Gas . Electric boilers use Resistance or Immersion type heating elements. Nuclear Fission is also used as a heat source for generating Steam . Heat Recovery Steam Generator s (HRSGs) use the heat rejected from other processes such as Gas Turbine s. Boilers can also be classified into:
The goal is to make the heat flow as completely as possible from the heat source to the water. For example, steam Locomotive s have Fire-tube Boiler s, where the fire is inside the tube and the water on the outside. These usually take the form of a set of straight tubes passing through the boiler through which hot combustion gases flow. In Water-tube Boiler s the water flows through tubes around a fire. The tubes frequently have a large number of bends and sometimes fins to maximize the surface area. This type of boiler is generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can contain the pressure with a thinner wall. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are mechanically assembled on site to create the finished boiler. There are other types of boilers, largely of historical interest. For example, the Cornish Boiler developed around 1812 by Richard Trevithick for generating steam for Steam Engine s. This was both stronger and more efficient than the simple boilers which preceded it. It was a cylindrical water tank around 27 feet long and 7 feet in diameter, and had a coal furnace placed in a single cylindrical tube about three feet wide which passed centrally along the long axis of the tank. The fire was tended from one end and the hot gases from it travelled along the tube and out of the other end, to be circulated back along flues running along the outside of the boiler before being expelled via the Chimney . This was later improved upon in the Lancashire Boiler which had a pair of furnaces in separate tubes side-by-side. This was an important improvement since each furnace could be stoked at different times, allowing one to be cleaned while the other was operating. These designs are really primitive fire tube boilers, and led on to the Scotch Boiler which remains a popular fire tube design. SUPERHEATED STEAM BOILERS Most boilers heat water until it boils, and then the steam is used at Saturation Temperature (i.e., saturated steam). Superheated steam boilers boil the water and then further heat the steam in a superheater. This provides steam at much higher temperature, and can decrease the overall thermal efficiency of the steam plant due to the fact that the higher steam temperature requires a higher flue gas exhaust temperature. However, there are advantages to superheated steam. For example, useful heat can be extracted from the steam without causing condensation, which could damage piping and turbine blades. Superheated steam presents unique safety concerns, however, if there is a leak in the steam piping, steam at such high pressure/temperature can cause serious, instantaneous harm to anyone entering its flow. Since the escaping steam will initially be completely superheated vapor, it is not easy to see the leak, although the intense heat and sound from such a leak clearly indicates its presence. The superheater works like coils on an air conditioning unit, however to a different end. The steam piping (with steam flowing through it) is directed through the flue gas path in the boiler furnace. This area typically is between 2500-3000 degrees fahrenheit. Some superheaters are radiant type (absorb heat by radiation), others are convection type (absorb heat via a fluid i.e. gas) and some are a combination of the two. So whether by convection or radiation the extreme heat in the boiler furnace/flue gas path will also heat the superheater steam piping and the steam within as well. It is important to note that while the temperature of the steam in the superheater is raised, the pressure of the steam is not. The process of superheating steam is most importantly designed to remove all moisture content from the steam to prevent damage to the turbine blading and/or associated piping. SUPERCRITICAL STEAM GENERATORS Supercritical steam generators are frequently used for the production of electric power. They operate at "supercritical pressure". In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3200 PSI, 22 MPa, 220 bar) that actual boiling ceases to occur, the boiler has no water - steam separation. There is no generation of steam bubbles within the water, because the pressure is above the "critical pressure" at which steam bubbles can form. It passes below the critical point as it does work in the high pressure turbine and enters the generator's condenser. This is more efficient resulting in slightly less fuel use and therefore less Greenhouse Gas production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" actually occurs in this device. HYDRONIC BOILERS Hydronic boilers are used in generating heat typically for residential uses. They are the typical power plant for Central Heating systems fitted to houses in northern Europe (where they are commonly combined with domestic water heating), as opposed to the Forced-air furnaces or wood burning stoves more common in North America . The hydronic boiler operates by way of heating water/fluid to a preset temperature (or sometimes in the case of Single Pipe Systems , until it boils and turns to steam) and circulating that fluid throughout the home typically by way of Radiator s, baseboard heaters or through the floors. The fluid can be heated by any means....gas, wood, fuel oil, etc, but in built-up areas where piped gas is available, Natural Gas is currently the most economical and therefore the usual choice. The fluid is in an enclosed system and circulated throughout by means of a motorized Pump . Most new systems are fitted with Condensing Boiler s for greater efficiency. Hydronic systems are being used more and more in new construction in North America for several reasons. Among the reasons are:
Forced-air heating does have some advantages, however. See Forced-air Heating . ACCESSORIES Boiler fittings
Steam accessories
Combustion accessories
Other essential items
CONTROLLING DRAFT Most boilers now depend on mechanical draft equipment rather than natural draft. This is because natural draft is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as the chimney height. All these factors make proper draft hard to attain and therefore make mechanical draft equipment much more economical. There are three types of mechanical draft:
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