A boiler is a closed vessel in which water or other fluid is heated under pressure. The hot fluid is then circulated out of the boiler for use in various process or heating applications. A safety valve is required to prevent over pressurisation and possible explosion of a boiler. Contents 1 Overview 2 Superheated Steam Boilers 3 Supercritical boilers 4 Hydronic boilers 5 Accessories 5.1 Boiler fittings 5.2 Steam accessories 5.3 Combustion accessories 6 Controlling draft 7 See also 8 References 9 External Links Overview Digram of a fire-tube boiler. Construction of boilers is mainly limited to copper, steel and cast iron. In live steam toys, brass is often used. Sources of heat for the boiler can be the combustion of fuels 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 generators, or HRSGs use the heat rejected from other processes such as gas turbines. Boilers can also be classified into fire-tube, water-tube boilers or cast iron sectional depending on whether the heat source is inside or outside the tubes or in the case of the cast iron sectional the design and manufacture of the boiler. The goal in all cases is to maximize the heat transfer between the water and the hot gases heating it. For example, steam locomotives have fire-tube boilers, 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 flows. Diagram of a water-tube boiler. In water-tube boilers the water flows through a large number of narrow tubes around the 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. Diagram of a Cornish 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 engines. 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 whilst 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 A superheated boiler on a steam locomotive. 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 pressure, and increases the overall thermal efficiency of the steam plant due to the fact that all of the added heat can be extracted to perform useful work. Superheated steam presents unique safety concerns, however, since the steam is at such high pressure (sometimes 1200 psi or higher). If there is a leak in the steam piping, water 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. Supercritical boilers Steam generation power plant. Supercritical boilers are used for the generation of electric power. They operate at "supercritical pressure". In contrast to a "subcritical boiler", a supercritical boiler has no water - steam separation. There is no generation of steam because the pressure is regulated above the "critical pressure" at which steam bubbles can form. Thus, the fluid generated is called "supercritical fluid". It passes below the critical point as it does work in the high pressure turbine and enters the generator's condensor. This is more efficient resulting in slightly less fuel use and therefore less greenhouse gas production. 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, as opposed to the forced-air furnaces or wood burning stoves more common in North America. "Boiler" is clearly a misnomer for this kind of device, which is really nothing but a large water heater in which the water is never intended to boil; but the name is universal and unlikely ever to change. The hydronic boiler operates by way of heating water/fluid to a preset temperature and circulating that fluid throughtout the home typically by way of radiators, 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 boilers for greater efficiency. Hydronic systems are being used more and more in new construction in North America for several reasons. Among the reasons are: They are more efficient and more economical than forced-air systems (although initial installation can be more expensive, because of the cost of the copper and aluminum). The baseboard copper pipes and aluminum fins take up less room and use less metal than the bulky steel ductwork required for forced-air systems. They provide more even, less fluctuating temperatures than forced-air systems. The copper baseboard pipes hold and release heat over a longer period of time than air does, so the furnace doesn't have to switch off and on as much. (Copper heats mostly through conduction and radiation, whereas forced-air heats mostly through forced convection. Air has much lower thermal conductivity and higher specific heat than copper; however, convection results in faster heat loss of air compared to copper. See also thermal mass.) They do not dry out the interior air as much. They do not introduce any dust, allergens, mold, or (in the case of a faulty heat exchanger) combustion byproducts into the living space. Forced-air heating does have some advantages, however. See forced-air heating. Accessories Boiler fittings Safety valve Water column: to help the operator tell if there is a satisfactory level of fluid in the boiler, a water gauge or water column is provided. Bottom blowdown valves Surface blowdown line Circulating pump Steam accessories Main steam stop valve Steam traps Main steam stop/Check valve used on multiple boiler instaltions. Combustion accessories Fuel oil system Gas system Coal system Automatic combustion systems 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: 1) Induced draft: This is obtained one of two ways, the first being through use of a steam jet. The steam jet forces steam into the stack and allows for a greater flue gas velocity increasing the overall draft in the furnace. This method was more common on older steam driven locomotives. The second method is by simply using an induced draft fan (ID fan) which sucks flue gases out of the furnace and up the stack. All induced draft furnaces have a negative pressure. 2) Forced draft: With this method draft is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is usually passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draft furnaces have a positive pressure. 3) Balanced draft: Balanced draft is obtained through use of both induced and forced draft. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric. See also Electric water boiler (for drinking water) Firebox (used by railway locomotives) Furnace Heating Heat-only boiler station Hot water heater Hydronics Pulverized coal-fired boiler Radiator Steam Steam generator (nuclear power) Thermoelectric References A short history of steamboat boilers Water Heater Resource Info, articles, and more on boilers and heaters Frederick M Steingress, Low pressure boilers. American Technical Publishers, 1986, ISBN 0-8269-4407-8 Different Types Of Boilers - The 3 Main Types Of Boiler. External Links Encyclopedia of Earth: Boilers Search Term: "Boiler" Categories: Boilers | Plumbing | HVAC

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