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A boiler is a closed vessel in which water or other fluid is heated under pressure. The fluid is then circulated out of the boiler for use in various processes or heating applications.

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 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 (HRSGs) use the heat rejected from other processes such as gas turbines.

Boilers can also be classified into:

Fire-tube boilers. Here, the heat source is inside the tubes and the water to be heated is outside.
Water-tube boilers. Here the heat source is outside the tubes and the water to be heated is inside.
A primitive, inefficient type where there are no tubes and the fire heats one side of the water container.

The goal is to make the heat flow as completely as possible from the heat source to the water. 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 flow.

In water-tube boilers 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.

File:Steam Boiler 1 english.png

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 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

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 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.

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 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 does not 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: used to relieve pressure and prevent possible explosion of a boiler
Water column: to show the operator the level of fluid in the boiler, a water gauge or water column is provided
Bottom blowdown valves
Surface blowdown line
Circulating pump
Check valve or clack valve: a nonreturn stop valve by which water enters the boiler

Steam accessories

Main steam stop valve
Steam traps
Main steam stop/Check valve used on multiple boiler installations

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 three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler. The more dense column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet oriented in the direction of flue gas flow induces flue gasses into the stack and allows for a greater flue gas velocity increasing the overall draft in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draft fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost all induced draft furnaces have a negative pressure.

2) Forced draft: Draft is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is often 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 usually 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.

References

Frederick M Steingress, Low pressure boilers. American Technical Publishers, 1986, ISBN 0-8269-4407-8

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 * [[Thermostat]]
 * [[Water heater]]
+http://kazanrao.googlepages.com/EfIM for boiler efficiency calculation
 ==References==