Boilers have many applications. They can be used in stationary applications to provide heat, hot water, or steam for domestic use, or in generators and they can be used in mobile applications to provide steam for locomotion in applications such as trains, ships, and boats. Using a boiler is a way to transfer stored energy from the fuel source to the water in the boiler, and then finally to the point of end use.
Construction of boilers is mainly in steel, stainless steel, and wrought iron. In live steam models, copper or brass is often used. Historically copper was often used for fireboxes (particularly for steam locomotives), because of its better thermal conductivity. The price of copper now makes this impractical.
Cast iron is used for domestic water heaters. Although these are usually termed "boilers", their purpose is to produce hot water, not steam, and so they run at low pressure and try to avoid actual boiling. The brittleness of cast iron makes it impractical for steam pressure vessels.
For much of the Victorian "age of steam", the only material for boilermaking was the highest grade of wrought iron, with assembly by rivetting. This iron was often obtained from specialist ironworks, such as Cleator Moor (UK), noted for the high quality of their rolled plate and its suitability for high reliability use in critical applications, such as high pressure boilers. 20th century practice moved towards steel and welding.
The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Electric steam 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 be classified into the following configurations:
"Pot boiler" or "Haycock boiler": a primitive "kettle" where a fire heats a partially-filled water container from below. 18th Century Haycock boilers generally produced and stored large volumes of very low-pressure steam, often hardly above that of the atmosphere. These could burn wood or most often, coal. Efficiency was very low.
Fire-tube boiler. Here, water partially fills a boiler barrel with a small volume left above to accommodate the steam (steam space). The heat source is inside a furnace or firebox that has to be kept permanently surrounded by the water in order to maintain the temperature of the heating surface just below boiling point. The furnace can be situated at one end of a fire-tube which lengthens the path of the hot gases, thus augmenting the heating surface which can be further increased by making the gases reverse direction through a second parallel tube or a bundle of multiple tubes (two-pass or return flue boiler); alternatively the gases may be taken along the sides and then beneath the boiler through flues (3-pass boiler). In the case of a locomotive-type boiler, a boiler barrel extends from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increase the heating surface compared to a single tube and further improve heat transfer. Fire-tube boilers usually have a comparatively low rate of steam production, but high steam storage capacity. Fire-tube boilers mostly burn solid fuels, but are readily adaptable to those of the liquid or gas variety.
Water-tube boiler. In this type,the water tubes are arranged inside a furnace in a number of possible configurations: often the water tubes connect large drums, the lower ones containing water and the upper ones, steam; in other cases, such as a monotube boiler, water is circulated by a pump through a succession of coils. This type generally gives high steam production rates, but less storage capacity than the above. Water tube boilers can be designed to exploit any heat source including nuclear fission and are generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can withstand the pressure with a thinner wall.
Flash boiler. A specialized type of water-tube boiler.
Fire-tube boiler with Water-tube firebox. Sometimes the two above types have been combined in the following manner: the firebox contains an assembly of water tubes, called thermic syphons. The gases then pass through a conventional firetube boiler. Water-tube fireboxes were installed in many Hungarian locomotives, but have met with little success in other countries.
Sectional boiler. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are assembled on site to create the finished boiler.
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 because, 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 1300-1600 degrees Celsius (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 turbine or moving pistons offer a "continuously expanding space" and the pressure remains the same as that of the boiler.The process of superheating steam is most importantly designed to remove all droplets entrained in the steam to prevent damage to the turbine blading and/or associated piping.
Supercritical steam generators
Supercritical steam generators (also known as Benson boilers) 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, and 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.
History of supercritical steam generation
Contemporary supercritical steam generators are sometimes referred as Benson boilers. In 1922, Mark Benson was granted a patent for a boiler designed to convert water into steam at high pressure.
Safety was the main concern behind Benson’s concept. Earlier steam generators were designed for relatively low pressures of up to about 100 bar, corresponding to the state of the art in steam turbine development at the time. One of their distinguishing technical characteristics was the riveted drum. These drums were used to separate water and steam, and were often the source of boiler explosions, usually with catastrophic consequences. However, the drum can be completely eliminated if the evaporation process is avoided altogether. This happens when water is heated at a pressure above the critical pressure and then expanded to dry steam at subcritical pressure. A throttle valve located downstream of the evaporator can be used for this purpose.
As development of Benson technology continued, boiler design soon moved away from the original concept introduced by Mark Benson. In 1929, a test boiler that had been built in 1927 began operating in the thermal power plant at Gartenfeld in Berlin for the first time in subcritical mode with a fully open throttle valve. The second Benson boiler began operation in 1930 without a pressurizing valve at pressures between 40 and 180 bar at the Berlin cable factory. This application represented the birth of the modern variable-pressure Benson boiler. After that development, the original patent was no longer used. The Benson boiler name, however, was retained.
Two current innovations have a good chance of winning acceptance in the competitive market for once-through steam generators:
A new type of heat-recovery steam generator based on the Benson boiler, which has operated successfully at the Cottam combined-cycle power plant in the central part of England,
The vertical tubing in the combustion chamber walls of coal-fired steam generators which combines the operating advantages of the Benson system with the design advantages of the drum-type boiler. Construction of a first reference plant, the Yaomeng power plant in China, commenced in 2001.
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. The name can be a misnomer in that, except for systems using steam radiators, the water in a properly functioning hydronic boiler never actually boils. These boilers are referred to as condensing boilers because they condense the water vapor in the flue gases to capture the latent heat of vaporization of the water produced during combustion.
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.
Boiler fittings and accessories
Water level indicators: to show the operator the level of fluid in the boiler, a water gauge or water column is provided
Bottom blowdown valves provide a means for removing solid particulates that condense and lay on the bottom of a boiler. As the name implies, this valve is usually located directly on the bottom of the boiler, and is occasionally opened to use the pressure in the boiler to push these particulates out.
Hand holes are steel plates installed in openings in the boiler shell to allow for inspections of the interior of the boiler.
Low- water cutoff is a mechanical means (usually a float switch) that is used to turn off the burner or shut off fuel to the boiler to prevent it from running once the water goes below a certain point. If a boiler is "dry-fired" (burned without water in it) it can cause rupture or catastophic failure.
Surface blowdown line provides a means for removing foam or other lightweight non-condensible substances that tend to float on top of the water inside the boiler.
Circulating pump is designed to circulate water back to the boiler after it has expelled some of its heat.
Feedwater check valve or clack valve: a nonreturn stop valve in the feedwater line. This may be fitted to the side of the boiler, just below the water level, or to the top of the boiler. A top-mounted check valve is called a top feed and is intended to reduce the nuisance of limescale. It does not prevent limescale formation but causes the limescale to be precipitated in a powdery form which is easily washed out of the boiler.
Main steam stop valve
Main steam stop/Check valve used on multiple boiler installations
Fuel oil system
Automatic combustion systems
Other essential items
Inspectors test pressure gauge attachment
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:
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.
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.
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.