Stirling boiler
Encyclopedia
The Stirling boiler is an early form of water-tube boiler
, used to generate steam in large land-based stationary plants. Although widely used around 1900, it has now fallen from favour and is rarely seen.
They are amongst the older, "large-tube" designs of water-tube boiler
s, having water-tubes that are around 3¼ inches (83 mm) in diameter. The tubes are arranged in near-vertical banks between a number of cylindrical, horizontal steam drums (above) and water drums (below). The number of drums varies, and the Stirling designs are categorized into 3-, 4- and 5-drum boilers. The number of tube banks is one less than this, i.e. 2, 3 or 4 banks.
Gas flow from the furnace passes through each bank in turn. Partial baffles of firebrick tiles are laid on each bank, so as to force the gasses to flow first up, and then down through each bank. Unusually, much of the gas flow is along the tubes' axis, rather than across them.
All circulation, both up and down, is through the heating tubes and there are no separate external downcomers. The steam drums and, (in a 5-drum boiler) the water drums, are however linked by short horizontal pipes and these form part of the circulation circuit.
The tubes themselves are seamless-drawn steel and mostly straight, with gently curved ends. The setting of the boiler is a large brick-built enclosure, but the steam drums are suspended from a separate girder framework inside this, so as to allow for expansion with heat. The tubes, and the water drums in turn, are hung from the steam drums, again to allow free expansion without straining the tube ends. Owing to their curved ends the water-tubes may enter the drums radially, allowing easy sealing, but this was also a feature considered, according to the fashion of the time, to be important on account of expansion
.
Stirling boilers may be made in very large sizes. It is usual for a standard design to be used, but in varying widths, according to need.
is fitted, it is installed as straight or hairpin
tubes in the upper part of the boiler between the first two steam drums. The baffles direct the gas-flow through this area first, so it may reach the highest temperature.
, but they have been used since to burn many sorts of wood or plant waste.
A chain-fed automatic stoker may also be fitted, where a heavy firing rate is required.
The three-drum form is also used as a heat-recovery boiler, using the exhaust gases from steelworks or other industrial processes.
Flow in the first bank is upwards as the tubes are heated, encouraged by their almost vertical position. Cooler water in the later water-tubes descends. Circulation is completed by the link pipes between the steam drums. Circulation is more active in the earlier, hotter circuits.
In the four-drum form, circulation within the middle bank may be split between a descending circuit with the first bank and an ascending circuit with the following bank.
Any precipitable deposits (colloquially, "mud") will emerge from solution in this circuit and accumulate in the final water drum. This keeps them away from the more active early tube banks, reducing the problems and inefficiencies of scale build-up within the major heating tubes. Since the final water drum may also be used to catch the "mud" (i.e. a "mud collector"), it is sometimes known as a "mud drum".
The marine version of the boiler is also of this form.
s, or where efficiency is most needed so as to gain the maximum heating from a limited fuel capacity.
The brick-built setting was replaced with a box-like steel housing, lined with firebrick. The water-tube diameter was reduced to between 2 and 2½ inches (51–64 mm). To avoid problems with the water levels shifting as the ship rolls, the water drums were arranged crosswise to the hull and provided with internal baffles.
Water-tube boiler
A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes...
, used to generate steam in large land-based stationary plants. Although widely used around 1900, it has now fallen from favour and is rarely seen.
Design
Stirling boilers are one of the larger arrangements for a water-tube boiler: acceptable for stationary use, but impractical for mobile use, except for large ships with modest power requirements. They consist of a large brick-built chamber with a sinuous gas path through it, passing over near-vertical water-tubes that zig-zag between multiple steam drums and water drums.They are amongst the older, "large-tube" designs of water-tube boiler
Water-tube boiler
A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes...
s, having water-tubes that are around 3¼ inches (83 mm) in diameter. The tubes are arranged in near-vertical banks between a number of cylindrical, horizontal steam drums (above) and water drums (below). The number of drums varies, and the Stirling designs are categorized into 3-, 4- and 5-drum boilers. The number of tube banks is one less than this, i.e. 2, 3 or 4 banks.
Gas flow from the furnace passes through each bank in turn. Partial baffles of firebrick tiles are laid on each bank, so as to force the gasses to flow first up, and then down through each bank. Unusually, much of the gas flow is along the tubes' axis, rather than across them.
All circulation, both up and down, is through the heating tubes and there are no separate external downcomers. The steam drums and, (in a 5-drum boiler) the water drums, are however linked by short horizontal pipes and these form part of the circulation circuit.
The tubes themselves are seamless-drawn steel and mostly straight, with gently curved ends. The setting of the boiler is a large brick-built enclosure, but the steam drums are suspended from a separate girder framework inside this, so as to allow for expansion with heat. The tubes, and the water drums in turn, are hung from the steam drums, again to allow free expansion without straining the tube ends. Owing to their curved ends the water-tubes may enter the drums radially, allowing easy sealing, but this was also a feature considered, according to the fashion of the time, to be important on account of expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
.
Stirling boilers may be made in very large sizes. It is usual for a standard design to be used, but in varying widths, according to need.
Superheaters
Where a superheaterSuperheater
A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired...
is fitted, it is installed as straight or hairpin
Hairpin turn
A hairpin turn , named for its resemblance to a hairpin/bobby pin, is a bend in a road with a very acute inner angle, making it necessary for an oncoming vehicle to turn almost 180° to continue on the road. Such turns in ramps and trails may be called switchbacks in American English, by analogy...
tubes in the upper part of the boiler between the first two steam drums. The baffles direct the gas-flow through this area first, so it may reach the highest temperature.
Fuels
A wide range of fuels may be burned, aided by a large grate area that may easily be increased further, should a poor quality fuel require it. The original boilers were developed to burn coalCoal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
, but they have been used since to burn many sorts of wood or plant waste.
A chain-fed automatic stoker may also be fitted, where a heavy firing rate is required.
The three-drum form is also used as a heat-recovery boiler, using the exhaust gases from steelworks or other industrial processes.
Circulation
As the gas flow passes through each tube bank in turn, the later banks are at a significantly lower temperature. This encourages an "extremely efficient" circulation by the thermosyphon effect. Water level is maintained with the steam drums approximately half-full, so the tubes operate in the "drowned" state with their upper ends permanently submerged.Flow in the first bank is upwards as the tubes are heated, encouraged by their almost vertical position. Cooler water in the later water-tubes descends. Circulation is completed by the link pipes between the steam drums. Circulation is more active in the earlier, hotter circuits.
In the four-drum form, circulation within the middle bank may be split between a descending circuit with the first bank and an ascending circuit with the following bank.
Feedwater
Feedwater is supplied to the final steam drum and distributed via an internal trough. The cold feedwater descends slowly through the last tube bank and into the last water drum.Any precipitable deposits (colloquially, "mud") will emerge from solution in this circuit and accumulate in the final water drum. This keeps them away from the more active early tube banks, reducing the problems and inefficiencies of scale build-up within the major heating tubes. Since the final water drum may also be used to catch the "mud" (i.e. a "mud collector"), it is sometimes known as a "mud drum".
Advantages
There are three advantages to the Stirling design:- Owing to the multiple passes through the tube banks and the careful arrangement of circulation and feedwater, the boiler is generally quite efficient.
- The grate area may easily be varied without affecting the design of the water circuit. This allows the use of large grates for burning low-grade fuels, such as waste and refuse. Where an industrial process both required process steam and also generated combustible waste products, the Stirling boiler could use one to generate the other. This was commonly done in sugar refining, burning bagasseBagasseBagasse is the fibrous matter that remains after sugarcane or sorghum stalks are crushed to extract their juice. It is currently used as a biofuel and as a renewable resource in the manufacture of pulp and paper products and building materials....
, or in paper pulp mills burning bark waste. The Stirling boiler was also used before World War IIWorld War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
to burn domestic waste in cities, the steam then being used for power generation. (Emissions controlsEmission standardEmission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment. Many emissions standards focus on regulating pollutants released by automobiles and other powered vehicles but they can also regulate emissions from industry, power...
in later years made this burning of 'uncontrolled' waste impractical).
- Finally, internal access to components is good for cleaning and maintenance. In particular, it is possible to replace individual water-tubes, without needing to dismantle an entire bank.
Disadvantages
- The boiler is physically large for its power.
- Use of large diameter tubes restricts its ability to deal with high pressures, as other designs were beginning to do around this time. A working pressure of 150psi was adequate for driving a compound steam engine, but inefficient for a steam turbineSteam turbineA steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....
.
- The arrangement of tubes encouraged efficiency in the boiler, but not superheater temperature. Again, as developments moved away from piston enginesReciprocating engineA reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
in favour of turbines, this became a significant limitation.
Variations
Although broadly similar, variations with different numbers of tube banks are produced.Three-drum or 'V' form
This simpler form is mainly used for low powers, or for heat-recovery from other furnace gases.Four-drum or 'B' form
This is the main form of the boiler and gives efficient results with economical construction.The marine version of the boiler is also of this form.
Five-drum or 'W' form
This is a more complex form, which uses an extra tube bank to gain efficiency. It is most popular for large installations, such as power stationPower station
A power station is an industrial facility for the generation of electric energy....
s, or where efficiency is most needed so as to gain the maximum heating from a limited fuel capacity.
Marine Stirling boilers
Although generally a land-based boiler, the four-drum form was also used as a marine boiler, to power large ships.The brick-built setting was replaced with a box-like steel housing, lined with firebrick. The water-tube diameter was reduced to between 2 and 2½ inches (51–64 mm). To avoid problems with the water levels shifting as the ship rolls, the water drums were arranged crosswise to the hull and provided with internal baffles.