Shell boiler
The shell boiler is a steam boiler and is used to generate steam or hot water in industrial companies and on board steam ships or steam locomotives . Characteristic for the boiler design is a cylindrical water-steam room with flat or cambered floors. The lighting was originally from the outside. With current constructions, the heating takes place exclusively from the inside via the flame tube and smoke tubes. In contrast to the water tube boiler , the flue gas is guided in the tubes.
Historical development
Suitcase boiler
The first boiler in pressure range at the end of the 18th century, the steam having a pressure of 0.5-1 bar produced were riveted to the shape due to so-called suitcase boiler , also known as Cart boiler or, after its inventor, Watt's boiler; James Watt designed the first suitcase kettle in 1770. Because of their large, flat walls, they can only withstand low steam pressures and were replaced by high-pressure boilers with a cylindrical cross-section as early as the early 19th century, after Oliver Evans designed the first roller boiler for his high-pressure steam engine , which was conceived in 1784 .
Roller bowl
This was followed by the roller boiler , the original form of the low-tension design with a cylindrical jacket and curved bottoms. As a rule, the boiler was operated horizontally. But there were also standing versions that were used in smelting works and were heated with the exhausting gases from flame and annealing furnaces.
The boiler was walled in and was fired from below. The flue gases were directed around the outer walls of the boiler. Trains were built into the masonry in order to use the heat of the flue gas as best as possible. Often two separate channels were created below the boiler. The boiler was set up with a slight incline towards the rear so that the sludge formed is drained back to the blowdown valve and the heat transfer from the boiler bottom is not further impeded.
The diameter of the roller bowl was up to 1.5 m with a length of up to 10 m. With this dimension, a heating surface of 25 m² is achieved. The area-specific steam output is 10–12 kg steam per m² and hour; so that the steam output of the drum boiler is limited to 300 kg / h.
The roller boiler was an inexpensive design. The scale could easily be loosened with the scale hammer, as there are no built-in components to hinder this work. The heat utilization is very unfavorable and the boiler requires a very long warm-up time and could therefore only be used for continuously operating companies.
Multiple roller shells
The multiple roller boiler consists of the main boiler or upper boiler located above and smaller boilers below, which are referred to as the lower boiler. The sub-kettles are about two-thirds the diameter of the main kettle. However, the diameter of the lower boiler should not be less than 55 cm in order to be able to drive on the part and clean the scale. The partial boilers are connected to one another by nozzles. There were arrangements with two lower boilers placed side by side. Depending on the steam requirement, several partial boilers have also been connected one above the other; these were then called battery boilers. The furnace was placed under the upper boiler, with plan or inclined grates being used. The flue gases were then directed around the lower boiler. The sludge settled in the lower kettle.
The roller boiler with a lower boiler has a heating surface of up to 50 m²; with 2 lower boilers it is 70 m². Battery boilers achieved heating surfaces of up to 150 m². The heating surface load is comparable to that of the simple roller boiler.
Flame tube / smoke tube boiler
As a further development step, the flame tube boiler or Cornwall boiler was created in 1811 . This consisted of a cylindrical jacket and a flame tube that was riveted between the two boiler floors. The first boilers of this type could be operated with a pressure of up to 7 bar. The boiler design made higher demands on the manufacturer. The firing took place exclusively with solid fuel . A flat grate is either built into the flame tube or the furnace is operated in an upstream combustion chamber, which can be designed as a sloping grate.
The heat transfer took place through radiation in the area of flame formation and convection. These boilers were walled in and the flue gases were led through brick partition walls around the jacket wall so that the heat content of the flue gas could be used as well as possible. A further development was the double flame tube boiler , also known as the Lancashire boiler . This design was widespread in industrial companies when steam with pressures of up to approx. 16 bar was required. These boilers were often used in mining boiler plants to generate steam for the hoisting machines or in industrial plants that used steam engines to drive a transmission . An important aspect in the dimensioning was the sufficient space in the boiler to be able to remove scale by manual picking. Flame tube boilers were always equipped with a steam dome.
Riveted steam boilers were built until around 1945. The disadvantage of the flame tube boilers was the suboptimal use of the heat content of the flue gases, as an increase in the heating surface on the boiler itself is not possible due to the design. Optimized utilization of the flue gas heat was only possible by installing secondary heating surfaces that can be used as superheaters or feed water preheaters .
By the middle of the 20th century, welding technology and the quality of the materials had developed to such an extent that welded joints could also be reliably produced for components subject to higher loads. In addition, seamless steel tubes have been available since the end of the 19th century ( Mannesmann ). The flame tube smoke tube boiler was developed, which is characterized by the following construction details:
- all connections are welded,
- The boiler has a flame tube and the flue gases are diverted several times in the longitudinal direction of the boiler. The boiler has several flues (2–5 flue gas flues), which are connected by turning chambers on the rear and front floors.
The so-called smoke tubes for the further use of the flue gas heat are drawn in above the flame tubes and are much smaller in diameter than the flame tubes. The convective heat transfer in a large number of smoke tubes is much better with a smaller diameter due to the larger Reynolds number than with a tube with a large diameter. In addition, with pipes with a small diameter, a larger heating surface is achieved in relation to the total cross-sectional area of the pipes. The flame tube / smoke tube boiler is only heated from the inside. There is no masonry and the walls are thermally insulated from the outside.
View into a riveted flame tube boiler, built in 1945
Double flame tube boiler from the Carl colliery (boiler house demolished)
Historical double flame tube boiler
Modern shell boiler with burner
Constructive details
Material stresses on the boiler components occur primarily due to the steam pressure. The jacket is loaded by the internal pressure, while the flame tube is exposed to external pressure. Therefore, the flame tube must be designed against denting. Flame tube smoke tube boilers with low loads often have a smooth flame tube and flat floors. Large capacity water boilers with higher operating pressures are designed with corrugated flame tubes (higher resistance to dents) and flat, cambered floors. The advantage of the crimped bottom is the position of the weld seam in the cylindrical area in front of the brim, so that the seam is only subjected to tensile stresses . In the case of flat disk bottoms, unfavorable bending stresses also occur. In addition, tensions occur due to temperature differences due to the heating. While the unheated surfaces (boiler shell) are at around saturated steam temperature, the temperature on the heating surfaces is higher. The temperature difference in the boiler without water-side deposits is up to 50 ° C. In the case of scale deposits, this temperature difference can be significantly higher and cause cracks.
In order to keep stresses in the boiler components as a result of thermal changes in length low, the smallest possible wall thicknesses are used for the boiler bottoms and tube plates. The flame tube and the smoke tubes absorb the internal pressure load on the boiler floors. In the non-tubed areas of the jacket (vapor space), corner or tie rods are also welded in, which partially absorb the bending stresses between the jacket and the floor. These are components that are critical in terms of voltage and susceptible to damage. The weld seams of the anchors must be welded through and the shape of the anchors must ensure a constant flow of tension.
More than three puffs are not common in today's shell boiler designs. The shell boilers are characterized by a high water content (1 - 30 t ) and thus a high heat storage capacity . Through the re-evaporation of the water at a saturated steam temperature, a briefly fluctuating steam consumption can be compensated. The required wall thicknesses for the jacket and the necessary anchoring of the floors limit the technically acceptable range of the operating pressure (up to approx. 38 bar ). Due to the design (large contiguous areas) shell boilers are sensitive to thermal stresses when heating up and cooling down. The boilers are therefore started up slowly in order to keep the temperature gradient of the boiler components low.
In order to further reduce fuel consumption, an economiser (Eco: feed water preheater) is nowadays installed downstream of the boiler. The boiler feed water is heated in the economiser before it is fed into the boiler. In the shell boiler, the economiser is a separate component through which the flue gas discharged from the boiler at temperatures of 200 - 300 ° C is passed. It consists of a pipe arrangement, which is often also provided with ribs. If a lot of cold make-up water is topped up, a second economiser is also connected, which preheats this water before it enters the feed water tank. In these cases, however, the condensation of water vapor from the flue gas with acid formation must be taken into account. The components must be made of stainless steel or a flue gas temperature control must be provided to ensure a minimum temperature.
Energy supply
Fossil fuels
The boilers were mostly heated with solid fuels ( coal , wood ) until the middle of the 20th century . In the 50s and 60s, the firing of many boilers was converted to heating oil because it was easier to use . The cheapest fuel was the heavy fuel oil S, which has to be heated for production. Due to the emissions (high sulfur content , NO x and dust pollution ) as well as the more complex operation ( burner and boiler have to be cleaned regularly, supply lines sticking if the auxiliary heating fails), heating oil S is hardly used anymore. In addition, additional measures are required (denitrification, dedusting) in order to comply with the current emission limit values . This is why almost only EL (extra light) heating oil is used. With the spread of natural gas in the 1960s, the gas is used for firing shell boilers, if it is available on site. The advantage of natural gas firing lies in the low-emission combustion and practically no soot formation occurs when the burner is correctly set.
electricity
A shell boiler can also be heated electrically. For this purpose, heating rods are used, which are usually used as a bundle in a connection piece of the boiler body. Due to the significantly higher specific costs of heating compared to the use of fossil fuels, electrical heating is usually only used for low steam outputs (less than 1 t / h). The investment costs and the space required are low. The exhaust system and the fuel supply are omitted, and the boiler can be set up in sterile areas. The boilers are used in hospitals and in the pharmaceutical industry (steam for sterilization) or in laboratories.
Feed water
The feed water from shell boilers must be treated to prevent corrosion and scale deposits. If the additional water used does not have a particularly high hardness, it is sufficient in most cases to replace the hardness components on a base exchanger regenerated with common salt with Na ions. The feed water should be thermally degassed. Conditioning agents (e.g. sodium phosphate , sodium sulfite ) must be added to the feed water to prevent the loss of residual hardness, to chemically bind residual oxygen and to raise the pH value. The feed water must also be de-oiled.
Ship boilers
The Scottish boilers were mostly used in steamships. These are two-compartment shell boilers with one to four flame tubes and one flue pipe. In order to achieve the necessary steam output for larger ships, the number of steam boilers was increased accordingly. The RMS Titanic had a total of 29 Scottish steam boilers.
If the ships were in port, they were heated through with a lay time of up to 3 weeks, i.e. at least one flame tube, usually the so-called " monkey ", i.e. the flame tube at the lowest point of the boiler, was kept on sparingly to keep the boiler water at to hold about 100 ° C and about 1 bar pressure.
As long as steam ships did not have desalination systems, reserve water had to be carried to compensate for inevitable losses due to blowdown, steam leakage or the steam whistle .
Locomotive steam boiler
A steam locomotive boiler is a type of mobile land steam boiler for generating steam to drive steam locomotives. Locomotive boilers are mostly tubular boilers. But there are also other designs known.
Quality requirements
Shell boilers are pressure equipment within the meaning of the Pressure Equipment Directive 2014/68 / EU and may only be placed on the market if the manufacturer has demonstrated through a conformity assessment procedure with the participation of a notified body that he has complied with the basic safety requirements of the directive. The manufacturer affixes the CE mark and issues a declaration of conformity . Harmonized product standards for shell boilers are:
- EN 12953-1 to 14: Shell boilers
- EN 14222: Stainless steel shell boiler
When using this standard, the manufacturer can assume that he meets the basic safety requirements of the directive (presumption of conformity).
Individual evidence
- ↑ Watt's suitcase kettle in the Deutsches Museum
- ^ Conrad Matschoss: The development of the steam engine. A history of the stationary steam engine and the locomobile, the ship engine and the locomotive, Springer 1908, page 606 (PDF file)
- ^ Meyers Konversationslexikon, Verlag des Bibliographisches Institut, Leipzig and Vienna, Fourth edition, 1885–1892, page 449: "Steam boiler (horizontal: Wattscher case boiler, cylinder boiler, flame tube boiler)"
Literature sources
- Joh. Eug. Mayer: Paperback for the modern stoker and boiler attendant , Berlin 1912, Verlag Hermann Schran & Co.
- RE Th. Schlippe: The steam boilers and their operation , 4th edition. Berlin 1923, Julius Springer publishing house.
- Wilhelm Leder: Marine engineering volume I: Ship steam boiler , 1956, Fachbuchverlag Leipzig.
- Fritz Mayr: Kesselbetriebstechnik , 10th edition. Graefelfing 2009, Resch, ISBN 3-930039-13-3 .