Scotch marine boiler
Encyclopedia
A "Scotch" marine boiler (or simply Scotch boiler) is a design of steam boiler best known for its used on ships.
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The general layout is that of a squat horizontal cylinder. One or more large cylindrical furnaces are in the lower part of the boiler shell. Above this is a large number of small-diameter fire-tubes. Gases and smoke from the furnace pass to the back of the boiler, then return through the small tubes and up and out of the chimney. The ends of these multiple tubes are capped by a smokebox
, outside the boiler shell.
The Scotch boiler is a fire-tube boiler
, in that hot flue gases pass through tubes set within a tank of water. As such, it is a descendant of the earlier Lancashire boiler and like the Lancashire it uses multiple separate furnaces to give greater heating area for a given furnace capacity. It differs from the Lancashire in two aspects: a large number of small diameter tubes (typically 3 or 4 inches diameter each) are used to increase the ratio of heating area to cross-section. Secondly the overall length of the boiler is halved by folding the gas path back on itself.
The front wall of the combustion chamber is supported against steam pressure by the tubes themselves. The rear face is stayed by rod stays through the rear shell of the boiler. Above the combustion chamber and tubes is an open steam collecting space. Larger long rod stays run the length of the boiler through this space, supporting the ends of the boiler shell.
With multiple furnaces, there is a separate combustion chamber for each furnace. A few small boilers did connect them into one chamber, but this design is weaker. A more serious problem is the risk of reversing the draught, where exhaust from one furnace could blow back and out of the adjacent one, injuring the stokers working in front of it.
's 'Wilberforce' class of 1830. This had a long cylindrical boiler shell similar to his earlier return-flued 'Royal George' , but with the return flue replaced by a number of small firetubes, as had been demonstrated so effectively by Stephenson with his 'Rocket
' a year earlier. The novel feature of an entirely internal combustion chamber was used. Unlike the later Scotch boiler though, this was self-supported by its own stays, rather than using stays through the walls of the boiler shell. This allowed the entire assembly of outer tubeplate, furnace tube, combustion chamber and firetubes to all be removed from the boiler shell as one unit, simplifying manufacture and maintenance. Although a valuable feature, this became impractical for larger diameter chambers that would require the support of the shell.
As with the Lancashire boiler, the furnace was often corrugated for strength. Various makers had their own particular ways of making these corrugations, leading to their classification for maintenance purposes under the broad titles of, Leeds
, Morrison, Fox
, Purves or Brown.
The "dry back" variation has the rear of the combustion chamber as an open box, backed or surrounded only by a sheetmetal jacket. This simplifies construction, but also loses much efficiency. It is only used for small boilers where capital cost outweighs fuel costs. Although the Scotch boiler is nowadays rarely the primary steam generator on a ship, small dry-back designs such as the Minipac are still encountered, for supporting secondary demands whilst alongside in port with the main boilers cold.
One interesting variant of the dry-back design has been a patent for burning ash-prone fuels. The rear of the combustion chamber is used as an access point for an ash separator, removing the ash before the small-diameter tubes.
" modification adds an extra combustion chamber where an additional single large flue returns from the rear to the front of the boiler. Flow through the multiple tubes is thus from front to back, and so the exhaust is at the rear. Multiple furnaces would share a single combustion chamber.
The major advantage of the Inglis is the extra heating area it adds, for a comparable shell volume, of perhaps 20%. Surprisingly this is not from the additional combustion chamber, but from lengthening the narrow firetubes. These can now run the full length of the boiler shell, rather than just the rather shorter distance from the inner combustion chamber to the front tubeplate. Despite this advantage, it is rarely used.
s with high-pressure water-tube boiler
s such as the Yarrow
.
Large or fast ships could require a great many boilers. The Titanic had 29 boilers: 24 double-ended and 5 smaller single-ended. The larger boilers were 15 feet 9 inch diameter and 20 feet long, the smaller were 11 feet 9 inch diameter. All had three corrugated Morrison furnaces of 3 feet 9 inch diameter, 159 furnaces in total, and a working pressure of 215 psi.
Smokebox
A smokebox is one of the major basic parts of a Steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is exhausted to the atmosphere through the chimney .To assist...
, outside the boiler shell.
The Scotch boiler is a fire-tube boiler
Fire-tube boiler
A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water...
, in that hot flue gases pass through tubes set within a tank of water. As such, it is a descendant of the earlier Lancashire boiler and like the Lancashire it uses multiple separate furnaces to give greater heating area for a given furnace capacity. It differs from the Lancashire in two aspects: a large number of small diameter tubes (typically 3 or 4 inches diameter each) are used to increase the ratio of heating area to cross-section. Secondly the overall length of the boiler is halved by folding the gas path back on itself.
Combustion chamber
The far end of the furnace is an enclosed box called the combustion chamber which extends upwards to link up with the firetubes.The front wall of the combustion chamber is supported against steam pressure by the tubes themselves. The rear face is stayed by rod stays through the rear shell of the boiler. Above the combustion chamber and tubes is an open steam collecting space. Larger long rod stays run the length of the boiler through this space, supporting the ends of the boiler shell.
With multiple furnaces, there is a separate combustion chamber for each furnace. A few small boilers did connect them into one chamber, but this design is weaker. A more serious problem is the risk of reversing the draught, where exhaust from one furnace could blow back and out of the adjacent one, injuring the stokers working in front of it.
Origins
The first recorded boiler of comparable form was used in a railway locomotive, HackworthTimothy Hackworth
Timothy Hackworth was a steam locomotive engineer who lived in Shildon, County Durham, England and was the first locomotive superintendent of the Stockton and Darlington Railway.- Youth and early work :...
's 'Wilberforce' class of 1830. This had a long cylindrical boiler shell similar to his earlier return-flued 'Royal George' , but with the return flue replaced by a number of small firetubes, as had been demonstrated so effectively by Stephenson with his 'Rocket
Stephenson's Rocket
Stephenson's Rocket was an early steam locomotive of 0-2-2 wheel arrangement, built in Newcastle Upon Tyne at the Forth Street Works of Robert Stephenson and Company in 1829.- Design innovations :...
' a year earlier. The novel feature of an entirely internal combustion chamber was used. Unlike the later Scotch boiler though, this was self-supported by its own stays, rather than using stays through the walls of the boiler shell. This allowed the entire assembly of outer tubeplate, furnace tube, combustion chamber and firetubes to all be removed from the boiler shell as one unit, simplifying manufacture and maintenance. Although a valuable feature, this became impractical for larger diameter chambers that would require the support of the shell.
Number of furnaces
Typical practice for ships was to have two furnaces in each boiler. Smaller boilers might only have one, larger boilers commonly had three. The limitation in boiler size was the amount of work each stoker could do, firing one furnace per man. Larger ships (meaning anything above the smallest) would have many boilers.As with the Lancashire boiler, the furnace was often corrugated for strength. Various makers had their own particular ways of making these corrugations, leading to their classification for maintenance purposes under the broad titles of, Leeds
Leeds Forge Company
The Leeds Forge Company manufactured corrugated furnaces for marine boilers and later, pressed steel railway vehicles, in Leeds, England.- Early history :The company was founded by Samson Fox, who was born in Bradford in 1838...
, Morrison, Fox
Leeds Forge Company
The Leeds Forge Company manufactured corrugated furnaces for marine boilers and later, pressed steel railway vehicles, in Leeds, England.- Early history :The company was founded by Samson Fox, who was born in Bradford in 1838...
, Purves or Brown.
Wet back and dry back
The typical design is the "wet back", where the rear face of the combustion chamber is water-jacketed as a heating surface.The "dry back" variation has the rear of the combustion chamber as an open box, backed or surrounded only by a sheetmetal jacket. This simplifies construction, but also loses much efficiency. It is only used for small boilers where capital cost outweighs fuel costs. Although the Scotch boiler is nowadays rarely the primary steam generator on a ship, small dry-back designs such as the Minipac are still encountered, for supporting secondary demands whilst alongside in port with the main boilers cold.
One interesting variant of the dry-back design has been a patent for burning ash-prone fuels. The rear of the combustion chamber is used as an access point for an ash separator, removing the ash before the small-diameter tubes.
Double-ended
The double-ended design places two boilers back-to-back, removing the rear wall of the boiler shell. The combustion chambers and firetubes remain separate. This design saves some structural weight, but it also makes the boiler longer and more difficult to install into a ship. For this reason they were not commonly used, although back-to-back arrangements of multiple single-ended boilers were common.Inglis
The "InglisJohn Inglis and Company
John Inglis and Company was a Canadian firm which made weapons for the United Kingdom and British Commonwealth military forces during the World War II era, then became a major appliance company....
" modification adds an extra combustion chamber where an additional single large flue returns from the rear to the front of the boiler. Flow through the multiple tubes is thus from front to back, and so the exhaust is at the rear. Multiple furnaces would share a single combustion chamber.
The major advantage of the Inglis is the extra heating area it adds, for a comparable shell volume, of perhaps 20%. Surprisingly this is not from the additional combustion chamber, but from lengthening the narrow firetubes. These can now run the full length of the boiler shell, rather than just the rather shorter distance from the inner combustion chamber to the front tubeplate. Despite this advantage, it is rarely used.
Use in ships
The Scotch marine boiler achieved near-universal use throughout the heyday of steam propulsion, particularly for the most highly developed piston engines such as the triple-expansion compounds. It lasted from the end of the low-pressure haystack boilers in the mid-19th century through to the early 20th century and the advent of steam turbineSteam turbine
A 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....
s with high-pressure 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 such as the Yarrow
Yarrow boiler
Yarrow boilers are an important class of high-pressure water-tube boilers. They were developed byYarrows and were widely used on ships, particularly warships....
.
Large or fast ships could require a great many boilers. The Titanic had 29 boilers: 24 double-ended and 5 smaller single-ended. The larger boilers were 15 feet 9 inch diameter and 20 feet long, the smaller were 11 feet 9 inch diameter. All had three corrugated Morrison furnaces of 3 feet 9 inch diameter, 159 furnaces in total, and a working pressure of 215 psi.
Shipboard working examples
Numerous Scotch boilers are in use on ships as of 2011 and new boilers can be built to replace life-expired ones. Examples of preserved steam boats employing Scotch boilers include:- Steam tug Mayflower, Bristol Industrial MuseumBristol Industrial MuseumThe Bristol Industrial Museum was a museum in Bristol, England, located on Prince's Wharf beside the Floating Harbour, and which closed in 2006. On display were items from Bristol's industrial past – including aviation, car and bus manufacture, and printing – and exhibits documenting Bristol's...
- Mayflowers boiler was recently removed for restoration and could easily be seen in close-up.
- Baltimore, of 1906, at the Baltimore Museum of IndustryBaltimore Museum of IndustryBaltimore Museum of Industry is a museum at Baltimore, Maryland, United States. Located in an old cannery, the museum has exhibits on various types of manufacturing and industry from the early 20th century...
, is the oldest working steam tug in the USA.
- Steamship ShieldhallSS ShieldhallSS Shieldhall is a preserved steamship that operates from Southampton. She spent her working life as one of the "Clyde sludge boats", making regular trips from Shieldhall in Glasgow, Scotland, down the River Clyde and Firth of Clyde past the Isle of Arran, to dump treated sewage sludge at sea...
based in SouthamptonSouthamptonSouthampton is the largest city in the county of Hampshire on the south coast of England, and is situated south-west of London and north-west of Portsmouth. Southampton is a major port and the closest city to the New Forest...
, UK, is fully operational and has two, oil-fired, scotch boilers.
- Steam powered icebreaker Stettin, operating on two coal-fired boilers