A "Scotch" marine boiler (or simply Scotch boiler) is a design of steam boiler best known for its use on ships.
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 (76 or 102 mm) 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 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.
The first recorded boiler of comparable form was used in a railway locomotive, 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.
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.
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.
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.
The "Inglis" 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 turbines with high-pressure water-tube boilers 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 inches (4.80 m) diameter and 20 feet (6.1 m) long, the smaller were 11 feet 9 inches (3.58 m) in length. All had three corrugated Morrison furnaces of 3 feet 9 inches (1.14 m) diameter, 159 furnaces in total, and a working pressure of 215 pounds per square inch (1,480 kPa).
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:
- Mayflower's boiler was recently removed for restoration and could easily be seen in close-up.
- Baltimore, of 1906, at the Baltimore Museum of Industry, is the oldest working steam tug in the USA.
- Steamship Shieldhall based in Southampton, UK, is fully operational and has two, oil-fired, scotch boilers.
- Steam powered icebreaker Stettin, operating on two coal-fired boilers
- Steam powered Drifter Lydia Eva, based in Lowestoft or Great Yarmouth, Fully operational coal fired scotch boiler. .
- The steam tug "KERNE" built 1913. Operated out of Liverpool by the Steam Tug Kerne Preservation Society. One coal fired wet back scotch marine boiler with two furnaces. Max working pressure 180 pounds per square inch (1,200 kPa).
- The steam yacht "LOUISE" built 1902. Operating on Lake Geneva, Wisconsin by Gage Marine Corporation. One diesel fired scotch marine boiler powering a double expansion steam engine, in excursion passenger service.
- The steam ship Trafik built 1892 in Stockholm Sweden by Bergsunds Mekaniska Verkstad as a passenger and freight ship on the Swedisk lake Vättern and with homeport Hjo. The machinery and its boiler are the original ones made for the ship in Stockholm.
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- "Baltimore"Steam Tug .
- "SS Shieldhall".
-  "Lydia Eva"].
- "s/s KERNE".
- "S/S Trafik Hjo".