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A vertical boiler is a type of steam generator where the boiler shell is situated vertically instead of horizontally. Much like all types of boilers, a vertical boiler can either be of a water tube or fire tube construction. A vertical boiler is very different from what one would find on a typical steam locomotive, as these boilers are usually oriented horizontally instead of vertically. Vertical boilers are most commonly found on smaller steam-driven vehicles, such as steam automobiles, boats, and wagons. On some rare occasions, however, vertical locomotive boilers have been used.
A vertical fire tube boiler, which is the least efficient, is most commonly found on earlier steam automobiles, steam launches, or boats, and some rarely-found locomotives and steam wagons. A fire tube boiler consists of a volume of water inside the shell with hollow, vertical tubes running through them. Hot gases from a burner below flow through the tubes, where the heated tubing generates enough heat to create steam. Water tube boilers are the exact opposite, as water runs through the tubes, which are heated by flames surrounding them.
Whether or not a vertical boiler employs a fire tube or water tube construction usually depends on the application. Most traditional steam launches and earlier steam automobiles used fire-tube boilers for their simplicity and low cost of materials. The only problem is that all of the water within the system needs to be heated at once; this makes building pressure a long process and carries an increased risk of a steam explosion. Water tube boilers, on the other hand, are known to heat water very quickly and efficiently and are most commonly found in advanced steam automobiles and power plants. The downside to these, however, is the expense of the materials used to manufacture them.
There are many advantages and disadvantages to vertical boilers, yet in most applications, the advantages seem to outweigh the disadvantages. One of the biggest advantages of a vertical boiler is the compact size—horizontal boilers take up far more room, as they are situated in a manner that takes up a great deal of length. On a vertical boiler, however, the entire casing is upright; because of this, they are far more compact from a length perspective compared to their horizontally-oriented counterparts. Such an arrangement makes them more practical for steam-driven vehicles, which demand maneuverability or a more compact steam plant. Another advantage is that the water level does not need to be so carefully maintained, as the volume of water being heated is always located above the firebox.
Depending on the arrangement of the tubes, however, vertical boilers can also have their disadvantages. In the case of fire-tube boilers, one of these is heat loss. In order to save on space, the tubes of a vertical boiler need to be shortened. This makes it so that gases from the burner cannot be completely utilized during the heating process. Another disadvantage is that the heating area is limited to the size of the boiler's base, which can limit steam output.