All steam engines (and other steam applications) have something in common. They all need a steam generating plant which is usually in the form of a steam boiler where water is heated in a large vessel and steam taken from the vessel to the steam engine itself. There are five main kinds of steam generator:
The Fixed Boiler.
This sort of unit is semi permanently imbedded into a brick structure which is designed to both support the boiler and circulate hot gases around the boiler to heat the water. This sort of boiler would typically be found in industry supplying one or more stationary steam engines. They can still be found today as heating plant for the building where they used to service engines. The firebox is below the boiler in front. Typically they were manually stoked and burnt either wood or coal. More modern units burnt oil or natural gas. A common feature of this sort of fixed boiler is the large access hole into the boiler space where a man can enter (when the unit is cool!) to clean and repair.
The Locomotive Boiler.
The most seen by the public, and hence seemingly most prevalent steam application is the mobile steam plant, whether it be a portable engine, traction engine, truck, bus or tram. All of these units have in common that they are mobile and cannot be tied to a fixed boiler. Arguably the most common type of boiler used in mobile applications is the Locomotive type. This boiler has the firebox and boiler as an integral unit with metal stays supporting the whole thing and holding all the parts together. The firebox is typically at the rear of the boiler and is fed by a fireman with coal, wood, or straw. The hot gases from the fire circulate through fire tubes within the boiler itself to emerge into the smoke box in front of the boiler and out the stack (chimney). The first picture shows the general layout of the locomotive boiler and the second picture shows some detail of tubes and stays. The tubes are the pipes running through the water space and the stays are the bolts (all thread bars) holding the various parts together.
The Return Flue Boiler
Similar in initial appearance to the locomotive type of boiler, the return flue has two major differences, visually different is that the chimney is in the back of the boiler above the rear of the firebox directly in front of the operator. Internally instead of having many small tubes to carry the hot gases through the water space, the hot gases circulate under the water space where they travel to the front of the boiler and back towards the rear with several large return flue pipes. The gases then discharge out through the smoke box and stack. The return flue boiler though not as common in preservation as the more popular locomotive type is thought to have both greater efficiency and safety over the locomotive type. In the return flue it is more difficult to expose metal not covered by water on hills making failure less likely. The first picture shows the general layout and the second picture gives a better idea of the flue layout.
The Vertical Boiler
The vertical boiler is common to steam trucks, buses, trams and very portable equipment such as donkey engines used for sawmilling and dockwork. The design of the vertical tube boiler lends itself well to rough handling while in steam making it ideal in situations where the work plant has to be moved frequently over non accomodating ground. They are popular for cargo trucks and people moving vehicles because they take up less room than the convential horizontal boiler styles. They are said to be the safest of all because it is almost impossible to expose bare uncovered metal to the fire. Their propensity for quick steaming in as little as 20 minutes made them popular with fire engine companies such as Shand Mason. The fire could be lit and steam raised while the boiler and pump were hauled to the fire in as little as twenty minutes. The fire is built in the bottom of the unit, with the hot gases circulating upwards though vertical tubes where they pass out through the stack. Most boilers have steam nozzles within the smokestack to help generate draft to keep the fire burning hot and bright. Vertical boilers rarely have or need this.
The Marine Boiler
The Marine Boiler in large vessels is typically a cross half breed between the fixed boiler and the locomotive boiler. In small vessels it can often be a vertical boiler adapted specially.
The Water Tube Boiler
John Byers and James Hansen reminded me about Water Tube Boilers. John pointed out that Westinghouse was one traction manufacturer that employed them. Most modern powerplants use them and they were popular in marine practice. Babcock and Wilcox units were/are very popular...they are still in business. On that happy day when I get my steam car project going, I hope to use a water tube along the lines of a Beckmann boiler: www.steamboating.net/boilers.html.
James added The water tube design is mostly used in stationary plants, but did see usage in one traction engine I know of, the westinghouse. The biggest advantage of a water tube boiler is that it is a very safe design. It consists of a header drum, to which the water tube pipes connect. This drum and the tubes are usually surrounded by an insulating jacket or brickwork, and the fire and products of combustion are directed so as to pass through the tubes a multiple number of times, either horizontally or vertically. There is another drum, a mud drum that receives the tubes on the bottom, and as it's name suggests, the mud, or particulate sediment of carbonates in the boiler feedwater settle out into this drum and can be removed when the boiler is blown down.
The vast majority of stationary plant boilers be they electrical gen stations or industrial plants use this type of boiler. Biggest advantage is safety. If a tube blows, it is extremely rare to have a boiler explosion ensue. That, and the design lend itself to be self cleaning- the tubes get very little lime deposit compared to the horizontal fire tube boiler. Also, if you replace the tubes, the majority of your heated surface gets renewed, unlike a locomotive boiler, however, that said, usually a new boiler is purchased these days, as they last so long that by the time the tubes are gone, so are the drums. and it is more cost effective and liability limiting to replace with new anyway.
The Flash Steam Boiler
Not strictly speaking a boiler at all, the concept of flash steam has been around since the 1800's and is common in instant on demand hot water systems today. The idea is to heat a long single pipe with a fire and pass only enough water through the pipe to create the amount of steam required at that moment by the engine. As only a small amount of water is heated at a time, this arrangement is efficient and effective. However it takes a skilled operator and is really only suited to smaller engines. This form of steam has been used effectively on such bizarre creatures as steam motorcylces. James Hansen added Flash boilers were also used very successfully in Stanley and White steam cars, to name only two. I have heard anecdotally that Henry Ford was originally torn between using steam or gasoline in his model T. Gas engines were new, and not well refined, while steam was essentially at it's peak, and worked extremely well and efficiently. The disadvantage was the time required to steam up- 5 minutes, but still, the petrol/gas engine was relatively instant. I say relatively, because I own a T. There is nothing instant about a model T...
Flash boilers have the advantage that they need little attention, and produce very dry steam, as the hottest part of the tubing effectively acts as a superheater. The Stanley brothers held the land speed record for some time with their modified steam car. The boilers were hydro tested in public demonstrations to 1000psi. Nothing unsafe about them. [:-)] Still used in steam jennies (cleaners) to my knowledge, just a copper coil with fire surrounding- poof you have steam. that's why they're called "flash" boilers, as the water tubes are at a heat sufficient to raise the feedwater to steam long before the water reaches the end of it's travel through the coil.
I received some good comments on this article from John Byers and James Hansen and I'm greatful to both guys for helping make the article better and more accurate. I'm always happy to receive constructive criticism.
You might like to read about more boiler terms.