When Heron of Alexandria created the first steam engine, he had no way of knowing what it would eventually lead to. It was almost two millenia later before labor levels and metal working technology were right to create the industrial revolution, but when it did happen, it was the steam engine that provided the driving force.
When you're looking at these industrial giants it's easy to forget that they don't actually produce any energy. While the steam engine is what provides the motion to run mechanical processes, it is the boiler that captures heat energy so that it can be applied to a wider range of uses. It breathes life into other machines.
At first glance it seems that boilers would be quite simple, or at least not much more complicated than a teakettle. However if you want a boiler to run something a little more powerful than a whistle, there's quite a bit of design and planning work that has to go into it.
Different machines require different volumes and pressures of steam. More steam requires a higher rate of heat transfer in the boiler, which requires a larger fire, more surface area to transfer the heat from the fire, and the fire will require more airflow in order to maintain efficient combustion. It also requires a larger volume to accommodate all the steam you need as a reservoir (in order to keep the pressure from dropping too quickly) and the large amount of water you'll want to keep the temperature stable when adding cold water to replace the steam leaving the boiler. If that sounds as if it's complicated to figure out, well, it can be. That doesn't mean the basics aren't fascinating.
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Vertical boilers have a straightforward and rugged design. If you have a gas powered water heater in your basement, it's basically built on this pattern. A tank is constructed with a flue (exhaust passage) going from the fire below the tank to the top of the tank. Boiler design is essentially the same only there are usually multiple flues in order to provide more surface area for heat transfer. The fact that the boiler is so much taller than the firebox means that there is plenty of water above the heated surface of the firebox, known as the crown sheet. If the crown sheet were to become dry they would get much hotter than usual, weakening it and making an explosion much more likely especially if water came back in contact with the overheated metal and began to boil rapidly. Luckily, since this design is much taller than it is wide, the boiler would have to be nearly empty or turned on it's side before that happened.
These boilers were often used for portable applications. Steam donkeys (portable engines for winching), early fire engines, earth moving equipment, small steam powered boats, and even small locomotives like the Tom Thumb and early Climax engines. They were well suited to situations where they were jostled about, but that's not the only thing that made them practical. Vertical boilers take advantage of natural convection (heat transfer with natural, not forced, air movement.) The hot air and smoke from the firebox naturally rise into the flues where they provide additional heat for the boiler. As the smoke and air rise, fresh air enters the firebox below, giving it more oxygen to burn, and making the fire burn hotter. This would keep the fire burning hot even without use of bellows or blowers to force air into the fire. (Although occasionally blowers were still used to get more steam out of these boilers.)
The speed with which these boilers could be steamed up made them ideal for situations where they were used infrequently or on a moments notice. That's why early fire engines used this type of boiler, it could be fired and ready to use in 30 minutes or less.
Horizontal boilers
These come in several different flavors.
Older, permanently mounted boilers were often built into brick enclosures. This gave them both support and insulation, helping to reduce the energy needed to keep them hot. An example like this has the firebox built directly into the end of the boiler. You might notice the space below the fire, where the ash settles. There is a grate between this and the rest of the firebox, and much of the air for the fire comes in through here. The smoke goes out through horizontal flues running to the other end of the boiler and out a chimney. This design is fairly simple, but not overly powerful because there is limited space for the fire, and a limited air supply In order to make a more powerful boiler, it's necessary to use a different design that can accommodate a larger firebox.
More complex installations like this one, were externally fired, meaning that the firebox was completely separate from the pressure vessel. This particular arrangement had the advantage of having doors for the firebox, the ash pan, and the flues all in the same location, making it possible to service the boiler almost entirely from one side. This also provided for a very large firebox and plenty of area to transfer heat to the pressure vessel.
The locomotive boiler, as you might guess, was most commonly seen on its namesake, but it could also be found on fixed boilers, portable steam engines, steam tractors and other applications.
The locomotive boiler is easily distinguished from other designs by the location of the firebox and smokebox. Like many boilers, the smokebox and firebox are at opposite ends of the boiler with flues running from one end to the other. A locomotive boiler has a large firebox that extends below the rest of the boiler with a grate and ash pan below, allowing air to travel directly up and through the firebox. The firebox is surrounded by water on all sides except the bottom in order to gain a larger surface area and keep the firebox sides from overheating.
Fireboxes came in varying sizes and shapes, usually depending upon the fuel that was used. Wood fired boilers typically need to have a tall narrow firebox that can be stacked full of wood. Coal requires a wide flat grate area so that a large amount of coal can be spread out in an even layer across the bottom of the firebox because a very deep bed of coals will become too dense for air to pass through and eventually suffocate itself.
The fact that a locomotive boiler has the firebox and smokebox on opposite ends is quite important. A smokestack right above the firebox would allow the boiler to steam up quickly because, like the vertical boiler, it would generate airflow due to the rising of the hot air and the natural convection type heat transfer, but it would not generate as much power. If the smoke goes up the stack too quickly, it actually reduces the fuel available for your boiler. To understand this better, think of a campfire. The bed of coals throws off quite a bit of heat, but if you add fresh fuel or force air through the coals you will get tall flames which rise up much higher than the solid fuel. This is the result of hydrocarbons and other chemicals being released from the fuel source. The heat of the fire causes the more volatile compounds in the fuel to turn into gases which are then burned in the form of visible flames.
In the case of a locomotive boiler there is usually a large volume of otherwise unused space in the firebox in which the gases from the fuel can be burned. Sending these gases down a set of flues to the other end of the boiler allows more time to burn the fuel and plenty of surface area to transfer this extra heat to the water. (This is more important when burning lignite and bituminous coal as they will have more volatile chemicals than anthracite, which burns with a very short flame.) Of course, in order to produce the gases from the solid fuel the fire must be quite hot. To achieve the necessary temperatures, most fuels require plenty of airflow. Since a locomotive boiler gets fairly little natural convective draft, it is necessary to add air to the fire through other means. To do this, exhaust steam which has been used by the steam engine is piped into the smokebox so that it travels up the smokestack. The rapid movement of this steam out the smokestack creates suction on the flues which causes suction on the firebox. The hot gases from the firebox are drawn into the flues, causing fresh air to be drawn into the firebox, helping the fire to burn hotter.
Like the firebox, the shape of the smokebox is also influenced by the fuel being used. If the draft up the smokestack is forced by a blower or the exhaust from a turbine it is relatively constant. But reciprocating steam engines create short bursts of exhaust causing fluctuation in the suction on the firebox. For engines burning hard coal or wood, this isn't an issue, but when using soft, powdery coal like lignite, this is can be problematic. Engineers have said that with a lignite coal, a strong draft can "suck the fire right out of the stack." While this may be a bit of an exaggeration, it's a pretty good description of what happens with softer coal. Small pieces of coal particles will continue to burn even while they are carried down the flues and through the smokebox. Early locomotives typically prevented this by way of special smokestacks that caught sparks and cinders. Larger boilers made it impossible to put tall smokestacks on top of locomotives so this had to be incorporated into the firebox.
The Chicago Burlington and Quincy incorporated smokeboxes of this design on western locomotives. The baffles and screens redirect the airflow so that burning embers collect in the bottom of the smokebox rather than going out the stack. In some cases smokeboxes like this were quite obvious because of their large size. The best example of this can be found on the CB&Q where locomotives that were otherwise identical would have dramatically different smokeboxes depending on where they were expected to operate.
Of course, all of this work on the smokebox would be wasted if the draft did not produce a proper air supply to the firebox. The fuel in the firebox rests on a grate which allows air to pass through it. This is vital to keeping a good fire. Drawing air through the coals provides oxygen directly to the solid fuel, helping it to burn hotter, and it warms the air so it is easier to consume the gases being released by the burning fuel. If cold air is allowed into the firebox, it will not thoroughly burn the volatile gases produced by most fuels. Even worse, the cold air will come into contact with portions of the firebox and the flues. This cools the metal causing it to shrink slightly. This shrinkage puts stress on any affected joint in the firebox and could eventually lead to leakage around flues and staybolts. The firebox door provides the most common source of cold air into the firebox, as it must be placed above the grate in order to feed the fire. A good fireman would keep the firebox door closed unless it was necessary to feed or check the fire. Sometimes a fireman would open the firebox door to reduce steam in a boiler, especially when the boiler was 'popping off' at an inopportune time (the pop valve releases pressure from the boiler with a very loud hiss or roar.) This was effective at reducing the rate of boiling in the boiler, but as I said, it can lead to leaks in the firebox which can be very difficult to fix.
There is one more key feature to a boiler that cannot be left out when discussing horizontal boiler design. Virtually all horizontal boilers are designed with a steam dome on top. The reason for steam domes is tied to the design of steam engines. Piston engines don't like water. They can handle a little condensation in the steam, but too much liquid is a problem because it can turn a steam piston into a hydraulic piston, which will invariably result in a failure somewhere in the cylinder or piston. Even without this problem, dry steam provides a much more efficient energy source for running any steam engine.
The simple answer to this problem is 'put the intake above the water level in the boiler. This is technically correct, but when water boils it splashes quite violently, throwing a mist of hot water into the air. If the intake is not sufficiently high above the water level of the boiler, the mist will get into the steam supply to the engine. Add considerations like, water being sloshed around in a running locomotive or moving steam tractor, and it's obvious that the intake needs to be a considerable height above the water in the boiler. The steam dome allows for this without the need for a drastic increase in boiler size. The dome can also be made with a baffle or separator plate that leaves very little open space between the boiler and the dome, which helps to prevent water from splashing into the top of the dome. Steam separators are also used in some applications to remove moisture from the steam, typically by forcing the steam to make a sudden change in direction, throwing water droplets out of the flow.
Most steam domes can be readily identified as a hump in the middle or back of a boiler. Locomotives often appear to have several domes, but only one of them is a true dome. The others are sand boxes used to give wheels more traction. You can tell the difference by looking for details like tubes leading from the sand dome down to the wheels of a locomotive (on sandboxes) or whistles, throttle linkages, and large steam pipes on the steam dome.
There is one other type of horizontal boiler worth noting, the return flue boiler. These were used in steam tractors and stationary applications, though I won't take the time to write up the details as True Blue Sam already has a good post on this type of boiler. The simple explanation is that the firebox is built into the bottom of the boiler, spanning virtually the entire length of the pressure vessel. A smokebox on the end opposite the fireman provides an opening to connect the firebox with the flues which take the smoke back to the fireman's end of the boiler, where they enter a crescent shaped smoke box and go up the smokestack. This design has potential efficiency benefits as well as maintenance advantages over locomotive boilers.
For more information on boilers I would recommend looking into the books Steam Power Plant Engineering by G F Gebhardt and the Cyclopedia of Locomotive Engineering by C F Swingle where I found some of the illustrations above. They are both in the public domain and available from Google.
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