Boilers are made mostly of metal with iron and steel predominating. Copper is also used for smaller units, but cost usually prevents its use for larger installations. Early boilers, 1745 to the 1830s, were mostly iron—either cast or wrought plate.
Cast iron is still in common use for stationary boilers. Natural gas burning, cast iron sectional boilers are probably the most efficient available today, often exceeding 98% fuel to heat to water transfer. A cast iron boiler is made by casting the water containment section and placing it over the firebox.
Wrought iron plate was made by taking cast iron pigs (billets of cast iron from the foundry) and heating them in furnaces called soaking pits. Once heated to near melting temperatures, the pigs were run between rollers and formed into plates. Often many passes were required, with the plates folded in half, fluxed and welded together with the rolling mill. Once the plates are of the desired shape, size, and malleability, they are formed on rollers to make the firebox, drum, sheets, and flues. Holes are then punched preparatory to riveting
Steel is prepared much the same as wrought iron, but is preferred as it has better strength and resistance to damage. Also staybolts and dimensions can be lighter, reducing the weight of the boiler.
Copper is also formed by rollers but must have larger dimensions for the same capacity due to the weakness of the material.
Lastly, we come to the high alloy steels, titanium, vanadium, etc. These steels are like steel compared to wrought iron, only more so. They allow even lighter dimensions for a given capacity of boiler. Sadly, the metallurgy required is advanced, and it may take years to create the physical plant needed to formulate these steels.
Boiler Types
Boilers come in many types. Mainly they can be divided into: a) Fire tube (exhaust gasses go through the tubes to the exhaust point), b) Water tube (the tubes are connected to "drums" and are filled with water with the exhaust gasses going around the water tubes to the exhaust point), and c) Sectional boilers (where prebuilt sections are assembled, contain the water, and the exhaust gasses pass between the sections). Other boiler types exist, even fireless types, but in the main they fall in to these broad categories
Fire tube boilers are as described earlier, where there is a firebox connected to the body of the boiler, with the exhaust gasses flowing through the tubes to the stack. The boiler type comes in a number of variations: vertical (firebox in the base, water drum on top, exhaust above that), locomotive style (firebox at the rear, horizontal tubes through the water drum, exhaust at the front), Scots marine (firebox contained within the water drum, exhaust through tubes also in the drum). All of these can be single- or multi-pass systems.
Vertical boilers are typically stationary, that is, used in the environmental systems of structures for heat and limited steam supply. The vertical boilers are simple to make and maintain, but are not very efficient.
Locomotive type boilers are used in stationary and mobile applications, are robust, and can be very efficient. Often called horizontal boilers, they are capable of producing steam in great quantities, and are the most common type in commercial applications where robustness and large steam production are needed.
Scots marine boilers are, as the title suggests, boilers in common use aboard ships. The fuel, typically oil or gas, is burned in a large fire tube in the base of the water drum, and the ends of the boiler are covered by doors or caps that are divided so as to reverse the flow of exhaust gasses through layers of tubes, usually two sets, so that the gasses make three passes through the boiler, hence the multi-pass name. Scots marine boilers are also very common in industrial use, (I have three at my facility) very efficient, and reliable.
Water tube boilers are made from a set of drums. The drums, usually one steam drum on top and a mud drum on the bottom, have holes in the bottom (steam drum) or the top (mud drum) where the water tubes are connected. The water tubes are not straight but curved and fill the space between the drums like spider legs. They are normally only inches apart. The whole assembly is mounted over the firebox and enclosed within an insulated case. Commonly used on large ships, they are efficient producers of steam and able to produce steam quickly due to the relatively small quantities of water (in each tube by cross section) being heated. While most common on ships, they are also used in large industrial plants where rapid steam production is needed.
Side note: I once worked on a set of five water tube boilers in a dairy in northern Utah that were thirty-plus feet tall, forty feet wide and sixty feet long. The work involved crawling inside the steam drum and using a water powered descaling drill to clean the scale off of the inside of the water tubes At the end of each day I had the "privilege" of crawling into the mud drum to remove the day's mud gleanings. This is of interest because the drums were four feet in diameter and show the size involved. They stick in my mind, though, because that was where a local maintenance guy cut our locks on the valves and proceeded to turn live steam in on us . . . I was irate when we got out, lucky to be only lightly toasted as the water in the lower half of the boiler cooled the steam somewhat.
Cast iron boilers are made of individual castings. These are bolted together. Early designs looked much like an oil drum or a water heater. These have the firebox at the bottom of the water tank. The water around the sides of the firebox are called "water legs," which go down to the grates.
Other cast iron boilers were made in hollow flat square sections bolted together like slices of toast on edge, and placed on the frame which has the burners and fire box beneath. Steam is withdrawn through a manifold connecting to the top of each section. Sections are ganged together by ports located in the top and bottom sides of each section. Old apartment radiators can be considered this type of sectional boiler. Gaskets, usually lead or bronze (or modernly, high temp silicon rubber), seal the sections together. Combustion gasses flow between the sections in channels and spaces provided for them when the sections were cast. The entire assembly is enclosed in an insulated shell with provision for the exhaust gasses to exit by means of a smoke stack. Unfortunately, cast iron does not respond well to sudden shock and tends to break up in mobile usage. Cast iron sections also require advanced casting methods, including cores, the handling of large molds, and manipulating large pours of molten iron.
Joint and Seam Methods
Boiler parts can be held together in a number of ways. Welding and riveting are the most common, but nuts and bolts are also used, and even drilled and tapped holes in the boiler shell are common.
Wrought iron welds well, and the primary form of welding wrought iron is hammer welding. Hammer welding, also called forge welding, is where the two pieces of metal to be welded are brought to a near molten state, (hotter than the temps used for rolling) fluxed, placed on each other, and compressed until the metal intermingles.
While hammer welds are easy in wrought iron, hammer welding steel is much more difficult. Oxidization caused by heating interferes with a good connection in the two steel parts being welded. Another problem with hammer welding is the amount of heat needed. Large components need more heat applied as the heat tends to travel and try to heat the whole part. Also, large parts can be difficult to handle as the weight is more than can be handled without machinery.
Welding can also be achieved by the application of heat in a localized spot in a short time. This heat can be created by gas torch or electrical resistance.
Gas welding, normally Oxy Acetylene, is very good for small parts (less than two inches in diameter), but has the problem of needing much more time when welding larger parts. Gas welding may have issues in the Ring of Fire, as production and storage would be difficult.
Electrical resistance welding comes in at least two forms. Stick welding, where an electrode attached to a handle uses an arc from the electrode to the work piece to create local intense heat and fuse the metal parts together. The electrode also supplies additional metal to use as filler in the joint. The second form is spot welding, where the two parts to be wel
ded are placed between two electrodes and high current is applied. This high current creates heat which fuses the metal together. This method is most suitable for sheet metal applications.
This resistance welding can be AC or DC and can be achieved under fairly low tech conditions. As an example, one time I was on a deployment in my two and a half ton truck. We were in some rough country and managed to break a bracket needed to keep the alternator in place and operational. While I had the mask, rod, and cables, the welding "box" was on another truck. Our solution was to weld the bracket on and continue our trip. This was accomplished by hooking our cables to the terminals of the truck's battery and making the weld. This was possible because an automotive battery has fifty or more amps, and resistance welding works well at that amperage. (We had four batteries, set up for 24vdc so we had in excess of 100 amps available). In the 1632 universe, the biggest problems will be insulating the welding cables and charging the batteries.
The best way to connect the boiler parts together is riveting. Rivets provide solid, dependable connections, have well understood properties, and are relatively easy to make. Riveting is still in common use for large steel fabrication. Even the locomotive our club is restoring (built in 1944) uses large numbers of riveted connections. Riveted joints come in a number of flavors. They are lap joints, lap joints with cover plates, and butt joints. They can be single, double, or triple riveted. The Machinery's Handbook (mine is the 1942 11th ed.) has the layout and math for setting up these joints (pp408-422). Any of the Grantville machine shops will have copies of this book, and will probably have multiple copies as new editions come out frequently (we are up to the 27th ed).
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Many areas that look like they would need complicated welds are really better made with rivets. The mud ring, that joint that runs around the base of the firebox, is still made by casting a "ring" of cast iron the width of the space between the inner and outer walls, and riveting through the ring. Firebox door ports (where they throw the coal in) are also simply made using a ring of cast iron around the opening. Stay bolts are also riveted over on the ends. The riveted joint gives solid, dependable connection.
Tubes are often installed into a boiler using a rolled fit. The rolled connection is accomplished by placing the tube in the sheet in its desired location, then placing the tube roller inside the tube. The roller is then turned and compresses the tube wall against the sheet, causing the tube end to expand and lock against the sheet. In a properly executed rolling operation, you can actually see ripples in the sheet as the roller expands the tube. Of note is that it is common practice that the firebox tubes are riveted down to the sheet (beaded) and then welded, while the exhaust end of the tube is left as rolled. This reduces the tendency of the combustion in the firebox to degrade the edges of the tubes and thus reduce the tubes' life span.
Threaded connections are also common in boiler construction. Tapered tapped holes (taper of 3/4 inch per foot) are used to put tapered threaded holes in boiler plate. These tapered holes are used to mount appliances (things needed to make the boiler/engine work) and other items (such as handrails and brackets) to the boiler. Mounting studs, tapered on the boilerside and straight threaded on the end not mounted in the boiler are a common device using this thread. Straight taps are used to mount long bolts not needing compression fit to the boiler and are similar to stay bolt taps that are used to tap the holes that the threaded staybolts put in to prior to them being riveted over or welded. Machinery's Handbook (pp1338-1339) have the common standards listed.
High vs. Low Pressure
Surprisingly, the demarcation of high pressure steam is atmospheric pressure, that is approximately fifteen pounds per square inch. Any pressure below that amount is "low" pressure and any pressure above that amount is "high" pressure.
In the early days of steam power, many machines were built to operate in the low pressure range, and do it well. Low pressure machines are, however, limited in the amount of work they can perform in comparison to their size.
High pressure machines give much greater horsepower to weight or size results and are more fuel efficient. Boilers tend to evaporate water at a given rate depending on their design. Higher pressures allow the use of smaller machinery to provide the same amount of work. Certainly higher pressures require stronger construction and thus heavier overall weights, but the tradeoff was generally that bigger was better. Another note about pressure, high altitude has little effect on boilers. Because they are regulated by pop valves, the internal pressure is what matters in a boiler; this pressure is what determines the transfer of energy to the water.
Steam Supply Control
To be useful, steam must be withdrawn from the boiler and sent to the machinery. The steam should contain as little liquid water as possible. Steam is therefore usually collected from the extreme top of the boiler in a space called the steam dome.
The steam dome extends above the boiler and contains the dry pipe. This dry pipe has an open top and goes down into the boiler and extends to wherever the steam is removed from the body of the boiler. The dome also usually has the pop valves mounted on it and may also be, through the use of a bolted on hatch, the entrance in to the inside of the boiler.
The dry pipe is also connected to a valve controlling the amount of steam allowed to exit the boiler. This valve is sometimes mounted at the base of the dry pipe below the steam dome or it may be mounted in the smoke box at the exhaust end of the tubes. Mounting the valve below the dome allows the use of the control rod and its housing as a support for the top of the boiler sheet, by connecting to the dry pipe, making a continuous link from the back head to the front sheet. If mounted in the smoke box the valve may be accessed without opening the pressure vessel of the boiler. Each type of installation has its benefits and the choice is really up to the design team and the purpose of the installation.
Figure 5
Super Heaters
Water is, for all intents and purposes, incompressible. This property of water presents some challenges to steam-powered machinery. For example, if you have water in your cylinder on the non-powered side, the water can blow the cylinder head right off the cylinder. Water, in comparison to dry steam, has mass and will slow down and otherwise make steam-powered machinery feel mushy in operation. And, last, the water takes the space that more energetic steam could be occupying.
The cure for water in your steam is simple, dry the steam out. Superheaters, sometimes called dryers, are tubes run from a header back through the flues (tubes) and then out to a collecting manifold that sends the steam on to the machinery. The superheater tubes also act as a sort of second pass ( like the Scots marine boiler type) and wrest more efficiency out of the fuel combusting in your firebox. At the end of the 1940s superheaters were so efficient that the machinery was really running on a pseudo-plasma of oxygen and hydrogen and actually causing new problems in cylinder wear.
Supplying Water and Fuel
Needless to say, a boiler that cannot be re-supplied with operating fluid (water) or fuel while in use is of limited utility. But, as in all else, this re-supply includes its own challenges. Injecting cold water into a hot boiler can cause problems. First, cold water on hot metal can cause shrinkage, stressing and even tearing the metal. At a minimum, it reduces the life of the boiler. Second, even if the water is put into the boiler so that it does no damage, it drops the overall temperature of the water in the boiler. This temperature drop can stop the boiler from steaming and will require extra heat to raise the water to operating temperature. Third, the water must be put into the boiler against the operating pressure of the system.
The cures for these ills are the feedwater injector, feedwater pump and the feedwater heater. Feedwater systems are always doubled. Two ways to put water in to the boiler are essential for safety reasons. Injectors work by using boiler pressure to force water into the boiler much like a waterjet-style well pump. Feedwater pumps are pis
ton arrangements that move the water against the boiler pressure by mechanical means. Heaters are heat exchangers that use waste heat from the boiler to bring the water closer to operating temperature before injection into the boiler.
Fuel can be fed into the firebox in a number of ways. Wood and coal can be thrown into the firebox through a door on the backhead. This was done primarily by hand, and is an exacting art as just dumping the fuel in the door will not provide the level of combustion needed for useful steam.
Oil is sprayed into the firebox and is normally done with steam bled off the boiler. Superpower locomotives (like our club's) needed mechanical stokers as the amount of fuel required far surpassed the ability of even three stokers to keep up. However, superpower is probably a long way down the road for Grantville.
Safety Controls
Safety appliances are those devices that protect, warn, and show the condition of a boiler in operation. If ignored and not maintained, the boiler will eventually kill the crew and destroy the locomotive. These devices were discussed at the beginning of this article.