The function of the boiler in the broadest sense is to make steam at a pressure and volume sufficient for the steam engine to perform its assigned work. To perform this function the boiler must be fed water at a sufficient rate to compensate for the steam being consumed. There must be sufficient heating surface to heat the water into steam at the operating pressure and rate of use demanded by the steam engine. A boiler’s construction must be as light and compact as possible while still offering good structural safety for the pressures present. And finally the boiler must make as efficient use as possible of the heat energy of the fuel used to heat it.

Internal combustion engines are thermodynamically classified as Otto-cycle engines because the fuel is consumed internally to the engine. Steam engines are thermodynamically classified as Rankine-cycle engines as the fuel is consumed external to the engine. The boiler is where the thermal energy of the fuel is converted to potential energy in the form of steam for later use by the steam engine. The use of steam energy allows it to be stored by the boiler in the form of water under pressure much the same way a battery stores electrical energy in the form of a chemical reaction. As steam is consumed from the boiler the water remaining in the boiler flashes into steam. As the steam is consumed water must be pumped into the boiler and quickly heated otherwise the boiler is similar to a battery that must be replaced (for a primary battery) or fully recharged (for a secondary battery) before it can be used again.

When steam powered much of the Industrial Revolution boilers were rated in horsepower. At that time it was generally accepted that a one horsepower steam engine in good working order consumed 30 pounds of water or steam per hour. A boiler that could evaporate 30 pounds of water per hour from an initial water temperature of 100º Fahrenheit into steam at 70 PSIG would be rated as one boiler horsepower. Today the definition has been changed to reflect a boiler’s ability to evaporate water. One boiler horsepower is now considered equal to the evaporation of 34.5 pounds of water at 212º Fahrenheit into saturated steam at the same temperature. In the case of marine and automotive applications the definition of boiler horsepower was again different. For these applications the horsepower of the boiler was related to the horsepower that could be continuously developed by the steam engine that the boiler powered at a given steam operating pressure.

Early Stanley cars had boilers that generated 400 PSIG steam pressures. With later models the boilers were strengthened and were routinely operated at 600 PSIG steam pressures. The earliest Stanley cars had steam engines that developed 4-1/2 to 8 horsepower at operating pressures of 400 PSIG. Their later cars and the vast majority of Stanley production were limited to three engine/boiler ratings; 10 horsepower, 20 horsepower, and 30 horsepower. Each of these boiler and engine combinations could generate well in excess of their horsepower rating by a factor of five or more for short periods of time. What is important is that the Stanley horsepower ratings reflect what the engine and boiler combination can sustain continuously (indefinitely with unlimited fuel and water supplies to the boiler). All Stanley Model 735 automobiles relied on a boiler and engine combination rated at 20 continuous horsepower.

All Stanley boilers are of a vertical fire tube design (a section of a Stanley boiler is pictured at the right). Fire tube boilers have the combustion gasses rising vertically through the cores of multiple tubes, called flues, while the water to be heated and turned to steam is on the outside of the tubes or flues. By being of vertical fire tube design the boiler is generally safer to operate. A common horizontal fire tube design like a boiler on a steam locomotive or steam traction engine by design must have heated steel surfaces (the crown sheet in particular) that can become uncovered with water under low water conditions. When this happens the steel becomes hot and weak and if things go really wrong a boiler explosion usually results.

With the vertical flue tube design the heated surface of the boiler is the bottom of the boiler where one end of the flues are attached. Should the boiler run out of water there is nothing from which to make steam. If the boiler continues to be heated (assuming the low water automatic fails to function as designed and shut down the burner) the flues will generally melt at the base of the boiler allowing any steam under pressure to be vented into the burner and generally causing havoc with the burner grate and fire. There are no known cases of a Stanley boiler ever exploding for any reason (there are numerous examples of "scorched boilers" where a low water condition has caused damage to the flue tubes).

One of the common ailments of a Stanley boiler is a boiler flue developing a leak.  Oxygen in the water and other contaminates along with imperfections in the steel or the flue tube all contribute to a flue developing a pin-hole leak.  The result is water and steam being sprayed into the tube and running down on the hot burner grate.  This not only wastes steam and water but the cold water hitting the hot grate can cause the grate to crack.  A solution is to use a boiler flue plug (shown at left).  The smoke box above the boiler is removed and the burner is dropped from under the boiler.  A tapered boiler flue plug can then be driven into the flue to seal it.  The leaking steam and water is then contained within the flue.  The boiler flue plugs are a handy item to carry in a Stanley toolbox as they could be needed at a moment's notice during a trip.  Generally two or three flues can be plugged and the boiler kept in service.  If more than three plugs are required it is time to consider either retubing the boiler or replacing it entirely.

The earliest 4-1/2 to 8 horsepower Stanley cars used boilers that were 14" and 16" in diameter by 13" high. With the introduction of the 10 horsepower boiler and engine combination the boilers were increased to 18" and 20" in diameter and 14" high. The 20 horsepower cars made up until 1917 used a 23" diameter boiler that was 14" high. With the introduction of the Model 735 car in 1918 the boilers were 23" in diameter and 18" high. All 30 horsepower rated cars including the Stanley’s famous Mountain Wagons used 26" diameter boilers that were 16" high.

The earliest boilers were hammer mill forged. The bottom flue sheet and the circular shell were made from a single sheet of steel. The two flue sheets were then drilled for the flues (a considerable accomplishment in itself considering one of the sheets resembled an inverted can). The top flue sheet was heat shrunk on and then gas welded. A pair of heavy steel bands was applied to the top and bottom sides of the boiler for strength and to serve as an attachment point for each of the three layers of wire that are wrapped around the circumference of the boiler. For later constructions the process was simplified to drilling a matching top and bottom flue sheets and then arc welding them to a circular shell. The flues were then installed and expanded tight in the flue sheets using a tapered punch. The final step was the application of three layers of piano wire to the outside of the boiler for strength.

The number of flue tubes in a Stanley boiler depends on the diameter of the boiler. The 18" boilers typically used for 10 horsepower cars contained around 470 flues providing 66 square feet of heating surface. 20 horsepower cars were the most common models manufactured and 105 square feet of heating surface was provided with approximately 750 tubes. For the Mountain Wagons and other 30 horsepower rated vehicles nearly 1000 flue tubes offered nearly 160 square feet of heating surface. These flue tube counts are for original boilers. Today’s boilers have different counts depending on who makes the boiler as the diameter of the flue tube can vary along with the spacing between adjacent flues.

The flue tubes in original Stanley boilers were 33/64" outside diameter, made of 18-gauge copper, and laid out on a 13/16" spacing between tubes. A small steel ferrule (shown at right) is inserted in the end of the flue to provide stiffness to the copper to insure the flue remains sealed to the flue sheet. When condensing cars were introduced Stanley soon experienced premature boiler failures primarily with the sealing of the flues on the lower flue sheet (the one closest to the burner). It was found that the steam cylinder oil would dissolve into the water as the steam was condensed. This water-oil mixture would be discharged into the water supply tank only to end up in the boiler. As the water was heated in the boiler the oil tended to collect on the lower flue sheet and cause hot spots (the oil would burn onto the bottom flue sheet and act as in insulator). This created uneven heating and allowed the flue tubes to start leaking. The solution that Stanley implemented was to use steel flue tubes on all their condensing car boilers and to weld (instead of expand) the flue tubes to the bottom flue sheet.

After a flue is inserted in a boiler it must be expanded into its hole to prevent leaking. Two accepted ways of doing this are to use a tapered expander or a roller expander. A tapered expander (upper two tools in the photograph below) is a hardened steel rod that has one end machined to a taper of 1/4" to 3/8" per foot. The tapered expander is coated with steam cylinder oil, inserted in the flue (and ferrule if it is a copper tube) and hammered into the flue to expand it tight in the flue sheet. The tapered expander is then twisted out of the flue. The roller expander is a lot easier to use in that it is inserted into the flue and a shaft on the expander is rotated with a drill motor. A set of tapered rods built into the roller taper rotates inside the flue tube to expand. The roller taper (shown at the bottom of the photo below) can be set to provide a specific amount of expansion to the flue thus providing a more uniform expansion of all the flues. Care needs to be exercised not to over-expand a flue as it can distort the flue sheet and under severe cases cause surrounding flues to become loose.  Where a flue is cut too long and needs to be trimmed, a trimming tool is used (as shown in the center of the photograph below).

Today when replacement boilers are constructed 1/2" tubing is used for the flues (either heavy walled Type-K copper or high-pressure steel hydraulic tubing). Generally 18-gauge wall or heavier tubing is used (18-gauge is equivalent to 0.049" wall thickness while 16-gauge is equivalent to 0.065" wall thickness). The flue-to-flue spacing is usually increased to 7/8". The flue sheets are constructed of 3/8" boiler steel while the side shell is rolled from 3/16" boiler steel. Stanley boilers do not require boiler inspections since they are used on highway vehicles that are exempted (if strict interpretation of the boiler code were applied to automotive vehicles then the high pressures present in internal combustion engine cylinders, tires, and other systems would also need to be regulated).

Stanley boilers operate at 600 PSIG pressures. If one calculates the force exerted on the total surface area of a flue sheet (the area between each of the flues) the numbers reveal over 80 tons of force are present on a 23" diameter boiler flue sheet. However the quantity of flues connecting the two flue sheets together act as stay bolts which are found in industrial, locomotive, and similar boiler constructions. A 23" diameter boiler can have between 550 and 700 flues depending on the flue-to flue spacing. Each of these flues provides support to the flue sheet against the steam pressure contained the boiler.

If the shell of a Stanley boiler were designed for the pressure contained in the boiler it would end up being an inch or more thick. The steam pressure within the boiler acting on the steel of the shell generates tensional lateral bursting stresses approaching 11 tons per square inch. This would make the boiler way too heavy for the car and not practical. As a way to keep the boiler light the shell thickness is reduced and then wrapped with wire for strength. Thus the Stanley boiler is built along the lines of some early artillery cannons where the barrels were wrapped with wire for strength.  A replacement Stanley condensing car boiler (steel tubes welded to the bottom flue sheet) is shown at right having its third layer of wire wound.  Stanley boilers are wrapped with high tensile strength wire having at least a minimum tensile strength of 300,000 pounds per square inch. At least three layers (although two layers are sufficient by calculation) and sometimes four are applied. This provides the boiler shell with great strength at a minimum of weight. The wire used for wrapping a boiler is 0.054" diameter minimum. Depending on the height of the boiler the wire length can for a single layer can vary. A 23" diameter by 16" high boiler can consume 1,600 feet of wire per layer or nine-tenths of a mile total.

Boilers are tested using a test known as a hydrostatic test.  A hydrostatic test is performed on a boiler when ever there is a question of it's integrity.  As part of the routine maintenance of a Stanley the boiler should be hydrostatically tested annually to insure they continue to be safe.  A hydrostatic test involves filling a boiler with water and then increasing the pressure to a minimum of 125% of the operating pressure of the boiler (750 PSIG for most Stanley boilers).  The reason for using water is that it is not highly compressible (like air or steam).  If a failure occurs there won't be an explosion but rather a rupture more along the lines of what happens with a water balloon.  If a compressible medium such as air or steam were used and a failure occurs then the result would be a dramatic version to what happens when an air-filled balloon breaks.  Once the pressure is raised to the test limit then all boiler valves are closed and the pressure is held on the boiler.  Generally the pressure should not drop more than a pound or two in ten minutes if the boiler is tight and sound.  If pressure can not be maintained then it probably won't be long before water will start dripping from somewhere indicating an area that needs attention.  Ideally a sound boiler should hold the test pressure with little or no drop in the pressure over a long period of time (if the water is under pressure in a perfectly tight vessel then there's no way for the pressure to bleed off).  While the boiler is being held at pressure it should be inspected for bulging of the sides and anything else that looks out of the ordinary.

When driving a Stanley it is recommended that as high a water level as practical be maintained in the boiler.  Stanley advertised the "reserve power" of their cars and that power comes from the boiler.  Similar to the battery associated with an electric starting motor, the energy storage source for the steam engine is the boiler.  There's a technical reason for carrying as much water in the boiler as possible ~ the water is where all the energy is stored and little reserve energy is contained in the steam!  A boiler sitting at 600 PSIG pressure has the water and steam at the temperature required to maintain the 600 PSIG pressure - about 483F or so.  When you draw steam off the boiler you don't decrease the pressure dramatically because some of the water instantly flashes to steam even without the burner lit underneath.  For the slight pressure drop from drawing off steam an even smaller amount of water flashes to steam.  Of course that water flashing to steam causes a slight temperature drop as well for the system.  Continued drawing off of steam reduces the water level in the boiler and drops the temperature of the complete system.  If that same boiler simply were full of steam only at 600 PSIG and you drew off the same volume of steam as above you'd see a much larger pressure drop since there's no water to convert to steam in the system. 

When a steamer is put away after a day's driving the owner always blows down the boiler to rid them of water impurities (they siphon full when they cool).  All of the water is all blown out but there's still 300 PSIG or so showing on the steam pressure gauge.  That's enough to move the car the 50 feet or so necessary to get it in the garage and parked.  If you have the boiler at 300 PSIG with water in it then you can go quite a distance more than the 50 feet or so.  The difference is the water flashes to steam and drawing off steam consumes energy from the stored water.  A boiler without any water at 300 PSIG has very little reserve energy available and the steam pressure drops quickly in moving the car.

Thus you want to operate a Stanley with the boiler full of water (but no so full that water is being carried over with the steam being used) because the water is where the reserve power in the form of energy is stored.  The water is the potential energy of the system and the steam is provides the kinetic energy of the system.  The reserve power that Stanley referred to is the water in the boiler and not the steam in the boiler.

Its often said that a steam car will get better water mileage running with the boiler full and at 600 PSIG than it will get with the boiler low and 600 PSIG.  With a low water level if you head up a hill you draw down the steam pressure faster than if you do the same hill with a full boiler.  It is easier to maintain 500 PSIG pressure with a full boiler than with a low boiler.  A observant steam car driver will observe that the pumps coming on to raise the boiler's water level don't bother a full boiler as much as they do a low boiler.  The reason for all this has to do with how changes to the boiler system (adding water, drawing off steam) affect the total BTU content stored in the water (remember it is the BTUs of energy added to the water that convert it to steam at a given pressure).  With a full boiler if you look at the BTUs stored in the water there's a lot more of them stored than if you have low boiler water level.  Thus any change made to the boiler (adding water, taking away steam with a certain BTU content for the pressure) is less of an impact on the full boiler since there's more BTUs stored in the water.