Ingenious Old Boilers

           I just helped cut up an old steam boiler--a 1913, Kewanee 300 hp coal fired, fire tube, low pressure steam boiler.  This is the boiler which originally heated the building where I now reside.  I live in an abandoned school building.  Not a cute little one-room school, but a three story brick building that once held classes for grades 1 thru 12.  As larger farms replaced small farms throughout the 20th century, rural areas such as the one where I live lost population, and many schools were shut down as the districts were re-organized.  My building was last used as a school in 1961, and I obtained it about a decade later.  I carved out a small apartment, about one fifth of the total square footage, which I then heavily insulated, and which I heat with a small LP gas furnace.  So since 1961, the old boiler has just sat there rusting.   A good thing, since it burned 300 tons of coal the last winter it was used.  The carbon footprint of operating such a monstrous teakettle (about the size of a small railroad locomotive) would be absurd.

            The thermal efficiency of coal boilers has improved since the days of the old fire tube boilers.  A modern "water tube" boiler in a coal fired power plant can yield over 90% efficiency, whereas the old fire tube boilers were lucky to get 15%.  So I was surprised to discover that these old boilers still showed some ingenious engineering. For instance, the thickness of the iron plate walls was not uniform throughout, but each area was only as thick as it had to be.  The main boiler walls were 1/2 inch thick, but the crown plates were only 3/8th".

              My boiler  was a "scotch boiler" design, but perhaps I should  digress a bit to explain exactly what that means.  A "straight through" boiler is simply a heavy walled horizontal barrel with a "crown plate"  welded or riveted across each end.  About 72  three inch tubes run through the length of  the barrel and through 3" holes in the plate, and then end. The tubes are secured and sealed to the plate around the outside of each tube, so that fire can flow through the tubes, but steam cannot get out of the chamber which surrounds the tubes.  The reason that the crown plates could be thinner is that the 72 tubes fastened to it would reinforce it.   A "straight through" boiler simply had a fire box at one end and the flames went through the boiler tubes to the other end and up the chimney.  The barrel, laid horizontally, had to be heavy enough to withstand the pressure of the steam, and long enough to allow the fire to travel far enough for the heat to be absorbed by the water and steam.

            But with a Scotch boiler,  the whole barrel is only half as long, but the flue gasses travel the same distance.  This is done by having the hot gasses first travel through only the lower 36 tubes, then be directed back through the upper 36 tubes.  So the fire goes through the boiler twice.  But this means that the last 5 ft before the fire goes up the chimney, it goes right over the fire box where it started. That means that the barrier between the fire box and the steam chamber is a flat plate that forms the roof of the fire box. So at that point, the steam chamber is a "D" shaped space with its floor as the ceiling of the fire box, and the ceiling being the top of the outer hull. So although the roof of the fire box was flat, it had steam pressure above it. But being flat, it did not have the strength of cylindrical walls found in the rest of the boiler.   To compensate for this, a forest of  3/4" iron rods connected this flat plate to the arched walls above.  When I discussed this with my brother, a former Navy boiler man, he explained that these rods would not only reinforce the walls, but also conduct heat.  And the fire box itself had double walls on three sides, with water circulating in between. Any heat which escaped from the fire box would still be heating water. Yet these walls, being flat,  also had to have 3/4" iron rods between them.

            Those who built boilers one hundred years ago did not have the sophisticated technology that we have today--but they were certainly not fools.

Note:  If this kind of post interests you, I have another post called, Ingenious Old Windmills