| Like the Cornish and Lancashire boilers, the Scotch Boiler utilizes internal furnaces with the fire box and primary flue traversing the lower portion of the water cylinder. Yet unlike the Lancashire boiler, the Scotch boiler does not utilize Galloway tubes. Instead, the designers choose to manufacture the water tank from corrugated plates. The end plates are reinforced by heavy through bolts. This combination of through bolts and corrugated plates provided an extremely strong boiler.
The Scotch Boiler is a "Fire Tube" design: a number of relatively small (3 1/2 inch diameter) metal tubes pass horizontally through the water cylinder and act as flues. A boiler 10 feet in diameter and 20 feet long would normally contain 137 individual horizontal tubes. These “fire tubes” were arranged above the furnaces, but below the water surface.
Fire and hot gasses pass from the furnace through the main flues which are surrounded by water. At the aft end of the boiler the hot gasses enter a chamber, or Dry Back which allow the end plate to be heated and also direct the gasses into the fire tubes. From there the hot gasses move forward through the numerous tubes to the chimney.
The Scotch Boiler was quite versatile. Designs were built to deliver anywhere from 6 to 300 BHP (boiler horse power). The largest were 10 feet in diameter, 20 feet long and contained four furnaces. The photo shows a Scotch type boiler with two furnaces.
Although it was said of this boiler type that “Their quick steaming ability and compactness make them particularly adaptable to marine and industrial use...”, even this design contained some fairly large flaws: Water circulation within the boiler was poor allowing cooler water to settle at the bottom of the boiler and remain there. This un-circulated water acted quite like an insulator decreasing the efficiency of the boiler. It also allowed mud andscales to be deposited on the outside of the main flues.
This accumulation amounted to an ever increasing insulation effect which lowered the heat transfer to the water. But it also meant that the metal tubes were not cooled adequately by the water. Eventually the insulation effect would allow the metal to heat to a point where it would plasticize (loose its strength) and begin to bend.
Engineers had to maintain a constant vigil on the condition of their boilers, to prevent a collapsed flue, or boiler explosion. To fix the problem of poor water circulation, designers added a rather large “steam and water drum” above the boiler. The modified unit became known as the Brady Scotch Boiler.
Another disadvantage of the Scotch design, as far as western steamboats were concerned, is their size. But keep in mind that the term “marine” as it was used then, and now, refers not steamboats but steamships. Deep water, ocean going vessels which have lots of room for such things as boilers. Smaller vessels used this design, too. A tugboat's small size and need for tremendous power made the Scotch boiler ideal for use aboard them. Alas, even this boiler was suceptable to catastrophic explosions if the engineer failed in his duties to maintain proper water level or the boiler itself held a flaw in its construction. |
