The boiler of the SP&S 700 has 186 flue tubes of 3.75" diameter and 54 more of 2.00" diameter. These 21' tubes carry the hot exhaust fumes from the firebox to the smokebox. Nestled within each of the larger tubes is another pair of pipes connected at the firebox end by a "U"-shaped return casting. This inner assembly is called a superheater. Within the smokebox, one of the pipes from each superheater is joined in a manifold called the superheater header while the other pipe leads to a similar manifold at the throttle box.
The short answer is that superheaters convert "wet" steam into "dry" steam. For a better understanding, let's follow the steam in an operating engine. We've already mentioned that the flue tubes channel the hot exhaust fumes from the firebox to the smokebox. As they do so, they transfer heat from fumes to the boiler water. As the water gets hotter and hotter, it eventually boils, creating "wet" steam and raising the pressure. Now when the engineer opens the throttle, this wet steam leaves the boiler, travels down the dry pipe, and enters the superheater header in the smokebox. The path of the steam there divides, leading through the 186 individual superheaters. While passing through the superheaters, the wet steam's temperature is raised several hundred degrees and it becomes known as "dry" steam because it is no longer on the verge of condensing into water. The dry steam next exits the superheater, passes through the throttle body and enters the steam chests and cylinders, where it provides the motive force of the locomotive. Dry steam is superior to wet steam for powering the locomotive because it provides higher thermal efficiency and is less likely to condense within the piping and cylinders.
The superheaters on the SP&S 700 are old, worn out by the caustic environment and the constant temperature swings within the flue tubes, and they need to be replaced. You can help keep the "First Lady of the Northwest" in service for many years to come by sponsoring one or more of her new superheaters. A donation of $300 will sponsor one superheater. All donations will be used for the inspection and rebuild of the SP&S 700, and as always all donations are 100% tax deductible.