See Railway age gazette, Volume 53, No. 24, 1912, p. 1148. I kid you not. http://books.google.com/books?id=QrElAAAAMAAJ&pg=PA1148&lpg=PA1148
This document says superheating is safe and effective for switching engines. I read somewhere else they tended to explode, so they stopped putting superheaters in them. Maybe that was before 1912. This says: Steam of such high degrees of superheat can be exposed to the cooling action of the steam chest and cylinder walls without condensation and at the same time has about 30 per cent, greater specific volume than saturated steam of the same pressure. A large part of this increased specific volume is again lost before expansion of the steam in the cylinders takes place on account of the cooling action of the steam chest and cylinder walls. While the superheat of the steam leaving the superheater may be 200 to 250 deg., the average superheat of the steam in the cylinder at the moment the cut-off takes place is hardly more than 100 deg.; but the entire elimination of all losses through condensation, together with the remaining increased volume of the steam, effects under average conditions a saving of 30 per cent, and more in the steam consumption per indicated horse power, which gain corresponds to a saving in fuel consumption of from 20 to 25 per cent., compared with a saturated steam locomotive working under the same conditions. . . . . . . "The second requirement assumes that the increased volume of steam be expanded as efficiently, or in other words, that the same cut-offs be used as in the.saturated steam engine. This would mean a corresponding increase in cylinder dimensions which in many cases is not possible on account of limitations in adhesive weight, strength of running gear and other limitations" There will always be an increase in hauling capacity obtainable, but whether the theoretical maximum can be obtained depends on the size of cylinders, and depends also on the quality of the saturated steam engines with which the superheater engine is compared, or to which the superheater has been applied, whether the engine is correctly proportioned or over cylindered, or deficient in boiler capacity, etc. It depends also on the service in which the locomotives are used; whether the service is such as to be favorable to developments of higher degrees of superheat and more or less unfavorable to the saturated steam locomotive. In switching service superheater engines make a very favorable showing, although only a moderate degree of superheat is being developed, but the improvement in efficiency is so remarkable because the saturated switch engine is the most inefficient locomotive type. Under all these varying service conditions the increased hauling capacity of superheater locomotives obtained in practical service varies between 20 and 30 per cent., and frequently even more. . . . Elsewhere it says that saturated steam caused a lot of wear and tear. I think the second dome at the top is the steam distributor, not the superheater. I hate to point to this but . . . see: http://en.wikipedia.org/wiki/Steam_locomotive_components Item 9. See also: http://en.wikipedia.org/wiki/Steam_locomotive QUOTE: Steam circuit The steam generated in the boiler fills the steam space above the water in the partially filled boiler. Its maximum working pressure is limited by spring-loaded safety valves. It is then collected either in a perforated tube fitted above the water level or from a dome that often houses the regulator valve, or throttle, the purpose of which is to control the amount of steam leaving the boiler. The steam then either travels directly along and down a steam pipe to the engine unit or may first pass into the wet header of a superheater, the role of the latter being to improve thermal efficiency and eliminate water droplets suspended in the "saturated steam", the state in which it leaves the boiler. . . . END QUOTE - Jed
