The above estimates are on the basis of a thoroughly dry saturated steam or steam just at the point of being superheated or containing a few degrees of superheat. If the saturated steam is moist, the saving due to superheat is more and ordinarily the gain in economy due to superheated steam, for equal boiler efficiencies, as compared with commercially dry steam is, say, 5 per cent for each 100 degrees of superheat. Aside from this gain, as already stated, superheated steam prevents erosion of the turbine buckets that would be caused by water in the steam, and for the reasons enumerated it is standard practice to use superheated steam for turbine work. The less economical the steam motor, the more the gain due to superheated steam, and where there are a number of auxiliaries that are run with superheated steam, the percentage of gain will be greater than the figures given above, which are the minimum and are for the most economical type of large steam turbines.
An example from actual practice will perhaps best illustrate and emphasize the foregoing facts. In October 1909, a series of comparable tests were conducted by The Babcock & Wilcox Co. on the steam yacht "Idalia" to determine the steam consumption both with saturated and superheated steam of the main engine on that yacht, including as well the feed pump, circulating pump and air pump. These tests are more representative than are most tests of like character in that the saving in the steam consumption of the auxiliaries, which were much more wasteful than the main engine, formed an important factor. A résumé of these tests was published in the Journal of the Society of Naval Engineers, November 1909.
The main engines of the "Idalia" are four cylinder, triple expansion, 11-1/2 × 19 inches by 22-11/16 × 18 inches stroke. Steam is supplied by a Babcock & Wilcox marine boiler having 2500 square feet of boiler heating surface, 340 square feet of superheating surface and 65 square feet of grate surface.
The auxiliaries consist of a feed pump 6 × 4 × 6 inches, an independent air pump 6 × 12 × 8 inches, and a centrifugal pump driven by a reciprocating engine 5-7/16 × 5 inches. Under ordinary operating conditions the superheat existing is about 100 degrees Fahrenheit.