vessels "Georgiana," "Adelaide," "Eutaw" and "Mackinaw," showing a saving 

 due to the use of superheat as high as 30 per cent. The tests on the "Eutaw" 

 were very carefully conducted and the gain here in steam economy was approxi- 

 mately 1 8 per cent, and in coal consumption 15 per cent. In view of these 

 results, Isherwood ultimately recommended the use of superheaters but suggested 

 that 100 degrees be placed as the limit of added temperature because of lubri- 

 cating difficulties. 



A very large proportion of the superheaters installed throughout this period 

 were in connection with marine boilers and it may be said that during the sixties 

 their use was general. These superheaters followed three general designs. The 

 simplest were of cylindrical form in which the steam was made to pass through 

 an annular space or a series of annular spaces between plates, either in or about 

 the uptake of a boiler. Such superheaters were built in 1859 by Messrs. 

 Dudgeon, James Watt & Co. and John Rowan & Sons. The second design was 

 tubular, the tubes in certain types, as in the Joshua Field (1859), being flattened, 

 horizontal tubes connecting boxes placed in the uptakes of two boilers, back to 

 back. John Penn used a similar design, though the tubes were not flattened. 

 By way of comparison with present-day practice, Penn's superheater had a heating 

 surface of 2 ^ square feet per nominal horse power, the boilers being rated on a 

 basis of 19 square feet per horse power. Penn and others also used vertical tube 

 superheaters where the products of combustion passed through the tubes and 

 around the shell and the steam around the tubes. This construction was 

 used with a view to easier cleaning of "the sediment from the steam which 

 collects within the tubes." The third design of superheater was constructed of 

 sheets and consisted of a box formed of flat plates bent to a sinuous form and 

 placed within the boiler uptake. A number of such superheaters were placed in 

 the Peninsular & Oriental Steam Navigation Company's steamships about 1865. 



It is to be remembered that all of the boilers to which the superheaters just 

 described were attached were operating at pressures considerably less than 50 

 pounds per square inch. Such being the case, 100 degrees of superheat repre- 

 sents an ultimate temperature of only 398 degrees. Practically all of the super- 

 heaters were located in the boiler uptakes, where the temperatures of the escaping 

 gases were sufficiently high to heat the steam to this point. While a great deal 

 of trouble was experienced in lubricating the cylinders of the engine, even with 

 the temperatures below 400 degrees, still the economy of superheaters with single 

 expansion engines was generally accepted. 



As early as 1845, M'Naught had compounded, or "M'Naughted," cylinders, 

 and Pole (1850), Elder (1854) and Cowper (1857) advanced the design of com- 

 pound engines. In the late sixties compound engines had come into very general 

 use. Their introduction was accompanied by the use of high pressure steam and 

 these two factors combined to force the superheater temporarily out of the field. 

 Such engines, using high pressure, showed an increase in efficiency over the single 

 expansion engines and the low pressures that had been in vogue, greater than 



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