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An Interesting Explanation of the Theory and Practice of the Superheater and an Account of Its Operation F LATE. years superheated O steam has attracted wide at- tention among marine engi- neers; and at the present time is ac- corded a prominent place, especially in the calculations of foreign ship owners and builders. Superheating is largely a question of degree or amount. For the purposes of this article the following divisions will be made: Low superheat, zero to 50 degrees Fahr. Moderate superheat, 50 de- grees to 125 degrees Fahr. High superheat, 125 degrees Fahr. upwards. Low degree superheat, perhaps be- ing the easiest to obtain, has, in many instances, been tried and found of some advantage. A common, and in some cases unintentional, development of low superheat has been by throt- tling or wire-drawing. A boiler pres- sure of 10 pounds or more above steam chest pressure permits, by wire-draw- ing through the throttle, a reduction in moisture, and if the moisture is in small amounts, slightly superheated steam may actually be obtained; how- ever, if some distance exists between the throttle and the steam _ chest, radiation losses will cause a loss of a part, or all, of the superheat. Effect of Temperature Changes Low degree superheaters are gen- erally exposed to relatively low tem- perature gases. In such cases a change in the gas temperature becomes a point more to be considered than where the gases are at high temper- atures. The difference between the gas and the steam temperature is, of course, small and a reduction in gas temperature which might be occasioned for example by opening the fire door, would easily bring the gas tempera- ture below the steam temperature. If such a condition existed, the super- heater would become a_ condenser, that is, it would give up heat to the gases. This should, of course, be guarded against. It should also be considered that, below certain tem- peratures, gases deposit soot in large quantities. This may be quite easily appreciated by those who have operat- ed feed water heaters or “econo- From a paper read before the Society of Naval Architects and Marine Engineers. The author, Henry B. Oatley, is chief engineer, Locomotive Superheater -Co., 30 Church street, New York. mizers”, many of these being provided with tube-scraping arrangements, either manually or automatically operated, which are mulated soot, thus enabling the appa- ratus to be operated at its designed efficiency. Moderate superheat has, during the last 20 years, been more widely used. Superheaters developing between 50 FIG. 1—WATKINSON TUBULAR SUPER- HEATER and 125 degrees of superheat have been constructed in a multitude of ways, and have been operated with a greater or less degree of satisfac- tion, but, in marine service at least, have not obtained any uniform de- gree of application. This is par- ticularly true of fire-tube boilers, as compared with water-tube construction. 198 intended to remove accu- . _By Henry B. Oatley High degree superheat has, during the past 10 years, been meeting with pronounced favor in merchant service, as well as in naval construction. This statement is believed to apply much more to England and_ continental countries than to the United States. In some tests recently made it was found that on a triple-expansion en- gine having 180 degrees of superheat in the high-pressure chest, 100 degrees of superheat was carried through to the intermediate chest. This prac- tically all disappeared before reaching the low-pressure chest. In another ship having quadruple engines a su- perheat above 210 degreés Fahr., was carried to the high pressure chest. This was decreased to about 125 degrees of superheat in the first in- termediate, and to just a trace, per- haps 10 degrees Fahr., of superheat, in the second intermediate, the low- pressure steam being very nearly dry. This point is considered of interest as showing that for triple and quadru- ple engines high superheat should be used if it is desired to obtain the large economies which are given in the tests reported within the past few years. Superheating Development To say that superheating is almost as old as the use of steam, and that it was from very early times known to be extremely efficient, is probably stating a platitude. Technical litera- ture, in many branches of engineering, has referred at length to Watt, Hirn and others, who many years ago worked assiduously to find satisfac- tory methods by which they could ob- tain and utilize this desirable qual- ity in steam. In 1859 technical writ- ers advocated strongly the use of superheaters and pointed out the ad- vantages to be attained, some refer- ence being made to difficulties ex- perienced at that early date. It was not lack of knowledge, but rather the lack of suitable materials and also methods for manufacturing steam containers, which prevented the wide- spread adoption of superheated steam. Watt had no steel mills -nor high- grade steel foundries. from. which to obtain the necessary plates, tubes, castings, etc., with which to build