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200 THe heat-receiving surfaces increases the rate of heat-transfer, because - this movement keeps the temperature dif- ference at its maximum.” Simplicity is perhaps an easy prob- lem. All superheaters are, in a sense, simple as compared with apparatus: that has moving parts. Simple con- struction is directly of value, in that it generally prevents excessive cost. This latter may appear either as first cost or installation cost. In either case it represents less money to be invested, and thought and study are required to make such investment a “minimum. Accessibility demands much of the ingenuity that the designer pos- sesses. An efficient and simple ap- paratus would be discarded, were it so inaccessible as to prevent inspec- tion, renewal and repairs. Durability, which is, of course, another way of saying “long life’, is a basic require- ment. Failure to provide this has caused the abandonment of a great number of superheater designs. The nature of the work which the super- heater has to do requires to be subjected to varying tempera- ture conditions, both from and without. Action upon the ma- terial of the superheater, from the products of combustion as well as from the steam, and, in some designs, from intermittent applications of wa- ter, has proved destructive. Conductivity Must Be Constant Superheaters are subjected to rela- tively high gas temperatures, and any change in the _ heat-transferring ability of the superheating surface will be towards a reduction in its efficiency. This, in turn, will mean an increase in the temperature of the metal, with a, consequent weakening. Tubes which are subjected to internal pressure are better able to resist defor- mation .than those subjected to exter- nal pressure. In general, it may be said that all of the widely adopted and successfully used superheaters have had one point in common, that superheater pipes are subjected to internal pressure. This has appeared to be a starting point in the minds of all designers, probably from the standpoint of safety. The modern tendency towards high degrees of superheat has, of course, meant subjecting superheating appa- ratus to higher gas temperatures. On first thought this would appear to aggravate destruction of the appa- ratus, as there are limiting tempera- tures. above which oxidization of all usable materials would take place. Experience, however, has shown the builders how far they may go without its parts’ within . harmful results, and progressive de- signers have taken full advantage ot this knowledge. It is, of course, evi- dent that, were many restrictions re- moved, different materials could be used and some advantages obtained thereby. Practical considerations have required compactness in the apparatus, which undoubtedly adds some _ cost by making it advisable to use high- priced material. Life of the Superheater From information obtained it ap- pears that a well-designed superheater, even one that is developing high de- grees of superheat, has at least a life as long as that of the boiler tubes. Many instances have come to the attention of the writer where the life of some of these superheaters is above eight years, and in some cases 12 years, without repairs of any extent. It would seem not at all unreasonable FIG. ‘3—TUBULAR. SUPERHEATER to expect superheater equipment to have a life practically as great as the boiler itself. If this is obtainable, many of the objections to superheat- ing should be removed. In. many cases, even if the life of the equip- ment were less than eight years, but where fuel economies of 10. per cent and upward were obtained, there would still be considerable on the credit side of the account. Superheaters may be broadly classi- fied on the basis of their location as follows: Separately fired superheaters, those subjected to gases coming from a source other than the furnaces of the main boilers. Waste-gas superheaters, those sub- jected to gases on their way from the boiler to the stack. Live-gas superheaters, those sub- jected to gases which have not left the main-boiler evaporating surface. MARINE REVIEW June, 191€ The superheaters in each of the three classes above mentioned may be and have been, constructed on one of the two. structural methods: tubu- lar or cellular. By tubular is meant a construction which requires the steam to pass through tubes for a greater part, or all, of its path dur-— ing which heat is added. By cellular is meant a construction which requires the steam to pass through a chamber, usually of irregular shape and to receive heat from gas flowing through tubes which pass through the steam chamber. In other words, with the tubu- lar construction steam is carried inside the tube, which is subject to bursting pressure, and with cellular construc- tion, gas is carried inside the tube, which is subject to collapsing pressure. ‘ Schmidt, about 1896, built a num- ber of tubular superheaters, and Wat- kinson, a few years later, constructed several different superheaters of this. general type. Fig. 1 illustrates Wat- kinson’s tubular design. Separately Fired Superheaters Separately fired superheaters have their advocates and opponents. It is not difficult with’ such construction to divide the superheater into parts. This makes possible an “initial” super- heater with one or more “intermedi- ate” superheaters. These intermedi- ate superheaters may be termed “re- heaters’, and there are certain ad- vantages in superheating steam _ be- tween the different cylinders. On the other hand, such a construction necessitates a very much longer path for the steam, which may result in greater losses in pressure than would be the case in a single high-degree. initial superheater. Separately fired superheaters, on shipboard, require space which can generally be other- wise used to advantage. Then, too, there are heat losses from this type of superheater, as it cannot use all the heat generated in its furnace. Fig. 3 shows another form of sep- arately fired tubular superheater, in general resembling a torpedo-boat boiler. This was installed in a num- ber of ships, but the writer has been unable to find the date on which this design first appeared. There can be no doubt as to this construction developing comparatively high degrees of superheat. Waste-gas superheaters have ap- peared in a large variety of forms. Perhaps the two most commonly known in connection with fire-tube boilers are the Watkinson and the Foster. The latter is a U-shaped pipe with separate saturated and su- perheated headers. It has been modi- fied in its detail by the introduction