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16 _tube boiler history on the Great Lakes ;and a somewhat longer period as ‘to ~scotch, but the number of examples scited is of course disproportionate in .view of the very great preponderance sof the latter. : That the water-tube boiler has ad- :vantages of its own is freely conceded ‘by all, but for the present pur- ‘pose we have to consider only the ‘economic features since several of the -advantages referred to assume rela- ‘tively little importance in the trade ‘under consideration The principal _feature which retains its value is ability to raise steam quickly from a cold condition, but even this, valu- ‘able as it is, entails some sacrifices. ‘It seems fair to say that the time ‘required to raise steam in a_ scotch ‘boiler from a cold condition is easily ‘overstressed so far as operation is “concerned. Practically invariably other ; boilers are under steam and the dead boiler may easily be at steam tem- perature by the time the _ heating surfaces are submerged. Water Tube Boiler Advantages Quick steaming, while partly due to the comparative freedom from in- ternal stresses and which permits of forcing fires from the start, is even more due to the small volumetric ca- pacity per unit of heating surface, or in other words the relatively small amount of water contained and the restricted volume of steam room which contributes to a rapid rise in pressure when once evaporation has begun. The penalty for this lies in the smali heat reserve due to the limited vol- ume of water and the relatively small weight of steam contained in the steam spaces. The great heat reserves stored up in the larger water content of the scotch boiler enable it to shoulder and carry load and fire fluctuations ‘without - noticeable. pressure.. drop on the one hand or blowing safety valves on the other. So well is the value of such heat reserves established Tarale 1. Comparative evaporation 0 Oo on g M 4 Ash ry ” * Combustible, " B.T.v. per lo. coal, ! 2 3 4 5 6 a 8 Numiset of bests averaged. Molsture incoal, percent, ra “ ” “ Yr MARINE REVIEW that the equipment of many shore plants includes what is known as a steam accumulator, which is merely a large cylindrical vessel about two- thirds filled with water and connected to the boilers and steam lines. At times when the boiler capacity is in excess of demand the surplus is ab- sorbed by the accumulator through its water content and when the de- mand exceeds boiler output at any time the surplus is restored to the lines. The boilers therefore are en- abled to work at a more nearly con- stant and economical rate and with a lower investment cost. Some of these accumulators reach very large dimensions, up to 7000 cubic feet or more, but their use is of course not feasible aboard ship. A flat pressure curve is of peculiar advantage in marine work in the maintenance of uniform speed and where pressures fluctuate the loss is easily perceptible in revolutions. It is believed that these statements are in accord with general observa- tion and are not controversial. Coming to the subject of compara- tive economy it must be admitted that data as to actual evaporation are far from satisfactory. There does not seem to be any good reason why, under parallel conditions as to working, fuel, air supply, draft and stack temperatures, etc., there should be any noticeable difference between the two types, but this parallelism practically never exists. Figures there are in abundance; there is no lack of data as to fuel weights which may be assumed to be fairly accurate, nor as to feed water weights which in most cases are either rough estimates based on pump displacements or are derived from meter measurements which are unreliable at best. In very few in- stances has feed water been actually weighed but these are hardly numerous ‘enough: to establish a comparative rate for such an extensive field. Data as to fuel values are also almost en- tirely lacking so that the prime es- Factor of evapotarion, Water per lo. dry coal, lbs. « £Qa:212°F. Efficiency on dry coal, percent, Firing Drake Stack rempetarure, deg.F Dratt " Induced} Positive | Natural: Unheated [Unheoted a + February, 1927 sentials for determination of boiler efficiency are sadly incomplete. Trial data from scores of ships are at hand but in the main so far as fuel > economy goes they are resolved into fuel per indicated horsepower and, in general, for all purposes. The situation therefore stands thus; we can (1) make comparisons as to evaporative efficiency between the few examples where feed water weights and fuel values are accurately known, and (2) we can take all available data as to fuel per indicated horse- power where such are reliable, and attempt to draw conclusions there- from. The latter group is quite nu- merous. In many instances _ fuel values are given but they are incom- plete and they might be assumed to average out. At all events the fuel quality is not under control and the figures must be accepted as the every- day work whether under best condi- tions or not. The first group is presented in Table I below and the second in Table II on page 58. Water Rates Inaccurate It may be said here that it is difficult to understand how some of the apparent duties based upon meter readings receive the consideration they do. Instances are found where the apparent evaporation is doubtful if not impossible and it is doubtless equally true that in other instances the apparent rate is too low. A fast meter gives a high water rate both for evaporation and for steam consumption at the engines while with a slow meter the exact reverse ob- tains. Furthermore, even meters spe- cially built for specified temperatures, velocities and pressures, and a def- inite error rate apparently ascertained upon calibration, do not function uni- formly. Some outstanding examples of this have occurred where the hourly rate varied as much as 30 per cent without any change whatever in any working condition. It is easy to see what marvelous efficiencies or sad de- ficiencies may result under such cir- cumstances. The lesson to be drawn is that data as to water consumption or evaporation based on other than actual weight are practically worth- less and ought to be eliminated. Any- one receiving such figures with confidence merely deceives himself. There are no difficulties of moment in the way of such determination; it is not even necessary to use a scale. Portable tanks of suitable pro- . portions are easily made and _ in- stalled; even calibrated barrels have served successfully. It is only neces- sary to establish the weight of the (Continued on Page 58)