A Portion of 9600 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters Being Erected at the South Boston, Mass., Station of the Boston Elevated Railway Co. This Company Operates in its Various Stations a Total of 46,400 Horse Power of Babcock & Wilcox Boilers
Fig. 7
It is evident that at the highest possible velocity of exit from the generating tubes, nothing but steam will be delivered and there will be no circulation of water except to supply the place of that evaporated. Let us see at what rate of steaming this would occur with the boiler under consideration. We shall have a column of steam, say 4 feet high on one side and an equal column of water on the other. Assuming, as before, the steam at 100 pounds and the water at same temperature, we will have a head of 866 feet of steam and an issuing velocity of 235.5 feet per second. This multiplied by 1.07 square feet of opening by 3,600 seconds in an hour, and by 0.258 gives 234,043 pounds of steam, which, though only one-eighth the weight of mingled steam and water delivered at the maximum, gives us 7,801 horse power, or 32 times the rated power of the boiler. Of course, this is far beyond any possibility of attainment, so that it may be set down as certain that this boiler cannot be forced to a point where there will not be an efficient circulation of the water. By the same method of calculation it may be shown that when forced to double its rated power, a point rarely expected to be reached in practice, about two-thirds the volume of mixture of steam and water delivered into the drum will be steam, and that the water will make 110 circuits while being evaporated. Also that when worked at only about one-quarter its rated capacity, one-fifth of the volume will be steam and the water will make the rounds 870 times before it becomes steam. You will thus see that in the proportions adopted in this boiler there is provision for perfect circulation under all the possible conditions of practice.
Fig. 8 [Developed to show Circulation]
In designing boilers of this style it is necessary to guard against having the uptake at the upper end of the tubes too large, for if sufficiently large to allow downward currents therein, the whole effect of the rising column in increasing the circulation in the tubes is nullified (Fig. 7). This will readily be seen if we consider the uptake very large when the only head producing circulation in the tubes will be that due to the inclination of each tube taken by itself. This objection is only overcome when the uptake is so small as to be entirely filled with the ascending current of mingled steam and water. It is also necessary that this uptake should be practically direct, and it should not be composed of frequent enlargements and [Pg 34]
[Pg 35] contractions. Take, for instance, a boiler well known in Europe, copied and sold here under another name. It is made up of inclined tubes secured by pairs into boxes at the ends, which boxes are made to communicate with each other by return bends opposite the ends of the tubes. These boxes and return bends form an irregular uptake, whereby the steam is expected to rise to a reservoir above. You will notice (Fig. 8) that the upward current of steam and water in the return bend meets and directly antagonizes the upward current in the adjoining tube. Only one result can follow. If their velocities are equal, the momentum of both will be neutralized and all circulation stopped, or, if one be stronger, it will cause a back flow in the other by the amount of difference in force, with practically the same result.
4880 Horse-power Installation of Babcock & Wilcox Boilers at the Open Hearth Plant of the Cambria Steel Co., Johnstown, Pa. This Company Operates a Total of 52,000 Horse Power of Babcock & Wilcox Boilers