Part 2 (1/2)
[Illustration: Fig 2]
But what is the object of facilitating the circulation of water in boilers? Why may we not safely leave this to the unassisted action of nature as we do in culinary operations? We may, if we do not care for the three most important aims in steam-boiler construction, namely, efficiency, durability, and safety, each of which is more or less dependent upon a proper circulation of the water As for efficiency, we have seen one proof in our kettle When we provided means to preserve the circulation, we found that we could carry a hotter fire and boil away the water much more rapidly than before It is the same in a stea but the unassisted circulation, the rising steam carried away so much water in the form of foam that the kettle boiled over, but when the currents were separated and an unier supply of steam was delivered in a comparatively dry state
Thus, circulation increases the efficiency in tays: it adds to the ability to take up the heat, and decreases the liability to waste that heat by what is technically known as pri There is yet another way in which, incidentally, circulation increases efficiency of surface, and that is by preventing in a greater or less degree the formation of deposits thereon Most waters contain some impurity which, when the water is evaporated, remains to incrust the surface of the vessel This incrustation becomes very serious sometimes, so much so as to almost entirely prevent the transmission of heat from the metal to the water
It is said that an incrustation of only one-eighth inch will cause a loss of 25 per cent in efficiency, and this is probably within the truth in many cases Circulation of water will not prevent incrustation altogether, but it lessens the amount in all waters, and alreatly to the efficiency of the surface
[Illustration: Fig 3]
A second advantage to be obtained through circulation is durability of the boiler This it securesall parts at a nearly uniforreatest freedom from unequal strains in a boiler is to provide for such a circulation of the water as will insure the same temperature in all parts
3rd Safety follows in the wake of durability, because a boiler which is not subject to unequal strains of expansion and contraction is not only less liable to ordinary repairs, but also to rupture and disastrous explosion By far the most prolific cause of explosions is this sa 4]
[Illustration: 386 Horse-power Installation of Babcock & Wilcox Boilers at B F Keith's Theatre, Boston, Mass]
Having thus briefly looked at the advantages of circulation of water in stea it under the most efficient conditions We have seen in our kettle that one essential point was that the currents should be kept fro with each other If we could look into an ordinary return tubular boiler when stea, we should see a curious co hither and thither, and shi+fting continually as one or the other contending force gained a momentary mastery The principal upward currents would be found at the two ends, one over the fire and the other over the first foot or so of the tubes Between these, the doard currents struggle against the rising currents of stea of the safety valve, the pressure being slightly reduced, the water ju lifted by the sudden generation of steahout the body of water You have seen the effect of this sudden generation of steam in the well-known experiment with a Florence flask, to which a cold application iswater under pressure is within You have also witnessed the geyser-like action ater is boiled in a test tube held vertically over a la 5]
If noe take a U-tube depending fro a circulation is at once set up within it, and no such spasmodic action can be produced Thus U-tube is the representative of the true method of circulation within a water-tube boiler properly constructed We can, for the purpose of securingincline (Fig 5), e have the well-known inclined-tube generator Now, by adding other tubes, we6), while it will still be the U-tube in effect and action In such a construction the circulation is a function of the difference in density of the two columns Its velocity is h){}, or, approxihter fluid This velocity will increase until the rising coluht circulated will attain a led stea colu column which is nearly coincident with the condition of half steaht co 6]
It becoiven boiler built on this principle, provided the construction is such as to permit a free flow of the water Of course, every bend detracts a little and so up the velocity, but when the boiler is well arranged and proportioned these retardations are slight
Let us take for example one of the 240 horse-power Babcock & Wilcox boilers here in the University The height of the colu from the surface of the water to about the center of the bundle of tubes over the fire, and the head would be equal to this height at the maximum of circulation We should, therefore, have a velocity of 8(4){} = 1697, say 17 feet per second There are in this boiler fourteen sections, each having a 4-inch tube opening into the drum, the area of which (inside) is 11 square inches, the fourteen aggregating 154 square inches, or 107 square feet This ives 1816 cubic feet ed per second, one-half of which, or 908 cubic feet, is steae pressure, it eigh 0258 pound per cubic foot Hence, 234 pounds of steaed per second, and 8,433 pounds per hour Dividing this by 30, the nuet 2811 horse power, about 17 per cent, in excess of the rated power of the boiler The water at the tehs 56 pounds per cubic foot, and the steam 0258 pound, so that the steaht, and consequently each particle of water willat this capacity, and circulating the h the tubes
[Illustration: 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]
[Illustration: Fig 7]
It is evident that at the highest possible velocity of exit fro 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 stea this would occur with the boiler under consideration We shall have a coluh on one side and an equal colu, as before, the steam at 100 pounds and the water at same temperature, ill have a head of 866 feet of stea velocity of 2355 feet per second Thisby 3,600 seconds in an hour, and by 0258 gives 234,043 pounds of stealed steaives 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 stea evaporated Also that orked 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
[Illustration: 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 alloard currents therein, the whole effect of the rising colu 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 s current of led steam and water It is also necessary that this uptake should be practically direct, and it should not be coements and 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 forular uptake, whereby the steam is expected to rise to a reservoir above You will notice (Fig 8) that the upward current of steaonizes 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
[Illustration: 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]
[Illustration: Fig 9]
In a well-known boiler, many of which were sold, but of which none are now made and a very few are still in use, the inventor claiainst the tubes were for the purpose of ”restricting the circulation” and no doubt they perfor for the ss they were not as efficient for that purpose as the arrange 10]