A Catechism of the Steam Engine Part 13

242. _Q._--What other gauges or instruments are there for telling the state, or regulating the power of an engine?

_A._--There is the counter for telling the number of strokes the engine makes, and the dynamometer for ascertaining the tractive power of steam vessels or locomotives; then there are the gauge c.o.c.ks, and gla.s.s tubes, or floats, for telling the height of water in the boiler; and in pumping engines there is the cataract for regulating the speed of the engine.

243. _Q._--Will you describe the mechanism of the counter?

_A._--The counter consists of a train of wheel work, so contrived that by every stroke of the engine an index hand is moved forward a certain s.p.a.ce, whereby the number of strokes made by the engine in any given time is accurately recorded. In most cases the motion is communicated by means of a detent,--attached to some reciprocating part of the engine,--to a ratchet wheel which gives motion to the other wheels in its slow revolution; but it is preferable to derive the motion from some revolving part of the engine by means of an endless screw, as where the ratchet is used the detent will sometimes fail to carry it round the proper distance. In the counter contrived by Mr. Adie, an endless screw works into the rim of two small wheels situated on the same axis, but one wheel having a tooth more than the other, whereby a differential motion is obtained; and the difference in the velocity of the two wheels, or their motion upon one another, expresses the number of strokes performed. The endless screw is attached to some revolving part of the engine, whereby a rotatory motion is imparted to it; and the wheels into which the screws work hang down from it like a pendulum, and are kept stationary by the action of gravity.

244. _Q._--What is the nature of the dynamometer?

_A._--The dynamometer employed for ascertaining the traction upon railways consists of two flat springs joined together at the ends by links, and the amount of separation of the springs at the centre indicates, by means of a suitable hand and dial, the force of traction. A cylinder of oil, with a small hole through its piston, is sometimes added to this instrument to prevent sudden fluctuations. In screw vessels the forward thrust of the screw is measured by a dynamometer constructed on the principle of a weighing machine, in which a small spring pressure at the index will balance a very great pressure where the thrust is employed; and in each case the variations of pressure are recorded by a pencil on a sheet of paper, carried forward by suitable mechanism, whereby the mean thrust is easily ascertained. The tractive force of paddle wheel steamers is ascertained by a dynamometer fixed on sh.o.r.e, to which the floating vessel is attached by a rope. Sometimes the power of an engine is ascertained by a friction break dynamometer applied to the shaft.

345. _Q._--What will determine the amount of thrust shown by the dynamometer?

_A._--In locomotives and in paddle steamers it will be determined by the force turning the wheels, and by the smallness of the diameter of the wheels; for with small wheels the thrust will be greater than with large wheels. In screw vessels the thrust will be determined by the force turning round the screw, and by the smallness of the screw's pitch; for with any given force of torsion a fine pitch of screw will give a greater thrust than a coa.r.s.e pitch of screw, just as is the case when a screw works in a solid nut.

246. _Q._--Will you explain the use of the gla.s.s gauges affixed to the boiler?

_A._--The gla.s.s gauges are tubes affixed to the fronts of boilers, by the aid of which the height of the water within the boilers is readily ascertainable, for the water will stand at the same height in the tube as in the boiler, with which there is a communication maintained both at the top and bottom of the tube by suitable stopc.o.c.ks. The c.o.c.ks connecting the gla.s.s tube with the boiler should always be so constructed that the tube may be blown through with the steam, to clear it of any internal concretion that may impair its transparency; and the construction of the sockets in which the tube is inserted should be such, that, even when there is steam in the boiler, a broken tube may be replaced with facility.

247. _Q._--What then are the gauge c.o.c.ks?

_A._--The gauge c.o.c.ks are c.o.c.ks penetrating the boiler at different heights, and which, when opened, tell whether it is water or steam that exists at the level at which they are respectively inserted. It is unsafe to trust to the gla.s.s gauges altogether as a means of ascertaining the water level, as sometimes they become choked, and it is necessary, therefore, to have gauge c.o.c.ks in addition; but if the boiler be short of steam, and a partial vacuum be produced within it, the gla.s.s gauges become of essential service, as the gauge c.o.c.ks will not operate in such a case, for though opened, instead of steam and water escaping from them, the air will rush into the boiler. It is expedient to carry a pipe from the lower end of the gla.s.s tube downward into the water of the boiler, and a pipe from the upper end upward into the steam in the boiler, so as to prevent the water from boiling down through the tube, as it might otherwise do, and prevent the level of the water from being ascertainable. The average level of water in the boiler should be above the centre of the tube; and the lowest of the gauge c.o.c.ks should always run water, and the highest should always blow steam.

248. _Q._--Is not a float sometimes employed to indicate the level of the water in the boiler?

_A._--A float for telling the height of water in the boiler is employed only in the case of land boilers, and its action is like that of a buoy floating on the surface, which, by means of a light rod pa.s.sing vertically through the boiler, shows at what height the water stands. The float is usually formed of stone or iron, and is so counterbalanced as to make its operation the same as if it were a buoy of timber; and it is generally put in connection with the feed valve, so that in proportion as the float rises, the supply of feed water is diminished. The feed water in land boilers is admitted from a small open cistern, situated at the top of an upright or stand pipe set upon the boiler, and in which there is a column of water sufficiently high to balance the pressure of the steam.

249. _Q._--What is the cataract which is employed to regulate the speed of pumping engines?

[Ill.u.s.tration: Fig. 38. ]

_A._--The cataract consists of a small pump-plunger _b_ and barrel, set in a cistern of water, the barrel being furnished on the one side with a valve, _c_, opening inwards, through which the water obtains admission to the pump chamber from the cistern, and on the other by a plug, _d_, through which, if the plunger be forced down, the water must pa.s.s out of the pump chamber. The engine in the upward stroke of the piston, which is accomplished by the preponderance of weight at the pump end of the beam, raises up the plunger of the cataract by means of a small rod,--the water entering readily through the valve already referred to; and when the engine reaches the top of the stroke, it liberates the rod by which the plunger has been drawn up, and the plunger then descends by gravity, forcing out the water through the c.o.c.k, the orifice of which has previously been adjusted, and the plunger in its descent opens the injection valve, which causes the engine to make a stroke.

250. _Q._--Suppose the c.o.c.k of the cataract be shut?

_A._--If the c.o.c.k of the cataract be shut, it is clear that the plunger cannot descend at all, and as in that case the injection valve cannot be opened, the engine must stand still; but if the c.o.c.k be slightly opened, the plunger will descend slowly, the injection valve will slowly open, and the engine will make a gradual stroke as it obtains the water necessary for condensation. The extent to which the c.o.c.k is open, therefore, will regulate the speed with which the engine works; so that, by the use of the cataract, the speed of the engine may be varied to suit the variations in the quant.i.ty of water requiring to be lifted from the mine. In some cases an air cylinder, and in other cases an oil cylinder, is employed instead of the apparatus just described; but the principle on which the whole of these contrivances operate is identical, and the only difference is in the detail.

251. _Q._--You have now shown that the performance of an engine is determinable by the indicator; but how do you determine the power of the boiler?

_A._--By the quant.i.ty of water it evaporates. There is, however, no very convenient instrument for determining the quant.i.ty of water supplied to a boiler, and the consequence is that this element is seldom ascertained.

CHAPTER V.

PROPORTION OF BOILERS.

HEATING AND FIRE GRATE SURFACE.

252. _Q._--What are the considerations which must chiefly be attended to in settling the proportions of boilers?

_A._--In the first place there must be sufficient grate surface to enable the quant.i.ty of coal requisite for the production of the steam to be conveniently burnt, taking into account the intensity of the draught; and in the next place there must be a sufficient flue surface readily to absorb the heat thus produced, so that there may be no needless waste of heat by the chimney. The flues, moreover, must have such an area, and the chimney must be of such dimensions, as will enable a suitable draught through the fire to be maintained; and finally the boiler must be made capable of containing such supplies of water and steam as will obviate inconvenient fluctuations in the water level, and abate the risk of water being carried over into the engine with the steam. With all these conditions the boiler must be as light and compact as possible, and must be so contrived as to be capable of being cleaned and repaired with facility.

253. _Q._--Supposing, then, that you had to proportion a boiler, which should be capable of supplying steam sufficient to propel a steam vessel or railway train at a given speed, or to perform any other given work, how would you proceed?

_A._--I would first ascertain the resistance which had to be overcome, and the velocity with which it was necessary to overcome it. I should then be in a position to know what pressure and volume of steam were required to overcome the resistance at the prescribed rate of motion; and, finally, I should allow a sufficient heating and fire grate surface in the boiler according to the kind of boiler it was, to furnish the requisite quant.i.ty of steam, or, in other words, to evaporate the requisite quant.i.ty of water.

254. _Q._--will you state the amount of heating surface and grate surface necessary to evaporate a given quant.i.ty of water?

_A._--The number of square feet of heating or flue surface, required to evaporate a cubic foot of water per hour, is about 70 square feet in Cornish boilers, 8 to 11 square feet in land and marine boilers, and 5 or 6 square feet in locomotive boilers. The number of square feet of heating surface per square foot of fire grate, is from 13 to 15 square feet in wagon boilers; about 40 square feet in Cornish boilers; and from 50 to 90 square feet in locomotive boilers. About 80 square feet in locomotives is a very good proportion.

255. _Q._--What is the heating surface of boilers per horse power?

_A._--About 9 square feet of flue and furnace surface per horse power is the usual proportion in wagon boilers, reckoning the total surface as effective surface, if the boilers be of a considerable size; but in the case of small boilers the proportion is larger. The total heating surface of a two horse power wagon boiler is, according to Boulton and Watt's proportions, 30 square feet, or 15 ft. per horse power; whereas, in the case of a 45 horse power boiler the total heating surface is 438 square feet, or 9.6 ft. per horse power. In marine boilers nearly the same proportions obtain. The original boilers of the Great Western steamer, by Messrs. Maudslay, were proportioned with about 10 square feet of flue and furnace surface per horse power, reckoning the total amount as effective; but in the boilers of the Retribution, by the same makers, but of larger size, a somewhat smaller proportion of heating surface was adopted. Boulton and Watt have found that in their marine flue boilers, 9 square feet of flue and furnace surface are requisite to boil off a cubic foot of water per hour, which is the proportion of heating surface that is allowed in their land boilers per horse power; but inasmuch as in most modern engines, and especially in marine engines, the nominal considerably exceeds the actual power, they allow 11 or 12 square feet of heating surface per nominal horse power in their marine boilers, and they reckon as effective heating surface the tops of the flues, and the whole of the sides of the flues, but hot the bottoms. For their land engines they still retain Mr.

Watt's standard of power, which makes the actual and the nominal power identical; and an actual horse power is the equivalent of a cubic foot of water raised into steam every hour.

256. _Q._--What is the proper proportion of fire grate per horse power?

_A._--Boulton and Watt allow 0.64 of a square foot area of grate bars per nominal horse power in their marine boilers, and a good effect arises from this proportion; but sometimes so large an area of fire grate cannot be conveniently got, and the proportion of half a square foot per horse power, which is the proportion adopted in the original boiler of the Great Western, seems to answer very well in engines working with a moderate pressure, and with some expansion; and this proportion is now very widely adopted. With this allowance, there will be 22 to 24 square feet of heating surface per square foot of fire grate; and if the consumption of fuel be taken at 6 lbs. per nominal horse power per hour, there will be about 12 lbs. of coal consumed per hour on each square foot of grate. The furnaces should not be more than 6 ft. long, as, if much longer than this, it will be impossible to work them properly for any considerable length of time, as they will become choked with clinker at the back ends.

257. _Q._--What quant.i.ty of fuel is usually consumed per hour on each square foot of fire grate?

_A._--The quant.i.ty of fuel burned on each square foot of fire grate per hour, varies very much in different boilers; in wagon boilers it is from 10 to 13 lbs.; in Cornish boilers from 3-1/2 to 4 lbs.; and in locomotive boilers from 80 to 150 lbs.; but about 1 cwt. per hour is a good proportion in locomotives, as has been already explained.

CALORIMETER AND VENT.

258. _Q._--In what manner are the proper sectional area and the proper capacity of the flue of a boiler determined?

_A._--The proper collective area for the escape of the smoke and flame over the furnace bridges in marine boilers is 19 square inches per nominal horse power, according to Boulton and Watt's practice, and for the sectional area of the flue they allow 18 square inches per horse power. The sectional area of the flue in square inches is what is termed the _calorimeter_ of the boiler, and the calorimeter divided by the length of the flue in feet is what is termed the _vent_. In marine flue boilers of good construction the vent varies between the limits of 20 and 25, according to the size of the boiler and other circ.u.mstances--the largest boilers having generally the largest vents; and the calorimeter divided by the vent will give the length of the flue in feet. The flues of all flue boilers diminish in their calorimeter as they approach the chimney, as the smoke contracts in its volume in proportion as it parts with its heat.

259. _Q._--Is the method of determining the dimensions of a boiler flue, by a reference to its vent and calorimeter, the method generally pursued?

_A._--It is Boulton and Watt's method; but some very satisfactory boilers have been made by allowing a proportion of 0.6 of a square foot of fire grate per nominal horse power, and making the sectional area of the flue at the largest part 1/7th of the area of fire grate, and at the smallest part, where it enters the chimney, 1/11th of the area of the fire grate. These proportions are retained whether the boiler is flue or tubular, and from 14 to 16 square feet of tube surface is allowed per nominal horse power.

260. _Q._--Are the proportions of vent and calorimeter, taken by Boulton and Watt for marine flue boilers, applicable also to wagon and tubular boilers?

_A._--No. In wagon and tubular boilers very different proportions prevail, yet the proportions of every kind of boiler are determinable on the same general principle. In wagon boilers the proportion of the perimeter of the flue which is effective as heating surface, is to the total perimeter as 1 to 3, or, in some cases as 1 to 2.5; and with any given area of flue, therefore, the length of the flue must be from 3 to 2.5 times greater than would be necessary if the total surface were effective, else the requisite quant.i.ty of heating surface will not be obtained. If, then, the vent be the calorimeter, divided by the length, and the length be made 3 or 2.5 times greater, the vent must become 3 or 2.5 times less; and in wagon boilers accordingly, the vent varies from 8 to 11 instead of from 21 to 25, as in the case of marine flue boilers. In tubular marine boilers the calorimeter is usually made only about half the amount allowed by Boulton and Watt for marine flue boilers, or, in other words, the collective sectional area of the tubes, for the transmission of the smoke, is from 8 to 9 square inches per nominal horse power. It is better, however, to make the sectional area larger than this, and to work the boiler with the damper sufficiently closed to prevent the smoke and flame from rus.h.i.+ng exclusively through a few of the tubes.

261. _Q._--What are the ordinary dimensions of the flue in wagon boilers?

_A._--In Boulton and Watt's 45 horse wagon boiler the area of flue is 18 square inches per horse power, but the area per horse power increases very rapidly as the size of the boiler becomes less, and amounts to about 80 square inches per horse power in a boiler of 2 horse power. Some such increase is obviously inevitable, if a similar form of flue be retained in the larger and smaller powers, and at the same time the elongation of the flue in the same proportion as the increase of any other dimension is prevented; but in the smaller cla.s.s of wagon boilers the consideration of facility of cleaning the flues is also operative in inducing a large proportion of sectional area. Boulton and Watt's 2 horse power wagon boiler has 30 square feet of surface, and the flue is 18 inches high above the level of the boiler bottom, by 9 inches wide; while their 12 horse wagon boiler has 118 square feet of heating surface, and the dimensions of the flue similarly measured are 36 inches by 13 inches. The width of the smaller flue, if similarly proportioned to the larger one, would be 6-1/2 inches, instead of 9 inches, and, by a.s.suming this dimension, we should have the same proportion of sectional area per square foot of heating surface in both boilers. The length of flue in the 2 horse boiler is 19.5 ft., and in the 12 horse boiler 39 ft., so that the length and height of the flue are increased in the same proportion.

262. _Q._--Will you give an example of the proportions of a flue, in the case of a marine boiler?