A Catechism of the Steam Engine Part 12

_A._--There cannot be any _different_ measure, but there are several equivalent measures. Thus the evaporation of a cubic foot of water in the hour, or the expenditure of 33 cubic feet of low pressure steam per minute, is reckoned equivalent to an actual horse power, or 528 cubic feet of water raised one foot high in the minute involves the same result.

[1] Tables of the horse power of both high and low pressure engines are given in the Key.

[2] Example.--What is the power of an engine of 42 inches diameter, 3-1/2 feet stroke, and making 85 strokes per minute? The speed of the piston will be 7 (the length of a double stroke) x 85 = 595 feet per minute. Now 42 x 42 = 1,764 x 595 = 1,049,580 / 6,000 = 175 horses power.

DUTY OF ENGINES AND BOILERS.

226. _Q._--What is meant by the duty of a engine?

_A._--The work done in relation to the fuel consumed.

227. _Q._--And how is the duty ascertained?

_A._--In ordinary mill or marine engines it can only be ascertained by the indicator, as the load upon such engines is variable, and cannot readily be determined; but in the case of engines pumping water, where the load is constant, the number of strokes performed by the engine will represent the work done, and the amount of work done by a given quant.i.ty of coal

represents the duty. In Cornwall the duty of an engine is expressed by the number of millions of pounds raised one foot high by a bushel, or 94 lbs.

of Welsh coal. A bushel of Newcastle coal will only weigh 84 Lbs.; and in comparing the duty of a Cornish engine with the performance of an engine in some locality where a different kind of coal is used, it is necessary to pay regard to such variations.

228. _Q._--Can you tell the duty of an engine when you know its consumption of coal per horse power per hour?

_A._--Yes, if the power given be the actual, and not the nominal, power.

Divide 166.32 by the number of pounds of coal consumed per actual horse power per hour; the quotient is the duty in millions of pounds. If you already have the duty in millions of pounds, and wish to know the equivalent consumption in pounds per actual horse power per hour, divide 166.32 by the duty in millions of pounds; the quotient is the consumption per actual horse power per hour. The duty of a locomotive engine is expressed by the weight of c.o.ke it consumes in transporting a ton through the distance of one mile upon a railway; but this is a very imperfect method of representing the duty, as the tractive efficacy of a pound of c.o.ke becomes less as the speed of the locomotive becomes greater; and the law of variation is not accurately known.

229. _Q._--What amount of power is generated in good engines of the ordinary kind by a given weight of coal?

_A._--The duty of different kinds of engines varies very much, and there are also great differences in the performance of different engines of the same cla.s.s. In ordinary rotative condensing engines of good construction, 10 lbs. of coal per nominal horse power per hour is a common consumption; but such engines exert nearly twice their nominal power, so that the consumption per actual horse power per hour may be taken at from 5 to 6 lbs. Engines working very expansively, however, attain an economy much superior to this. The average duty of the pumping engines in Cornwall is about 60,000,000 lbs. raised 1 ft. high by a bushel of Welsh coals, which weighs 94 lbs. This is equivalent to a consumption of 3.1 lbs. of coal per actual horse power per hour; but some engines reach a duty of above 100,000,000, or 1.74 lbs. of coal per actual horse power per hour.

Locomotives consume from 8 to 10 lbs. of c.o.ke in evaporating a cubic foot of water, and the evaporation of a cubic foot of water per hour may be set down as representing an actual horse power in locomotives as well as in condensing engines, if expansion be not employed. When the locomotive is worked expansively, however, there is of course a less consumption of water and fuel per horse power, or per ton per mile, than when the full pressure is used throughout the stroke; and most locomotives now operate with as much expansion as can be conveniently given by the slide valves.

230. _Q._--But is not the evaporative power of locomotives affected materially by the proportions of the boiler?

_A._--Yes, but this may be said of all boilers; but in locomotive boilers, perhaps, the effect of any misproportion becomes more speedily manifest. A high temperature of the fire box is found to be conducive to economy of fuel; and this condition, in its turn, involves a small area of grate bars.

The heating surface of locomotive boilers should be about 80 square feet for each square foot of grate bars, and upon each foot of grate bars about 1 cwt. of c.o.ke should be burnt in the hour.

231. _Q._--Probably the heat is more rapidly absorbed when the temperature of the furnace is high?

_A._--That seems to be the explanation. The rapidity with which a hot body imparts heat to a colder, varies as the square of the difference of temperature; so that if the temperature of the furnace be very high, the larger part of the heat pa.s.ses into the water at the furnace, thereby leaving little to be transmitted by the tubes. If, on the contrary, the temperature of the furnace be low, a large part of the heat will pa.s.s into the tubes, and more tube surface will be required to absorb it. About 16 cubic feet of water should be evaporated by a locomotive boiler for each, square foot of fire grate, which, with the proportion of heating surface already mentioned, leaves 5 square feet of heating surface to evaporate a cubic foot of water in the hour. This is only about half the amount of surface usual in land and marine boilers per cubic foot evaporated, and its small amount is due altogether to the high temperature of the furnace, which, by the rapidity of transmission it causes, is tantamount to an additional amount of heating surface.

232. _Q._--You have stated that the steam and vacuum gauges are generally gla.s.s tubes, up which mercury is forced by the steam or sucked by the vacuum?

_A._--Vacuum gauges are very often of this construction, but steam gauges more frequently consist of a small iron tube, bent like the letter U, and into which mercury is poured. The one end of this tube communicates with the boiler, and the other end with the atmosphere; and when the pressure of the steam rises in the boiler, the mercury is forced down in the leg communicating with the boiler and rises in the other leg, and the difference of level in the legs denotes the pressure of the steam. In this gauge a rise of the mercury one inch in the one leg involves a difference of the level between the two legs of two inches, and an inch of rise is, therefore, equivalent to two inches of mercury, or a pound of pressure. A small float of wood is placed in the open leg to show the rise or fall of the mercury, and this leg is surmounted by a bra.s.s scale, graduated in inches, to the marks of which the float points.

233. _Q._--What other kinds of steam and vacuum gauges are there?

_A._--There are many other kinds; but probably Bourdon's gauges are now in more extended use than, any other, and their operation has been found to be satisfactory in practice. The principle of their action may be explained to be, that a thin elliptical metal tube, if bent into a ring, will seek to coil or uncoil itself if subjected to external or internal pressure, and to an extent proportional to the pressure applied. The end of the tube is sharpened into an index, and moves to an extent corresponding to the pressure applied to the tube; but in the more recent forms of this apparatus, a dial and a hand, like those of a clock, are employed, and the hand is moved round by a toothed sector connected to the tube, and which sector acts on a pinion attached to the hand. Mr. Shank, of Paisley, has lately introduced a form of steam gauge like a thermometer, with a flattened bulb; and the pressure of the steam, by compressing the bulb, causes the mercury to rise to a point proportional to the pressure applied.

THE INDICATOR.

234. _Q._--You have already stated that the actual power of an engine is ascertained by an instrument called the indicator, which consists of a small cylinder with a piston moving against a spring, and compressing it to an extent answerable to the pressure of the steam. Will you explain further the structure and mode of using that instrument?

[Ill.u.s.tration: Fig. 36]

_A._--The structure of the common form of indicator will be most readily apprehended by a reference to fig. 36, which is a McNaught's indicator.

Upon a movable barrel A, a piece of paper is wound, the ends of which are secured by the slight bra.s.s clamps shown in the drawing. The barrel is supported by the bracket _b_, proceeding from the body of the indicator, and at the bottom of the barrel a watch spring is coiled with one end attached to the barrel and the other end to the bracket, so that when the barrel is drawn round by a string wound upon its lower end like a roller blind, the spring returns the barrel to its original position, when the string is relaxed. The string is attached to some suitable part of the engine, and at every stroke the string is drawn out, turning round the barrel, and the barrel is returned again by the spring on the return stroke.

235. _Q_--But in what way can these reciprocations of the barrel determine the power of the engine?

_A._--They do not determine it of themselves, but are only part of the operation. In the inside of the cylinder _c_ there is a small piston moving steam tight in a cylinder of which _d_ is the piston rod, and _e_ a spiral spring of steel, which the piston, when forced upwards by the steam or sucked downwards by the vacuum, either compresses or extends; _f_ is a c.o.c.k attached to the cylinder of the indicator, and which is screwed into the cylinder cover. It is obvious that, so soon as this c.o.c.k is opened, the piston will be forced up when the s.p.a.ce above the piston of the engine is opened to the boiler, and sucked down when that s.p.a.ce is opened to the condenser--in each case to an extent proportionate to the pressure of the steam or the perfection of the vacuum, the top of the piston _c_ being open to the atmosphere. A pencil, _p_, with a knife hinge, is inserted into the piston rod, at _e_, and the point of the pencil bears upon the surface of the paper wound upon the drum A. If the drum A did not revolve, this pencil would merely trace on the paper a vertical line; but as the drum A moves round and back again every stroke of the engine, and as the pencil moves up and down again every stroke of the engine, the combined movements trace upon the paper a species of rectangle, which is called an indicator diagram; and the nature of this diagram determines the nature of the engine's performance.

236. _Q._--How does it do this?

_A._--It is clear that if the pencil was moved up instantaneously to the top of its stroke, and was also moved down instantaneously to the bottom of its stroke, and if it remained without fluctuation while at the top and bottom, the figure described by the pencil would be a perfect rectangle, of which the vertical height would represent the total pressure of the steam and vacuum, and therefore the total pressure urging the piston of the engine. But in practice the pencil will neither rise nor fall instantaneously, nor will it remain at a uniform height throughout the stroke. If the steam be worked expansively the pressure will begin to fall so soon as the steam is cut off; and at the end of the stroke, when the steam comes to be discharged, the subsidence of pressure will not be instantaneous, but will occupy an appreciable time. It is clear, therefore, that in no engine can the diagram described by an indicator be a complete rectangle; but the more nearly it approaches to a rectangle, the larger will be the power produced at every stroke with any given pressure, and the area of the s.p.a.ce included within the diagram will in every case accurately represent the power exerted by the engine during that stroke.

237. _Q._--And how is this area ascertained?

_A._--It may be ascertained in various ways; but the usual mode is to take the vertical height of the diagram at a number of equidistant points on a base line, and then to take the mean of these several heights as representative of the mean pressure actually urging the piston. Now if you have the pressure on the piston per square inch, and if you know the number of square inches in its area, and the velocity with which it moves in feet per minute, you have obviously the dynamical effort of the engine, or, in other words, its actual power.

238. _Q._--How is the base line you have referred to obtained?

_A._--In proceeding to take an indicator diagram, the first thing to be done is to allow the barrel to make two or three reciprocations with the pencil resting against it, before opening the c.o.c.k attached to the cylinder. There will thus be traced a horizontal line, which is called the _atmospheric line_, and in condensing engines, a part of the diagram will be above and a part of it below this line; whereas, in high pressure engines the whole of the diagram will be above this line. Upon this line the vertical ordinates may be set off at equal distances, or upon any base line parallel to it; but the usual course is to erect the ordinates on the atmospheric line.

239. _Q._--Will you give an example of an indicator diagram?

[Ill.u.s.tration: Fig. 37]

_A._--Fig. 37 is an indicator diagram taken from a low pressure engine, and the waving line _a b c_, forming a sort of irregular parallelogram, is that which is described by the pencil. The atmospheric line is represented by the line o o. The scale at the side shows the pressure of the steam, which in this engine rose to about 9 lbs. per square inch, and the vacuum fell to 11 lbs. The steam begins to be cut off when, about one-fourth of the stroke has been performed, and the pressure consequently falls.

240. _Q._--Is this species of indicator which you have just described applicable to locomotive engines?

_A._--It is no doubt applicable under suitable conditions; but another species of indicator has been applied by Mr. Gooch to locomotive engines, which presents several features of superiority for such a purpose.

This indicator has its cylinder placed horizontally; and its piston compresses two elliptical springs; a slide valve is subst.i.tuted for a c.o.c.k, to open or close the communication with the engine. The top of the piston rod of this indicator is connected to the short arm of a smaller lever, to the longer arm of which the pencil is attached, and the pencil has thus a considerably larger amount of motion than the piston; but it moves in the arc of a circle instead of in a straight line. The pencil marks on a web of paper, which is unwound from one drum and wound on to another, so that a succession of diagrams are taken without the necessity of any intermediate manipulation.

241. _Q._--These diagrams being taken with a pencil moving in an arc, will be of a distorted form?

_A._--They will not be of the usual form, but they may be easily translated into the usual form. It is undoubtedly preferable that the indicator should act immediately in the production of the final form of diagram.

DYNAMOMETER, GAUGES, AND CATARACT.