Page images
PDF
EPUB

gines, much in use in paddle steamers, the beam ited and unvarying pressure of the atmosphere, being placed below the level of the cylinder introduced a source of power susceptible of cover, and worked by a rod or rods descending indefinite increase, and restricted only by conto it from the cross head of the piston rod; B, siderations of safety and of practical advantage. direct-acting engines, in which the piston rod The question, within what limits of steam acts directly upon the crank; as 1, direct- pressure the maximum of advantage is to be acting horizontal engines, stationary; 2, direct- attained, is however one involving so many acting vertical engines, stationary or marine, variable quantities, dependent on the construcin some forms known as “steeple” engines; tion of the engine and the conditions under 3, oscillating engines. Rotatory, disk, and cer- which it is worked, that no general determinatain other peculiar forms of engine are also tion of this problem has been obtained; and direct-acting. Among the advantages of di- the desired information must be arrived at, if rect-acting over beam engines are the saving of at all, by repeated'experiments for each sort space, of liability of damage to the cylinder from of engine and each set of conditions under breaking down and fall of the beam, and of some which it may be worked. The tendency of useless load and friction; the parts transmit- such experiments has been to show a gai ting the power in direct-acting being generally from the use of comparatively high pressures; less than in beam engines, in the ratio of 2 : 5. so that, in condensing engines, the working The Corliss steam engine company, of Provi- pressure (in excess over one atmosphere) per dence, construct a non-condensing horizontal square inch on the piston has been carried up engine, which is worked by Corliss's peculiar from about 8 or 12 to an average of 25 or 30 lbs. valve gear. These valves are segmental in form, Steam, as commonly employed, is drawn directand in their movement rotative-reciprocating. ly from the boiler, and generally in the boiler The steam passages, four in number, are re- is saturated, or at maximum density. So exduced to the shortest practicable length, and isting, the slightest fall of the temperature, each is controlled by a separate valve. The owing to abstraction of heat by the surfaces steam is employed expansively, the point of of the cylinder, by radiation, or otherwise, cutting off being controlled and regulated unavoidably determines the condensation of a (without appreciable resistance) by the gover- portion of the steam. It is this instantaneous nor, so as to proportion the total pressure of sensitiveness to cold and facility of condensathe stroke continually to the desired rate of tion that most frequently prevent our attainmovement of the machinery; while a peculiar- ing the working power of which theoretically ity of the action of the valves is the complete the boiler pressure and the engine should be and almost instantaneous manner in which capable, and that oftenest defeat the special they open and close the passages, thus admit- expedients resorted to for increasing their ting the steam at or near the full boiler pres- efficiency. When steam first enters the cylinsure, and preventing the effect known as “wire- der, the space it exists in is at once enlarged drawing." Some of these engines have re- by that through which the piston moves; and cently been furnished to orders from Scotland, if the steam space in the boiler and the heat the centre of steam engine manufacture. Non- for generating fresh supplies did not much condensing stationary engines are direct-acting, exceed the capacity of the cylinder, the conseand have two principal plans of construction, quence would be a rapid reduction of density the horizontal and the vertical. Of these the and pressure of the acting body of steam. former are most common. The oblong form of With adequate boiler and furnace, however, base is now mainly superseded by a base in L the steam removed into the cylinder is continform, of which the chief strength is in a verti- ually replaced; and if the pressure be at first cal web; and the cylinder, crank, and fly somewhat above that of the air, and the steam wheel are so attached to this that the strains pipe kept open, the initial pressure of the enarising in working the engine are best met, tering steam may be regarded as, so far as and the relative positions of the parts the this cause is concerned, maintained from bemost accurately preserved. Pumping engines ginning to end of each stroke of the piston. are of various construction; among them the The disturbances in the actual pressure spring most noted and efficient are the so called Corn- mainly from other sources. Even though it ish engines. These engines economize the were uniform, the pressure on the piston is power they produce by dispensing with cranks, not equal to that in the boiler; a result due to the fly-wheel, and many other parts to which length and winding of passages, to friction, with ordinary forms of engine must impart move- usually some condensation. Upon the piston ment; while for facility of admitting steam of the steam works in a twofold manner: first, very high pressure, for the great ratio of ex- by the tension it possesses as delivered freely pansive working they allow, and the small and continuously into the cylinder; secondly, amount of friction involved, they have for after the supply is cut off, by the expansion of pumping taken precedence of all others. For the volume previously delivered, until in so Worthington's “duplex engine," for pumping, expanding its pressure may fall to and be see Pump. IV. Behavior of Steam in the En- balanced by that of the atmosphere, or by the gine, and horo known. The substitution of the back pressure of the exhaust steam on the elastic force of a vapor for the practically lim- other side of the piston. Actuated in the

Let us

former manner through the entire stroke, the return of the piston constitutes a stroke; the piston should advance under a uniform impul- respective half strokes, however, being termed sion, its speed being constantly accelerated, forward stroke and return stroke. and its momentum at last suddenly expended trace the distribution of the steam during & on some fixed parts of the machine, involving stroke starting from either end of the cylinder. injury to the latter, and waste of power; when Suppose the piston, fig. 4, to have risen althe latter method is, at the proper point in the ready to its highest position in the cylinder; stroke, made to supersede the former, the pres- now, in obedience to the position and throw sure gradually falling may be caused to approx- of the eccentric, or to a connection with the imate so nearly to, or to fall so slightly below, governor, three events have just previously the back pressure, that the impulsion of the occurred: the returning valve has closed the piston and its velocity shall gradually decline, exhaust from before or above the piston, lockand terminate naturally at or near the end of ing up before it a residue of steam; it has at the stroke. So much cutting off and expansive the same moment, or a little before, opened working of steam is practically desirable in all the exhaust beneath the piston, relieving it of engines; but this is not what is technically in- the pressure that impelled it forward; and tended by “cutting off” and “expansion.” very shortly after these two events it has In the mode of working specially so named, opened the steam communication above the the steam being of comparatively high tension, piston by the amount of lead allowed, and the supply to the cylinder is cut off at an commenced the steam supply. Under this last, earlier period in the stroke; and expansion is having come to rest, the piston commences its availed of, not merely for avoiding waste, but forward stroke; the exhaust from before (beas a positive means of deriving from a given neath) it, having been previously opened, is volume of steam an augmented total of pres- maintained during nearly this whole movesure, and so of performance. In strict lan- ment; but first, and at the proper fraction of guage, the whole process is expansive acting; the movement, the steam supply behind (above) since so long as its pressure is in excess of that the piston is cut off. Shortly before compleof the bodies that confine it, the steam must tion of this forward stroke the exhaust in adcontinue to push these bodies before it, in the vance of it is closed, and that from behind it tendency to arrive at equilibrium; and, with opened; and at a very small distance from the open ports, its expansive energy acts from the end the lead or admission of steam from beFater in the boiler as its fixed point or ful- neath occurs, and in a moment more the arrestcrum, as, after cutting off,_ it acts from the ed piston is ready for the return stroke. Thus, fired end of the cylinder. . For practical pur- on the two sides of the piston there are at the poses, however, the distinction to be drawn at same time proceeding two complete cycles of the point between full and expansive working events, but in different parts of their course. is a real and important one. In actual work- In each cycle there are these four events, in ing, again, the pressure of full stroke is seldom their order: 1, admission of steam; 2, supor never maintained quite uniform; owing to pression or cutting off; 3, release, or exhaust; time consumed in shutting the ports, or to 4, arrest, or lock-up, prior to readmission of great speed of the piston, or to both, the steam. The pressure, and generally the bedensity of the steam is reduced, and the pres- havior of the steam during a stroke in either sore begins to decline before cutting off is end of the cylinder, is known by use of the incomplete; if this result is marked, “wire- dicator. This, in a usual form, fig. 6, is a drawing" of the steam is said to occur, the

long, small cylinder, haveffect being as if the entering charge were

ing within it a piston, gradually drawn or spun into steam of reduced

which is fixed to the density and pressure. In order that the steam

lower end of a spiral may gain admission within the cylinder by the S 80

spring that attaches. very moment at which the stroke should com- 20

above and within to the mence, the valve motion—the eccentric on the

top of the cylinder, and shaft, for example-must be so set in advance

which, moving with little of the crank that the steam port shall be al

w friction and carrying outready uncovered or opened by a small fraction

side the cylinder case an of the movement of the valve by the time the

index, is made by this to piston is prepared to return. This anticipative

с

show and also to register opening of the steam port is called the lead;

the pressures of steam and it may be greater or less, even to com

exerted upon it; thus mencing the steam supply in front of the piston

it can indicate for any while nearing the end of its stroke, for the

steam space the work the purpose of “cushioning" it, i, e., arresting its

steam in it is capable of, movement against the steam itself, as against

or is performing. O is 3 spring; the point of effecting this being sim

FIG. 6.

the cylinder; 8 a tube ply determined by the adjustment of the valve with screw thread for fitting into an orifice gear, with the form and dimensions of the in the cylinder cover, or in any passage, or valves Properly, one complete advance and the boiler, as may be desired; ĉ the cock

50

if

[ocr errors]
[ocr errors]
[ocr errors]

opening or closing the tube. The index, P, points. Of course, with high pressure engines fixed to the piston, rises and descends with it, the diagram will never run below the atmosone of its ends moving along a scale, S, show- pheric line. In fig. 7 are shown two diagrams, ing the pressure in pounds per square inch on the heavy-lined and dotted-lined figures, taken the piston; the other end having a joint by from a locomotive engine, under different conwhich a pencil is brought down in contact ditions. A B, the atmospheric line, may also with a sheet of paper called the card, rolled represent the length of stroke; the periods of about a vertical drum, D, and held upon it by the several events in the distribution of steam clasps, as f. The spring is of such length that are here noted for comparison. The diagram when the atmosphere entering above rests is a picture of the operations in one end of the upon the piston, and steam of one atmosphere cylinder; and the indicator has been aptly balances it from beneath, the piston shall rest said, like the stethoscope, to reveal what is in its natural or unstrained position; this point transpiring beyond the reach of the eye. When is the 0 of the scale. If the steam pressure accuracy is desired, diagrams are taken for both exceed one atmosphere, it forces up the pis- ends of a cylinder. A B, fig. 7, may stand for the ton and index, compressing the spring; if it length of stroke, and the space above this line fall short, the piston and index are by the air represent the interior of the cylinder. In the carried proportionally down, elongating the heavy-lined diagram, taken with the slow avspring. The drum can rotate about half way erage speed of piston, 40 feet per minute, the around a vertical axis, and when released is piston is seen here to start under the uniform returned with a like uniform movement by a pressure of 61 lbs. above the atmosphere; to spring within. By the wheel, W, and cord, ?, preserve this nearly until cutting off of steam; the connection needed to work the drum is the pressure during expansion then rapidly demade with some part of the machinery. To clining to about 23 lbs. ; on release, still more use the indicator : Connect it with the interior rapidly; and before the end of stroke to come of the cylinder or other steam space; leave down completely to one atmosphere. During the cock open during a few strokes of the en- the return stroke, the back pressure remains gine piston, to bring the indicator cylinder to thus low, until, upon lock-up, the pressure a like temperature; and the pressures upon curve mounts rapidly; and at a, when the the indicator piston and, say, the engine pis- “lead” takes place, it sweeps still more rapidton may now be considered equal. Now, at be- ly up, regaining the full head by beginning of ginning of a stroke, bring down the pencil to the next forward stroke. Here, with admistouch the card; the latter semi-rotating and sion through about | the forward stroke, and returning, and the pencil rising and falling with expansion through slightly more, inspection the varying pressures through the stroke, will show that about the whole work of the there is traced on the card a curved figure, steam has been that secured by expansion. approaching more or less to an oblong, which The dotted-lined diagram shows the behavior is the “indicator diagram.” Closing the cock of the steam in the same cylinder, with speed at beginning of the next stroke, let the drum of piston equal to 310 feet per minute, other turn once or more while the pencil rests sta- conditions remaining the same. Here, the steam tionary; thus will be traced through or be- entering at initial pressure of 62 lbs., the quick neath the diagram, as the case may be, at the recedence of the piston before it allows the O level, the "atmospheric line." The lengths pressure curve to fall slightly; from wireof ordinates drawn from this line to points of drawing near cutting off, it falls still more rapthe curve above it will show the excess above idly; after release, however, keeping higher one atmosphere of the steam pressure, and to than before, since the speed of the piston

does not allow time for exhaustion; and at no time in the return

stroke quite falling to 0. If, when -60 lbs. a slide valve is in middle position, 50 the advancing edge on which

ever side already overlaps the port 40 on that side, it must have closed

that port previous to its reach-30 ing such position, i. e., to cut

off earlier than in full working, -20

and to work the steam expan10

sively. The effect, the opposite of А

"lead,” is called the “lap;" and B

the amount of the lap determines -EXHAUST ( CONTINUED

<COMPRESSION

the ratio in which expansion shall occur. If the cylinder be colder

than the admitted steam, & very Fig. 7.

sensible condensation occurs, both points below the line the degree of exhaustion, during admission and the early part of the for parts of the stroke corresponding to those expansion; and though during the latter part

EORWARD STROKE

FEXHAUST-EXPANSION

ADMISSION

RELEASE

-SUPPRESSIONS

COMPRESSION

RETURN STROKE

15 14 12 10 8

Average Pressure 49 hs. Sam

NON

[ocr errors]
[ocr errors]

143

Fig. 8.

[ocr errors]

of the expansion the remaining steam, be- the total indicator power must be found by coming more tenuous and dry, re-absorbs part adding the results given by the cylinders sepaof this water due to condensation, yet there is rately. This is the work upon the piston, unan absolute and considerable loss, which in- der the total of resistances of every kind that creases as the steam is earlier cut off. In or- must be overcome during, and to allow of, its dinary engines the waste due to this cause has movements; but to find what part of this been found often to exceed 12 per cent. ; in work is expended upon the useful resistance exposed locomotive engines, to amount some- overcome, or at any other connection between times to nearly 40 per cent. And it is doubt- the piston and the useful resistance, so as to less this source of loss, where the steam is not learn how much of the retardation of the pisat all superheated and the heat of the cylinder

10 9 9 7 6 5 4 3 2 1 not kept in, that often defeats attempts at expansive working of steam, and leads to a prejudice against the method, when the fault is in the unfitness of the conditions under which it is tried. No doubt one of the chief actual benefits of superheating steam through a few degrees, before admission to the cylinder, arises from its thus being supplied with a surplus of heat, by parting with a portion of which it keeps up the temperature of the cylinder, while another portion serves to prevent condensation or speedily to re-convert into steam the water due to its momentary occurrence. Back pressure in condensing engines is in part due to air liberated from the boiler water; ton is due to the machinery, or possibly to imbot, on the principle that the pressure in com- perfections in it, it becomes necessary to intermunicating vessels is never less than that in pose some form of dynamometer at the connecthe coldest part, it is chiefly that of the va- tion where the observation is to be made, so por in the condenser, its temperature being as to find by its indications how much of the about 104° F., and its pressure 1.06 lbs. per total work upon the piston during a given time square inch; in practice, the total back pressure reaches and is expended at that connection. is 1 to 3 lbs. or more. Back pressure is made Nominal horse power" is a conventional unit less by enlarging the exhaust port. As to the of size of cylinder, not of observed power of measurement of the work of the engine : the the engine. The rules of estimating it have indicator diagram represents, for each given differed with different localities, and they have point in the advance and return of the piston, usually allowed a larger unit of capacity for the effective steam pressure or the back pres- condensing engines. It is not much in use as sure exerted per square inch on the correspond- & measure of value of engines in the United ing face of the piston. As the lines of the States; when it is so, the following is a usual diagram are curved, its area must be found by a process of reduction or average. Divide form: H. P. = V stroke in feet x diameter ? in inches the diagram into horizontal sections answer- Actual horse power should be reckoned in acing to the pressures, and into a convenient tual units of pressure, to be known either by number of vertical sections, as shown in fig. use of the indicator, or of a dynamometer 8; take the mean effective pressure in each showing the power delivered at the crank of the vertical sections, add these together, shaft. For the former, the rule may have the divide by the number of such divisions, and following expressions: Ind. H. P.= the quotient is the effective mean pressure mean pross. X diam.? x.7854 x stroke X 2 x No. of turns, per unit of surface for the whole diagram;

83,000 multiply this by the area of the piston in like

pressure x diam. ' x stroke x No. of turns units, and the product is the whole effective

21,000 work upon one surface of the piston for one stroke. Proceed in the same way with a dia

33,000 gram for the other end of the cylinder; add V. Work of Steam in the Engine. The presthe two results; take their mean; multiply sure of steam in the boiler, and within the steam by the number of single or half strokes of the chest or the cylinder, is commonly brought engine per minute, and divide by 33,000 (see under direct observation by means of pressure MECHAsics); the resulting quotient is the “in- gauges of various sorts (see PNEUMATICs), while dicator horse power” of the engine. In aver- the degree of vacuum in the condenser is indisging the diagram, fig. 8, add the average preg- cated by the vacuum gauge. What is at differsures for the 10 divisions made in the stroke; ent times called the “vacuum” of the condenser their sum, 204.5, divided by 10, gives 20.45 may be either the degree of actual vacuum Ibs., the mean unbalanced pressure per square produced, or the residual pressure. Thus, when inch on the piston throughout the stroke. If the latter sustains 5 inches of mercury, it may there be two or more engines acting together, be said in one sense that a vacuum equivalent

2

=

47

[ocr errors]

pressure x area x stroke X 2 X No. of turns

[ocr errors]

*

to 25 inches of mercury is obtained ; in an- has usually not sufficient advantage over the other and less correct sense, that the vacuum ordinary throttle-valve, to compensate for its corresponds to 5 inches of mercury. In double- cost and the attention it may require. It may acting condensing engines, the piston is releas- be considered doubtful whether these conclued from effect of the external atmosphere, and sions will be fully sustained ; but they will at its work is performed and estimated indepen- least have the good effect to call attention to dently of it. The indicated power of every the extreme to which expansive working has steam engine is greater than the available been carried, steam having been cut off in power by the amount of energy expended in some cases at 32, or even o'r of the stroke. overcoming the resistance of the engine. The The ideal “cut-off” arrangement would be available power is the useful work the engine that which, first, should close the ports instancan perform in a given time, or rather the taneously at the proper moments, so that the power it can impart in such time to the me- steam should be admitted unreduced by wirechanism to be moved by it. The useful effect, drawing, and enabled to act as it were with or net available power, is a quantity involving explosive force upon the piston; and secondly, three others, the velocity, the load, and the which should be completely under control of rate of evaporation in the boiler; and this net the governor. The cut-off's now in use are available power can be expressed and calcu- very numerous, among them those of Sickels, lated in six different ways: 1, in foot-pounds Stevens, Allen and Wells, Corliss, Woodruff and per unit of time; 2, in horse power; 3, in Beach, and others. In respect to superheating weight raised per pound of fuel; 4, do. do. per of steam within limits from 10° to 40° above the cubic foot of water evaporated; 5, in number temperature of saturation, it may be proper to of pounds of fuel, or cubic feet of water to a add that the fears once entertained of its dehorse power; 6, in number of horse power to stroying lubrication, burning the surface of the & pound of fuel or a cubic foot of water. For cylinder or passages, &c., have proved quite the investigations by which formulas for these groundless in practice; while at the same time, calculations are obtained, and for the modes of the notion that high superheating would greatobtaining the requisite numerical solutions by ly increase the effective work of the steam has them, the reader is referred to the more com- also been discarded; so that superheating withprehensive works presenting the mathematical in moderate limits is now resorted to, mainly theory of the steam engine, especially to those as a desirable condition for successful expansive of De Pambour, Tredgold, Bourne, and Ran- working, or otherwise, merely as a means of kine. The limit beyond which expansion can- preventing loss of the steam pressure. In pracnot be advantageously carried seems to be that tice, indeed, owing to want of heat conducnumber of volumes found by dividing the initial tion in fluids, with radiation of heat into the pressure of the steam by the pressure in the con- dome from the water surface, or from other denser. This result of theory, however, pro- causes, the steam in a high dome, and especially ceeds on the supposition that the steam is main- when the water beneath is for some time but tained in the perfectly gaseous condition. Prac- slightly agitated, is in effect lated from the tical results seem to sustain Prof. Rankine's water, and actually very often becomes superestimate, to the effect that the gain of efficiency heated, unknown to the engineer. In some in an ordinary engine, cutting off at one fifth, marine engines, also, steam is in practice surwith superheating by heat from the flues steam charged with heat in the dome, by carrying of 34 lbs. pressure, is about 15 per cent. ; if by fues through or around it. It is customary to heat otherwise wasted, as by carrying the steam estimate the efficiency of steam in a rough way pipe through the chimney, about 23 per cent. by considering the effective mechanical force Though the principle of working steam ex- of a cubic foot of water vaporized as 60 horse pansively is very simple, and has long been power. If, then, this quantity of water be conaccepted, the subject is not yet exempt from verted into steam in an hour, it will give a discussion or differences of opinion among en- horse power per hour; and the boiler and engineers. Mr. King, in his “Practical Notes on gine that could generate and employ the steam Steam" (New York, 1861), estimates that by of 10 cubic feet of water per hour, would give cutting off at half stroke the saving in fuel continually 10 horse power of work upon the may be made nearly 20 per cent.; and for piston. The high boiling point of water, but other ratios of expansion, within certain limits, more especially the large degree of latent heat in proportion. Engineers Isherwood, Stimers, required to vaporize it, renders steam power and others, as the result of numerous and it expensive through necessity of a proportionwould appear carefully conducted experiments ately great consumption of fuel. Accordingly, with the engines of the steamer Michigan, at various other vapors, as well as gases generatErie, Penn., were led to conclude that the ed by explosion, have been tried as substitutes maximum gain by expansion is secured by cut- for steam. A comparison of the boiling points ting off at io stroke; that to eat off much and latent heat of certain other liquids, with short of this affords no gain ; that the loss by the relative volume and density (air being 1) condensation in the cylinder, and by increased of their vapors, will show theoretically their friction and back pressure, is generally under- eligibility thus to serve as more economical rated; and that the use of a variable cut-off substitutes :

[ocr errors]
« ՆախորդըՇարունակել »