Imágenes de página

when empty, the vessel a is brought up to its original position by the weight of the lever f, &c. The whole is contained in a box, from which the water is led away after working the cataract. The number of times the injection-valve is opened determines the number of strokes the engine makes in a given time; and the falling of the box or vessel a, which opens the injection-valve, is regulated by the quantity of water allowed to pass through the pipe b; this quantity of water is therefore proportioned to the quantity to be drawn from the mine, or the work to be done by the engine. The reader will find descriptions of forms of improved cataracts in the Artisan Treatise on the Steam-Engine.


In cotton-mills, and in cases where the engine is loaded to a certain extent, accidents frequently occur by parts of the machinery breaking down. The engine, thus relieved of a certain part of its load, "runs away," as it is termed, at a greatly increased speed, endangering the stability of the whole apparatus. The ordinary mechanism of the throttle-valve and governor is not found capable of making the engine recover its usual speed. ingenious contrivance, highly esteemed, is given in the following diagram, fig. 113. The main object of this invention is to prevent damage by the accidental alteration of load, and to regulate the speed of steam-engines. It is intended to supersede the common throttle-valve, and prevent variation of speed and the many breakages arising from accidental alterations of weight. The valves can readily be connected with ordinary governors; and when attached, are so sensitive, that should a shaft break, or any weight be suddenly thrown on or off, the engine will recover its usual speed in less than two revolutions, without any interference by the engineer. This will be obvious, when it is considered that there are two valves fitted to the same spindle; and consequently, that each of them need be opened to only one half the extent, as if there were one valve. The result is, a throttle-valve of extreme sensibility, half an inch being the utmost amount of play allowed to the spindle between wide open and quite shut; therefore, a very slight change in the position of the governor-balls produces considerable change in the amount of opening afforded by the valves.

The governor is connected with the valve-spindle by a fork, in the usual manner, which, by means of a crank-lever, gives a slight motion on its axis to a bar, provided at the other end with another crank-lever connected with the valve-spindle. It may be attached to all ordinary governors. In the case of two fifty-horse engines coupled together in Sir C. Armitage's mill, at Pendleton, near Manchester, the result of a trial by throwing off the whole weight was the recovery of the usual speed in 1 revolutions. The load on these two engines consisted of 20,000 throstlespindles, 13,000 mule-spindles, and 250 power-looms, with all the necessary apparatus for working the mill.

In fig. 113 we give a diagram illustrative of this apparatus, so capable of instantaneously regulating the performance of engines. a is the steam-pipe from boiler, e c the double-beat valve; when this is raised by the lever actuated on by the lever d of the connecting-rod, the steam passes from ɑ in the direction of the arrows to the steam delivery-pipe b.

The "hydraulic motion regulator" is an American invention, recently introduced into this country, and which has already taken a high place as an effective governor for steam-engines; we give a diagram and description of this apparatus, taken from the inventor's circular (Mr. Pitcher).

the water

The principle of the invention consists in working a small pump, delivered by which acts in the plunger of a second cylinder or barrel: the

[ocr errors][merged small][merged small][merged small]

water from the pump escapes through an aperture of a certain area; this is calculated so as to maintain the plunger at a certain height in the cylinder. Should, however, the speed of the engine increase beyond its regular working speed, the pump is in consequence worked faster, and a greater quantity of water delivered to the plunger cylinder: but the aperture for its escape remaining unchanged, it cannot get away, it therefore fills the cylinder to a higher level; this raises the plunger, which, connected with the regulating-valve or throttle-valve, lessens the supply of steam to the cylinder and reduces the speed. The pump is worked by the engine by any of the ordinary methods of connection. In fig. 114, e is the pump worked by the engine; c the suction-valve, through which the supply of water is obtained, the delivery-valve, supplying the cylinder f. To prevent the plunger from rising higher than necessary, a small hole, g, is made in the cylinder; when the plunger rises above this the water escapes by it, and the plunger rises no higher. The piston-rods pass through the bonnet or cover of the external casing jj; cups ik are used to return any water that may be drawn up through the stuffing-boxes; the piston-rods are thus lubricated, or made to work smoothly, with water surrounding them. The whole apparatus is enclosed in the casing jj; so that the same water is used over and over again. A spring is coiled round the piston-rod of plunger f at z; this prevents its falling further than necessary. The opening by which the water ordinarily escapes is made just above the delivery-valve b; and the amount of opening is regulated by a handle or lever which passes through the cover h, according to the speed at which the engine is desired to run. In place of the ordinary throttle-valve, the inventor prefers to use a regulating valve similar in principle to the disc-valve employed as the regulator" in locomotive engines.


In calculating the power of steam-engines, there are two terms usedthe "nominal power" and the "actual, or effective power." By the term "nominal power," reference is made to an engine having a cylinder of

given diameter, a given length of stroke, with a uniform pressure upon the piston of 7 lbs. per inch. Watt calculated the effective pressure on the piston in small engines at 6.8 lbs.

per inch, and in large-as 100 horse-power-at 6.94; 7lbs., however, is the effective pressure calculated. By the term "actual" power, is meant the number of times 33,000lbs. the engine is capable of lifting 1 foot high per minute. By the term "duty" of an engine is meant the amount of work done in relation to the amount of fuel consumed.

In calculating the nominal horse-power of an engine, the following rule is adopted: "Square the diameter of the cylinder, and multiply the number of square inches thus found by the cube-root of the length of the stroke in feet, and divide the product by 47; the quotient is the number of nominal horse-power of the engine."

By the term "horse-power," as introduced by Watt, was meant the mechanical force necessary to lift 33,000 lbs. 1 foot high per minute. Prior to the extended introduction of steam-engines, the work at mines, &c. was frequently done with horse-power; it became, then, of importance to be able to state the amount of work done by a steam-engine, as compared with a given number of horses. The power of a horse, as calculated by Watt, was equal to the raising of 33,000lbs. 1 foot high in a minute. Engines now, however, calculated at this rate, really exert a greater power than the nominal power; it is, therefore, of importance to be able to cal


fig. 114.

culate the effective or actual power of an engine, without reference to its nominal power. This is ascertained by means of the "indicator," which gives the effective pressure on the cylinder of the engine: from this is deducted a pound and a half of pressure absorbed in friction, &c.; the velocity of the motion of the piston in feet per minute is thus ascertained: this is done by multiplying the number of revolutions of the engine per minute by the length of stroke. These data having been ascertained, the

following rule will give the effective power of the engine, calculated on Watt's data: "Multiply the area of the piston in square inches by the effective pressure (found as above) and by the motion of the piston in feet per minute, and divide this by 33,000; the quotient is the actual number of horse-power." For each horse-power of an engine it is calculated that 33 cubic feet of steam is expended per minute, or an evaporation of 1 cubic foot of water per hour. The combustion of 1lb. of coal is calculated to raise 6 or 8lbs. of water into steam; Watt reckoned 7 lbs. of water to be evaporated by the combustion of 1lb. of coal. In the modern Cornish engines, the same quantity of fuel evaporates 10lbs. of water. Land engines are generally calculated to consume 10lbs. of fuel per hour for every nominal horse-power, or 5 or 6 lbs. for each actual horse-power. In the Cornish engines the duty of an engine is "expressed by the number of millions of pounds raised one foot high by a bushel, or 94lbs. of Welsh coal;" a bushel of Newcastle coal will only weigh 84lbs; and, in comparing the duty of a Cornish engine with the performance of an engine in some locality where a different quality of coal is used, it is necessary to pay regard to such variations." In the engine at Long Benton Colliery, erected by Smeaton, the duty performed was equal to 9.45 millions of pounds, raised 1 foot high by the consumption of 1 bushel of Newcastle coal. In the present time, what with improved engines and boilers, and the extensive adoption of the principle of expansive working, the duty of Cornish engines is estimated at 60,000,000lbs.; and in some instances the duty has increased to the large amount of 100,000,000 lbs. raised 1 foot by the consumption of 94 lbs. of fuel. For much valuable practical information on the power of engines, the heating surface for each horse-power, &c. &c. we refer the reader to Bourne's Catechism of the Steam Engine.






[ocr errors]



We now, in concluding the present division, give a description and diagrams of the important instrument, the "indicator," so essentially necessary in computing the effective power of steam-engines. A small cylinder, as a, fig. 115, is placed in connection with the interior of the cylinder, either above or below the piston, generally it is screwed into the aperture made in the cylinder-cover; a stop-cock is placed in the pipe b, by which the connection between the interior of steam-engine cylinder and that of the indicator can be closed or opened as required. Into the cylinder a a piston works; within the interior of the rod c of this, which is made hollow on purpose, a spiral spring is placed; the lower end of this is fixed to the piston, and the upper to a small cross-head, e, supported by side-rods, connected with the cylinder, a a. To the top of the piston-rod a pencil, fg, is attached; the point of the pencil works in contact with a slip of paper wrapped round the small cylinder, h, and kept in contact with it by the vertical strip of brass, ii, on which is marked a scale. The axis of this cylinder, h, is continued downwards, and provided with a pulley, k. This pulley is connected with the parallel motion, or other reciprocating

fig. 115.

part of the engine, by which motion is given to it, causing it to revolve only in one direction: the cylinder, h, makes its return motion to its original position by a spring coiled up in the bottom near ss; the direction of the cord, after leaving the pulley, is changed by a guide-pulley not shown in our diagram. The effect of the two motions, namely, the up and down motion of the piston, and the revolution of the cylinder, h, is to cause the pencil, g, to describe a curve, varying in its outline. Before the connection is made between the interior of cylinder a a and the steam-engine cylinder, what is called the atmospheric line is drawn on the paper; this is effected by pulling the cord, or allowing the engine to act on the pulley. The cock at the pipe b is then opened, when the piston is at the top of its stroke: the steam acting on the cylinder (steam-engine) piston acts also on that of the indicator; this will therefore rise, and with it the pencil, which is made to press slightly on the surface of the paper by a small spring; at the same time the roller, or cylinder, revolves. A line is thus traced on the paper, which rises higher up on the cylinder as the pressure of the steam increases, and comes lower upon it as the steam-pressure subsides."




fig. 116.


The method of ascertaining the pressure on the piston of the engine, from the diagram thus obtained, is very simple, and will be easily understood by reference to fig. 116. Suppose abcd to be the slip of f paper, on which the indicator diagram has been taken, and ef the atmospheric line, the divisions at the ends, as at bc, correspond to the divisions of the scale i on the indicator, fig. 115. Divide the length of the diagram into any number of equal divisions, and through these draw lines at right angles to the atmospheric line ef; measure the lengths of the spaces thus formed by the intersection of the diagram with the lines, as the length from m to n, and from r to s, and so on (the measurements must be taken from a scale corresponding to that in the indicator-scale), and all the lengths together; divide this by the number of spaces, and the quotient is the mean effective pressure on the piston in pounds per square inch. We have already described the rule for calculating the effective pressure of the engine. The indicator is not only useful to ascertain the amount of power exerted by the strokes of a steam-engine, but it serves also to point out particular defects in the working. Thus the nearer the diagram attains to the form of a parallelogram, the more perfect is the working of the engine. Where certain deviations from the square at the corners are indicated on the diagram, certain defects are made known.

For full information on the practical working of the indicator, and the method of ascertaining the defects as indicated by the diagram, we must refer the reader to other and more practical works-as the Indicator and Dynamometer, by Professor Main and D. Brown; published by Hebert, Cheapside.

« AnteriorContinuar »