Modern Machine-Shop Practice Part 248

Heat also pa.s.ses from a body in straight lines or rays, which do not heat the air through which they pa.s.s to their own temperature, but do impart that temperature to a solid body, as iron or water; the heat that pa.s.ses from a body in this manner is termed radiant heat, or the heat of _radiation_.

In the cylinder of a steam engine, therefore, the heat contained in the steam is disposed of as follows:

A certain portion of it is converted into work through the medium of the piston.

Another portion is conveyed away by the walls of the cylinder, this portion including the heat of convection and that of radiation.

Yet another portion is converted into internal work. Referring to the latter, suppose that steam is permitted to expand and its atoms will be in motion, which motion has been derived at the expense of or from the conversion of a certain quant.i.ty of heat.

The amount of the heat so converted obviously depends upon the amount of the motion. Suppose, for example, that steam is generated in a closed vessel as in a steam boiler, and that a certain pressure having been attained, the steam is permitted to pa.s.s off as fast as it is formed from the boiler, then the amount of atomic motion will remain constant, because the pressure remains constant; but suppose instead of the steam pa.s.sing off, it be confined within the boiler, then the pressure will increase and there will be a greater resistance to the motion of the atoms, hence their motion will be less, and less of their heat will therefore be converted into atomic motion, and, as a consequence, more of it will exist in the form of sensible heat; hence while the pressure of steam continues to increase, its heat is increased, not only by reason of the heat it receives from the furnace, but also by reason of that abandoned by the steam, because it is prevented by the pressure, from expending it in atomic motion.

CHAPTER XL.--THE INDICATOR.

The indicator is an instrument which marks or draws a figure, or diagram as it is called, which shows the pressure there is in the cylinder at every point in the piston stroke, while it also shows the resistance offered by the same body of steam to the piston on its return stroke.

From the form of this figure or diagram, the engineer is enabled to discover whether those parts of the engine whose operation regulates the admission of the steam to and its exhaust from the cylinder are correctly adjusted.

From the diagram the engineer may find the average or mean effective pressure of steam on the piston throughout the stroke, for use in calculating the power of the engine.

He may also locate the point of cut off, of release, the amount of back pressure, the degree of perfection of the vacuum in a condensing engine, and the amount of compression.

From the area of the diagram the engineer may also estimate the quant.i.ty of steam that is used, and supposing it to be dry steam, he may calculate the amount of water used to make the steam, and a.s.suming one pound of coal to evaporate so much water, he may calculate the amount of coal used to produce the steam.

The indicators commonly used upon steam cylinders contain two princ.i.p.al mechanical movements; first, a drum revolving the piece of paper upon which the diagram is to be marked, and second, a piston and parallel motion for moving the pencil to mark the diagram upon the revolving paper.

The drum is given a motion that, to insure a correct diagram, is exactly timed with the piston motion.

The pencil is given a vertical movement; this movement must bear a constant and uniform relation to the pressure of the steam in the engine cylinder.

[Ill.u.s.tration: Fig. 3358.]

An indicator may be attached to each end of the cylinder or in the middle, with a pipe pa.s.sing to each end of the cylinder, as in Fig.

3358, but an indicator of the usual construction and such as here referred to, can take a diagram, or _card_ as it is sometimes called, from but one end of the cylinder at a time. The stop valves A and B are used, so that the communication between the indicator and one end of the cylinder may be shut off while a diagram is being taken from the other end, while both ends may be shut off when the indicator is not being used.

In the figure a piece of paper (or card, as it is commonly called) is shown in place upon the drum with a diagram upon it.

[Ill.u.s.tration: Fig. 3359.]

[Ill.u.s.tration: Fig. 3360.]

The Thompson Indicator is shown in Fig. 3359, and in section in Fig.

3360.

The Tabor Indicator is shown in Fig. 3361, and in section in Fig. 3362.

Both are made with the piston and parallel motion as light as possible, in order to enable the taking of diagrams at as high a speed of engine revolution as possible.

Each consists of a cylinder and piston, the bottom surface of the latter being in communication with the bore of the engine cylinder, so as to receive whatever steam pressure there may be in the cylinder.

This indicator piston receives, on its upper surface, the pressure of a spiral spring, which acts to resist the steam pressure.

The indicator piston rod actuates an arm or line on the end of which is a pencil, which, by means of a parallel motion, is caused to move in a straight line.

The paper or _card_ being in place upon the drum, and steam let into the indicator, the pencil lever is moved until the pencil touches the paper as lightly as possible, and as a result of the combined movements of the pencil and drum, the diagram is marked, its form being ill.u.s.trated in Fig. 3363, which represents a diagram placed above a cylinder, and the engine piston in three positions; first at the beginning of the stroke; second, at the point of cut off (which is supposed to be at one-third of the stroke); and third, at the point of release where the valve first opens the port for the exhaust. For convenience, the diagram is shown as long as the cylinder, but the actual diagram usually measures about 2-1/2 inches high and 4-1/4 inches long.

[Ill.u.s.tration: Fig. 3361.]

[Ill.u.s.tration: Fig. 3362.]

[Ill.u.s.tration: Fig. 3363.]

Supposing the cylinder to be filled with air, and the engine piston in position 1, and the indicator piston would be at the corner A of the diagram; but if steam were admitted, the pencil would rise vertically, marking the line from A to B, which is therefore called the _admission line_, or by some, the _induction line_.

If on reaching B the pressure was enough to move the engine piston, that piston and the indicator drum would move simultaneously, and as long as live steam was admitted the line from B to C would be drawn, hence this is called the _steam line_, its length denoting the live steam period.

The cut off occurs when the engine piston is in position 2, and the indicator pencil at C.

From this point the pencil will fall, in proportion as the steam pressure falls from expansion until the exhaust begins, the piston then being in position 3, and the pencil at D.

The line from C to D is therefore called the _expansion line_ or _expansion curve_, and the point D the _point of release_ or _point of exhaust_.

We have now to explain that in reality the whole of the remainder of the line of the diagram is, in reality, the exhaust line, yet there is a difference between the part of the line from point D to the end E of the diagram, and that part from E to A, inasmuch as that during the period of exhaust from D to E, the pressure is helping to propel the piston, while after E is reached, whatever steam pressure there may remain in the cylinder acts to r.e.t.a.r.d the piston.

The line from D to E is therefore the exhaust line, and that from E to A is the _back pressure line_ or _counter pressure line_.

In this example it has been supposed that while the piston was moving from position 3 to the end of its stroke, and the pencil from D to E, the indicator piston would have a steam pressure on it equal to atmospheric pressure, hence the line from E to A, in this case, represents the atmospheric line, and also the back pressure line.

The atmospheric line is a line drawn when there is no steam admitted to the indicator, and represents a pressure above a perfect vacuum equal to the pressure of the atmosphere. Its use is to show the amount of back pressure, and in a condensing engine to show the degree of vacuum obtained.

It also forms a line wherefrom the line of perfect vacuum, or that of full boiler pressure, may be marked.

The steam pressure at any point in the stroke is denoted by the height of the diagram above the atmospheric line, but the steam pressure thus taken is obviously above atmospheric, and is thus the same as the pressure of a steam gauge, which is also above the atmospheric pressure, and therefore represents the pressure that produces useful effect in a non-condensing engine.

This is what may be called a theoretical diagram, because, first, it supposes the steam not to be admitted to the cylinder until the piston was at the end of its stroke, and to attain its full pressure in the cylinder before the piston lead begins to move, whereas, in order to attain a full steam pressure at the beginning of the stroke, the valve must have lead.

Second, it supposes the cut off to be effected simultaneously, whereas the valve must have time to move and close the port, and during this time the steam pressure will fall, and the curve C of the diagram will therefore be rounded more or less according to the rapidity with which the valve closed.

Third, it supposes the steam to have exhausted down to atmospheric pressure by the time the piston had reached the end of the stroke, whereas the piston will have moved some part of the back or return stroke before the steam will have had time to exhaust down to atmospheric pressure; and,

Fourth, it supposes the steam to remain at atmospheric pressure until the piston arrives at the end of its return stroke, whereas the valve will begin to close the port and cause the steam remaining in the cylinder to compress before the piston has completed its return stroke.

In practice the diagram will, under favorable conditions, accord nearer to the shape shown in the lower part of Fig. 3363, in which the closure of the port for the cut off is shown by the curve at F. At the point denoted by _g_ the valve began to close, and at the point denoted by _h_ the cut off was completely effected, and the expansion curve began.

The curve beginning at D is caused by the gradual opening of the exhaust port.