Marine engine indicating; a complete treatise on the indicator and indicator diagrams as applied to marine engines by Linch Charles Sutterley 1869-

Author:Linch, Charles Sutterley, 1869-
Language: eng
Format: epub
Tags: Indicators for steam-engines, Marine engines
Publisher: Boston, American Steam Gauge and Valve Mfg. Co.
Published: 1910-03-25T05:00:00+00:00

Cylindtvi/Qiumt i/i

Fig. 15

Expansion of Steam

The steam in the cyUnder of a steam engine during expansion is supposed to follow substantially a law known as the law of Boyle and Mariotte. This law states that the pressure varies as the volume in an

inverse ratio. That is to say: As the volume increases the pressure suffers a decrease.

Symbolically, if P = pressure, and V = volume, then P. V. = C.

We say substantially, because the actual changes of pressure do not follow the law exactly. The pressure may, and in the majority of cases it does fall more rapidly in the early stages of the expansion, and less rapidly in the latter portion than indicated by the law of inverse ratio. Therefore, the finial pressure is as a rule greater than that which would be deduced from the ratio of expansion.

Now the fullness of the expansion curve depicted on the indicator diagram, near the end, compensates for the hoUowness near the beginning, and hence we find that the area bounded by the curve is practically equal to that bounded by a hyperboUc curve according to the law.

We, therefore, assume that for all practical purposes, and for general investigation, the steam expands according to the law, P. V. =C.

The curve which represents diminishing pressures due to increasing volume is a portion of a h)rperbola.

The rectangular hyperbola used as a curve of expansion is constructed as follows: (See Fig. 15.)

Let OY' = P, the initial pressure.

Let Y'U=V, the volume up to cut-off.

Let 0X7 =Vo, the volume at end of stroke.

Produce the Une Y'U to U?; divide UU7 into any number of parts, say 7. Draw a series of radiating lines from 0 to Ui, U2, Ua U7.

Now where the radiating lines OUi, OU2 OU7 intersect the

ordinate UX, such as points 1, 2, 3, etc., these points of intersection give points through which are drawn lines parallel to OX7, as 1, 1,-2, 2, - 3, 3, etc.

Drawing a fair curve through the corresponding points of intersection with the ordinates Ui Xi, U2 X2, Ua Xa U7 X7, we

have the curve known as the rectangular hyperbola, or curve of P. V.=C.

To determine the pressure at any point of the expansion curve, say for volume Y'U3=OX3. Draw the diagonal line OUa, then through point 3 the intersection of U, X and OUa draw the horizontal Une 3,3 parallel to OX7. Point 3 is a point on the expansion curve and the vertical line 3, Xa gives the absolute pressure corresponding to the voliune OXa.

Should we desire to obtain the finial pressure after expansion: Draw the diagonal line OU7; then through the point 7, the intersection of UX and OU7, draw the horizontal fine 7, 7, parallel to OX7. The vertical line 7, X7 gives the required finial absolute pressure. We can conversely find the volume which a quantity of steam V. would

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MARINE INDICATING

35

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occupy at the pressure P. if it were compressed to the pressure Pa. To obtain the volume, draw the diagonal line OU' (see Fig. 16) now where OU' intersects Y'U, draw A, A parallel to Y"0.

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