CHAPTER VIII.

TO FIND THE MEAN PRESSURE.

DIVIDE the length of the stroke by the length of the space into which the steam is admitted; find in the table the logarithm of the number nearest to the quotient, to which add 1—the sum is the ratio of the gain; then find the terminal pressure by dividing the initial pressure by the proportion of the stroke during which the steam is admitted, and multiply it by the logarithms + 1, found as above; the product will be the mean pressure through the stroke.

Example 1. Suppose the length of the stroke to be 48 inches, the initial pressure to be 40 pounds per square inch, and the steam to be cut off at 12 inches of the stroke, what will be the mean pressure?

48÷12=4. Hyp. log. of 4,= 1.38629+1= 2.38629. Then 40÷4=10X 2. 38629=23.8629 pounds, the mean pressure required.

Example 2. Suppose the length of the stroke to be 36", initial pressure to be 50 pounds per square inch, and the steam to be cut off at 9′′ of the stroke, what will be the average pressure?

36÷9=4. Hyp. log. of 4= 1.38629 + 1 = 2.38629. Then 50 ÷ 4 = 12.5 × 2.38 = 29.75, mean pressure required.

This is correct without taking the clearance into account.

With the clearance added, the mean pressure would be slightly greater.

Or 2d, by means of the following table. The pressures per table are 15 pounds greater than the pressure that would be shown by a correct steam-gauge.

In the "Average Pressure" no deduction is made for back-pressure. Deducting, say six pounds for a condensing engine, we have the mean effective pressure.

For example it is required to find the power of an engine twenty inches diameter of cylinder, running at a piston speed of 600 feet per minute,

admitting steam of seventy pounds pressure, by gauge, to the cylinder, and cutting off at a quarter stroke. Add fifteen to seventy to find the absolute pressure; then against eighty-five and under a quarter cut-off the average pressure is found to be 50.65 pounds. This, less six pounds, 44.65 pounds is the mean effective pressure condensing.

MEAN PRESSURE OF STEAM AT DIFFERENT RATES
OF EXPANSION.

Initial pressure in pounds per square inch.

Average pressure in pounds per square inch for
the whole stroke.

Points in the stroke at which steam is cut off.

29.67 30.66 33.86 36.14 33.38 34.89 38.09 40.66 37.07 38.32 42.32 45.18 40.83 42.08 46.47 49.91 44.49 45.98 50.73 54.41 48.35 49.89 54.98 58.96 52.00 53.52 59.07 63.25 55.73 57.36 63.38 68.00 59.41 61.13 67.47 72.45 63.17 65.00 71.84 77.00 66.94 68.83 76.00 81.47 70.52 72.68 80.16 86.00 74.23 76.47 84.37 90.57 81.82 84.12 93.00 99.95 89.10 91.98 101.44 108.63 96.55 99.47 110.00 117.89 104.00 107.21 118.36 126.77 111.63 114.56 126.89 136.00

CHAPTER IX.

SLIDE VALVES.

THIS is a branch of the subject deserving the special attention of the engineer.

Its importance in regard to the economical working of the steam engine cannot be overestimated.

The slide valve ordinarily used in steam engines, and the manner of its operation, are well known to nearly every practical mechanic and engineer. It will be remembered that the operations of admitting the fresh steam, and releasing the waste steam, are alternately performed by the same valve and the same motion.

The valve being made to slide backwards and forwards upon the face of the ports, opens and closes the several passages in their turn.

The two extreme ones, called the steam ports, communicate with each end of the cylinder.

The middle one is called the exhaust port, and its corresponding passage terminates in a pipe open to the atmosphere.

Steam is admitted freely into the steam chest from the boiler, and the valve is made of sufficient length to cover all the ports, when it is placed in the centre of the stroke.

When it is in this position no steam can enter the cylinder, but as the valve moves on one of the ports opens, the arrangement of the valve gearing being such that when the piston is ready to begin its stroke, the steam port begins to open.

During the stroke of the piston, the valve not only travels to the end of its stroke, but also returns to the point from whence it set out, and its continued motion in the same direction finally closes the port and prevents any further

S

admission of steam.

E

FIG. 9.

The steam has now done its work, and must be removed. In the middle of the valve a hollow chamber is formed of sufficient length to open between the ports.

As soon as the edge of this chamber passes the edge of the steam port, the pent-up steam finds vent and rushes through the exhaust port and escapes through the exhaust pipe into the atmosphere.

Now looking at Fig. 9 it will be noticed that the exhaust port opens when the steam port