In [Fig. 53], GH is the atmospheric line; OX is the line of vacuum or zero pressure, drawn so that GO = 14.7 pounds on the scale; and OY is the clearance line, so drawn that ON = 0.06 NX. The line PQ is drawn from OX to some point on the compression line, as at Q. From C, a point on the expansion line, just before release, the line CF is drawn perpendicular to OX. The following dimensions are now carefully measured from the actual diagram (not the one shown in the illustration), with the results given:
| OX = 3.71 | OP = 0.42 |
| NX = 3.50 | CF = 0.81 |
| OF = 3.20 | QP = 0.81 |
On the indicator diagram, being taken with a 60-pound spring, all vertical distances represent pounds per square inch, in the ratio of 60 pounds per inch of height. The stroke of the engine is 30 inches or 2.5 feet. The length of the diagram NX is 3.5 inches; hence, each inch in length represents 2.5⁄3.5 = 0.71 feet. From the above it is evident that vertical distances in [Fig. 53] must be multiplied by 60 to reduce them to pounds pressure per square inch, and that horizontal distances must be multiplied by 0.71 to reduce them to feet. Making these reductions gives:
| OX = 2.63 feet. | OP = 0.30 foot. |
| NX = 2.49 feet. | CF = 48.6 pounds. |
| OF = 2.27 feet. | QP = 48.6 pounds. |
As a card from the head end of the cylinder is taken to avoid corrections for the piston rod, the area is 201 square inches or 1.4 square foot. With the above data the volume and weight of the steam in the cylinder can be computed at any point in the stroke. When the piston is at C, the volume is 1.4 × 2.27 = 3.18 cubic feet. When the piston is at Q, the volume is 1.4 × 0.30 = 0.42 cubic foot. From a steam table the weight of a cubic foot of steam at 48.6 pounds absolute pressure is found to be 0.116 pounds. Therefore, the weight of steam present when the piston is at C is 3.18 × 0.116 = 0.369 pounds. The weight of steam present when the piston is at Q is 0.42 × 0.116 = 0.049 pound. That is the weight of steam in the cylinder at release is 0.369 pound, and the weight kept at exhaust closure for compression is 0.049 pound.
The weight exhausted per stroke is therefore 0.369 - 0.049 = 0.32 pound. The number of strokes per hour is 150 × 2 × 60 = 18,000, from which the steam accounted for by the diagram is found to be 18,000 × 0.32 = 5760 pounds, or 5760 ÷ 217 = 26.5 pounds per indicated horsepower per hour. If the cylinder condensation for this type of engine is 20 per cent of the total steam consumption, the water rate will be 26.5 ÷ 0.8 = 33.1 pounds per indicated horsepower per hour.
In the present case it has been assumed, for simplicity, that the head- and crank-end diagrams were exactly alike, except for the piston rod. Ordinarily, the above process should be carried out for both head and crank ends, and the results averaged.
Transcriber's note:
- Inconsistencies have not been corrected (hyphenated vs non-hyphenated or spaced words), except horse-power (changed to horsepower) and cut off (changed to cut-off) as elsewhere.
- Minor typographical errors have been corrected.
- In-line multi-line formulas have been changed to single-line formulas, where necessary with the addition of brackets to prevent ambiguity.
- Most tables and illustrations have been moved to the paragraph where they are first discussed or mentioned.