Multiply the area of the piston by the average
pressure, less 5; multiply this product by the number of feet the
piston travels per minute; divide the product by 33,000; the
result will be horse power of the engine.
Q. How will you find the area of piston?
A. Square the diameter of piston and multiply it by .7854.
Q. What do you mean by squaring the diameter?
A. Multiplying it by itself. If a cylinder is 6 inches in
diameter, 36 multiplied by .7854, gives the area in square
inches.
Q. What do you mean by average pressure?
A. If the pressure on boiler is 60 pounds, and the engine is
cutting off at 1/2 stroke, the pressure for the full stroke would
be 50 pounds.
Q. Why do you say less 5 pounds?
A. To allow for friction and condensation.
Q. What is the power of a 7 x 10 engine, running 200
revolutions, cutting off at 1/2 stroke with 60 pounds steam?
A. 7 x 7 = 49 x .7854 = 38.4846. The average pressure of
60 pounds would be 50 pounds less 5 = 45 pounds; 38-4846 x
45 = 1731.8070 x .333 1/3, (the number of feet the piston
travels per minute) 577,269.
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