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HORSE-POWER. 317
HORSE=POWER.
The term horse-power, as applied in mechanical calcula*
tions, is 33,000 foot-pounds of work performed per minute, or
550 foot-pounds of work per second.
To Calculate the Horse=Power of a Steam Engine.
Rule.
Multiply the area of piston in square inches by the mean
effective steam pressure, and this by the piston speed in feet
per minute, and divide this product by 33,000. The quotient is
the horse-power of the engine.
Formula
:
Horse-power = 0-7854 Z* X j»_X 2 , X »
33000
D = Diameter of piston in inches.
p — Mean effective steam pressure in pounds per square
inch.
s = Length of stroke in feet.
n = Number of revolutions per minute.
Example.
What is the horse-power of a steam engine of the following
dimensions ?
Cylinder, 20 inches diameter ; length of stroke, 3 feet
;
number of revolutions per minute, 75 ; mean effective steam
pressure in cylinder during the stroke, 60 pounds per square
inch.
Horse-power = 2°2
X ’ 7854 X 2 X 3 X 75 X 60
33000
Horse-power = 314.16 X 450 X
33000
Horse-power = 257.04
To Calculate the Horse=Power of a Compound or Triple
Expansion Engine.
Rule.
Calculate the mean effective pressure of the steam (accord-
ing to its number of expansions and initial pressure), and cal-
culate the horse-power exactly as if it was a single cylinder
engine of the same size as the size of the last cylinder.
Another way is to take indicator diagrams of each cylinder,
and calculate the power of each cylinder separately.
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