P.L.A.N.
= The indicated horse power. 33,000 Where P is the mean effective pressure (as shown by a steam engine indicator) per square inch, acting on the piston. L is the length of the stroke in feet.
A is the area of the piston in sq. inches (area of the piston rod to be deducted for the crank end).
N is the number of working strokes per minute (in double acting steam engines it will be twice the number of revolutions).
Indicated Horse Power of Gas Engines.
The same formula is used for this calculation, except that the symbol N represents the total num ber of explosions per minute in all cylinders.
In computations for four cycle single cylinder, single-acting gas engines firing every fourth stroke, this will be the revolutions per minute, divided by two. For two cycle single cylinder, single-acting engines, firing every second stroke, or once every rev olution, the value of N will be represented by the number of revolutions per minute.
Electrical Horse Power: Multiply the volts by the amperes and the product will be watts. Since 746 watts is equivalent to an
electrical horse power, dividing this product by 746 gives electrical horse power. If the circuit is an alternating one, multiply the product of volts and amperes by the power factor before dividing by 746. Boiler Horse Power: The evaporation of 34.5 pounds of water per hour from a feed water temperature of 212° F. into steam at the same temperature is a standard commercial boiler horse power and is considered as equivalent to the evaporation of 3o pounds of water per hour from a feed water temperature of ioo° F. into steam at 7o pounds pressure. The former is called "The equivalent evaporation from and at 212° F.," and is obtained by multiplying the actual evaporation by the "factor of evaporation" for any given feed water temperature and boiler pressure.