One should be familiar with a number of terms which are frequently used in calculations.
Heating surface means all surface having water on one side and fire or heated gases on the other.
Grate surface means the surface of the grate bars, or the area of the surface which supports the burning fuel.
Horse-power. There is no such thing as the horse-power of a boiler. The term horse-power refers to the measurement of power or energy produced in a given time. A boiler does not produce energy; therefore, the work of a boiler cannot be measured by horse-power. Energy is the product of a given force in pounds multiplied by the distance in feet through which it moves; horse-power is obtained by dividing the energy thus obtained in one second by 550; in one minute, by 33,000; and in one hour by 1,980,000. A boiler contains a force only. Therefore the term horse-power is merely relative, and when applied to a boiler conveys to the mind the horse-power of an engine which a boiler of a given size is capable of supplying with steam.
Priming is that process by which the water is carried up into the steam pipes in considerable quantities and frequently over into the engine. The most common cause is a high water line, which may be the effect of a faulty boiler design. Too many tubes, the forcing of a boiler, irregular firing, or sudden opening of the stop valve may also cause it.