129. The amount of heating surface required per horsepower varies greatly in different kinds of boilers. The following are the heating surfaces commonly allowed:

Square Feet

Cylinder............

6 to 10

Return tubular.........

14 to 18

Vertical tubular........

15 to 20

Water-tube..........

10 to 12

Locomotive..........

12 to 16

Cast-iron sectional.......

10 to 14

In practice, the rate of heat transmission per square foot of heating surface varies from 800 B. T. U. per hour and upwards, in cast-iron sectional boilers for domestic heating, to 15,000 B. T. U. per hour in the high-pressure water-tube boilers used in torpedo boats.

The actual heating surface of a boiler includes all parts which have water on one side of the metal, and gas having a higher temperature than the water on the other. Surfaces which are in contact with steam only have very-little value as heating surfaces.

130. The area of grate required in any given case depends almost entirely upon the type of boiler employed, and the rate of combustion. Assuming that the heating surfaces are ample for the work to be done, the proper area of the grates, in square feet, may be computed by multiplying the horsepower of the boiler by certain factors, which are as follows:

Cylinder boiler...........

.60

Flue................

.45

Return tubular...........

.50

Water-tube.............

.30

Vertical......................

.65

Locomotive (stationary).......

.40

These factors are suitable for a rate of combustion of about 12 pounds of coal per square foot, per hour, and should be modified for other rates.

Practice has shown that, with this rate of combustion, the ratio of heating surface to grate surface, in a return-tubular boiler, should be about 45 to 1 with bituminous coal, and 36 to 1 with anthracite.