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FORCED DRAFT..

Marine boilers, 60-130 lbs. per sq. ft. per hour.

Locomotive boilers, 40-120 lbs. per sq. ft. per hour.

The evaporation per square foot of grate surface depends upon the type, the rate of combustion, condition of boiler and care in firing. The highest rate is obtained with slow rate of combustion, care and skill in firing, and clean plates and tubes. The table gives the equivalent evaporation per pound of coal for several types.

Plain cylindrical 8 pounds.

Vertical pounds.

Cornish pounds.

Lancashire pounds.

Galloway pounds.

Multitubular pounds.

Water tube 6 —12 pounds.

Marine return tube pounds.

Locomotive pounds.

Experiment shows that an increase in the amount of coal burned per square foot of grate per hour gives an increase in the amount of water evaporated ; but a decrease in amount evaporated per pound of fuel, or a decrease in economy.

To find the area of grate for a boiler. Let G = area of grate in square feet, R = rate of combustion in pounds per square foot per hour, E = evaporation per pound of coal.

Then G = (Pounds of water evaporated)/(ExR) Let us take an example. Suppose we have an externally fired multitubular boiler ; assume the rate of combustion to be 12 pounds, and that our type of boiler will evaporate 9 pounds of water per pound of coal. How large must the grate be, if 2400 pounds-of water are evaporated per hour? G = 2400/(ExR) = 2400/(9x12) = 22.2 square feet.

Then 22.2 square feet of grate surface are necessary. In this case the grate probably would be made 6 feet by 4 fees or 24 square feet.