64

MODERN STEAM PRACTICE.

ture of heat by the total cylinder capacity, and by twice the numberof revolutions per minute.

First Method.

Probable indicated power,. 743

x foot-pounds per hour (in indicated horse-power),. 1,980,000

1,471,140,000

The above divided by the probable efficiency of the steam, 0*12, givesthe available heat required in foot-pounds per hour,. = 12,259,500,000

Second Method.

Estimated pressure equivalent to rate of expenditure of heat in steam(lbs. on the square inch) 108^ x estimated total cylinder capacity inprism of x foot x 1 inch x inch by twice the number of revolutions per

hour >. 39.033

4,228,575

28,992

12,259,484,650

“Available Heat required in foot-pounds per hour .—The availableheat of combustion of i lb. of fuel (or rather coal) is to be estimatedby multiplying the total heat of combustion of I lb. of fuel by theefficiency of the furnace.

“The total heat of combustion of I lb. of coal of a good quality formarine purposes may be estimated at from 9,000,000 to 10,000,000foot-pounds, and that of the very best at 12,000,000 foot-pounds.Inferior qualities about two-thirds of the above estimates.

“The efficiency of the furnaces may be roughly estimated as fol-lows—Divide the intended number of square feet of heating surfaceper lb. of fuel per hour, by the same number + 0'5, eleven-twelfthsof the quotient will be the probable efficiency nearly. The followingare examples:—

Square FeetHeating Surfaceper Lb. of Fuelper Hour.

Efficiency

of

Furnace.

Available Heatper Lb. of Coalof Total Heat,10,000,000.

Small Value for Marine Boilers.

o'5°

0'4Ö

4,600,000

r

075

°'55

5,500,000

I 'OO

o-6i

§,100,000

Ordinary Values in Marine Boilers 4

1-25

0 65

6,500,000

1-50

0'6g

6,900,000

l

2 ’OO

073

7,300,000

Water Tube and Cellular Boilers, j

3' 00

079

7,900,000

6'oo

o'84

8,400,000

“The most common values of the available heat of a pound of good