bortions enable us to establish the following rule, cable to flue-boilers of every class:

E THE PROPER SECTIONAL AREA OF THE FLUE IN

FLUE-BOILERS.

ply the square root of the number of pounds of med per hour by the constant number 300, and diroduct by the square root of the height of the chim: the quotient is the proper sectional area of the are inches.

-What is the proper sectional area of the flue in urning 100 lbs. of coal per hour, the chimney being

0010, and 10 × 300 :

3000; which divided by root of 49) = 428 square inches, which is the f the flue in this boiler.

.-What is the proper sectional area of the flue in -urning 30 lbs. of coal per hour, the chimney being

=

5= 5·48, and 5·48 × 300: 1644; which divided are root of 81) = 183 nearly, which is the proper

e in square inches.

squar It

Large that a

of lar

for pu

the eff

Th

power

PRO

Horse P

9

10

11

12

13

14

15

16

17

18

19

20

30

45

60

75

100

BOULTON AND WATT'S PRACTICE.

311

Example 3.-What is the proper area of the flue in a flueboiler burning 1,000 lbs. of coal per hour, and with the chimney 49 feet high?

Here

=

1000 31.78, which × 300 9534, and dividing by we get 1,362, as the proper

7 (which is the square root of 49), area of the flue in square inches. square inches per horse-power.

This is equivalent to 13.62

It is the universal experience with boilers of every class, that Large boilers are more economical than small, or, in other words, that a given quantity of coal will boil off more water in boilers of large power than in boilers of small power. Nevertheless, for purposes of classification, it may be convenient to assume the efficiencies as equal.

The proper proportions of flue-boilers from 1 to 100 horses power are given in the following Table :

PROPER PROPORTIONS OF FLUE-BOILERS OF DIFFERENT POWERS.

Mr. Watt reckoned that in his boilers 8 lbs. of coal would evaporate a cubic foot of water in the hour, which is equivalent to an actual horse-power in the case of engines working without expansion. Good Welsh coal, however, it has been found, will evaporate 10 lbs. of water for each pound of coal, which is equivalent to 1.6 cubic feet of water, or 1·6 horse's power in the case of an engine working without expansion; and if such a measure of expansion be used as will double the efficiency of the steam, then 10 lbs. of coal burned in the furnace will generate 3.2 actual horses' power. To attain this measure of efficiency, however, the steam would have to be cut off between

and of the stroke, and in the best boilers and engines working with the usual rates of expansion it will not be safe to reckon more than 2 (or at most 21) actual horses' power as obtainable by the evaporation of a cubic foot of water. When, therefore, engines work up to five times their nominal power, as they now often do, it can only be done by passing through them twice the quantity of steam that answers to their nominal power -or, in other words, by making the boilers of twice the proportionate size, unless where some expedient for producing an artificial draught is employed.

The proper height of chimney where the sectional area of the flue is known can easily be deduced from the foregoing rule.

which formula put into words is as follows:

TO FIND THE PROPER HEIGHT OF A CHIMNEY IN FEET WHEN THE NUMBER OF POUNDS OF COAL CONSUMED PER HOUR AND ALSO THE SECTIONAL AREA OF THE FLUE ARE KNOWN.

RULE.-Multiply the square root of the number of pounds of coal consumed per hour by the constant number 300, and divide the product by the sectional area of the flue in square inches; the square of the quotient is the proper height of the chimney in feet.

DIMENSIONS OF CHIMNEYS FOR GIVEN POWERS. 313

Example 1.-What is the proper height of the chimney of a boiler consuming 100 lbs. of coal per hour, and with a sectional area of flue of 428 square inches.

=

Here /100 = 10, and 10 x 300 3000, which divided by 4287, the square of which is 49, which is the proper height of the chimney in feet.

Example 2.-What is the proper height of the chimney of a flue-boiler consuming 100 lbs. of coal per hour, and with a sectional area of flue of 333 square inches?

=

Here /100=10, and 10 × 300 3000, which divided by 3339, the square of which is 81, which is the proper height of the chimney in feet.

In flue-boilers, the sectional area of the chimney will be the same as that of the flue of a boiler of half the power. Hence in the foregoing Table the proper sectional area of the chimney of a 20-horse boiler-the chimney being 49 feet high-will be the same as the sectional area of the flue of a 10-horse boiler, namely 428 square inches, with a height of chimney of 49 feet; and the proper sectional area of the chimney of a 30-horse boiler will be the same as that of the flue of a 15-horse boiler, namely, 524 square inches, with a height of chimney of 49 feet. If the chimney be 91 feet high, then the values will become 333 and 408 square inches respectively. As then the area of the chimney should be the same as that of the flue of the boiler of half the power, it is needless to give a separate rule for finding the area of the chimney, as such rule will be in all respects the same as that for finding the proper area of the flue, except that we take half the number of pounds of coal burned per hour instead of the whole.

In marine tubular boilers the total capacity or bulk of the boiler, exclusive of the chimney, is about 8 cubic feet for each cubic foot of water evaporated per hour-divided in the proportion of 6.5 cubic feet devoted to the water, furnaces, and tubes, and 1.5 cubic foot occupied as a receptacle or repository for the steam. The common diameter of tube in marine boilers is about 3 inches, and the length is 28 or 30 times the diameter. In locomotive toilers the usual diameter of the tubes is 2 inches, and

The area of the blast th of the area of the chimney.

the length is about 60 times the diameter. orifice in locomotives is about The fire-bars are commonly inch thick, and the air-spaces are made 1 inch wide for fast trains. The main dimensions of marine and locomotive boilers required for the evaporation of a cubic foot of water, are given in the following Table :

:

PROPORTIONS OF MODERN BOILERS REQUIRED TO EVAPORATE A CUBIC FOOT OF WATER PER HOUR.

The quantity of coal or coke burned on each square foot of fire-grate in the hour to evaporate a cubic foot of water will of course very much depend on the goodness of the coal or coke. In the above Table the average working result of 8 lbs. of water evaporated by 1 lb. of coal, or a cubic foot of water evaporated by 7.8 lbs. of coal, is taken.

The efficiency of a steam vessel is measured by the expenditure of fuel necessary to transport a given weight at a given speed through a given space, and one of the most efficient steam vessels of recent construction is the steamer Hansa, built by Messrs. Caird & Co., to ply between Bremen and America. In this vessel there are two inverted direct-acting engines, with cylinders 80 inches diameter and 3 feet stroke. There are four tubular boilers, with four furnaces in each, containing a total grate surface of 350 square feet, and a heating surface of 9,200 square feet; besides which there is a superheater, containing a heating surface of 2,100 square feet. The steam is of 25 lbs. pressure on the square inch, and it is condensed by being discharged into a vessel traversed by 3,584 brass tubes, 1 inch external diameter, and 7 feet long. Each tube having 1·75 square