Temperature of the steam.

Temperature of the feed-water.

32°

50°

68°

86°

104°

122°

140°

158°

176°

194°

212°

212°....

1.19

1.17

1.15

1.13

1.11

1.10

1.08

1.06

1.04

1.02

1.00

230°.....

1.20

1.18

l.l6

1.14

1.12

1.10

1.08

1.06

1.04

1.02

1.01

248°.....

1.20

1.18

1.16

1.14

1.13

1.11

1.09

1.07

1.05

1.03

1.01

266°.....

1.21

1.19

1.17

1.15

1.13

1.11

1.09

1.07

1.06

1.04

1.02

284°. ...

1.21

1.20

1.18

1.16

1.14

1.12

1.10

1.08

1.06

1.04

1.02

302°....

1.22

1.20

1.18

1.16

1.14

1.12

1.11

1.09

1.07

1.05

1.03

320°.....

1.22

1.21

1.19

1.17

1.15

1.13

1.11

1.09

1.07

1.05

1.03

338°.....

1.23

1.21

1.19

1.17

1.15

1.14

1.12

1.10

1.08

1.06

1.04

356°.....

1.23

1.22

1.20

1.18

1.16

1.14

1.12

1.10

1.08

1.06

1.04

374°.....

1.24

1.22

1.20

1.18

1.17

1.15

• 1.13

1.11

1.09

1.07

1.05

392°.....

1.24

1.23

1.21

1.19

1.17

1.15

1.13

1.11

1.09

1.07

1.06

410°....

1.25

1.23,

1.22

1.20

1.18

1.16

l.:14

1.12

1.10

1.08

1.06

To illustrate the use of the tables, suppose an engine of 6 horse-power is supplied with steam at a pressure of 50 pounds per square inch, and that the temperature of the feed-water supplied to the boiler is 160°. It is required to find how much water must be evaporated per hour" from and at 2120 for the engine. The temperature of steam having a pressure of 50 pounds is 298°. In the table of " Factors of Evaporation, " the factor corresponding to a steam temperature of 302° and a feed temperature of 158° (which are the numbers in the table nearest to the given ones), is 1.09. Now this engine requires an evaporation of 6 times 79, or 474 pounds of water per hour, at a pressure of 50 pounds, or an equivalent evaporation " from and at 212°" of 1.09 times 474, which is equal to 516.66 pounds.

IV.-To find the proportions suitable for a boiler which is to have a given evaporation, (a) To ascertain the grate-surface in square feet: Divide the number of pounds of water to be evaporated per hour, from and at 212°, by 75, for cylinder boilers ; by 77, for flue boilers ; by 78, for tubular boilers ; by 80, for locomotive and vertical boilers.

Example.-Suppose that a cylinder boiler is to be proportioned for an evaporation of 500 lbs. of water per hour, at a pressure of 75 lbs., the temperature of the feed-water being 80°. The equivalent evaporation will be 1.17 times 500, or 585 lbs., and the grate-surface 585 divided by 75, or 7-8/10 square feet.

(b) To ascertain the heating surface in square feet: Multiply the grate-surface by 11, for cylinder boilers ; by 17, for flue boilers ; by 30, for tubular, locomotive, and vertical boilers. (c) To ascertain the cross-section of flues or tubes in square feet: Multiply the grate-surface by 0.134. This is an average value for good practice, and it can be varied between the limits of 0.125 and 0.143, as may be most convenient, (d) To ascertain the length of hotter: Cylinder boilers should be from 10 to 12 times the diameter; flue boilers from 5 to 6 times the diameter; tubular boilers, and the shells of locomotive and vertical boilers, from 3 to 3-1/2 times the diameter. There is very great variation from these figures in practice; but the numbers given above represent the most general limits, so far as they can conveniently be classified.

Pressure by gauge.

Temperature Fahrenheit.

170........

...........375°

180........

.........379°

190........

...........384°

200........

............388°

Example.-What are the dimensions of a tubular boiler for an engine that is to develop 4-1/2| horse-power, with a steam-pressure of 100 lbs., the temperature of the feed-water being 100°?

The equivalent evaporation required per horse-power per hour is 1.1 times 63, or 69-3/10 lbs. The total equivalent evaporation is 4-1/2 times 69-3/10, or about 312 lbs. Hence the grate-surface, being the quotient arising from dividing 312 by 78, is 4 square feet. The heating surface is 30 times 4, or 120 square feet. The cross-section of the tubes should be about 0.536 square feet (4 times 0.134), or it should vary between the limits of 0.5(4 times 0.125) and 0.572 (4 times 0.143) square feet.

While the rules relating to engines given above are generally only applicable for cases within the limits mentioned at the beginning of this article, those for the proportions of boilers give safe average values for the majority of cases that are met with in practice. B.