rs lies in the fact that the former is composed of parts of
relatively small diameter as against the large diameters necessary in
the latter.

The factor of safety of the boiler parts which come in contact with the
most intense heat in water-tube boilers can be made much higher than
would be practicable in a shell boiler. Under the assumptions considered
above in connection with the thickness of plates required, a number 10
gauge tube (0.134 inch), which is standard in Babcock & Wilcox boilers
for pressures up to 210 pounds under the same allowable stress as was
used in computing Table 1, the safe working pressure for the tubes is
870 pounds per square inch, indicating the very large margin of safety
of such tubes as compared with that possible with the shell of a boiler.

TABLE 1

PLATE THICKNESS REQUIRED
FOR VARIOUS CYLINDER
DIAMETERS

ALLOWABLE STRESS,
12000 POUNDS PER
SQUARE INCH,
200 POUNDS GAUGE
PRESSURE, NO JOINTS

+---------+-----------+
|Diameter | Thickness |
|Inches | Inches |
+---------+-----------+
| 4 | 0.033 |
| 36 | 0.300 |
| 48 | 0.400 |
| 60 | 0.500 |
| 72 | 0.600 |
| 108 | 0.900 |
| 120 | 1.000 |
| 144 | 1.200 |
+---------+-----------+

A further advantage in the water-tube boiler as a class is the
elimination of all compressive stresses. Cylinders subjected to external
pressures, such as fire tubes or the internally fired furnaces of
certain types of boilers, will collapse under a pressure much lower than
that which they could withstand if it were applied internally. This is
due to the fact that if there exists any initial distortion from its
true shape, the external pressure will tend to increase such distortion
and collapse the cylinder, while an internal pressure tends to restore
the cylinder to its original shape.

Stresses due to unequal expansion have been a fruitful source of trouble
in fire-tube boilers.

In boilers of the shell type, the riveted joints of the shell, with
their consequent double thickness of metal exposed to the fire, gives
rise to serious difficulties. Upon these points are concentrated all
strains of unequal expansion, giving rise to frequent leaks and
oftentimes to actual ruptures. Moreover, in the case of such rupture,
the whole body of contained water is liberated instantaneously and a
disastrous and usually fatal explosion results.

Further