In selecting a boiler, the most efficient design will be found to be
that in which _the greatest amount of shell surface is exposed to
direct heat_. It is the direct heating surface that does the bulk of
the work and every tendency to reduce it, either in the construction
or setting of the boiler, should be avoided. The smaller the amount of
surface enclosed by or in contact with the setting, the better results
will be obtained.
A boiler with a bad circulation is the bane of an engineer’s existence.
Proper circulation facilities constitute one of the chief factors in
the construction of a successful and economical boiler. In tubular
boilers the best practice places the tubes in vertical rows, leaving
out what would be the centre row. The circulation is up the sides of
the boiler and down the centre. Tubes set zig-zag to break spaces
impede the circulation and are not considered productive of the best
results.
The surface from which evaporation takes place should be made greater
as the steam pressure is reduced, that is to say, as the size of the
bubbles of steam become greater. To produce 100 lbs. of steam per hour
at atmospheric pressure this surface should not be less than 732 square
feet, which may be reduced to 146 square feet for steam at 75 lbs.
pressure, and to 73 feet for steam at a pressure of 150 lbs. It is for
this reason that triple-expansion engines can be worked with smaller
boilers than are required with engines using steam of lower pressure.
The amount of steam space to be permitted depends upon the volume
of the cylinders and the number of revolutions made per minute. For
ordinary engines it may be made a hundred times as great as the average
volume of steam generated per second.
A volume of heated water in a boiler performs the same office in
furnishing a steady supply of steam as a fly-wheel does to an engine in
insuring uniformity of speed; hence the centre space of a boiler should
be ample, in order to take advantage of this reserve force.