, unequal strains result in shell or fire-tube boilers due to the
difference in temperature of the various parts. This difference in
temperature results from the lack of positive well defined circulation.
While such a circulation does not necessarily accompany all water-tube
designs, in general, the circulation in water-tube boilers is much more
defined than in fire-tube or shell boilers.

A positive and efficient circulation assures that all portions of the
pressure parts will be at approximately the same temperature and in this
way strains resulting from unequal temperatures are obviated.

If a shell or fire-tubular boiler explodes, the apparatus as a whole is
destroyed. In the case of water-tube boilers, the drums are ordinarily
so located that they are protected from intense heat and any rupture is
usually in the case of a tube. Tube failures, resulting from blisters or
burning, are not serious in their nature. Where a tube ruptures because
of a flaw in the metal, the result may be more severe, but there cannot
be the disastrous explosion such as would occur in the case of the
explosion of a shell boiler.

To quote Dr. Thurston, relative to the greater safety of the water-tube
boiler: "The stored available energy is usually less than that of any of
the other stationary boilers and not very far from the amount stored,
pound for pound, in the plain tubular boiler. It is evident that their
admitted safety from destructive explosion does not come from this
relation, however, but from the division of the contents into small
portions and especially from those details of construction which make it
tolerably certain that any rupture shall be local. A violent explosion
can only come from the general disruption of a boiler and the liberation
at once of large masses of steam and water."

Economy--The requirement probably next in importance to safety in a
steam boiler is economy in the use of fuel. To fulfill such a
requirement, the three items, of proper grate for the class of fuel to
be burned, a combustion chamber permitting complete combustion of gases
before their escape to the stack, and the heating surface of such a
character and arrangement that the maximum amount of available heat may
be extracted, must be co-ordinated.

Fire-tube boilers from the nature of their design do not permit the
variety of combinations of grate surface, heating surface, and
combustion space possible in practically any water-tube boiler.