COMBUSTION AND BOILER SETTINGS 335
long as a portion of the perimeter of the tube is covered with
water there is no danger of the tube becoming overheated. The
hydrostatic head causing the flow of water into the tube will be
due to any difference of temperature between the front and rear
tube headers and the height of the water column above the tube.
Some experiments have indicated that this velocity decreases as
.the boiler is forced above rating. Should the flow of water be
interrupted it will only require a few seconds for the tube to
become full of dry steam, it would then rapidly heat until it burst.
This would throw full boiler pressure against any slight obstruc-
tion and immediately remove all evidence of the cause of the
trouble. With tube pitches of less than that assumed, the liability
to interruption in the water circulation increases. The slight
differential head that exists through a tube might readily oe
sufficient to hold a piece of loose scale over the end of a tube and
be insufficient to break it; that is, a very slight obstacle would
suffice to stop the flow of water or so reduce it that a considerable
portion of the tube surface would become dry. Another possible
cause of burst tubes is the forcing of the boiler to such an extent
that the water inflow into a tube becomes insufficient to provide
for the evaporation taking place. This last condition would be
more likely to occur in a clean tube, free from scale internally and
from dust externally, than in a dirty tube.
When the boiler tubes are vertical or nearly vertical the
water circulation is enhanced by the " air-lift effect " of the steam
bubbles. Like the air lift, this is a problem that involves so
many uncontrollable variables that it is doubtful whether any
rational expression for this circulation will ever be worked out.
The investigation of this circulation, however, will throw con-
siderable light upon the rational design of steam boilers. It is
very possible that a modification of the design of the Niclausse
boiler, having the field tubes set vertically with the manifold at
the top, will offer almost unlimited forcing possibilities, greatly
exceeding the evaporative capacity of existing designs. To secure
increased circulation in any design it is rather important that the
course of the water and the steam bubbles should be arranged in
such a manner that they do not impede each other. Theoretically,
there should be no limitation to the amount of forcing which a
vertical-tube boiler can stand except the heat-absorption capacity
of its heating surface. With inclined tubes the boiler can be
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