Steam, Its Generation and Use | Page 7

was so small in comparison to the number of shell boilers. The reason
for this is found in the difficulties involved in the design and
construction of water-tube boilers, which design and construction
required a high class of engineering and workmanship, while the plain
cylindrical boiler is comparatively easy to build. The greater skill
required to make a water-tube boiler successful is readily shown in the
great number of failures in the attempts to make them.
[Illustration: Partial View of 7000 Horse-power Installation of Babcock
& Wilcox Boilers at the Philadelphia, Pa., Plant of the Baldwin
Locomotive Works. This Company Operates in its Various Plants a
Total of 9280 Horse Power of Babcock & Wilcox Boilers]

REQUIREMENTS OF STEAM BOILERS
Since the first appearance in "Steam" of the following "Requirements
of a Perfect Steam Boiler", the list has been copied many times either
word for word or clothed in different language and applied to some
specific type of boiler design or construction. In most cases, although
full compliance with one or more of the requirements was structurally
impossible, the reader was left to infer that the boiler under
consideration possessed all the desirable features. It is noteworthy that
this list of requirements, as prepared by George H. Babcock and
Stephen Wilcox, in 1875, represents the best practice of to-day.
Moreover, coupled with the boiler itself, which is used in the largest
and most important steam generating plants throughout the world, the
list forms a fitting monument to the foresight and genius of the
inventors.

REQUIREMENTS OF A PERFECT STEAM BOILER
1st. Proper workmanship and simple construction, using materials
which experience has shown to be the best, thus avoiding the necessity
of early repairs.
2nd. A mud drum to receive all impurities deposited from the water,
and so placed as to be removed from the action of the fire.
3rd. A steam and water capacity sufficient to prevent any fluctuation in
steam pressure or water level.
4th. A water surface for the disengagement of the steam from the water,
of sufficient extent to prevent foaming.
5th. A constant and thorough circulation of water throughout the boiler,
so as to maintain all parts at the same temperature.
6th. The water space divided into sections so arranged that, should any
section fail, no general explosion can occur and the destructive effects
will be confined to the escape of the contents. Large and free passages
between the different sections to equalize the water line and pressure in
all.
7th. A great excess of strength over any legitimate strain, the boiler
being so constructed as to be free from strains due to unequal
expansion, and, if possible, to avoid joints exposed to the direct action
of the fire.
8th. A combustion chamber so arranged that the combustion of the
gases started in the furnace may be completed before the gases escape
to the chimney.
9th. The heating surface as nearly as possible at right angles to the
currents of heated gases, so as to break up the currents and extract the
entire available heat from the gases.

10th. All parts readily accessible for cleaning and repairs. This is a
point of the greatest importance as regards safety and economy.
11th. Proportioned for the work to be done, and capable of working to
its full rated capacity with the highest economy.
12th. Equipped with the very best gauges, safety valves and other
fixtures.
The exhaustive study made of each one of these requirements is shown
by the following extract from a lecture delivered by Mr. Geo. H.
Babcock at Cornell University in 1890 upon the subject:

THE CIRCULATION OF WATER IN STEAM BOILERS
You have all noticed a kettle of water boiling over the fire, the fluid
rising somewhat tumultuously around the edges of the vessel, and
tumbling toward the center, where it descends. Similar currents are in
action while the water is simply being heated, but they are not
perceptible unless there are floating particles in the liquid. These
currents are caused by the joint action of the added temperature and
two or more qualities which the water possesses.
1st. Water, in common with most other substances, expands when
heated; a statement, however, strictly true only when referred to a
temperature above 39 degrees F. or 4 degrees C., but as in the making
of steam we rarely have to do with temperatures so low as that, we may,
for our present purposes, ignore that exception.
2nd. Water is practically a non-conductor of heat, though not entirely
so. If ice-cold water was kept boiling at the surface the heat would not
penetrate sufficiently to begin melting ice at a depth of 3 inches in less
than about two hours. As, therefore, the heated water cannot impart its
heat to its neighboring particles, it remains expanded