In securing the best results in fuel economy, the draft area in a boiler
is an important consideration. In fire-tube boilers this area is limited
to the cross sectional area of the fire tubes, a condition further
aggravated in a horizontal boiler by the tendency of the hot gases to
pass through the upper rows of tubes instead of through all of the tubes
alike. In water-tube boilers the draft area is that of the space outside
of the tubes and is hence much greater than the cross sectional area of
the tubes.

Capacity--Due to the generally more efficient circulation found in
water-tube than in fire-tube boilers, rates of evaporation are possible
with water-tube boilers that cannot be approached where fire-tube
boilers are employed.

Quick Steaming--Another important result of the better circulation
ordinarily found in water-tube boilers is in their ability to raise
steam rapidly in starting and to meet the sudden demands that may be
thrown on them.

In a properly designed water-tube boiler steam may be raised from a cold
boiler to 200 pounds pressure in less than one-half hour.

For the sake of comparison with the figure above, it may be stated that
in the U. S. Government Service the shortest time allowed for getting up
steam in Scotch marine boilers is 6 hours and the time ordinarily
allowed is 12 hours. In large double-ended Scotch boilers, such as are
generally used in Trans-Atlantic service, the fires are usually started
24 hours before the time set for getting under way. This length of time
is necessary for such boilers in order to eliminate as far as possible
excessive strains resulting from the sudden application of heat to the
surfaces.

Accessibility--In the "Requirements of a Perfect Steam Boiler", as
stated by Mr. Babcock, he demonstrates the necessity for complete
accessibility to all portions of the boiler for cleaning, inspection and
repair.

Cleaning--When the great difference is realized in performance, both as
to economy and capacity of a clean boiler and one in which the heating
surfaces have been allowed to become fouled, it may be appreciated that
the ability to keep heating surfaces clean internally and externally is
a factor of the highest importance.

Such results can be accomplished only by the use of a design in boiler
construction which gives complete accessibility to all portions. In
fire-tube boilers the tubes are frequently nested together with a space
between them often less than 1