eighed against the advantages
of high capacity developed from small heating surfaces. Briefly stated,
these factors are as follows:

1st. Efficiency. As the capacity increases, there will in general be a
decrease in efficiency, this loss above a certain point making it
inadvisable to try to secure more than a definite horse power from a
given boiler. This loss of efficiency with increased capacity is treated
below in detail, in considering the relation of efficiency to capacity.

2nd. Grate Ratio Possible or Practicable. All fuels have a maximum rate
of combustion, beyond which satisfactory results cannot be obtained,
regardless of draft available or which may be secured by mechanical
means. Such being the case, it is evident that with this maximum
combustion rate secured, the only method of obtaining added capacity
will be through the addition of grate surface. There is obviously a
point beyond which the grate surface for a given boiler cannot be
increased. This is due to the impracticability of handling grates above
a certain maximum size, to the enormous loss in draft pressure through a
boiler resulting from an attempt to force an abnormal quantity of gas
through the heating surface and to innumerable details of design and
maintenance that would make such an arrangement wholly unfeasible.

3rd. Feed Water. The difficulties that may arise through the use of poor
feed water or that are liable to happen through the use of practically
any feed water have already been pointed out. This question of feed is
frequently the limiting factor in the capacity obtainable, for with an
increase in such capacity comes an added concentration of such
ingredients in the feed water as will cause priming, foaming or rapid
scale formation. Certain waters which will give no trouble that cannot
be readily overcome with the boiler run at ordinary ratings will cause
difficulties at higher ratings entirely out of proportion to any
advantage secured by an increase in the power that a definite amount of
heating surface may be made to produce.

Where capacity in the sense of overload is desired, the type of boiler
selected will play a large part in the successful operation through such
periods. A boiler must be selected with which there is possible a
furnace arrangement that will give flexibility without undue loss in
efficiency over the range of capacity desired. The heating surface must
be so arranged that it will be possible to install in a