practical
manner, sufficient grate surface at or below the maximum combustion rate
to develop the amount of power required. The design of boiler must be
such that there will be no priming or foaming at high overloads and that
any added scale formation due to such overloads may be easily removed.
Certain boilers which deliver commercially dry steam when operated at
about their normal rated capacity will prime badly when run at overloads
and this action may take place with a water that should be easily
handled by a properly designed boiler at any reasonable load. Such
action is ordinarily produced by the lack of a well defined, positive
circulation.

Relation of Efficiency and Capacity--The statement has been made that in
general the efficiency of a boiler will decrease as the capacity is
increased. Considering the boiler alone, apart from the furnace, this
statement may be readily explained.

Presupposing a constant furnace temperature, regardless of the capacity
at which a given boiler is run; to assure equal efficiencies at low and
high ratings, the exit temperature in the two instances would
necessarily be the same. For this temperature at the high rating, to be
identical with that at the low rating, the rate of heat transfer from
the gases to the heating surfaces would have to vary directly as the
weight or volume of such gases. Experiment has shown, however, that this
is not true but that this rate of transfer varies as some power of the
volume of gas less than one. As the heat transfer does not, therefore,
increase proportionately with the volume of gases, the exit temperature
for a given furnace temperature will be increased as the volume of gases
increases. As this is the measure of the efficiency of the heating
surface, the boiler efficiency will, therefore, decrease as the volume
of gases increases or the capacity at which the boiler is operated
increases.

Further, a certain portion of the heat absorbed by the heating surface
is through direct radiation from the fire. Again, presupposing a
constant furnace temperature; the heat absorbed through radiation is
solely a function of the amount of surface exposed to such radiation.
Hence, for the conditions assumed, the amount of heat absorbed by
radiation at the higher ratings will be the same as at the lower ratings
but in proportion to the total absorption will be less. As the added
volume of gas does not increase the rate of heat transfer, there are
therefor