the plant should be made of such size as to enable it to carry these
peaks at the maximum possible overload on the boilers, sufficient margin
of course being allowed for insurance against interruption of service.
With the boilers operating at this maximum overload through the peaks a
large sacrifice in boiler efficiency is allowable, provided that by such
sacrifice the overload expected is secured.

[Illustration: Portion of 4890 Horse-power Installation of Babcock &
Wilcox Boilers at the Billings Sugar Co., Billings, Mont. 694 Horse
Power of these Boilers are Equipped with Babcock and Wilcox Chain Grate
Stokers]

Some methods of handling a load of this nature are given below:

Certain plant operating conditions make it advisable, from the
standpoint of plant economy, to carry whatever load is on the plant at
any time on only such boilers as will furnish the power required when
operating at ratings of, say, 150 to 200 per cent. That is, all boilers
which are in service are operated at such ratings at all times, the
variation in load being taken care of by the number of boilers on the
line. Banked boilers are cut in to take care of increasing loads and
peaks and placed again on bank when the peak periods have passed. It is
probable that this method of handling central station load is to-day the
most generally used.

Other conditions of operation make it advisable to carry the load on a
definite number of boiler units, operating these at slightly below their
rated capacity during periods of light or low loads and securing the
overload capacity during peaks by operating the same boilers at high
ratings. In this method there are no boilers kept on banked fires, the
spares being spares in every sense of the word.

A third method of handling widely varying loads which is coming somewhat
into vogue is that of considering the plant as divided, one part to take
care of what may be considered the constant plant load, the other to
take care of the floating or variable load. With such a method that
portion of the plant carrying the steady load is so proportioned that
the boilers may be operated at the point of maximum efficiency, this
point being raised to a maximum through the use of economizers and the
general installation of any apparatus leading to such results. The
variable load will be carried on the remaining boilers of the plant
under either of the methods just given, that is, at the high ratings of
all boilers in se