es to be
operated when any of the regular boilers must be taken off the line.
With the steam requirements determined for a plant of moderate size and
a reasonably constant load, it is highly advisable to install at least
two spare boilers where a continuity of service is essential. This
permits the taking off of one boiler for cleaning or repairs and still
allows a spare boiler in the event of some unforeseen occurrence, such
as the blowing out of a tube or the like. Investment in such spare
apparatus is nothing more nor less than insurance on the necessary
continuity of service. In small plants of, say, 500 or 600 horse power,
two spares are not usually warranted in view of the cost of such
insurance. A large plant is ordinarily laid out in a number of sections
or panels and each section should have its spare boiler or boilers even
though the sections are cross connected. In central station work, where
the peaks are carried on the boilers brought up from the bank, such
spares are, of course, in addition to these banked boilers. From the
aspect of cleaning boilers alone, the number of spare boilers is
determined by the nature of any scale that may be formed. If scale is
formed so rapidly that the boilers cannot be kept clean enough for good
operating results, by cleaning in rotation, one at a time, the number of
spares to take care of such proper cleaning will naturally increase.

In view of the above, it is evident that only a suggestion can be made
as to the number and size of units, as no recommendation will hold for
all cases. In general, it will be found best to install units of the
largest possible size compatible with the size of the plant and
operating conditions, with the total power requirements divided among
such a number of units as will give proper flexibility of load, with
such additional units for spares as conditions of cleaning and insurance
against interruption of service warrant.

In closing the subject of the selection of boilers, it may not be out of
place to refer to the effect of the builder's guarantee upon the
determination of design to be used. Here in one of its most important
aspects appears the responsibility of the manufacturer. Emphasis has
been laid on the difference between test results and those secured in
ordinary operating practice. That such a difference exists is well known
and it is now pretty generally realized that it is the responsible
manufacturer who, where guarantees are nec