ng at which a boiler plant should be run is dependent
solely upon the load to be carried by that individual plant and the
nature of such load. The economical load for each individual plant can
be determined only from the careful study of each individual set of
conditions or by actual trial.

The controlling factor in the cost of the plant, regardless of the
nature of the load, is the capacity to carry the maximum peak load that
may be thrown on the plant under any conditions.

While load conditions, do, as stated, vary in every individual plant, in
a broad sense all loads may be grouped in three classes: 1st, the
approximately constant 24-hour load; 2nd, the steady 10 or 12-hour load
usually with a noonday period of no load; 3rd, the 24-hour variable
load, found in central station practice. The economical load at which
the boiler may be run will vary with these groups:

1st. For a constant load, 24 hours in the day, it will be found in most
cases that, when all features are considered, the most economical load
or that at which a given amount of steam can be produced the most
cheaply will be considerably over the rated horse power of the boiler.
How much above the rated capacity this most economic load will be, is
dependent largely upon the cost of coal at the plant, but under ordinary
conditions, the point of maximum economy will probably be found to be
somewhere between 25 and 50 per cent above the rated capacity of the
boilers. The capital investment must be weighed against the coal saving
through increased thermal efficiency and the labor account, which
increases with the number of units, must be given proper consideration.
When the question is considered in connection with a plant already
installed, the conditions are different from where a new plant is
contemplated. In an old plant, where there are enough boilers to operate
at low rates of capacity, the capital investment leads to a fixed
charge, and it will be found that the most economical load at which
boilers may be operated will be lower than where a new plant is under
consideration.

2nd. For a load of 10 or 12 hours a day, either an approximately steady
load or one in which there is a peak, where the boilers have been banked
over night, the capacity at which they may be run with the best economy
will be found to be higher than for uniform 24-hour load conditions.
This is obviously due to original investment, that is, a given amount of
invested capital ca