rnace will to a certain point increase with an increase in
capacity. This makes the ordinary point of maximum combined efficiency
somewhat above the rated capacity of the boiler and in many cases the
combined efficiency will be practically a constant over a considerable
range of ratings. The features limiting the establishing of the point of
maximum efficiency at a high rating are the same as those limiting the
amount of grate surface that can be installed under a boiler. The
relative efficiency of different combinations of boilers and furnaces at
different ratings depends so largely upon the furnace conditions that
what might hold for one combination would not for another.

In view of the above, it is impossible to make a statement of the
efficiency at different capacities of a boiler and furnace which will
hold for any and all conditions. Fig. 40 shows in a general form the
relation of efficiency to capacity. This curve has been plotted from a
great number of tests, all of which were corrected to bring them to
approximately the same conditions. The curve represents test conditions.
The efficiencies represented are those which may be secured only under
such conditions. The general direction of the curve, however, will be
found to hold approximately correct for operating conditions when used
only as a guide to what may be expected.

[Graph: Combined Efficiency of Boiler and Furnace Per Cent
against Per Cent of Boiler's Rated Capacity Developed

Fig. 40. Approximate Variation of Efficiency with Capacity under Test
Conditions]

Economical Loads--With the effect of capacity on economy in mind, the
question arises as to what constitutes the economical load to be
carried. In figuring on the economical load for an individual plant, the
broader economy is to be considered, that in which, against the boiler
efficiency, there is to be weighed the plant first cost, returns on such
investment, fuel cost, labor, capacity, etc., etc. This matter has been
widely discussed, but unfortunately such discussion has been largely
limited to central power station practice. The power generated in such
stations, while representing an enormous total, is by no means the
larger proportion of the total power generated throughout the country.
The factors determining the economic load for the small plant, however,
are the same as in a large, and in general the statements made relative
to the question are equally applicable.

The economical rati