n be made to earn a larger return through the higher
overload, and this will hold true to a point where the added return more
than offsets the decrease in actual boiler efficiency. Here again the
determining factors of what is the economical load are the fuel and
labor cost balanced against the thermal efficiency. With a load of this
character, there is another factor which may affect the economical plant
operating load. This is from the viewpoint of spare boilers. That such
added capacity in the way of spares is necessary is unquestionable.
Since they must be installed, therefore, their presence leads to a fixed
charge and it is probable that for the plant, as a whole, the economical
load will be somewhat lower than if the boilers were considered only as
spares. That is, it may be found best to operate these spares as a part
of the regular equipment at all times except when other boilers are off
for cleaning and repairs, thus reducing the load on the individual
boilers and increasing the efficiency. Under such conditions, the added
boiler units can be considered as spares only during such time as some
of the boilers are not in operation.

Due to the operating difficulties that may be encountered at the higher
overloads, it will ordinarily be found that the most economical ratings
at which to run boilers for such load conditions will be between 150 and
175 per cent of rating. Here again the maximum capacity at which the
boilers may be run for the best plant economy is limited by the point at
which the efficiency drops below what is warranted in view of the first
cost of the apparatus.

3rd. The 24-hour variable load. This is a class of load carried by the
central power station, a load constant only in the sense that there are
no periods of no load and which varies widely with different portions of
the 24 hours. With such a load it is particularly difficult to make any
assertion as to the point of maximum economy that will hold for any
station, as this point is more than with any other class of load
dependent upon the factors entering into the operation of each
individual plant.

The methods of handling a load of this description vary probably more
than with any other kind of load, dependent upon fuel, labor, type of
stoker, flexibility of combined furnace and boiler etc., etc.

In general, under ordinary conditions such as appear in city central
power station work where the maximum peaks occur but a few times a year,