overs, there is to be added
that of the auxiliaries. Again it is impossible to make a definite
statement of what this allowance should be, the figure depending wholly
upon the type and the number of such auxiliaries. For approximate work,
it is perhaps best to allow 15 or 20 per cent of the steam requirements
of the main engines, for that of auxiliaries. Whatever figure is used
should be taken high enough to be on the conservative side.

When any such figures are based on the actual weight of steam required,
Table 60, which gives the actual evaporation for various pressures and
temperatures of feed corresponding to one boiler horse power (34.5
pounds of water per hour from and at 212 degrees), may be of service.

With the steam requirements known, the next step is the determination of
the number and size of boiler units to be installed. This is directly
affected by the capacity at which a consideration of the economical load
indicates is the best for the operating conditions which will exist. The
other factors entering into such determination are the size of the plant
and the character of the feed water.

The size of the plant has its bearing on the question from the fact that
higher efficiencies are in general obtained from large units, that labor
cost decreases with the number of units, the first cost of brickwork is
lower for large than for small size units, a general decrease in the
complication of piping, etc., and in general the cost per horse power of
any design of boiler decreases with the size of units. To illustrate
this, it is only necessary to consider a plant of, say, 10,000 boiler
horse power, consisting of 40-250 horse-power units or 17-600
horse-power units.

The feed water available has its bearing on the subject from the other
side, for it has already been shown that very large units are not
advisable where the feed water is not of the best.

The character of an installment is also a factor. Where, say, 1000 horse
power is installed in a plant where it is known what the ultimate
capacity is to be, the size of units should be selected with the idea of
this ultimate capacity in mind rather than the amount of the first
installation.

Boiler service, from its nature, is severe. All boilers have to be
cleaned from time to time and certain repairs to settings, etc., are a
necessity. This makes it necessary, in determining the number of boilers
to be installed, to allow a certain number of units or spar