aching of the
potential efficiencies made possible by the refinement of design, and a
systematic supervision of the operation assisted by a detailed record of
performances and conditions. The question of supervision will be taken
up later in the chapter on "Operation and Care of Boilers".

The efficiencies that may be expected from the combination of
well-designed boilers and furnaces are indicated in Table 59 in which
are given a number of tests with various fuels and under widely
different operating conditions.

It is to be appreciated that the results obtained as given in this table
are practically all under test conditions. The nearness with which
practical operating conditions can approach these figures will depend
upon the character of the supervision of the boiler room and the
intelligence of the operating crew. The size of the plant will
ordinarily govern the expense warranted in securing the right sort of
supervision.

The bearing that the type of boiler has on the efficiency to be expected
can only be realized from a study of the foregoing chapters.

Capacity--Capacity, as already defined, is the ability of a definite
amount of boiler-heating surface to generate steam. Boilers are
ordinarily purchased under a manufacturer's specification, which rates a
boiler at a nominal rated horse power, usually based on 10 square feet
of heating surface per horse power. Such a builders' rating is
absolutely arbitrary and implies nothing as to the limiting amount of
water that this amount of heating surface will evaporate. It does not
imply that the evaporation of 34.5 pounds of water from and at 212
degrees with 10 square feet of heating surface is the limit of the
capacity of the boiler. Further, from a statement that a boiler is of a
certain horse power on the manufacturer's basis, it is not to be
understood that the boiler is in any state of strain when developing
more than its rated capacity.

Broadly stated, the evaporative capacity of a certain amount of heating
surface in a well-designed boiler, that is, the boiler horse power it is
capable of producing, is limited only by the amount of fuel that can be
burned under the boiler. While such a statement would imply that the
question of capacity to be secured was simply one of making an
arrangement by which sufficient fuel could be burned under a definite
amount of heating surface to generate the required amount of steam,
there are limiting features that must be w