he present time 10 square feet of
heating surface is ordinarily considered the equivalent of one boiler
horse power among manufacturers of stationary boilers. In view of the
arbitrary nature of such rating and of the widely varying rates of
evaporation possible per square foot of heating surface with different
boilers and different operating conditions, such a basis of rating has
in reality no particular bearing on the question of horse power and
should be considered merely as a convenience.

The whole question of a unit of boiler capacity has been widely
discussed with a view to the adoption of a standard to which there would
appear to be a more rational and definite basis. Many suggestions have
been offered as to such a basis but up to the present time there has
been none which has met with universal approval or which would appear
likely to be generally adopted.

With the meaning of boiler horse power as given above, that is, a
measure of evaporation, it is evident that the capacity of a boiler is a
measure of the power it can develop expressed in boiler horse power.
Since it is necessary, as stated, for boiler manufacturers to adopt a
standard for reasons of convenience in selling, the horse power for
which a boiler is sold is known as its normal rated capacity.

The efficiency of a boiler and the maximum capacity it will develop can
be determined accurately only by a boiler test. The standard methods of
conducting such tests are given on the following pages, these methods
being the recommendations of the Power Test Committee of the American
Society of Mechanical Engineers brought out in 1913.[61] Certain changes
have been made to incorporate in the boiler code such portions of the
"Instructions Regarding Tests in General" as apply to boiler testing.
Methods of calculation and such matter as are treated in other portions
of the book have been omitted from the code as noted.

[Illustration: Portion of 2600 Horse-power Installation of Babcock &
Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at
the Peter Schoenhofen Brewing Co., Chicago, Ill.]

1. OBJECT

Ascertain the specific object of the test, and keep this in view not
only in the work of preparation, but also during the progress of the
test, and do not let it be obscured by devoting too close attention to
matters of minor importance. Whatever the object of the test may be,
accuracy and reliability must underlie the work from beginnin