+----------------------------------------------------------------------+
|+--------------------------------------------------------------------+|
|| |B. t. u.|Per Cent||
|+--------------------------------------------------+--------+--------+|
||Heat absorbed by Boiler | 11,363 | 79.88 ||
||Loss due to Evaporation of Moisture in Fuel | 22 | 0.15 ||
||Loss due to Moisture formed by Burning of Hydrogen| 618 | 4.34 ||
||Loss due to Heat carried away in Dry Chimney Gases| 1311 | 9.22 ||
||Loss due to Incomplete Combustion of Carbon | 61 | 0.43 ||
||Loss due to Unconsumed Carbon in the Ash | 261 | 1.83 ||
||Loss due to Radiation and Unaccounted Losses | 589 | 4.15 ||
|+--------------------------------------------------+--------+--------+|
||Total | 14,225 | 100.00 ||
|+--------------------------------------------------+--------+--------+|
+----------------------------------------------------------------------+
Application of Heat Balance--A heat balance should be made in connection
with any boiler trial on which sufficient data for its computation has
been obtained. This is particularly true where the boiler performance
has been considered unsatisfactory. The distribution of the heat is thus
determined and any extraordinary loss may be detected. Where accurate
data for computing such a heat balance is not available, such a
calculation based on certain assumptions is sometimes sufficient to
indicate unusual losses.
The largest loss is ordinarily due to the chimney gases, which depends
directly upon the weight of the gas and its temperature leaving the
boiler. As pointed out in the chapter on flue gas analysis, the lower
limit of the weight of gas is fixed by the minimum air supplied with
which complete combustion may be obtained. As shown, where this supply
is unduly small, the loss caused by burning the carbon to CO instead of
to CO_{2} more than offsets the gain in decreasing the weight of gas.
The lower limit of the stack temperature, as has been shown in the
chapter on draft, is more or less fixed by the temperature necessary to
create sufficient draft suction for good combustion. With natural draft,
this lower limit is probably between 400 and 450 degrees.
Capacity--Before the capacity of a boiler is considered, it is necessary
to define the basis to which such a term may be referred. Such a basis
is the so-called boiler horse power.
The unit of motive power in general use among steam engineers is the
"horse power" which is equivalent to 33,000 foot pounds per minute.
Stationary boilers are at the present time rated in horse power, though
such a basis of rating may lead and has often led to a misunderstanding.
_Work_, as the term is used in mechanics, is the overcoming of
resistance through space, while _power_ is the _rate_ of work or the
amount done per unit of time. As the operation of a boiler in service
implies no motion, it can produce no power in the sense of the term as
understood in mechanics. Its operation is the generation of steam, which
acts as a medium to convey the energy of the fuel which is in the form
of heat to a prime mover in which that heat energy is converted into
energy of motion or work, and power is developed.
If all engines developed the same amount of power from an equal amount
of heat, a boiler might be designated as one having a definite horse
power, dependent upon the amount of engine horse power its steam would
develop. Such a statement of the rating of boilers, though it would
still be inaccurate, if the term is considered in its mechanical sense,
could, through custom, be interpreted to indicate that a boiler was of
the exact capacity required to generate the steam necessary to develop a
definite amount of horse power in an engine. Such a basis of rating,
however, is obviously impossible when the fact is considered that the
amount of steam necessary to produce the same power in prime movers of
different types and sizes varies over very wide limits.
To do away with the confusion resulting from an indefinite meaning of
the term boiler horse power, the Committee of Judges in charge of the
boiler trials at the Centennial Exposition, 1876, at Philadelphia,
ascertained that a good engine of the type prevailing at the time
required approximately 30 pounds of steam per hour per horse power
developed. In order to establish a relation between the engine power and
the size of a boiler required to develop that power, they recommended
that an evaporation of 30 pounds of water from an initial temperature of
100 degrees Fahrenheit to steam at 70 pounds gauge pressure be
considered as _one boiler horse power_. This recommendation has been
generally accepted by American engineers as a standard, and when the
term boiler horse power is used in connection with stationary
boilers[58] throughout this country,[59] without special definition, it
is understood to have this meaning.
Inasmuch as an equivalent evaporation from and at 212 degrees Fahrenheit
is the generally accepted basis of comparison[60], it is now customary
to consider the standard boiler horse power as recommended by the
Centennial Exposition Committee, in terms of equivalent evaporation from
and at 212 degrees. This will be 30 pounds multiplied by the factor of
evaporation for 70 pounds gauge pressure and 100 degrees feed
temperature, or 1.1494. 30 × 1.1494 = 34.482, or approximately 34.5
pounds. Hence, _one boiler horse power is equal to an evaporation of
34.5 pounds of water per hour from and at 212 degrees Fahrenheit_. The
term boiler horse power, therefore, is clearly a measure of evaporation
and not of power.
A method of basing the horse power rating of a boiler adopted by boiler
manufacturers is that of heating surfaces. Such a method is absolutely
arbitrary and changes in no way the definition of a boiler horse power
just given. It is simply a statement by the manufacturer that his
product, under ordinary operating conditions or conditions which may be
specified, will evaporate 34.5 pounds of water from and at 212 degrees
per definite amount of heating surface provided. The amount of heating
surface that has been considered by manufacturers capable of evaporating
34.5 pounds from and at 212 degrees per hour has changed from time to
time as the art has progressed. At the 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.]