40 Cost of coal per ton of......pounds delivered in boiler room......dollars
41 Cost of coal required for evaporating 1000 pounds of water
from and at 212 degrees........................................dollars
[Illustration: Portion of 3600 Horse-power Installation of Babcock &
Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at
the Loomis Street Plant of the Peoples Gas Light & Coke Co., Chicago,
Ill. This Company has Installed 7780 Horse Power of Babcock & Wilcox
Boilers]
THE SELECTION OF BOILERS WITH A CONSIDERATION OF THE FACTORS DETERMINING
SUCH SELECTION
The selection of steam boilers is a matter to which the most careful
thought and attention may be well given. Within the last twenty years,
radical changes have taken place in the methods and appliances for the
generation and distribution of power. These changes have been made
largely in the prime movers, both as to type and size, and are best
illustrated by the changes in central station power-plant practice. It
is hardly within the scope of this work to treat of power-plant design
and the discussion will be limited to a consideration of the boiler end
of the power plant.
As stated, the changes have been largely in prime movers, the steam
generating equipment having been considered more or less of a standard
piece of apparatus whose sole function is the transfer of the heat
liberated from the fuel by combustion to the steam stored or circulated
in such apparatus. When the fact is considered that the cost of steam
generation is roughly from 65 to 80 per cent of the total cost of power
production, it may be readily understood that the most fruitful field
for improvement exists in the boiler end of the power plant. The
efficiency of the plant as a whole will vary with the load it carries
and it is in the boiler room where such variation is largest and most
subject to control.
The improvements to be secured in the boiler room results are not simply
a matter of dictation of operating methods. The securing of perfect
combustion, with the accompanying efficiency of heat transfer, while
comparatively simple in theory, is difficult to obtain in practical
operation. This fact is perhaps best exemplified by the difference
between test results and those obtained in daily operation even under
the most careful supervision. This difference makes it necessary to
establish a standard by which operating results may be judged, a
standard not necessarily that which might be possible under test
conditions but one which experiment shows can be secured under the very
best operating conditions.
The study of the theory of combustion, draft, etc., as already given,
will indicate that the question of efficiency is largely a matter of
proper relation between fuel, furnace and generator. While the
possibility of a substantial saving through added efficiency cannot be
overlooked, the boiler design of the future must, even more than in the
past, be considered particularly from the aspect of reliability and
simplicity. A flexibility of operation is necessary as a guarantee of
continuity of service.
In view of the above, before the question of the selection of boilers
can be taken up intelligently, it is necessary to consider the subjects
of boiler efficiency and boiler capacity, together with their relation
to each other.
The criterion by which the efficiency of a boiler plant is to be judged
is the cost of the production of a definite amount of steam. Considered
in this sense, there must be included in the efficiency of a boiler
plant the simplicity of operation, flexibility and reliability of the
boiler used. The items of repair and upkeep cost are often high because
of the nature of the service. The governing factor in these items is
unquestionably the type of boiler selected.
The features entering into the plant efficiency are so numerous that it
is impossible to make a statement as to a means of securing the highest
efficiency which will apply to all cases. Such efficiency is to be
secured by the proper relation of fuel, furnace and boiler heating
surface, actual operating conditions, which allow the approaching 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 weighed against the advantages
of high capacity developed from small heating surfaces. Briefly stated,
these factors are as follows:
1st. Efficiency. As the capacity increases, there will in general be a
decrease in efficiency, this loss above a certain point making it
inadvisable to try to secure more than a definite horse power from a
given boiler. This loss of efficiency with increased capacity is treated
below in detail, in considering the relation of efficiency to capacity.
2nd. Grate Ratio Possible or Practicable. All fuels have a maximum rate
of combustion, beyond which satisfactory results cannot be obtained,
regardless of draft available or which may be secured by mechanical
means. Such being the case, it is evident that with this maximum
combustion rate secured, the only method of obtaining added capacity
will be through the addition of grate surface. There is obviously a
point beyond which the grate surface for a given boiler cannot be
increased. This is due to the impracticability of handling grates above
a certain maximum size, to the enormous loss in draft pressure through a
boiler resulting from an attempt to force an abnormal quantity of gas
through the heating surface and to innumerable details of design and
maintenance that would make such an arrangement wholly unfeasible.
3rd. Feed Water. The difficulties that may arise through the use of poor
feed water or that are liable to happen through the use of practically
any feed water have already been pointed out. This question of feed is
frequently the limiting factor in the capacity obtainable, for with an
increase in such capacity comes an added concentration of such
ingredients in the feed water as will cause priming, foaming or rapid
scale formation. Certain waters which will give no trouble that cannot
be readily overcome with the boiler run at ordinary ratings will cause
difficulties at higher ratings entirely out of proportion to any
advantage secured by an increase in the power that a definite amount of
heating surface may be made to produce.
Where capacity in the sense of overload is desired, the type of boiler
selected will play a large part in the successful operation through such
periods. A boiler must be selected with which there is possible a
furnace arrangement that will give flexibility without undue loss in
efficiency over the range of capacity desired. The heating surface must
be so arranged that it will be possible to install in a practical
manner, sufficient grate surface at or below the maximum combustion rate
to develop the amount of power required. The design of boiler must be
such that there will be no priming or foaming at high overloads and that
any added scale formation due to such overloads may be easily removed.
Certain boilers which deliver commercially dry steam when operated at
about their normal rated capacity will prime badly when run at overloads
and this action may take place with a water that should be easily
handled by a properly designed boiler at any reasonable load. Such
action is ordinarily produced by the lack of a well defined, positive
circulation.
Relation of Efficiency and Capacity--The statement has been made that in
general the efficiency of a boiler will decrease as the capacity is
increased. Considering the boiler alone, apart from the furnace, this
statement may be readily explained.
Presupposing a constant furnace temperature, regardless of the capacity
at which a given boiler is run; to assure equal efficiencies at low and
high ratings, the exit temperature in the two instances would
necessarily be the same. For this temperature at the high rating, to be
identical with that at the low rating, the rate of heat transfer from
the gases to the heating surfaces would have to vary directly as the
weight or volume of such gases. Experiment has shown, however, that this
is not true but that this rate of transfer varies as some power of the
volume of gas less than one. As the heat transfer does not, therefore,
increase proportionately with the volume of gases, the exit temperature
for a given furnace temperature will be increased as the volume of gases
increases. As this is the measure of the efficiency of the heating
surface, the boiler efficiency will, therefore, decrease as the volume
of gases increases or the capacity at which the boiler is operated
increases.
Further, a certain portion of the heat absorbed by the heating surface
is through direct radiation from the fire. Again, presupposing a
constant furnace temperature; the heat absorbed through radiation is
solely a function of the amount of surface exposed to such radiation.
Hence, for the conditions assumed, the amount of heat absorbed by
radiation at the higher ratings will be the same as at the lower ratings
but in proportion to the total absorption will be less. As the added
volume of gas does not increase the rate of heat transfer, there are
therefore two factors acting toward the decrease in the efficiency of a
boiler with an increase in the capacity.