_________________________________________________________________
| | | |
| | | Testing Segments |
| | Round Mesh | Standard Square |
| | | Mesh |
| Trade Name |__________________|__________________|
| | | | | |
| | Through | Over | Through | Over |
| | Inches | Inches | Inches | Inches |
|___________________________|_________|________|_________|________|
| | | | | |
| Broken | 4-1/2 | 3-1/4 | 4 | 2-3/4 |
| Egg | 3-1/4 | 2-3/8 | 2-3/4 | 2 |
| Stove | 2-3/8 | 1-5/8 | 2 | 1-3/8 |
| Chestnut | 1-5/8 | 7/8 | 1-3/8 | 3/4 |
| Pea | 7/8 | 5/8 | 3/4 | 1/2 |
| No. 1 Buckwheat | 5/8 | 3/8 | 1/2 | 1/4 |
| No. 2 Buckwheat or Rice | 3/8 | 3/16 | 1/4 | 1/8 |
| No. 3 Buckwheat or Barley | 3/16 | 3/32 | 1/8 | 1/16 |
|___________________________|_________|________|_________|________|
Anthracite--Anthracite coal is ordinarily marketed under the names and
sizes given in Table 40.
The larger sizes of anthracite are rarely used for commercial steam
generating purposes as the demand for domestic use now limits the
supply. In commercial plants the sizes generally found are Nos. 1, 2 and
3 buckwheat. In some plants where the finer sizes are used, a small
percentage of bituminous coal, say, 10 per cent, is sometimes mixed with
the anthracite and beneficial results secured both in economy and
capacity.
Anthracite coal should be fired evenly, in small quantities and at
frequent intervals. If this method is not followed, dead spots will
appear in the fire, and if the fire gets too irregular through burning
in patches, nothing can be done to remedy it until the fire is cleaned
as a whole. After this grade of fuel has been fired it should be left
alone, and the fire tools used as little as possible. Owing to the
difficulty of igniting this fuel, care must be taken in cleaning fires.
The intervals of cleaning will, of course, depend upon the nature of the
coal and the rate of combustion. With the small sizes and moderately
high combustion rates, fires will have to be cleaned twice on each
eight-hour shift. As the fires become dirty the thickness of the fuel
bed will increase, until this depth may be 12 or 14 inches just before a
cleaning period. In cleaning, the following practice is usually
followed: The good coal on the forward half of the grate is pushed to
the rear half, and the refuse on the front portion either pulled out or
dumped. The good coal is then pulled forward onto the front part of the
grate and the refuse on the rear section dumped. The remaining good coal
is then spread evenly over the whole grate surface and the fire built up
with fresh coal.
A ratio of grate surface to heating surface of 1 to from 35 to 40 will
under ordinary conditions develop the rated capacity of a boiler when
burning anthracite buckwheat. Where the finer sizes are used, or where
overloads are desirable, however, this ratio should preferably be 1 to
25 and a forced blast should be used. Grates 10 feet deep with a slope
of 1½ inches to the foot can be handled comfortably with this class of
fuel, and grates 12 feet deep with the same slope can be successfully
handled. Where grates over 8 feet in depth are necessary, shaking grates
or overlapping dumping grates should be used. Dumping grates may be
applied either for the whole grate surface or to the rear section. Air
openings in the grate bars should be made from 3/16 inch in width for
No. 3 buckwheat to 5/16 inch for No. 1 buckwheat. It is important that
these air openings be uniformly distributed over the whole surface to
avoid blowing holes in the fire, and it is for this reason that
overlapping grates are recommended.
No air should be admitted over the fire. Steam is sometimes introduced
into the ashpit to soften any clinker that may form, but the quantity of
steam should be limited to that required for this purpose. The steam
that may be used in a steam jet blower for securing blast will in
certain instances assist in softening the clinker, but a much greater
quantity may be used by such an apparatus than is required for this
purpose. Combustion arches sprung above the grates have proved of
advantage in maintaining a high furnace temperature and in assisting in
the ignition of fresh coal.
Stacks used with forced blast should be of such size as to insure a
slight suction in the furnace under any conditions of operation. A blast
up to 3 inches of water should be available for the finer sizes supplied
by engine driven fans, automatically controlled by the boiler pressure.
The blast required will increase as the depth of the fuel bed increases,
and the slight suction should be maintained in the furnace by damper
regulation.
The use of blast with the finer sizes causes rapid fouling of the
heating surfaces of the boiler, the dust often amounting to over 10 per
cent of the total fuel fired. Economical disposal of dust and ashes is
of the utmost importance in burning fuel of this nature. Provision
should be made in the baffling of the boiler to accommodate and dispose
of this dust. Whenever conditions permit, the ashes can be economically
disposed of by flushing them out with water.
Bituminous Coals--There is no classification of bituminous coal as to
size that holds good in all localities. The American Society of
Mechanical Engineers suggests the following grading:
_Eastern Bituminous Coals_--
(A) Run of mine coal; the unscreened coal taken from the mine.
(B) Lump coal; that which passes over a bar-screen with openings 1¼
inches wide.
(C) Nut coal; that which passes through a bar-screen with 1¼-inch
openings and over one with ¾-inch openings.
(D) Slack coal; that which passes through a bar-screen with ¾-inch
openings.
_Western Bituminous Coals_--
(E) Run of mine coal; the unscreened coal taken from the mine.
(F) Lump coal; divided into 6-inch, 3-inch and 1¼-inch lump, according
to the diameter of the circular openings over which the respective
grades pass; also 6 × 3-inch lump and 3 × 1¼-inch lump, according as
the coal passes through a circular opening having the diameter of
the larger figure and over that of the smaller diameter.
(G) Nut coal; divided into 3-inch steam nut, which passes through an
opening 3 inches diameter and over 1¼ inches; 1¼ inch nut, which
passes through a 1¼-inch diameter opening and over a ¾-inch
diameter opening; ¾-inch nut, which passes through a ¾-inch
diameter opening and over a 5/8-inch diameter opening.
(H) Screenings; that which passes through a 1¼-inch diameter opening.
As the variation in character of bituminous coals is much greater than
in the anthracites, any rules set down for their handling must be the
more general. The difficulties in burning bituminous coals with economy
and with little or no smoke increases as the content of fixed carbon in
the coal decreases. It is their volatile content which causes the
difficulties and it is essential that the furnaces be designed to
properly handle this portion of the coal. The fixed carbon will take
care of itself, provided the volatile matter is properly burned.
Mr. Kent, in his "Steam Boiler Economy", described the action of
bituminous coal after it is fired as follows: "The first thing that the
fine fresh coal does is to choke the air spaces existing through the bed
of coke, thus shutting off the air supply which is needed to burn the
gases produced from the fresh coal. The next thing is a very rapid
evaporation of moisture from the coal, a chilling process, which robs
the furnace of heat. Next is the formation of water-gas by the chemical
reaction, C + H_{2}O = CO + 2H, the steam being decomposed, its oxygen
burning the carbon of the coal to carbonic oxide, and the hydrogen being
liberated. This reaction takes place when steam is brought in contact
with highly heated carbon. This also is a chilling process, absorbing
heat from the furnaces. The two valuable fuel gases thus generated would
give back all the heat absorbed in their formation if they could be
burned, but there is not enough air in the furnace to burn them.
Admitting extra air through the fire door at this time will be of no
service, for the gases being comparatively cool cannot be burned unless
the air is highly heated. After all the moisture has been driven off
from the coal, the distillation of hydrocarbons begins, and a
considerable portion of them escapes unburned, owing to the deficiency
of hot air, and to their being chilled by the relatively cool heating
surfaces of the boiler. During all this time great volumes of smoke are
escaping from the chimney, together with unburned hydrogen,
hydrocarbons, and carbonic oxide, all fuel gases, while at the same time
soot is being deposited on the heating surface, diminishing its
efficiency in transmitting heat to the water."
To burn these gases distilled from the coal, it is necessary that they
be brought into contact with air sufficiently heated to cause them to
ignite, that sufficient space be allowed for their mixture with the air,
and that sufficient time be allowed for their complete combustion before
they strike the boiler heating surfaces, since these surfaces are
comparatively cool and will lower the temperature of the gases below
their ignition point. The air drawn through the fire by the draft
suction is heated in its passage and heat is added by radiation from the
hot brick surfaces of the furnace, the air and volatile gases mixing as
this increase in temperature is taking place. Thus in most instances is
the first requirement fulfilled. The element of space for the proper
mixture of the gases with the air, and of time in which combustion is to
take place, should be taken care of by sufficiently large combustion
chambers.
Certain bituminous coals, owing to their high volatile content, require
that the air be heated to a higher temperature than it is possible for
it to attain simply in its passage through the fire and by absorption
from the side walls of the furnace. Such coals can be burned with the
best results under fire brick arches. Such arches increase the
temperature of the furnace and in this way maintain the heat that must
be present for ignition and complete combustion of the fuels in
question. These fuels too, sometimes require additional combustion
space, and an extension furnace will give this in addition to the
required arches.
As stated, the difficulty of burning bituminous coals successfully will
increase with the increase in volatile matter. This percentage of
volatile will affect directly the depth of coal bed to be carried and
the intervals of firing for the most satisfactory results. The variation
in the fuel over such wide ranges makes it impossible to definitely
state the thickness of fires for all classes, and experiment with the
class of fuel in use is the best method of determining how that
particular fuel should be handled. The following suggestions, which are
not to be considered in any sense hard and fast rules, may be of service
for general operating conditions for hand firing:
Semi-bituminous coals, such as Pocahontas, New River, Clearfield, etc.,
require fires from 10 to 14 inches thick; fresh coal should be fired at
intervals of 10 to 20 minutes and sufficient coal charged at each firing
to maintain a uniform thickness. Bituminous coals from Pittsburgh Region
require fires from 4 to 6 inches thick, and should be fired often in
comparatively small charges. Kentucky, Tennessee, Ohio and Illinois
coals require a thickness from 4 to 6 inches. Free burning coals from
Rock Springs, Wyoming, require from 6 to 8 inches, while the poorer
grades of Montana, Utah and Washington bituminous coals require a depth
of about 4 inches.
In general as thin fires are found necessary, the intervals of firing
should be made more frequent and the quantity of coal fired at each
interval smaller. As thin fires become necessary due to the character of
the coal, the tendency to clinker will increase if the thickness be
increased over that found to give the best results.
There are two general methods of hand firing: 1st, the spreading method;
and 2nd, the coking method.
[Illustration: Babcock & Wilcox Chain Grate Stoker]
In the spreading method but little fuel is fired at one time, and is
spread evenly over the fuel bed from front to rear. Where there is more
than one firing door the doors should be fired alternately. The
advantage of alternate firing is the whole surface of the fire is not
blanketed with green coal, and steam is generated more uniformly than if
all doors were fired at one time. Again, a better combustion results due
to the burning of more of the volatile matter directly after firing than
where all doors are fired at one time.
In the coking method, fresh coal is fired at considerable depth at the
front of the grate and after it is partially coked it is pushed back
into the furnace. The object of such a method is the preserving of a bed
of carbon at the rear of the grate, in passing over which the volatile
gases driven off from the green coal will be burned. This method is
particularly adaptable to a grate in which the gases are made to pass
horizontally over the fire. Modern practice for hand firing leans more
and more toward the spread firing method. Again the tendency is to work
bituminous coal fires less than formerly. A certain amount of slicing
and raking may be necessary with either method of firing, but in
general, the less the fire is worked the better the results.
Lignites--As the content of volatile matter and moisture in lignite is
higher than in bituminous coal, the difficulties encountered in burning
them are greater. A large combustion space is required and the best
results are obtained where a furnace of the reverberatory type is used,
giving the gases a long travel before meeting the tube surfaces. A fuel
bed from 4 to 6 inches in depth can be maintained, and the coal should
be fired in small quantities by the alternate method. Above certain
rates of combustion clinker forms rapidly, and a steam jet in the ashpit
for softening this clinker is often desirable. A considerable draft
should be available, but it should be carefully regulated by the boiler
damper to suit the condition of the fire. Smokelessness with hand firing
with this class of fuel is a practical impossibility. It has a strong
tendency to foul the heating surfaces rapidly and these surfaces should
be cleaned frequently. Shaking grates, intelligently handled, aid in
cleaning the fires, but their manipulation must be carefully watched to
prevent good coal being lost in the ashpit.
Stokers--The term "automatic stoker" oftentimes conveys the erroneous
impression that such an apparatus takes care of itself, and it must be
thoroughly understood that any stoker requires expert attention to as
high if not higher degree than do hand-fired furnaces.
Stoker-fired furnaces have many advantages over hand firing, but where a
stoker installation is contemplated there are many factors to be
considered. It is true that stokers feed coal to the fire automatically,
but if the coal has first to be fed to the stoker hopper by hand, its
automatic advantage is lost. This is as true of the removal of ash from
a stoker. In a general way, it may be stated that a stoker installation
is not advantageous except possibly for diminishing smoke, unless the
automatic feature is carried to the handling of the coal and ash, as
where coal and ash handling apparatus is not installed there is no
saving in labor. In large plants, however, stokers used in conjunction
with the modern methods of coal storage and coal and ash handling, make
possible a large labor saving. In small plants the labor saving for
stokers over hand-fired furnaces is negligible, and the expense of the
installation no less proportionately than in large plants. Stokers are,
therefore, advisable in small plants only where the saving in fuel will
be large, or where the smoke question is important.
Interest on investment, repairs, depreciation and steam required for
blast and stoker drive must all be considered. The upkeep cost will, in
general, be higher than for hand-fired furnaces. Stokers, however, make
possible the use of cheaper fuels with as high or higher economy than is
obtainable under operating conditions in hand-fired furnaces with a
better grade of fuel. The better efficiency obtainable with a good
stoker is due to more even and continuous firing as against the
intermittent firing of hand-fired furnaces; constant air supply as
against a variation in this supply to meet varying furnace conditions in
hand-fired furnaces; and the doing away to a great extent with the
necessity of working the fires.
Stokers under ordinary operating conditions will give more nearly
smokeless combustion than will hand-fired furnaces and for this reason
must often be installed regardless of other considerations. While a
constant air supply for a given power is theoretically secured by the
use of a stoker, and in many instances the draft is automatically
governed, the air supply should, nevertheless, be as carefully watched
and checked by flue gas analyses as in the case of hand-fired furnaces.
There is a tendency in all stokers to cause the loss of some good fuel
or siftings in the ashpit, but suitable arrangements may be made to
reclaim this.
In respect to efficiency of combustion, other conditions being equal,
there will be no appreciable difference with the different types of
stokers, provided that the proper type is used for the grade of fuel to
be burned and the conditions of operation to be fulfilled. No stoker
will satisfactorily handle all classes of fuel, and in making a
selection, care should be taken that the type is suited to the fuel and
the operating conditions. A cheap stoker is a poor investment. Only the
best stoker suited to the conditions which are to be met should be
adopted, for if there is to be a saving, it will more than cover the
cost of the best over the cheaper stoker.
Mechanical Stokers are of three general types: 1st, overfeed; 2nd,
underfeed; and 3rd, traveling grate. The traveling grate stokers are
sometimes classed as overfeed but properly should be classed by
themselves as under certain conditions they are of the underfeed rather
than the overfeed type.
Overfeed Stokers in general may be divided into two classes, the
distinction being in the direction in which the coal is fed relative to
the furnaces. In one class the coal is fed into hoppers at the front end
of the furnace onto grates with an inclination downward toward the rear
of about 45 degrees. These grates are reciprocated, being made to take
alternately level and inclined positions and this motion gradually
carries the fuel as it is burned toward the rear and bottom of the
furnace. At the bottom of the grates flat dumping sections are supplied
for completing the combustion and for cleaning. The fuel is partly
burned or coked on the upper portion of the grates, the volatile gases
driven off in this process for a perfect action being ignited and burned
in their passage over the bed of burning carbon lower on the grates, or
on becoming mixed with the hot gases in the furnace chamber. In the
second class the fuel is fed from the sides of the furnace for its full
depth from front to rear onto grates inclined toward the center of the
furnace. It is moved by rocking bars and is gradually carried to the
bottom and center of the furnace as combustion advances. Here some type
of a so-called clinker breaker removes the refuse.
Underfeed Stokers are either horizontal or inclined. The fuel is fed
from underneath, either continuously by a screw, or intermittently by
plungers. The principle upon which these stokers base their claims for
efficiency and smokelessness is that the green fuel is fed under the
coked and burning coal, the volatile gases from this fresh fuel being
heated and ignited in their passage through the hottest portion of the
fire on the top. In the horizontal classes of underfeed stokers, the
action of a screw carries the fuel back through a retort from which it
passes upward, as the fuel above is consumed, the ash being finally
deposited on dead plates on either side of the retort, from which it can
be removed. In the inclined class, the refuse is carried downward to the
rear of the furnace where there are dumping plates, as in some of the
overfeed types.
Underfeed stokers are ordinarily operated with a forced blast, this in
some cases being operated by the same mechanism as the stoker drive,
thus automatically meeting the requirements of various combustion rates.
Traveling Grates are of the class best illustrated by chain grate
stokers. As implied by the name these consist of endless grates composed
of short sections of bars, passing over sprockets at the front and rear
of the furnace. Coal is fed by gravity onto the forward end of the
grates through suitable hoppers, is ignited under ignition arches and is
carried with the grate toward the rear of the furnace as its combustion
progresses. When operated properly, the combustion is completed as the
fire reaches the end of the grate and the refuse is carried over this
rear end by the grate in making the turn over the rear sprocket. In some
cases auxiliary dumping grates at the rear of the chain grates are used
with success.
Chain grate stokers in general produce less smoke than either overfeed
or underfeed types, due to the fact that there are no cleaning periods
necessary. Such periods occur with the latter types of stokers at
intervals depending upon the character of the fuel used and the rate of
combustion. With chain grate stokers the cleaning is continuous and
automatic, and no periods occur when smoke will necessarily be produced.
In the earlier forms, chain grates had an objectionable feature in that
the admission of large amounts of excess air at the rear of the furnace
through the grates was possible. This objection has been largely
overcome in recent models by the use of some such device as the bridge
wall water box and suitable dampers. A distinct advantage of chain
grates over other types is that they can be withdrawn from the furnace
for inspection or repairs without interfering in any way with the boiler
setting.
This class of stoker is particularly successful in burning low grades of
coal running high in ash and volatile matter which can only be burned
with difficulty on the other types. The cost of up-keep in a chain
grate, properly constructed and operated, is low in comparison with the
same cost for other stokers.
The Babcock & Wilcox chain grate is representative of this design of
stoker.
Smoke--The question of smoke and smokelessness in burning fuels has
recently become a very important factor of the problem of combustion.
Cities and communities throughout the country have passed ordinances
relative to the quantities of smoke that may be emitted from a stack,
and the failure of operators to live up to the requirements of such
ordinances, resulting as it does in fines and annoyance, has brought
their attention forcibly to the matter.
The whole question of smoke and smokelessness is to a large extent a
comparative one. There are any number of plants burning a wide variety
of fuels in ordinary hand-fired furnaces, in extension furnaces and on
automatic stokers that are operating under service conditions,
practically without smoke. It is safe to say, however, that no plant
will operate smokelessly under any and all conditions of service, nor is
there a plant in which the degree of smokelessness does not depend
largely upon the intelligence of the operating force.
[Illustration: Fig. 26. Babcock & Wilcox Boiler and Superheater Equipped
with Babcock & Wilcox Chain Grate Stoker. This Setting has been
Particularly Successful in Minimizing Smoke]
When a condition arises in a boiler room requiring the fires to be
brought up quickly, the operatives in handling certain types of stokers
will use their slice bars freely to break up the green portion of the
fire over the bed of partially burned coal. In fact, when a load is
suddenly thrown on a station the steam pressure can often be maintained
only in this way, and such use of the slice bar will cause smoke with
the very best type of stoker. In a certain plant using a highly volatile
coal and operating boilers equipped with ordinary hand-fired furnaces,
extension hand-fired furnaces and stokers, in which the boilers with the
different types of furnaces were on separate stacks, a difference in
smoke from the different types of furnaces was apparent at light loads,
but when a heavy load was thrown on the plant, all three stacks would
smoke to the same extent, and it was impossible to judge which type of
furnace was on one or the other of the stacks.
In hand-fired furnaces much can be accomplished by proper firing. A
combination of the alternate and spreading methods should be used, the
coal being fired evenly, quickly, lightly and often, and the fires
worked as little as possible. Smoke can be diminished by giving the
gases a long travel under the action of heated brickwork before they
strike the boiler heating surfaces. Air introduced over the fires and
the use of heated arches, etc., for mingling the air with the gases
distilled from the coal will also diminish smoke. Extension furnaces
will undoubtedly lessen smoke where hand firing is used, due to the
increase in length of gas travel and the fact that this travel is
partially under heated brickwork. Where hand-fired grates are
immediately under the boiler tubes, and a high volatile coal is used, if
sufficient combustion space is not provided the volatile gases,
distilled as soon as the coal is thrown on the fire, strike the tube
surfaces and are cooled below the burning point before they are wholly
consumed and pass through as smoke. With an extension furnace, these
volatile gases are acted upon by the radiant heat from the extension
furnace arch and this heat, together with the added length of travel
causes their more complete combustion before striking the heating
surfaces than in the former case.
Smoke may be diminished by employing a baffle arrangement which gives
the gases a fairly long travel under heated brickwork and by introducing
air above the fire. In many cases, however, special furnaces for smoke
reduction are installed at the expense of capacity and economy.
From the standpoint of smokelessness, undoubtedly the best results are
obtained with a good stoker, properly operated. As stated above, the
best stoker will cause smoke under certain conditions. Intelligently
handled, however, under ordinary operating conditions, stoker-fired
furnaces are much more nearly smokeless than those which are hand fired,
and are, to all intents and purposes, smokeless. In practically all
stoker installations there enters the element of time for combustion,
the volatile gases as they are distilled being acted upon by ignition or
other arches before they strike the heating surfaces. In many instances
too, stokers are installed with an extension beyond the boiler front,
which gives an added length of travel during which, the gases are acted
upon by the radiant heat from the ignition or supplementary arches, and
here again we see the long travel giving time for the volatile gases to
be properly consumed.
To repeat, it must be emphatically borne in mind that the question of
smokelessness is largely one of degree, and dependent to an extent much
greater than is ordinarily appreciated upon the handling of the fuel and
the furnaces by the operators, be these furnaces hand fired or
automatically fired.
[Illustration: 3520 Horse-power Installation of Babcock & Wilcox Boilers
at the Portland Railway, Light and Power Co., Portland, Ore. These
Boilers are Equipped with Wood Refuse Extension Furnaces at the Front
and Oil Burning Furnaces at the Mud Drum End]