________________________________________________________________________
| | | | |
| Characteristics | Class A | Class B | Class C |
|_____________________|________________|________________|________________|
| | | | |
| Fuse Point, Degrees | Safe at Degrees| Safe at Degrees| Safe at Degrees|
| Fahrenheit | 3200-3300 | 2900-3200 | 2900-3000 |
| | | | |
| Compression Pounds | 6500-7500 | 7500-11,000 | 8500-15,000 |
| | | | |
| Hardness Relative | 1-2 | 2-4 | 4-6 |
| | | | |
| Size of Nodules | Medium | Medium to |Medium to Large |
| | | Medium Large | |
| | | | |
| Ratio of Nodules | High | Medium to High | Medium Low |
| | | | to Medium |
|_____________________|________________|________________|________________|
An approximate determination of the quality of a fire brick may be made
from the appearance of a fracture. Where such a fracture is open, clean,
white and flinty, the brick in all probability is of a good quality. If
this fracture has the fine uniform texture of bread, the brick is
probably poor.
In considering the heavy duty of brick in boiler furnaces, experience
shows that arches are the only part that ordinarily give trouble. These
fail from the following causes:
Bad workmanship in laying up of brick. This feature is treated below.
The tendency of a brick to become plastic at a temperature below the
fusing point. The limits of allowable plastic temperature have already
been pointed out.
Spalling. This action occurs on the inner ends of combustion arches
where they are swept by gases at a high velocity at the full furnace
temperature. The most troublesome spalling arises through cold air
striking the heated brickwork. Failure from this cause is becoming rare,
due to the large increase in number of stoker installations in which
rapid temperature changes are to a great degree eliminated. Furthermore,
there are a number of brick on the market practically free from such
defects and where a new brick is considered, it can be tried out and if
the defect exists, can be readily detected and the brick discarded.
Failures of arches from the expansive power of brick are also rare, due
to the fact that there are a number of brick in which the expansion is
well within the allowable limits and the ease with which such defects
may be determined before a brick is used.
Failures through chemical disintegration. Failure through this cause is
found only occasionally in brick containing a high percentage of iron
oxide.
With the grade of brick selected best suited to the service of the
boiler to be set, the other factor affecting the life of the setting is
the laying. It is probable that more setting difficulties arise from the
improper workmanship in the laying up of brick than from poor material,
and to insure a setting which will remain tight it is necessary that the
masonry work be done most carefully. This is particularly true where the
boiler is of such a type as to require combustion arches in the furnace.
Red brick should be laid in a thoroughly mixed mortar composed of one
volume of Portland cement, 3 volumes of unslacked lime and 16 volumes of
clear sharp sand. Not less than 2½ bushels of lime should be used in the
laying up of 1000 brick. Each brick should be thoroughly embedded and
all joints filled. Where red brick and fire brick are both used in the
same wall, they should be carried up at the same time and thoroughly
bonded to each other.
All fire brick should be dry when used and protected from moisture until
used. Each brick should be dipped in a thin fire clay wash, "rubbed and
shoved" into place, and tapped with a wooden mallet until it touches the
brick next below it. It must be recognized that fire clay is not a
cement and that it has little or no holding power. Its action is that of
a filler rather than a binder and no fire-clay wash should be used which
has a consistency sufficient to permit the use of a trowel.
All fire-brick linings should be laid up four courses of headers and one
stretcher. Furnace center walls should be entirely of fire brick. If the
center of such walls are built of red brick, they will melt down and
cause the failure of the wall as a whole.
Fire-brick arches should be constructed of selected brick which are
smooth, straight and uniform. The frames on which such arches are built,
called arch centers, should be constructed of batten strips not over 2
inches wide. The brick should be laid on these centers in courses, not
in rings, each joint being broken with a bond equal to the length of
half a brick. Each course should be first tried in place dry, and
checked with a straight edge to insure a uniform thickness of joint
between courses. Each brick should be dipped on one side and two edges
only and tapped into place with a mallet. Wedge brick courses should be
used only where necessary to keep the bottom faces of the straight brick
course in even contact with the centers. When such contact cannot be
exactly secured by the use of wedge brick, the straight brick should
lean away from the center of the arch rather than toward it. When the
arch is approximately two-thirds completed, a trial ring should be laid
to determine whether the key course will fit. When some cutting is
necessary to secure such a fit, it should be done on the two adjacent
courses on the side of the brick away from the key. It is necessary that
the keying course be a true fit from top to bottom, and after it has
been dipped and driven it should not extend below the surface of the
arch, but preferably should have its lower ledge one-quarter inch above
this surface. After fitting, the keys should be dipped, replaced
loosely, and the whole course driven uniformly into place by means of a
heavy hammer and a piece of wood extending the full length of the keying
course. Such a driving in of this course should raise the arch as a
whole from the center. The center should be so constructed that it may
be dropped free of the arch when the key course is in place and removed
from the furnace without being burned out.
[Illustration: A Typical Steel Casing for a Babcock & Wilcox Boiler
Built by The Babcock & Wilcox Co.]
Care of Brickwork--Before a boiler is placed in service, it is essential
that the brickwork setting be thoroughly and properly dried, or
otherwise the setting will invariably crack. The best method of starting
such a process is to block open the boiler damper and the ashpit doors
as soon as the brickwork is completed and in this way maintain a free
circulation of air through the setting. If possible, such preliminary
drying should be continued for several days before any fire is placed in
the furnace. When ready for the drying out fire, wood should be used at
the start in a light fire which may be gradually built up as the walls
become warm. After the walls have become thoroughly heated, coal may be
fired and the boiler placed in service.
As already stated, the life of a boiler setting is dependent to a large
extent upon the material entering into its construction and the care
with which such material is laid. A third and equally important factor
in the determining of such life is the care given to the maintaining of
the setting in good condition after the boiler is placed in operation.
This feature is discussed more fully in the chapter dealing with general
boiler room management.
Steel Casings--In the chapter dealing with the losses operating against
high efficiencies as indicated by the heat balance, it has been shown
that a considerable portion of such losses is due to radiation and to
air infiltration into the boiler setting. These losses have been
variously estimated from 2 to 10 per cent, depending upon the condition
of the setting and the amount of radiation surface, the latter in turn
being dependent upon the size of the boiler used. In the modern efforts
after the highest obtainable plant efficiencies much has been done to
reduce such losses by the use of an insulated steel casing covering the
brickwork. In an average size boiler unit the use of such casing, when
properly installed, will reduce radiation losses from one to two per
cent., over what can be accomplished with the best brick setting without
such casing and, in addition, prevent the loss due to the infiltration
of air, which may amount to an additional five per cent., as compared
with brick settings that are not maintained in good order. Steel plate,
or steel plate backed by asbestos mill-board, while acting as a
preventative against the infiltration of air through the boiler setting,
is not as effective from the standpoint of decreasing radiation losses
as a casing properly insulated from the brick portion of the setting by
magnesia block and asbestos mill-board. A casing which has been found to
give excellent results in eliminating air leakage and in the reduction
of radiation losses is clearly illustrated on page 306.
Many attempts have been made to use some material other than brick for
boiler settings but up to the present nothing has been found that may be
considered successful or which will give as satisfactory service under
severe conditions as properly laid brickwork.