A consideration of the losses in boiler efficiency, due to the effects
of excess air, clearly indicates the necessity of maintaining the brick
setting of a boiler tight and free from air leaks. In view of the
temperatures to which certain portions of such a setting are subjected,
the material to be used in its construction must be of the best
procurable.
Boiler settings to-day consist almost universally of brickwork--two
kinds being used, namely, red brick and fire brick.
The red brick should only be used in such portions of the setting as are
well protected from the heat. In such location, their service is not so
severe as that of fire brick and ordinarily, if such red brick are
sound, hard, well burned and uniform, they will serve their purpose.
The fire brick should be selected with the greatest care, as it is this
portion of the setting that has to endure the high temperatures now
developed in boiler practice. To a great extent, the life of a boiler
setting is dependent upon the quality of the fire brick used and the
care exercised in its laying.
The best fire brick are manufactured from the fire clays of
Pennsylvania. South and west from this locality the quality of fire clay
becomes poorer as the distance increases, some of the southern fire
clays containing a considerable percentage of iron oxide.
Until very recently, the important characteristic on which to base a
judgment of the suitability of fire brick for use in connection with
boiler settings has been considered the melting point, or the
temperature at which the brick will liquify and run. Experience has
shown, however, that this point is only important within certain limits
and that the real basis on which to judge material of this description
is, from the boiler man's standpoint, the quality of plasticity under a
given load. This tendency of a brick to become plastic occurs at a
temperature much below the melting point and to a degree that may cause
the brick to become deformed under the stress to which it is subjected.
The allowable plastic or softening temperature will naturally be
relative and dependent upon the stress to be endured.
With the plasticity the determining factor, the perfect fire brick is
one whose critical point of plasticity lies well above the working
temperature of the fire. It is probable that there are but few brick on
the market which would not show, if tested, this critical temperature at
the stress met with in arch construction at a point less than 2400
degrees. The fact that an arch will stand for a long period under
furnace temperatures considerably above this point is due entirely to
the fact that its temperature as a whole is far below the furnace
temperature and only about 10 per cent of its cross section nearest the
fire approaches the furnace temperature. This is borne out by the fact
that arches which are heated on both sides to the full temperature of an
ordinary furnace will first bow down in the middle and eventually fall.
A method of testing brick for this characteristic is given in the
Technologic Paper No. 7 of the Bureau of Standards dealing with "The
testing of clay refractories with special reference to their load
carrying capacity at furnace temperatures." Referring to the test for
this specific characteristic, this publication recommends the following:
"When subjected to the load test in a manner substantially as described
in this bulletin, at 1350 degrees centigrade (2462 degrees Fahrenheit),
and under a load of 50 pounds per square inch, a standard fire brick
tested on end should show no serious deformation and should not be
compressed more than one inch, referred to the standard length of nine
inches."
In the Bureau of Standards test for softening temperature, or critical
temperature of plasticity under the specified load, the brick are tested
on end. In testing fire brick for boiler purposes such a method might be
criticised, because such a test is a compression test and subject to
errors from unequal bearing surfaces causing shear. Furthermore, a
series of samples, presumably duplicates, will not fail in the same way,
due to the mechanical variation in the manufacture of the brick. Arches
that fail through plasticity show that the tensile strength of the brick
is important, this being evidenced by the fact that the bottom of a
wedge brick in an arch that has failed is usually found to be wider than
the top and the adjacent bricks are firmly cemented together.
A better method of testing is that of testing the brick as a beam
subjected to its own weight and not on end. This method has been used
for years in Germany and is recommended by the highest authorities in
ceramics. It takes into account the failure by tension in the brick as
well as by compression and thus covers the tension element which is
important in arch construction.
The plastic point under a unit stress of 100 pounds per square inch,
which may be taken as the average maximum arch stress, should be above
2800 degrees to give perfect results and should be above 2400 degrees to
enable the brick to be used with any degree of satisfaction.
The other characteristics by which the quality of a fire brick is to be
judged are:
Fusion point. In view of the fact that the critical temperature of
plasticity is below the fusion point, this is only important as an
indication from high fusion point of a high temperature of plasticity.
Hardness. This is a relative quality based on an arbitrary scale of 10
and is an indication of probable cracking and spalling.
Expansion. The lineal expansion per brick in inches. This characteristic
in conjunction with hardness is a measure of the physical movement of
the brick as affecting a mass of brickwork, such movement resulting in
cracked walls, etc. The expansion will vary between wide limits in
different brick and provided such expansion is not in excess of, say,
.05 inch in a 9-inch brick, when measured at 2600 degrees, it is not
particularly important in a properly designed furnace, though in general
the smaller the expansion the better.
Compression. The strength necessary to cause crushing of the brick at
the center of the 4½ inch face by a steel block one inch square. The
compression should ordinarily be low, a suggested standard being that a
brick show signs of crushing at 7500 pounds.
Size of Nodules. The average size of flint grains when the brick is
carefully crushed. The scale of these sizes may be considered: Small,
size of anthracite rice; large, size of anthracite pea.
Ratio of Nodules. The percentage of a given volume occupied by the flint
grains. This scale may be considered: High, 90 to 100 per cent; medium,
50 to 90 per cent; low, 10 to 50 per cent.
The statement of characteristics suggested as desirable, are for arch
purposes where the hardest service is met. For side wall purposes the
compression and hardness limit may be raised considerably and the
plastic point lowered.
Aside from the physical properties by which a fire brick is judged, it
is sometimes customary to require a chemical analysis of the brick. Such
an analysis is only necessary as determining the amount of total basic
fluxes (K_{2}O, Na_{2}O, CaO, MgO and FeO). These fluxes are ordinarily
combined into one expression, indicated by the symbol RO. This total
becomes important only above 0.2 molecular equivalent as expressed in
ceramic empirical formulae, and this limit should not be exceeded.[75]
From the nature of fire brick, their value can only be considered from a
relative standpoint. Generally speaking, what are known as first-grade
fire brick may be divided into three classes, suitable for various
conditions of operation, as follows:
Class A. For stoker-fired furnaces where high overloads are to be
expected or where other extreme conditions of service are apt to occur.
Class B. For ordinary stoker settings where there will be no excessive
overloads required from the boiler or any hand-fired furnaces where the
rates of driving will be high for such practice.
Class C. For ordinary hand-fired settings where the presumption is that
the boilers will not be overloaded except at rare intervals and for
short periods only.
Table 61 gives the characteristics of these three classes according to
the features determining the quality. This table indicates that the
hardness of the brick in general increases with the poorer qualities.
Provided the hardness is sufficient to enable the brick to withstand its
load, additional hardness is a detriment rather than an advantage.