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The ASTM C-155 standard
classification for IFB establishes a maximum
bulk density limit and a reheat change maximum
of +/-2% when tested 5000°F below the
temperature rating. The maximum bulk density
assumes low conductivity or good insulating
value. The reheat test ensures suitability at
the temperature rating but only with
nonaggressive atmospheres. There are no
specifications for chemistry or dimensional
changes other than the reheat test. The maximum
density for a Group 26 is 54 pounds per cubic
foot.
The conventional approach of switching to a
higher-temperature-class IFB, such as Group 28,
when there is a problem is not a reasonable
alternative. Higher-temperature grades typically
have higher density and conductivity. The higher
density might help slightly on resistance to
alkali attack, but the higher conductivity would
result in higher heat loss and energy costs
during operations.
There are several ways a manufacturer’s Group 28
IFB maydiffer from its Group 26 IFB.
• From some suppliers, Group 28 IFB have the
same chemical composition as Group 26 IFB. The
only differences are higher density and firing
to result in an acceptable 2750°F reheat
shrinkage. This offers no significant
improvement to Group 26 IFB for resistance to
chemical attack.
• Other suppliers manufacture a Group 28 IFB
that are lowfired. These pass the 2750°F reheat
shrinkage test by relying on expansion during
mullite formation to counteract the
high-temperature related shrinkage and allow the
net change to be in the +/-2% range after the
reheat test. If the IFB are heated only into the
temperature range to experience the mullite
formation and below the temperature where
shrinkage occurs, they expand as much as 7%. A
large tunnel kiln cannot accommodate these large
dimensional changes, even if chemical attack
does not occur.
• Some Group 28 IFB are both high-fired and of a
higher alumina composition. The chemistry might
appear to be more suitable to the alkali-rich
atmosphere; however, high alumina content is not
always an indication of alkali resistance.
Prefabricated side wall sections
of a sanitaryware tunnel kiln before shipping to
the job site. BNZ 26-60 IFB is used on the hot
face and BNZ 2300 HS IFB is used as back-up
insulation.
Some of these have a high free-silica content.
This is thought to be more reactive with alkali
vapors. The higher thermal conductivity is also
a drawback.
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Wanting to supply kilns of the highest quality,
Swindell Dressler
approached BNZ Materials, Inc., with the
problem. A higher alumina content
(60.4%) was chosen to withstand the combination
of temperature and atmosphere. Special raw
materials and firing conditions were also
selected to yield high mullite content and low
free-silica content, as manufactured. The result
became a specialty grade (BNZ 26-60®) that meets
the ASTM C-155 specifications for group 26.
An additional benefit of the resulting IFB is an
18% lower thermal conductivity than a
conventional group 26 IFB. Thermal
conductivity at 2000°F has been measured as 2.3
Btu-in/ft2/hr/F (0.33 W/mk). Better resistance
to damage from thermal cycling has also been
observed. In thermal cycling tests (from room
temperature to around 2000°F), the
specialty-grade brick has lasted three times
longer before cracking compared to the
conventional material.
This specialty grade has been adopted by
Swindell-Dressler as the standard hot zone
lining material in its tunnel kilns for
sanitaryware, tile and other whiteware
applications. This commitment to using quality
materials has helped Swindell Dressler become
one of the largest suppliers of kilns to the
sanitaryware industry in the world, supplying
kilns to 10 countries on four continents. |
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