Page 263 - Fluid mechanics, heat transfer, and mass transfer

CONDUCTION HEAT TRANSFER 243

& High-porosity refractories have low thermal conduc- & Ceramic fiber insulation panels. Ceramic fibers are
tivities due to entrapment of air in the pores. replacing customary refractory insulation. Ceramic
& High-porosity refractories, on the other hand, are fiber insulation has lower thermal conductivities than
structurally weak and cannot cope with flame im- refractories. Consequently, the insulation thick-
pingement in the furnaces and therefore not desirable nesses have reduced considerably for the same duty,
to be used in hotter zones. They shrink on exposure to making the furnace construction lighter and more
high temperatures, which adds to their structural compact. Construction times have reduced and main-
instability. tenance has become easier.
& Dense refractories are used in hotter zones and high- . What are geopolymers? What are their characteristics
porosity refractories are used as backup lining layer and applications?
to serve insulation requirements. & Inorganic polymers are different and show promise
. How are refractory layers organized in furnace for use in elevated temperature insulation applica-
construction? tions. Inorganic polymers made from aluminosili-
cates are termed geopolymers. They are amorphous
& Furnaces may use a number of layers of different
to semicrystalline and consist of two- or three-di-
refractories in their construction. For instance, a
mensional aluminosilicate networks, dependent on
dense hot face material may be used on the inner
the composition. Geopolymers can be formed using a
surfaces that are exposed to the highest temperatures.
relatively low-temperature processing techniques.
& On the outside, a low-density highly insulating re-
& Physical behavior of geopolymers is similar to those
fractory layer may be employed to conserve energy.
of Portland cement. Consequently, they have been
& Between the hot face refractory and the low-density
considered as a possible improvement on conven-
insulation, a number of different layers of interme-
tional cements with respect to compressive strength,
diate materials may also be employed.
resistance to fire, heat, and acidity as well as a
& Each successive layer (moving away from the hot
medium for the encapsulation of hazardous or low/
face) would have an increasingly lower density and
intermediate level radioactive wastes.
more than likely a lower refractoriness compared to
& These are suitable as refractory coatings, heat insu-
the previous layer.
lation materials for continuous use at temperatures in
. What type of insulation is used in modern furnaces?

the range from 1000 to 1400 C.