Page 257 - Fluid mechanics, heat transfer, and mass transfer

CONDUCTION HEAT TRANSFER 237

& To protect site personnel against burns. Temperatures gases at 130 C. It can also be used for high-

of outer surfaces of insulation should not be allowed temperature applications up to 750 C.

to rise above 60 C to prevent burns. & High-Temperature Applications:
& To prevent condensation. ➢ Mineral/slag wool, glass wool.
& To protect against frost. ➢ Diatomaceous earth, silica powder (up to 820 C).

& To protect equipment from corrosion. ➢ Diatomaceous earth, asbestos, and bonding
& To provide fire protection. material (up to 870 C).

& To provide acoustic insulation. ➢ Glass blocks (up to 650 C).

. What are the desirable characteristics for the selection ➢ 85% magnesia (up to 340 C).
of an insulating material? ➢ Ceramic refractories (for higher temperatures).
& Low thermal conductivity.
➢ Insulating firebrick.
& Resistance to moisture absorption.
➢ Zirconia grain (up to 1650 C).
& High porosity. ➢ Microquartz fibers (up to 1650 C).

& Ease of application on surfaces. ➢ Calcium silicate products are known for excep-
& Nontoxic and nonflammable. tional strength and durability in both intermediate-
& Noncorrosive. and high-temperature applications.
& Good strength. . What is a Supertherm ceramic insulation? What are its
characteristics?
. “Air has very low thermal conductivity and therefore is
one of the best insulators.” Why then heat transfer & Supertherm is essentially a ceramic coating type of
surfaces are provided with various types of solid in- insulation consisting of different types of ceramic
sulators when these surfaces are surrounded by atmo- powders with combinations of acrylic and urethane
spheric air? polymers to provide elasticity, adhesion, toughness,
& Atmospheric air involves convection currents that and moisture repulsion. The ceramic components
remove heat from the hot surfaces or add heat to cold repel radiation with resistance for heat conduction.
surfaces through convection process. Air, in the & It can be used as spray-on coating on irregular
absence of convection currents, will only act as a surfaces.
good insulating material. Therefore, in order to make & Ceramic is more expensive than fiberglass insulation.
air free from convection currents, it has to be broken Therefore, where fiberglass or other conventional
down into tiny globules by entrapping it by noncon- insulation can be effectively used, ceramic is not an
ducting cellular solid structures to make an effective economically attractive insulation material.
insulation. & Ceramic is also a good corrosion inhibitor and more
. Name some thermal insulating materials for low- and
resistant to impact damage and moisture.
high-temperature insulation applications.
& There are types of ceramic that can be applied to
& Low-Temperature Applications:
hot surfaces, reducing or eliminating downtime
➢ Cork board.
application.
➢ Fiberglass with asphalt coating.
& Supertherm is also a good soundproofing material.
➢ Expanded foam glass, glass blocks.
. “Asbestos is a good insulating material.” What is the
➢ Polyurethane, polyisocyanurate, phenolic, polyi- important limitation involved in its application?
mide, and polystyrene foams and elastomeric & Asbestos is found to be a carcinogenic material and
cellular plastic.
many countries banned its manufacture and use.
➢ Vegetable fiberboard with asphalt coating. Asbestos dust resulting from demolition of structures
➢ Mineral wool board or rock wool. with asbestos-containing materials is particularly
➢ Expanded rubber board or Rubatex. harmful to exposed workers.
➢ Silica aerogel powder or Santocel. & Due to this reason, asbestos is discontinued as an
insulating material.
➢ Perlite: This involves fine pores of trapped air.
Attributes include low thermal conductivity, non- . What is the utility of reusable insulation pads? What are
flammable, ease of application, and low moisture their applications?
retention and relatively low cost. Expanded perlite & Reusable insulating pads are commonly used in
is used as a loose-filling material in the annulus of industrial facilities for insulating flanges, valves,
a double-walled vessel for storage of cryogenic expansion joints, heat exchangers, pumps, turbines,