Page 262 - Fluid mechanics, heat transfer, and mass transfer

DIMENSIONLESS NUMBERS, TEMPERATURE MEASUREMENT, AND CONDUCTION HEAT TRANSFER
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& True. For example, at 100 C, the thickness is 1.27 cm materials such as carbides, nitrides, borides, and

(0.5 in.); at 200 C, the thickness is 2.54 cm (1.0 in.); graphite. The actual composition of a refractory
and at 300 C, the thickness is 3.17 cm (1.25 in.) material is dependent on operating factors such as

. What is the effect of windy conditions on insulation temperature, atmosphere, and the materials it will be
requirements? in contact with.
& Where windy conditions (12 km/h) prevail, 10–20% . What is refractoriness? How is refractoriness of a
greater insulation thickness is justified. material measured?
& What is the minimum thickness suitable for using & The refractoriness of a material is a measure of its
multilayer insulation? What precautions are taken in ability to withstand exposure to elevated tempera-
applying multilayer insulation? tures without undergoing appreciable deformation.
& It is generally recommended that insulation thick- & It is generally measured using what are known as
nesses greater than 75 mm are applied in multilayers. Sager cones, which are cone-shaped ceramic objects
of differing compositions. When heated to different
& Where insulation is applied in two or more layers, all
temperatures, these cones will slump as they soften in
joints, both longitudinal and circumferential, should
response to the temperature, with the degree of
be staggered.
slumping being dependent on the composition.
& Metal cladding used outdoors should have joints
& A similar cone-shaped object is made from the
overlapped and be sealed with a suitable mastic.If
material to be measured and heated along with
securing screws penetrate the metal finish, these
standard Sager cones. After the conclusion of the
should be sealed.
heating cycle, the sample material is compared to the
. Give an example where the purpose of using insulation
Sager cones to gain a comparative measure of its
is to increase rather than decrease heat losses.
refractoriness.
& Electrical transmission lines are often lagged to
& However, it should be noted that when a material is
increase heat dissipation rate.
subject to a mechanical load, it may well soften at
➢ In this case, as wires are of very small diameter
well below the temperature indicated by the Sager
compared to pipes, thickness of the insulation
cone test.
exceeds the critical thickness resulting in in-
. What types of refractories are used in steel making?
creased heat transfer rates due to surface increase
compared to reduction of heat transfer rates due to & In steel making, basic refractories are used be-
increased thickness, that is, increased resistance to cause the refractories often come into contact with
heat flow. Net effect is increased heat losses. basic slags containing magnesium and calcium
oxides. If the refractory lining was made from
& Heat pickup by insulated small diameter refrigerated
acidic refractories, it would be eroded quickly by
lines can be more than that by the bare pipes.
the chemical interaction of the basic slag and the
. How does an ablative material work as a thermal
acidic lining (e.g., silica) forming low melting
insulation/shield?
point compounds.
& Ablative materials, coated on the surfaces to be
. Where are (i) magnesia (basic) and (ii) silica bricks
insulated, will expand into a foam structure on
(acidic) used?
exposure to high temperatures, for example, fire
& Due to its excellent corrosion resistance, refractory
exposure, acting as effective insulation, protecting
grade fused magnesia is used in high wear areas in
the surfaces from yielding due to softening or melting
steel making, for example, basic oxygen and electric
before remedial measures take over the job of pro-
arc furnaces, converters, and ladles.
tection. These materials, initially developed for
& Silica bricks are widely used in glass furnaces, glass
spacecraft applications, are frequently used to protect
kiln vaults, and so on.
equipment on fire exposure. Once such an exposure
occurs, the material must be replaced. . What are the applications of graphite refractories?
. What are refractories? & Graphite refractories can operate at temperatures of
up to several thousand degrees Celsius under reduc-
& Refractories are generally ceramic materials that are
ing conditions, or oxygen-free conditions, such as
capable of withstanding high temperatures. The bulk
vacuum. However, they may begin to sublime at
of refractory materials consist of single or mixed
approximately 1000 C under oxidizing conditions.

high-melting point oxides of elements such as
silicon, aluminium, magnesium, calcium, and zirco- . What is the effect of porosity of refractories on their
nium. Nonoxide refractories also exist and include insulating and high-temperature characteristics?