Page 258 - Fluid mechanics, heat transfer, and mass transfer

DIMENSIONLESS NUMBERS, TEMPERATURE MEASUREMENT, AND CONDUCTION HEAT TRANSFER
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tanks, and other irregular surfaces. The pads are . What type of insulating material is used as insulation in
flexible and vibration resistant and can be used with domestic refrigerators? What are the other applications
equipment that is horizontally or vertically mounted of rigid insulation?
or that is difficult to access. & Rigid polyurethane foams, cellular polystyrene, and
& The pads are made of a noncombustible inside cover, other rigid insulations arewidelyused as insulation in
insulation material, and a noncombustible outside domestic refrigerators.
cover that resists tears and abrasion. . What are the applications of rigid insulations?
. Polyurethane foams can be used as insulating materials. & Rigid insulation is resistant to deformation when
Name some applications for these materials. subjected to foot traffic.
& Polyurethane foams are good insulating materials & Areas that experience loads or repetitive personnel
that combine low thermal conductivities of polyur- access/use will require a firmer rigid insulation than
ethanes with low thermal conductivity of air that is inaccessible areas. Piping used as ladders/walkways
entrapped in the micropores of polyurethane. and horizontal surfaces subject to vibration/loads are
& These foams are available as rigid foams in molded examples where rigid insulation is required.
form and in liquid form for spraying on surfaces for . Name some commonly used rigid insulations and give
subsequent setting into highly porous solid, adhering their characteristics.
on to the solid surfaces on which they are sprayed. & Calcium silicate insulation is a rigid, dense material
& Rigid foams are molds made into panels in different
used for above ambient to 650 C. Commonly used
shapes to suit the application to different types for high-temperature applications. It has good com-
of equipment operating at ambient and low pressive strength and is noncombustible.
temperatures. & Calcium silicate block and pipe insulation is used on
& Foams in liquid form are sprayed on external walls of
surfaces with temperatures up to 870 C.
houses in cold climates, cold storages, and on other & Expanded or exfoliated vermiculite thermal insulat-
surfaces to prevent heat ingress and conserve energy. ing cement in the form of dry cement or plaster is
. What is mineral/slag wool? Compare this with glass mixed with a suitable proportion of water, applied as
wool for insulating purposes. a plastic mass, and dried in place, for use as insulation
& Mineral/slag wool is a rawglass fiberinsulation made on surfaces operating at temperatures 35–1000 C.

out of molten blast furnace slag, which is forced The cement is not suitable where it will be exposed to
through multiple hole nozzles made out of heat combustion conditions, such as the hot face lining of
resistant materials, to give rise to solidified fibers. a furnace. Vermiculite cement is also used as loose-
Blast furnace slag, which is a waste mineral matter fill insulation.
from steel plants, provides a cheap insulating mate- & Cellular glass is also rigid, dense material normally
rial entrapping air to give high-efficiency insulation. used in the temperature range from 230 to
Slag wool insulation is widely used in industrial þ 200 C. Its closed cell structure makes it preferable

insulation applications. for low-temperature applications and for use on
& Glass wool, on the other hand, is made out of molten services where fluid absorption into the insulation
glass that is equally good insulation but expensive could be a problem. This is particularly useful where
compared to slag wool. leaks can introduce fluids into the insulation.
. What is the important application of ceramic fiber ➢ Fires due to leaked organic fluids that self-heat
insulation? through slow oxidation inside the insulation are
common phenomena. The insulation prevents the
& High-temperature systems, for example, ceramic
dissipation of heat from such oxidation, increasing
fiber blankets are used to line fired heaters.
temperatures inside the insulation to the point
. Cork is a good cold insulating material. Is it being used
where self-ignition occurs, leading to the so-called
for the present day cold insulation applications?
lagging fires.
Comment.
. What are the measures that are useful in preventing fires
& Cork has become a scarce material with consequent
in insulations?
increases in its price. Equivalent insulating materials,
& Use of cellular glass insulation that resists fluid
namely, plastic foams, have replaced cork. These
absorption by the insulation.
materials are cheaper and easily available and last
longer in the moist atmospheres where cork has been & If a leak develops, the insulation must be removed
used earlier. and replaced by new insulation, after the leaks are
contained.