Page 261 - Fluid mechanics, heat transfer, and mass transfer

CONDUCTION HEAT TRANSFER 241

➢ Repairs giving rise to gaps for moisture entry. & Because of their sound absorption characteristics,
. What type of insulation one would recommend for some insulations provide greater sound attenuation
insulating cold storages? than others.
& Mineral fiber products are among the best thermal
& Spray-on foam insulations.
insulation materials for sound attenuation.
. What insulation is recommended for insulating hot oil
& The jacketing material used to cover the insulation
storage tanks?
can play an important role in sound attenuation.
& Spray-on rigid urethane foam.
& A fabric-reinforced mastic finish over insulation has
. Explain how a thermos flask maintains temperature of
better sound absorption properties than metal
its contents.
jacketing.
& Thermos flask is a double-walled glass vessel, with
. What is critical radius? If a thicker insulation exceed-
vacuum inside the sealed outer annulus and highly
ing the critical radius is used, what happens to heat
reflective surface on the outer glass wall.
losses?
& Vacuum and the reflective surface are the effective
& As thickness of insulation increases, resistance for
means of maintaining inside temperature of the
heat conduction through the insulation increases and
vessel.
hence heat losses decrease.
& Glass, with reasonably low thermal conductivity, is
& Also, as insulation thickness increases on cylindrical
used to enhance insulating power of the flask.
surfaces, exposed surface area increases.
& Additionally, the flask is protected by a plastic or a
➢ Heat losses are a direct function of exposed
metal container that also provides nearly stagnant air
surface.
gap between the glass and the outer containers,
➢ So, effect of increased insulation thickness is to
adding to the insulation effectiveness.
decrease heat losses due to increased resistance
. What type of construction is used for storage tanks for
and increased heat losses due to increased surface
liquid ammonia with the objective of preventing heat
area.
leakages into the system?
➢ There will be a maximum insulation thickness
& Double-walled construction is used with the outside
beyond which losses due to increased surface
of the external wall coated with alumina paint to
will overtake the reduction of heat losses due to
reflect back the heat incident on it.
increased resistance.
. Why is it necessary to insulate inside steel surface of an
➢ This maximum thickness is called critical thick-
ammonia reactor?
ness and the radius of the cylinder at this thickness
& Ammonia reactors operate at temperatures of over
is called critical radius.
500 C and 250 bar pressures. . Explain why heat losses increase when thermal insu-

➢ Inside surfaces of the steel wall must be kept at
lation is applied to thin wires.
temperatures below 370 C, temperature at which

& Surface area increases rapidly as insulation is applied
steels begin to decline in strength and also to
to wires, increasing heat losses.
prevent hydrogen access to steel shell through
which it diffuses causing hydrogen embrittlement. . What is optimum insulation thickness?
& Providing an air gap (about 2 cm) between the outer & As insulation thickness increases, savings in energy
shell and the insulating liner significantly improves costs (operating costs) will increase. Exposed surface
overall insulating quality. area of the insulation on piping or walls with a
curvature, such as in the case of tanks, increases
. “Entrapping air within an insulation will substantially
contributing to heat losses.
increase insulation power. Convection currents within
& Also as insulation thickness increases, cost of insu-
entrapped air will decrease the insulating power.” True/
lation material as well as costs involved in applying
False? Comment.
the material increases (fixed costs).
& True. Dry still air is a very good insulator. Any
& Therefore, there will be a maximum thickness be-
convection currents increase heat transfer reducing
yond which fixed costs overtake the costs involved in
insulating power of the insulation.
energy savings. This thickness is called optimum
. “It is said that insulation of surfaces reduces noise.”
thickness.
Comment.
. “Optimum insulation thickness varies with
& Sound attenuation is a natural by-product of the
temperature.” True/False? Illustrate.
insulation design.