Page 251 - Chemical Process Equipment - Selection and Design

8.12. INSULATION OF EQUIPMENT 221

TABLE 8.18. Program for Finding the Radiant Gas calculations. Optimum thicknesses of pipe insulation also are
Temperature by Steps 22 and 23 of Table 8.17 tabulated in Chemical Engineers Handbook (1984, 11.56); they
cover both indoor and outdoor conditions, temperature ranges of
150-1200°F and energy costs of 1-8 dollars/million Btu.
For very large tanks storing volatile liquids and subject to
pressure buildup and breathing losses, it is advisable to find eco-
nomic thickness of insulation by economic analysis. The influence
of solar radiation should be taken into account; a brief treatment
of this topic is in the book of Threlkeld (Thermal Environmental
Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1970). In at least
one application, rigid urethane foam sprayed onto storage tanks in
2in. thickness and covered with a 4mil thickness of neoprene
rubber for weather proofing was economically attractive.
Although resistance to heat transfer goes up as the thickness of
pipe insulation is increased, the external surface also increases; a
thickness may be reached at which the heat transfer becomes a
minimum and then becomes larger. In accordance with this kind of
behavior, heat pickup by insulated refrigerated lines of small
diameters can be greater than that of bare lines. In another
instance, electrical transmission lines often are lagged to increase
the rate of heat loss. An example worked out by Kreith (Principles
of Heal Transfer, Pntext, New York, 1973, p. 44) reveals that an
insulated 0.5 in. OD cable has a 45% greater heat loss than a bare
one.
LOW TEMPERATURES
Insulants suited to cryogenic equipment are characterized by
multiple small spaces or pores that occlude more or Iess stagnant air
of comparatively low thermal conductivity. Table 8.19 lists the most
common of these materials. In application, vapor barriers are
provided in the insulating structure to prevent inward diffusion of
atmospheric moisture and freezing on the cold surface with resulting
increase in thermal conductivity and deterioration of the insulation.
Sealing compounds of an asphalt base are applied to the surface of
the insulation which then is covered with a weatherproof jacket or
cement coating. For truly cryogenic operations such as air
liquefaction and rectification in which temperatures as low as
-300°F are encountered, all of the equipment is enclosed in a box,
and then the interstices are filled with ground cork.
MEDIUM TEMPERATURES
Up to about 600"F, 85% magnesia has been the most popular
material. It is a mixture of magnesia and asbestos fibers so
constructed that about 90% of the total volume is dead air space.
Equivalents are available for situations where asbestos is
undesirable. Such insulants are applied to the equipment in the
form of slabs or blankets which are held in place with supports and
clips spotwelded to the equipment. They are covered with cement
to seal gaps and finished off with a canvas cover that is treated for
resistance to the weather. A galvanized metal outer cover may be
preferred because of its resistance to mechanical damage of the
insulation.
A mixture of diatomaceous earth and an asbestos binder is
suitable for temperatures up to the range of 1600-1900°F.
Johns-Manville "Superex" is one brand. Since this material is more
expensive than 85% magnesia, a composite may be used to save
money: sufficient thickness of the high temperature resistant mate-
rial to bring its external surface to below 600"F, finished off with 85%
magnesia in appropriate thickness. Table 8.22(c) is one standard speci-
insulation depend on the process temperature according to: fication of this type.
200 400 600
T (OF) REFRACTORIES
Thickness (in.) 0.5 1.0 1.25
Equipment made of metal and subject to high temperatures or
The data of Table 8.22 are roughly in agreement with these abrasive or corrosive conditions often is lined with ceramic material.