Page 197 - Chemical Process Equipment - Selection and Design

8

TRANSFER AND HEAT EXCHANGERS

asic concepts of heat transfer are reviewed in this equipment, some of which are treated in the next and other
chaptslr and applied primarily to heat exchangers, chapters. The three recognized modes of heat transfer are by
which are equipment for the transfer of heat conduction, convection, and radiation, and may occur
between two fluids through a separating wall. simultaneously in some equipment.
Heat transfer also is a key process in other specialized

8.q. CONDUCTION OF NEAT which demonstrates that use of a value at the average temperature
gives an exact result. Thermal conductivity data at several
In a solid wall such as Figure 8.l(a), the variation of temperature temperatures of some metals used in heat exchangers are in Table
with time and position is represented by the one-dimensional 8.1. The order of magnitude of the temperature effect on k is
Fourier equation illustrated in Example 8.1.

For the most part, only the steady state condition will be of concern
here, in which the case the partial integral of Eq. (8.1) becomes

assuming the thermal conductivity k to be independent of
temperature. Furthermore, when both k ad A are independent of
position,

in the notation of Figure 8.l(a).
Equation (8.3) is the basic form into which more complex
situations often are cast. FOP example,

when the area is ,variable and

in certain kinds of heat exchangers with variable temperature
difference.
THERMAL CQ N [IUCTIVITY
Thermal conductivity is a fundamental property of substances that
basically is obtained experimentally although some estimation
methods also are available. It vanes somewhat with temperature. In
many heat transfer situations an average value over the prevailing
temperature range often is adequate. When the variation is linear
with
(e) (f)
k = ko(l + aT), (8.6)
Figure 8.1. Temperature profiles in one-dimensional conduction of
heat. (a) Constant cross section. (b) Hollow cylinder. (c) Composite
the integral of Eq. (8.2) becomes
flat wall. (d) Composite hollow cylindrical wall. (e) From fluid A to
fluid F through a wall and fouling resistance in the presence of
Q(L/A) = k,,[T, - T2 + O.SCY(TT - Tz)] eddies. (f) Through equivalent fluid films, fouling resistances, and
= ko(Tl - Ti)[l + 0.5a(TI + TJ], (8.7) metal wall.
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