Page 464 - Handbook of Energy Engineering Calculations

discussed by Lord, Minton, and Slusser earlier in this section).
Primarily, the method combines into one relationship the classical
empirical equations for film heat-transfer coefficients with heat-balance
equations and with correlations that describe the geometry of the heat
exchanger. The resulting overall equation is recast into three separate groups

that contain factors relating to the physical properties of the fluid, the
performance or duty of the exchanger, and the mechanical design or
arrangement of the heat-transfer surface. These groups are then multiplied

together with a numerical factor to obtain a product that is equal to the
fraction of the total driving force—or log mean temperature difference (δT M
or LMTD)—that is dissipated across each element of resistance in the heat-

flow path.
When the sum of the products for the individual resistances equals 1, the
trial design may be assumed satisfactory for heat transfer. The physical
significance is that the sum of the temperature drops across each resistance is

equal to the total available ΔT . The pressure drop for both fluid-flow paths
M
must be checked to ensure that they are within acceptable limits. Usually,
several trials are necessary to get a satisfactory balance between heat transfer

and pressure drop.
Table 11 summarizes the equations used with spiral-tube exchangers. The
column on the left presents the conditions to which each equation applies,

and the second column gives the standard form of the film-coefficient
correlation found in texts. The remaining columns tabulate the numerical,
physical-property, work, and mechanical-design factors—all of which
together form the recast dimensional equation. The product of these factors
gives the fraction of the total temperature drop or driving force (ΔT /ΔT )
M
f
across the resistance.
As stated, the sum of ΔT ΔT (the tube-wall factor) ΔT /ΔT (the shellside
M
i/
o
M
factor), ΔT /ΔT (the fouling factor), and Δ T /ΔT (the tube-wall factor)
w
M
s
M
determine the adequacy of heat transfer. Any combinations of ΔT /ΔT may
f
M
be used as long as the orientation specified by the equation matches that of
the exchanger’s flow path.
The units in the pressure-drop equations are consistent with those used for
heat transfer. Pressure drop is calculated directly in psi.
For many organic liquids, thermal conductivity data are either not available