Page 441 - Handbook of Energy Engineering Calculations

fluid, performance or duty of the exchanger, and 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
(LMTD or τT )—that is dissipated across each element of resistance in the
M
heat-flow path.
When the sum of the products for the individual resistance equals one, the

trial design may be assumed to be satisfactory for heat transfer. The physical
significance is that the sum of the temperature drops across each resistance is
equal to the total available LMTD. The pressure drop on both tubeside and
shellside must be checked to ensure that both are within acceptable limits. As

shown in the sample calculation above, usually several trials are necessary to
obtain a satisfactory balance between heat transfer and pressure drop.
Tables 6 and 7, respectively, summarize the equations used with the
method for heat transfer and for pressure drop. The column on the left lists

the conditions to which each equation applies. The second column lists the
standard form of the correlation for film coefficients that is found in texts.
The remaining columns then 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 total
temperature drop or driving force (ΔT /ΔT ) across the resistance.
f
M
As described above, the addition of ΔT /ΔT , tubeside factor; plus ΔT /
i
M
o
ΔT , shellside factor; plus ΔT /T , fouling factor; plus ΔT /ΔT , tube-wall
s
M
M
w
M
factor, determine the heat-transfer adequacy. Any combination of ΔT /ΔT M
i
and ΔT /ΔT may be used, as long as a horizontal orientation on the tubeside
M
o
is used with a horizontal orientation on the shellside, and a vertical tubeside
orientation has a corresponding shellside orientation.
The units in the pressure-drop equations (Table 7) are consistent with those
used for heat transfer. The pressure drop in psi is calculated directly. Because
the method is a shortcut approach to design, certain assumptions pertaining to
thermal conductivity, tube pitch, and shell diameter are made.

For many organic liquids, thermal conductivity data are either not available
or difficult to obtain. Since molecular weights (M) are known, for most
design purposes the Weber equation, which follows, yields thermal
conductivities with quite satisfactory accuracies: