Page 454 - Handbook of Energy Engineering Calculations

phenolic-resin coatings, among others, protect against corrosion from cooling
water. Electrodes may also be wound into the assembly to anodically protect
surfaces against corrosion.
Spiral-plate exchangers are normally designed for the full pressure of each
passage. Because the turns of the spiral are of relatively large diameter, each

turn must contain its design pressure, and plate thickness is somewhat
restricted—for these three reasons, the maximum design pressure is 150
2
lb/in (1033.5 kPa), although for smaller diameters the pressure may
sometimes be higher. Limitations of materials of construction govern design
temperature.
The shortcut rating method for spiral-plate exchangers depends on the
same technique as that for shell-and-tube heat exchangers (which were

discussed by Lord, Minton, and Slusser in the previous procedure).
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 resistance equals 1, 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 ΔT . The pressure drops for both fluid-flow paths
M
must be checked to ensure that both are within acceptable limits. Usually,

several trials are necessary to get a satisfactory balance between heat transfer
and pressure drop.
Table 9 summarizes the equations used with the method for heat transfer
and pressure drop. The columns on the left list the conditions to which each

equation applies, and the second columns gives the standard forms of the
correlations for film coefficients that are found in texts. The remaining