Page 439 - Handbook of Energy Engineering Calculations

not exceed the outlet process-stream temperature if a single body having
one shell pass—but more than one tube pass—is used.

• When cooling or condensing a fluid, the inlet coolant temperature should
not be less than 5°C (9°F) above the freezing point of the highest freezing
component of the fluid.

• For cooling reactors, a 10°C (18°F) to 15°C (27°F) difference should be

maintained between reaction and coolant temperatures to permit better
control of the reaction.

• A 20°C (36°F) approach to the design air temperature is the minimum for
air-cooled exchangers. Economic justification of units with smaller

approaches requires careful study. Trim coolers or evaporative coolers
should also be considered.

• When condensing in the presence of inerts, the outlet coolant temperature
should be at least 5°C (9°F) below the dew point of the process stream.
In an exchanger having one shell pass and one tube pass, where two fluids

may transfer heat in either cocurrent or countercurrent flow, the relative
direction of the fluids affects the value of the mean temperature difference.
This is the log mean in either case, but there is a distinct thermal advantage to

counterflow, except when one fluid is isothermal.
In concurrent flow, the hot fluid cannot be cooled below the cold-fluid
outlet temperature; thus, the ability of cocurrent flow to recover heat is
limited. Nevertheless, there are instances when cocurrent flow works better,
as when cooling viscous fluids, because a higher heat-transfer coefficient

may be obtained. Cocurrent flow may also be preferred when there is a
possibility that the temperature of the warmer fluid may reach its freezing
point.

These factors are important in determining the performance of a shell-and-
tube exchanger:
Tube Diameter, Length. Designs with small-diameter tubes [5/8 (15.8 mm)
to 1 in (25.4 mm)] are more compact and more economical than those with
larger-diameter tubes, although the latter may be necessary when the

allowable tubeside pressure drop is small. The smallest tube size normally