Page 340 - Mechanical Engineers' Handbook (Volume 4)

5 Use of Computers in Thermal Design of Process Heat Exchangers 329

ing. A relatively new option available from the Brown Fin Tube Company is the twisted
tube. This tube provides spiral corrugations through which fluids flow in spiral counterflow
on the shellside and tubeside. No baffles are needed.
Tube Length. This is usually limited by plant requirements. In general, longer exchangers
are economically preferable within pressure drop restrictions, except possibly for vertical
thermosiphon reboilers.

Tube Diameter. Small diameters are more economical in the absence of restrictions. Cleaning
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restrictions normally limit outside diameters to not less than ⁄8 or ⁄4 in. However, some
manufacturers now offer microchannel exchangers, which are very effective for some fluids,
such as clean gases. Pressure drop restrictions, especially in vacuum, may require larger
1
sizes. Vacuum vertical thermosiphon reboilers often require 1 ⁄4-in. tubes, and vacuum falling
film evaporators frequently use as large as 2-in. tubes. Excessive pressure drop can be quickly
decreased by going to the next standard tube diameter, since pressure drop is inversely
proportional to the fifth power of the inside diameter.
Tube Pitch. Tube pitch for shellside flow is analogous to tube diameter for tubeside flow.
Small pitches are more economical and also can cause pressure drop or cleaning problems.
In laminar flow, here too-small tube pitch can prevent bundle penetration and force more
bypassing and leakage. A pitch-to-tube diameter ratio of 1.25 or 1.33 is often used in absence
of other restrictions depending on allowable pressure drop. For shellside reboilers operating
at high heat flux, a ratio of as much as 1.5 is often required. Equation (54) shows that the
maximum heat flux for kettle reboilers increases with increasing tube pitch.
Tube Layout. Performance is not critically affected by tube layout, although some minor
differences in pressure drop and vibration characteristics are seen. In general, either 30 or
60 layouts are used for clean fluids, while 45 or 90 layouts are more frequently seen for
fluids requiring shellside fouling maintenance.
Tube Material. The old standby for noncorrosive moderate-temperature hydrocarbons is the
less expensive and sturdy carbon steel. Corrosive or very high-temperature fluids require
stainless steel or other alloys. Titanium and hastelloy are becoming more frequently used
for corrosion or high temperature despite the high cost, as a favorable economic balance is
seen in comparison with severe problems of tube failure.

Exchanger Orientation. Exchangers normally are horizontal except for tubeside thermosi-
phons, falling film evaporators, and tubeside condensers requiring very low pressure drop or
extensive subcooling. However, it is becoming more frequent practice to specify vertical
orientation for two-phase feed-effluent exchangers to prevent phase separation, as mentioned
in Section 4.3.

Fouling
All programs require the user to specify a fouling factor, which is the heat-transfer resistance
across the deposit of solid material left on the inside and/or outside of the tube surface due
to decomposition of the fluid being heated or cooled. Considerations involved in the deter-
mination of this resistance are discussed in Section 4.1. Since there are presently no thermal
design programs available that can make this determination, the specification of a fouling
resistance, or fouling factor, for each side is left up to the user. Unfortunately, this input is
probably more responsible than any other for causing inefficient designs and poor operation.