Page 332 - Mechanical Engineers' Handbook (Volume 4)

4 Common Operational Problems 321

by a cage of rods. A proprietary design of this type exchanger (RODbaffle) is licensed by
Phillips Petroleum Co. Calculation methods are published in Ref. 52.

4.3 Flow Maldistribution

Several types of problems can occur when the flow velocities or fluid phases become dis-
tributed in a way not anticipated by the designer. This occurs in all types of exchangers, but
the following discussion is limited to shell and tube and air-cooled exchangers, in which
maldistribution can occur on either shellside or tubeside.

Shellside Flow
Single-phase flow can be maldistributed on the shellside owing to bypassing around the tube
bundle and leakage between tubes and baffle and between baffle and shell. Even for typical
well-designed heat exchangers, these ineffective streams can comprise as much as 40% of
the flow in the turbulent regime and as much as 60% of the flow in the laminar regime. It
is especially important for laminar flow to minimize these bypass and leakage streams, which
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cause both lower heat-transfer coefficients and lower effective MTD. This can, of course,
be done by minimizing clearances, but economics dictate that more practical methods include
use of bypass sealing strips, increasing tube pitch, increasing baffle spacing, and using an
optimum baffle cut to provide more bundle penetration. Methods for calculating the effects
of these parameters are described by Taborek. One method to minimize leakage and bypass
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inefficiencies is to use helical baffles, which cause flow to proceed through the exchanger
along a spiral path. Elimination of sharp flow reversals provides a much more uniform
shellside distribution. A proprietary version of the helical baffle option is provided by the
ABB Lummus Company.
Another type of shellside maldistribution occurs in gas–liquid two-phase flow in hori-
zontal shells when the flow velocity is low enough that the vapor and liquid phases separate,
with the liquid flowing along the bottom of the shell. For condensers this is expected and
taken into account. However, for some other types of exchangers, such as vapor–liquid
contactors or two-phase reactor feed-effluent exchangers, separation may cause unacceptable
performance. For such cases, if it is important to keep the phases mixed, a vertical heat
exchanger is recommended. Improvement in mixing is obtained for horizontal exchangers if
horizontal rather than vertical baffle cut is used.

Tubeside Flow
Several types of tubeside maldistribution have been experienced. For single-phase flow with
axial nozzles into a single-tubepass exchanger, the dynamic head of the entering fluid can
cause higher flow in the central tubes, sometimes even producing backflow in the peripheral
tubes. This effect can be prevented by using an impingement plate on the centerline of the
axial nozzle.
Another type of tubeside maldistribution occurs in cooling viscous liquids. Cooler tubes
in parallel flow will tend to completely plug up in this situation, unless a certain minimum
pressure drop is obtained, as explained by Mueller. 53
For air-cooled single pass condensers, a backflow can occur owing to the difference in
temperature driving force between bottom and top tube rows, as described by Berg and
54
Berg. This can cause an accumulation of noncondensables in air-cooled condensers, which
55
can significantly affect performance, as described by Breber et al. In fact, in severe cases,
this effect can promote freezeup of tubes, or even destruction of tubes by water hammer.
Backflow effects are eliminated if a small amount of excess vapor is taken through the main