The main problems affecting the performance are usually due to one of the following:
+ Fouling
+ Tube vibrations
+ Leakage
+ Dead Zones
Fouling
This
can be generally defined as the precipitation of unwanted
material within the heat exchanger over time which hamper the
performance.
The principal types of fouling encountered in process heat exchangers include:
• Particulate fouling
• Corrosion fouling
• Biological fouling
• Crystallisation fouling
• Chemical reaction fouling
• Freezing fouling
In
the case of corrosion, the surfaces of the heat exchanger can become
corroded as a result of the interaction between the process fluids
and the materials used in the construction of the heat exchanger. The
situation is made even worse due to the fact that various fouling types
can interact with each other to cause even more fouling. Fouling can and
does result in additional resistance with respect to the heat transfer
and thus decreased performance with respect to heat transfer. Fouling
also causes an increased pressure drop in connection with the
fluid flowing on the inside of the exchanger.
To improve the
performance of fouled heat exchangers requires that the tubes be cleaned
periodically. Tube cleaning procedures for shell and tube heat
exchangers are performed off-line, the most frequently chosen and
fastest method being mechanical cleaning. Among other off-line methods
is the use of very high pressure water but, since the jet can only be
moved along the tube slowly, the time taken to clean a heat exchanger
can become extended. Chemicals are also used for the off-line cleaning
of heat exchanger tubes. Several mildly acidic products are available
and will remove more deposit than most other methods; but it is
expensive, takes longer for the operation to be completed, and the
subsequent disposal of the chemicals, an environmental hazard, creates
its own set of problems.
Tube vibrations
Another
problem that often arises in connection with the use of heat exchangers
is tube vibration damage. Tube vibration is most intense and damage is
most likely to occur in cross flow implementations where fluids flow is
perpendicular to the tubes, although tube vibration damage can also
occur in non cross flow (i.e. axial) implementations in the case of very
high fluid velocities. Vibration may be eliminated by reducing
velocities, decreasing the unsupported span or, in some cases,
by altering the method of fixing or pinning the ends of the unsupported
span.
This problem can cause significant damage to the exchanger if within high limits.
Leakage
Sometimes the fluid of the tube side can leak to shell side or vice versa, This problem can cause huge production loss.
Leaks may develop at the tube to tube sheet joints of fixed tube sheet exchangers because differential thermal
expansion between the tubes and the shell causes overstressing of the
rolled joints. Or, thermal cycling caused by frequent shutdowns or batch
operation of the process may cause the tubes to loosen in the tube
holes. Floating heads or U-bend exchangers would be considered first for
this type of service. If a fixed tube sheet
unit is required, an
expansion joint will be specified. An exchanger that will be thermally
cycled two or three times a day will require superior
mechanical construction such as the strength welding of tubes to the
tube sheet, complete inspection of the shell and channel welds during
fabrication. Welding the tubes to the tube sheets does not guarantee
that a leak will not occur as sometimes weld failure due to porosity in
the welds or just one poorly welded tube out of the hundreds of welds
can cause a leakage. The use of double tube sheets to minimise
the
chances of leakage between the tube side and shell side can be a good
solution to the problem. Nevertheless, double tube sheet can cause
considerable maintenance problems because the outboard and inboard tube
sheets may be subjected to considerably different process temperatures
and this can have
differential expansion between the tube sheets resulting in bending the tubes
Dead zones
Areas
that have the flow to minimal or even non existent and usually produce
poor heat transfer and can lead ultimately to excessive fouling.
Existing
shell and tube heat exchangers suffer from the fact that they must
typically use baffles to maintain the required heat transfer. This,
however, results in "dead zones" within the heat exchanger where flow is
minimal or even non existent. These dead zones generally lead to
excessive fouling. Other types of heat exchangers may
or may not
employ baffles. If they do, the same increased fouling problem
exists. Further, in heat exchangers fitted with baffles, for example,
the cross flow implementation results in the additional problem of
potential damage to tubes as a result of flow induced vibration. In the
case of such damage, processes must often be interrupted or shut down in
order to perform costly and time consuming repairs to the device.
