Chemical Engineering November 2022 - 29

to prevent larvae attaching.
Final filtered effluent. In
some wastewater-treatment
sites, final filtered effluent
(FFE) taken after the
filter press is used as a free
cooling medium. Due to the
high level of biological material
contained in FFE, it has
a high fouling potential and
fouling can quickly occur,
depending on the exact nature
of both the FFE and the
heat exchanger design.
Ultraviolet
(UV)
FIGURE 4. Struvite (magnesium ammonium phosphate)
is one of the most commonly encountered fouling agents.
Companies, such as Ostara, recover it as a valuable nutrient
for reuse
added to the sludge stream, although
this is usually determined by
other factors. Cleaning is very difficult,
often relying on the use of hydrochloric
acid solutions which may
not be compatible with the materials
used in heat exchanger and system
construction.
Biological fouling
Living organisms can also cause fouling
when untreated water is used.
Algae. Algal fouling is particularly
encountered where untreated river
or canal water is used for cooling.
Environmental regulations
prevent
the use of chemical additives and
also limit the temperature increase
and so algae quickly grow in what is
an ideal environment.
Using high velocities or even
scraped-surface
heat
exchangers
can prevent fouling, as can the use
of construction materials such as
copper or brass. A regular cleaning
regime is usually necessary.
Zebra mussels. Zebra mussels are
an invasive species found across
the world. They are now established
in freshwater bodies in the
Southeast of England and are found
in sewers and sewage-treatment
works. They enter pipework as larvae
and then colonize and grow. In
the worst cases, hundreds of tons
of mussels have been removed from
some water-treatment works. They
are relatively uncommon in
heat
exchangers and can be controlled
by keeping the velocity of the fluid
through the exchanger above 2 m/s
radiation
treatment of the FFE can
sometimes help to reduce
the biological load, and
therefore the potential for
fouling, but it is expensive
and not 100% effective.
These systems are normally left to
run, then are cleaned when necessary,
with cleaning usually relying
on caustic cleaning-in-place (CIP)
systems. It is therefore important to
specify heat exchangers and other
equipment that can cope with such
caustic cleaning materials.
Deposition fouling
Solid particles in the media settling
onto the heat-exchange surface is
deposition fouling.
Sediment. This is the most common
type of heat-exchanger fouling
and is caused by particulate matter
in the treated fluid settling out onto
the surface of the heat exchanger.
It will usually be prevented by
good heat-exchanger design and
choosing the right heat exchanger
for the job. For example, making
sure that the fluid has sufficient velocity,
while the use of corrugated
tubes can prevent sedimentation, or
scraped-surface
heat
exchangers
can continually remove it to ensure
efficient operation.
Burn-on. Burn-on occurs where
the heat exchanger wall temperature
is too high, causing particles
in the fluid (particularly organic materials)
to become baked onto the
tube walls. It often occurs where a
malfunction has arisen; for example,
heating has continued while product
flow has stopped, resulting in overheating
of the material.
The likelihood of burn-on can
be reduced by good design of the
overall system and interlocking the
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controls for both water and sludge
pumps, so that if one stops, so
does the other one. Control of the
water temperature (ideally keeping
it below 90°C) will also help prevent
burn-on. Where it occurs, it can
usually be removed by physical or
chemical cleaning.
Corrosion fouling. This type of
fouling usually occurs in specific circumstances
where either the material
being treated, or the construction
of the heat exchanger itself, is
particularly susceptible to corrosion.
For example, aluminum and
copper can be highly reactive and
frequently suffer from galvanic corrosion,
or the formation of oxides on
the tube surface where they have
been used for the manufacture of
heat-exchanger tubes.
Using a material that is resistant to
such corrosion, yet maintains good
thermal-transfer properties, such as
stainless steel, will overcome most
of these issues. Good system design
(for example, to regularly remove
grit) and regular cleaning will
also help to prevent the formation
of corrosion.
Another form of corrosion fouling
is crystallization where, due to
cooling or increasing concentration,
components in the fluid are deposited
on the heat-exchanger surface.
Scraping the heat-transfer surface
to remove these layers of fouling
maintains high heat-transfer rates in
such situations.
Preventing fouling
As in many aspects of life, prevention
is better than cure. Preventing
or reducing fouling is less expensive
and more effective in maintaining
heat-exchanger performance than
cleaning or removing fouling that
has already occurred.
Two of the main methods for preventing
fouling are material choice
and heat-exchanger design. The
surface of the heat exchanger will
have an effect on fouling, and rough
surfaces are known to collect particulate
matter (which increases fouling).
The smooth, polished surfaces
that can be achieved on tubes made
from 304 or 316 stainless steel
therefore help to minimize fouling.
Using corrugated tubes in heatexchanger
construction is beneficial
in increasing heat transfer and
29
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