Page 322 - Fluid mechanics, heat transfer, and mass transfer
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HEAT EXCHANGERS 303
& Identifying the Fouling Mechanism(s): Eliminating ➢ Minimization of corrosion through selection of
some types of corrosion (e.g., naphthenic acid appropriate materials of construction.
corrosion) may reduce fouling while minimizing ➢ Growth of oxide scale offers resistance to heat
fouling might reduce some types of corrosion transfer that is equivalent to fouling.
(e.g., sulfide corrosion in hydrotreating of hydro- ➢ Use of coatings on tube surfaces. For example,
carbon streams). Use of dispersant-type chemical
epoxy coatings are of great success in seawater
additives might keep insolubles from forming scales
cooling exchangers in preventing corrosion and
on tube walls.
reducing fouling.
➢ Fouling due to soluble corrosion products can be
& Design Steps:
curbed by preventing the chemical reaction(s)
➢ Pretreatment of feed streams to a heat exchanger
involved in the formation of fouling products.
by filtration, softening (water), desalting, and so
➢ Biofouling is dependent on the tube wall tempera-
on.
tures, rather than cooling water temperatures. As
➢ Use of high fluid velocities (>1.8 m/s) to prevent
tube wall temperatures in the range of 25–35 C
settling of particles on heat transfer surfaces.
give rise to biofouling, these temperatures should
- For high-viscosity liquids, at above velocities,
be avoided by adjusting water flow rates in sum-
DP can be prohibitive.
mer and winter conditions. Biofouling decreases
➢ Minimizing areas of lower velocities, that is,
with increased tubewall temperatures above 50 C.
stagnant regions, for example, on shell side of a
& Developing a Fouling Mitigation Strategy: This
heat exchanger where stagnant areas are possible,
should involve some of the solutions discussedearlier
especially behind baffles, where deposition can
as well as determining cleaning schedules, alternat-
occur.
ing blending of feed stocks, changing operating
conditions, use of appropriate additives, adopting ➢ Regions of recirculation are also possible, which
alternative tube bundle designs, changing number can result in extended residence times, with a
of tube passes, changing baffle designs, use of tube chance for chemical reactions to take place influ-
inserts, changes in process conditions, modifying encing incidence of fouling.
piping networks, and use of alternative heat ➢ Use of helical baffles, rod baffles, or twisted tubes
exchangers. can minimize or eliminate shell side fouling.
➢ Guidelines are available from different organiza- ➢ Improvements on the tube side may be made by
tions such as Heat Exchanger Institute Standards, using vibrating or fixed inserts to promote turbu-
Engineering Sciences Data Unit Guide data, and lence and enhance heat transfer, which, in turn,
others. Best practice is the organizations them- may also reduce incidence of fouling.
selves developing their own performance models. ➢ Polymerization, precipitation, and freezing foul-
Many organizations follow this practice. ing are direct results of temperature extremes at
➢ The simplest way to develop a model is to com- heat transfer surface.
pare heat transfer just before and just after clean- - Reduced by lowering DT between the surface
ing and then calculating the increase in efficiency and bulk flow region.
due to the gain in heat transfer, assigning costs to - Lowering DT is done by increasing velocity that
the heat transfer gain and costs involved in clean- increases h.
ing, process interruption (if any), maintenance,
- Increased h in turn results in decrease in DT.
and so on.
- There is a limit for increasing velocities.
➢ An example is provided by a 500 MW power plant
- Better approach would be use of adequate ve-
condenser, which was not cleaned for a year,
locities þ some form of extended surface.
amounting to $265,000 in 1 year as costs due to
- Extended surface is used only when construction
fouling. One cleaning would have paid off seven
permits access for cleaning.
times the direct fuel costs. This example involves
only the direct losses due to fouling and not the ➢ Shell side fouling is greatest for segmentally
costs of corrosion of materials due to fouling and baffled bundles in the regions of low velocities.
loss of equipment life due to corrosion. ➢ Keeping heat transfer surface temperatures as low
. What are the techniques used to minimize fouling heat as possible (for heating applications).
transfer surfaces? ➢ Use of cocurrent flow where rise in temperature is
& Corrosion Control: detrimental to keeping surfaces clean of fouling.
