Page 325 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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➢ EDTA is not as hazardous as HCl, although . Are fouling factors affected for steam contaminated
ammonia smell will be evident. with traces of oil? If so, compare fouling factors for oil-
➢ Copper removal often does not require a prelim- free steam and steam with traces of oil.
inary stage or a completely separate second step. & Yes. Fouling factors for oil-free steam are much less
➢ The spent solvent may be evaporated in the boiler, than for oil contaminated steam.
greatly reducing project costs. . What are the sources for obtaining fouling factors?
& Disadvantages: & TEMA standards.
➢ Boiler must be fired during the cleaning process. & Published literature.
➢ Chemical must be manually circulated because & Company generated data or in-house experience for
soaking does not work well. use in specific situations.
➢ Although EDTA is not as hazardous as HCl, safety & Other sources.
issues regarding the high temperature of the liquid . What are the uncertainties involved in specifying foul-
must be considered.
ing factors while designing or evaluating the perfor-
- If boiler temperatures exceed 150 C, EDTA

mance of a heat exchanger?
decomposes.
& TEMA recommended values are based on the fouling
& During firing, the boiler water volume swells, which
propensity of the service fluid and the resistance
may require partial draining of the solution, which
caused by that fouling. They do not account for the
should be done under nitrogen blanketing.
type of material of construction and the smoothness
& EDTA is more expensive than HCl. of its finished surfaces. Other sources of error include
differences between the process fluid and ostensibly
. ‘‘Organic heat transfer fluids give rise to oxidation and
identical fluid on which fouling data are based and
decomposition products on usage.’’ How are such
any additional out of context application arising
fluids cleaned to prevent fouling deposits arising from
from plant revamping. Microbial activity, tempera-
them?
ture, and fluid velocity might be different.
& For in-system filtration, disposable glass fiber-wound
& Published fouling factors do not reflect true perfor-
filter cartridges are generally the most satisfactory
mance, as for some services they are too high while
since they withstand system temperatures up to
400 C and have adequate solids handling capacity. for others they are too low.

Sintered metal filters, while satisfying high- & Specified fouling factors are static values, while in
temperature capability, are more expensive and dif- actual situations, they vary based on the operational
ficult to clean. variations during service. During the initial stages of
the operational life of the exchanger, it will overper-
. What are fouling factors? How do they influence heat
form, which may have process control implications,
transfer rates?
as stated earlier.
& For making an allowance for fouling in the design of
& Temperature and velocity are two very significant
heat exchangers, it is customary to assign a value to
variables that affect fouling rates.
the anticipated heat transfer resistance, which is often
referred to as fouling factor. & Crude oils, for example, foul by different mechan-
isms including sedimentation, crystallization/precip-
& At the start of commissioning, there will not be
itation, corrosion, asphaltene flocculation, and
fouling resistance and hence, the exchanger will
coking. It will foul by one mechanism, or combina-
initially overperform, which has implications for
tion of mechanisms, at one point in a refinery preheat
control:
train but will foul via a different mechanism at
➢ Depending on the duty of the exchanger, one may
another point. Many crude oils foul very heavily
be tempted to use a bypass so that the mixed exit
whereas others foul hardly at all. Level of desalting
temperature from the exchanger system has the
of the crude influences fouling rates.
desired value.
➢ Specific crude oil, wall temperature, shear stress,
➢ If the flow rate of the stream is not critical (e.g.,
run time, exchanger design, and many other vari-
cooling water), this variable may be reduced.
ables influence fouling rates. Therefore, alterna-
➢ Both the above alternatives effectively reduce the
tives to fouling factors are needed.
stream velocity through the exchanger, with the
& Despite the constraints mentioned above, TEMA and
potential to accelerate fouling process.
other fouling factor data used do provide qualitative