Page 323 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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➢ Use of low fouling exchangers that make use of ➢ Use of an additive in conjunction with a physical
elimination of cross-flow plate baffle. method may be more effective than either tech-
- Examples are plate heat exchangers, spiral plate nique alone.
exchangers, twisted tube exchangers, and so on. ➢ Use of sonic vibrations.
These involve no baffles and provide 25–30%
& Chemical Cleaning: Weak acids, solvents, and so
excess surface to compensate for fouling.
on.
& Other Designs: To develop easily removable fouling
& Loose Deposits: Flushing with high-velocity steam
layers on which further deposition occurs.
or water jets.
& Use of Additives: Periodic or continuous injection of ➢ Circulating hot wash oil or light distillate through
small amounts of additives. tubes or shell at high velocity.
➢ Different additives reduce fouling problem by ➢ Sand blasting or use of sand–water slurry.
chemical or physicochemical effects. ➢ Periodic use of high fluid velocities.
➢ Controlled acid dosing (H 2 SO 4 , HCl taking care
➢ Some salt deposits may be washed out by circu-
that there is no corrosion) of hard waters to combat
lating hot freshwater.
calcium carbonate deposits.
& Scales: Mechanical action.
➢ Antiscalants for aqueous systems, for example,
➢ Rodding.
chelates such as ethylene diamine tetra acetic
➢ Turbining.
acid, which combine with scaling material and
give rise to soluble compounds. ➢ Scraping and rotating wire brushing. These meth-
ods are limited to surfaces that can be reached by
➢ Threshold agents, for example, polyphosphates
the tools. In shell and tube exchangers, use of large
that retard precipitation.
clearances between tubes and/or the use of rotated
➢ Crystal habit modifiers, for example, polycarbolic
square tube pitch. Scraping should not be used on
acids, which distort crystal habits of scaling com-
finned tubes.
pounds that prevent adherence to hot surfaces.
➢ Powered rotating drills cut and propel their way
➢ Dispersants, for example, polyelectrolytes to keep
through hard, dense deposits.
scaling materials in suspension.
➢ Water jets þ drills are used for hard, dense/thick
& Biofouling is controlled by injection of (1) biocides
deposits.
that kill microorganisms or (2) biostats that arrest
➢ Hydraulic jets at pressures up to 700 bar. For shell
growth of microorganisms.
side, the jets will not beveryeffective deep inside a
➢ Chlorine is a common biocide but gives rise to
large tube bank.
corrosion. Less corrosive substitutes are used.
& Mechanical cleaning sometimes requires pulling out
➢ Use of copper alloys and/or chemical treatment
tube bundle for tube outside scale removal.
of fluid to prevent organism growth and
reproduction. & Inside scales need not require removal of bundle.
. What are the ways by which fouling deposits are . What are the precautions to be taken while cleaning
removed? deposits?
& Tubes should not be cleaned by blowing steam
& Cleaning: Periodic cleaning is necessary.
through individual tubes since this heats the tube
➢ Off-line cleaning methods: Mechanical methods
and may result in severe expansion strain, deforma-
(rodding, drilling, brushing, etc.), hydraulic or
tion of the tube, or loosening of the tube-to-tube sheet
pneumatic methods, or chemical cleaning
joint.
methods.
& When mechanically cleaning a tube bundle, care
➢ Onstream methods: Cleaning of inside tubes/pipes
should be exercised to avoid damaging the tubes.
by the use of recirculating sponge rubber balls or
& Cleaning compounds must be compatible with the
by flow-driven brushes, wherever high degree of
metallurgy of the exchanger.
cleanliness is desirable. The moving balls remove
deposits and corrosion products as they pass . What are the precautions taken while cleaning heat
through the tubes. A mesh basket in the outlet transfer surfaces on which pyrophoric iron sulfide scales
piping collects the balls and a ball pump reinjects are formed?
them into the inlet fluid. & Such scales, which might form on petroleum crude
➢ Use of tube inserts, ultrasound, and circulation of heat exchangers, should never be removed in dry
polymer fibers. condition as it creates fire hazards.