Page 317 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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etc.), suspended in process fluid, onto heat transfer ➢ This type of fouling occurs in refrigeration
surfaces where velocities are low. and cryogenic systems and those involving high
melting point components, for example, waxy
& Sedimentation Fouling: Due to gravity settling.
components.
Deposits formed through sedimentation processes
are generally loose and do not adhere strongly to the & Combination Mechanisms: In most cases, fouling
heat transfer surfaces. These deposits are self- occurs involving a combination of the above mechan-
limiting, that is, as they grow they get washed off isms. A common example is fouling of cooling water
by the flowing fluid. Sedimentation fouling is involving sedimentation and inverse solubility, as
strongly affected by fluid velocity and less so by most surface waters used for cooling contain sedi-
temperature. However, a deposit can get baked on to ment and calcium carbonates. Macrofouling cuts
a hot wall and become very difficult to remove. down on cooling water flow and thus allows more
sedimentary deposits, which in turn, can lead to
& Chemical Reaction Fouling: Formation of insoluble
microfouling and corrosion.
products by chemical reaction and subsequent
deposit formation on heat transfer surfaces, for ➢ Cooling waters are not normally once-through
example, petroleum coke deposits in cracking pro- flows but involve recirculation. Evaporative cool-
cesses and asphaltenes, polymer, and food derived ing reduces quantities of waters, thus increasing
products. concentration of solids and, in addition, contains
treatment chemicals. Use of makeup water in
➢ For example, coking and polymerization reactions
cooling water systems is to be carefully planned.
take place on hot metal surfaces, producing an
Blowdown requirements should also be properly
adhering solid product of reaction.
calculated and implemented.
& Corrosion Fouling: Accumulation of corrosion
. What are the different contributors for scale formation
products.
in equipment?
➢ Corrosion layer (less conducting than metal) is
produced by reaction of heat transfer surface with & Oxidation Products:
flowing fluids. ➢ Metal oxide scales. Oxides are more stable than
➢ Sometimes a corrosion layer acts as a shield for metals themselves.
further corrosion, in which case attempts to clean ➢ Mill scales.
the surface may only result in accelerated corro- ➢ Magnetite scales (Fe 3 O 4 ) are formed during forg-
sion and eventual failure of the exchanger. ing, hot rolling, and other high-temperature
& Biofouling: Attachment of macroorganisms manufacturing operations.
(macrobiofouling) and/or microorganisms (microbi- & Fluid oxidation due to air infiltration mainly causes
al or microbiofouling) present in fluid stream towarm solids formation and fouling in organic thermal
heat transfer surfaces, where they adhere, grow, and liquid heat transfer systems. Oxidation of the organic
reproduce. These biofouling processes can be further fluid creates insoluble solids. The combination of
classified as aerobic and nonaerobic types. corrosion products and oxidized heat transfer fluid
➢ Microbial slime and algae are examples of insolubles usually deposits on heat transfer surfaces.
microbiofoulants. Rapid thermal decomposition of a heat transfer fluid
➢ Snails, barnacles, and mussels are examples of occurs when the fluid is exposed to excessively
macrobiofoulants. high temperatures. Off-design operating conditions
(e.g., high heat flux or reduced flow conditions) are
➢ For tackling biofouling, the usual solution is to kill
common causes of excessive fluid temperatures.
the life forms by chlorination or to discourage their
These conditions may result in surface fouling and,
settling on the surfaces by using 90–10 copper–-
in severe cases, rapid coking.
nickel alloy or other high copper alloy tubes.
Intermittent shock chlorination is an alternative & Corrosion:
solution than continuous chlorination. ➢ Magnetite scales crack, producing fissures in the
& Solidification Fouling: Overcooling of a fluid below scales. Exposed metal acts as anode and mill
freezing point at heat transfer surface resulting in scales act as cathodes, accelerating corrosion
solidification of the process fluid and coating of heat process.
transfer surface. ➢ Rust or Red Iron Scales (Fe 2 O 3 ) on Metal
➢ Higher melting components of a multicomponent Surfaces: In water transfer lines, heat exchangers
solution freeze on colder surfaces. on water cooling side, boilers, and so on.