Page 320 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 301

counterbalanced by anions such as bicarbonates, & It is normal to consider high flow velocities to
chlorides, and sulfates. minimize fouling deposits. High flow velocities give
& Groundwaters usually contain higher concentrations rise to erosion problems on equipment, particularly
of dissolved ions. wherever direction changes for flow occurs. Heat
exchanger failures occur in U-bends and where
& Surface waters are softer but contain suspended
solids, silica, organics, and microorganisms. impingements are involved. Materials such as tita-
nium or stainless steels resist erosion better. A
& Inverse solubility of many compounds formed from
cupronickel alloy with 83% copper, 17% nickel, and
different ions (CaCO 3 , silicates including MgSiO 3 )
0.4% chromium is claimed to be having much greater
contained in cooling waters precipitate on tempera-
resistance to erosion attack, being capable of oper-
ture increases on heat transfer surfaces.
ating with seawater at velocities of 7–8 m/s.
& These give rise to scaling in condensers, which gets
. ‘‘For a particular service, use of carbon steel tubes
aggravated due to recirculation. Phosphate-based
serves the purpose from corrosion point of view.’’ Is
corrosion inhibitors give rise to scale forming
there anyadvantage of using more expensivealloy tubes
Ca 3 (PO 4 ) 2 .
or smooth surfaces?
& Microbial activity in cooling waters gives rise to
& Yes.
protective sticky films on tube walls, which absorbs
& Alloy tubes are smoother and result in less fouling.
silt and sediment, further increasing deposit volume.
Biofouling may also induce corrosion with the & Highly polished surfaces (mechanical or electro-
dissolved oxygen in cooling water diffusing toward chemical polishing) are used in food industry.
the oxygen-deficient zone under the deposit near the & Surface treatment by coating the surface chemically
tube walls. Many microbes produce acids in the or modification of the surface itself to reduce adhe-
presence of oxygen as part of their metabolic pro- sion of deposits.
cesses. These acids attack the metal walls. & Easy to clean.
. What are the effects of fouling on finned surfaces? & Better heat transfer rates, although equipment is
& Finned surfaces are on outside of tubes in tubular more expensive.
exchangers. Effects of fouling on these surfaces are & Longer life.
more complicated than inside tubes, because in ex-
. Name the conditions in the operation of plant equipment
treme situations there is a possibility that the finite
that change fouling rates.
thickness of the fouling layer can effectively block off
& When plant operating conditions change too fast.
flowthroughthefins,negatingtheusefulnessofthefins.
& When a process upset occurs.
& On the other hand, finned surfaces are sometimes
& When pumps are switched on.
found to be more resistant to fouling than normal
plain surfaces. One reason for this could be the & During unloading tank cars.
expansion and contraction of the surface during nor- & During addition of new catalysts.
maloperational cyclesthattendtobreakoffthebrittle & When surfaces become rough due to corrosion or due
deposits. to roughness from earlier fouling.
& Normally, it is not recommended to have finned & Due to excessive cooling water temperatures causing
surfaces for heat transfer involving dirty liquids as scale formation on the waterside surfaces.
cleaning becomes a constraint. . Name some of the considerations involved for
& High finned surfaces are commonly used with air and
the choice of heat exchanger design with respect to
other low pressure and relatively clean gases. Dust is fouling.
the main fouling agent in such cases and is easily & Table 10.8 gives considerations involved in the
removed by blowing.
selection of a heat exchanger with respect to fouling.
. How does selection of materials influence fouling?
. What are the indications that an exchanger has fouled
& Corrosion-type fouling is minimized by proper and how maintenance handles this?
selection of material for heat transfer surfaces. Such
& A light sludge or scale coating on the tube greatly
material should also withstand corrosive attack on
reduces its efficiency. A marked increase in pressure
fluids used for chemical cleaning.
drop and/or reduction in performance usually indi-
& Biological fouling can be largely eliminated by the cates cleaning is necessary.
use of copper-based alloys containing over 70% of & The unit should first be checked for air or vapor
copper.
binding to confirm that this is not the cause for the