Page 318 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 299

➢ Waters with sulfide contaminants give rise to TABLE 10.6 Influence of Flow Velocity on Fouling
metallic sulfide þ iron scales. Mechanisms
➢ Water Insolubles: Precipitation of insolubles on Particulate fouling decreases with flow and stops above 1.7 m/s
heating of water, for example, Crystalline fouling slightly decreases with flow over 0.5 m/s
Corrosion fouling decreases very slightly with flows above 1.2 m/s
CaðHCO 3 Þ þ heat ! CaCO 3 þ H 2 O þ CO 2 : Biological fouling decreases with flow over 0.7 m/s and stops
2
over 2.2 m/s
ð10:13Þ
MgðHCO 3 Þ þ heat ! MgCO þ H 2 O þ CO 2 :
3
2
providing more chances for deposition to occur on
ð10:14Þ
the heat transfer surface.
➢ CaCO 3 forms thick scales in boilers, cooling
➢ Simultaneously, the shear forces acting at the
engine jackets, steam-heated kettles, and so on.
fluid–heat transfer surface interface increase,
➢ Sulfates in water give rise to calcium sulfate mixed
which aids deposit removal.
with CaCO 3 scales (inverse solubility at higher
➢ The actual amount of fouling is a balance between
temperatures).
these two opposing effects.
& Conversion Products:
➢ Effect of flow velocity on fouling mechanisms is
Additives used in water treatment, for example,
given in Table 10.6.
phosphates used to precipitate calcium as soft sludge.
& Temperature: The prevailing temperature of a fluid
➢ Removal of sludge by blowdown prevents accu-
passing through a heat exchanger and temperature of
mulation, but with time deposits of Ca or Mg
the heat transfer surface can have a profound influ-
hydroxylapatite form.
ence on the fouling mechanisms.
➢ Dispersants used thermally degrade giving rise to
➢ Table 10.7 gives the influence of temperature on
residues of heavy chalk like accumulations.
fouling mechanisms.
& Silicate Scales: Thin, tightly adherent, and very hard
➢ The presence of a deposit will affect temperature
scales.
distribution across the exchanger, which, in turn,
➢ Require special consideration in descaling.
changes temperature at the point of deposition,
➢ Formed by a combination or rearrangement of ions
thereby influencing rate of temperature-dependent
in water to form highly complex insoluble fouling.
deposits. & Conceptual Design and Geometry of the Exchanger:

➢ For example, water with Mg, Na, SO 4 , and
It is known that shell and tube heat exchangers are
silicate ions in solution. If pH is raised by addition more sensitive to fouling than, for example, a plate
of alkali, hard glassy scales will form. and frame or double pipe heat exchangers. This is
mostly because velocities and turbulence levels are
3MgSO þ 2Na 2 SiO 3 þ 2NaOH þ H 2 O
4
higher for the latter one.
! 3MgO 2SiO 2 2H 2 O þ 3Na 2 SO 4
ð10:15Þ
& Process Deposits: Due to degradation of products in TABLE 10.7 Influence of Temperature on Fouling
flow lines, coolers, evaporators, and so on. Mechanisms
➢ Polymers may form from processing hydrocar- Mechanism Temperature Dependence
bons. These range from thin oily substances to Particulate Little effect except physical conditions
thick black gummy matrices. deposition are affected
➢ Coke and rubber-like scales. Precipitation or Solubility; crystalline fouling involving
& Electrochemical Action: Water with traces of cop- crystallization inverse solubility salts greatly
increases
per is stripped of copper content whenpassed through
Chemical reaction Reaction rate
steel piping due to electrolytic action. Copper is
Corrosion Corrosion rate; corrosion fouling
deposited electrolytically on steel.
slightly decreases above 50 C
. What are the most significant process variables that Biofouling Metabolic activity; biofouling

affect the fouling process? Explain. decreases above 50 C and absent
above 140 C

& Flow Velocity: As velocity increases, entrainment
Freezing Solidification
and transport of the fouling species increases,