Page 327 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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& Whether the fouling factors specified denote asymp- . What is a self-cleaning heat exchanger? State its oper-
totic values or values after a fixed operating time is ating principle.
not clearly stated. & Self-cleaning heat exchanger is essentially a vertical
& TEMA-specified fouling factors do not account for shell and tube heat exchanger through which a foul-
the effect of material of construction, which could ing fluid flows upward inside the tubes charged with
result in inaccurate design. For example, copper alloy solid particles that are swept upward along with the
tubes in exchangers used in crude oil processing units fluid, producing a scouring action on the walls of the
are highly susceptible to coking resulting in higher tubes as they move up.
fouling resistances than TEMA-specified values. & A distributing system in the inlet channel provides
Stainless tubes havebeen found to offer lower fouling uniform distribution of the fluid and the particles into
resistances than copper alloy tubes. In one study on the tubes.
fuel oil, it was found that fouling resistances for & From the outlet channel, the particles and fluid are
copper tubes were 0.005 whereas for 316 stainless
carried into a separator. From the separator, the
steel tubes were found to be 0.0025. The implications
particles are returned to the inlet channel through
are less cleaning and maintenance requirements for
a control system.
stainless steel than copper, although copper having
& Existing heat exchangers such as evaporators and
higher thermal conductivity offers less resistance
reboilers can be retrofitted with the solids circulation
than stainless steel.
system, reducing fouling problems.
& Another aspect is that since the exchanger does not
& The disadvantage with such systems is erosion pro-
exhibit the specified fouling resistance at the start of
blems for the tubes and separation of fouling solids
its operational life, it will initially overperform,
from the circulating particles. Certain percentages of
which may involve implications on process control.
the fouled solids are removed and fresh solids intro-
For example, the overdesign, with respect to initial
duced into the system as practiced, for example, in
operation, may require lower initial fluid velocities,
boiler blowdown.
which in turn gives rise to higher surface tempera-
tures than prescribed by design, thus resulting in
more or faster fouling than would be the case 10.1.4 Pressure Drop
otherwise.
. What are the normally assumed pressure drops in the
& In spite of the above limitations, no clear and effec-
design of heat exchangers?
tive alternative to TEMA-specified fouling factors is
& 0.2–0.62 bar (3–9 psi) for most services and 0.1 bar
available and these are continued to be used.
(1.5 psi) for boiling.
. What are the effects of specifying larger fouling factors
& For low-viscosity liquids, allowable pressure drops
than necessary in the design of a heat exchanger?
are lower compared to high-viscosity liquids. This is
& Results in oversize exchanger:
because flow velocities should be kept higher to
➢ Low Fluid Velocities: More fouling on two
increase heat transfer coefficients for high-viscosity
counts, namely, increased settling rates of solids
liquids, which are otherwise low and sometimes flow
due to decreased velocities and increased surface
blockages occur. When a high pressure drop is used
temperatures that can give rise to decomposition
in the design, care should be taken to prevent erosion
and polymerization reactions of the fluids and
of equipment surfaces.
also higher corrosion rates giving rise to more
& For gases and vapors, allowable pressure drops for
corrosion products.
design purposes depend on system pressure. Very low
& Net effect is decreased heat transfer in spite of using
pressure drops are a necessity for vacuum systems
larger and more expensive exchanger.
compared to atmospheric and high-pressure
➢ For example, if overall coefficient, U, is 100 W/
exchangers.
2
(m C)foracleanexchangerandfoulingfactorsof
. Give equations for estimation of DP in the tube side of a
0.001 are specified on either side, required area of
heat exchanger.
theexchangerincreasesby20%.Ifcleanexchanger
& For straight circular tubes, Fanning equation can be
U is 100 and total fouling factor (both sides com-
used for DP:
bined) specified as 0.01, required A will double.
2
. ‘‘Overdesign of a heat exchanger leads to good perfor- P in P out ¼ 2fG L=g c rD i : ð10:16Þ
mance.’’ True/False?
& Including expansion, contraction, or changes in
& False (See previous question.)
direction (nonisothermal, multipass exchangers):