Page 291 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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to mechanical scraping as in ladles used, for example, & Recuperators and Regenerators: There are two
in the kitchen for making sweets or other products types of air preheaters in furnace systems, namely,
involving solidification or thickening on being recuperators and regenerators.
heated. Surfaces are constantly renewed to prevent ➢ Recuperators are gas–gas heat exchangers placed
charring of products with increased residence times on the furnace stack. Internal tubes or plates
and consequent quality deterioration of heat- transfer heat from the outgoing exhaust gas to
sensitive materials. It typically consists of a jacketed the incoming combustion air. The two streams are
cylinder with rotating rows of scraper blades. The prevented from mixing by the tubes or plates.
product is pumped through the cylinder while a ➢ Regenerators include two or more separate heat
heating or cooling medium is circulated between the
storage sections. Flue gases and combustion air
cylinder and the insulating jacket. Typically used for
take turns flowing through each regenerator,
freezing of ice cream and in the cooling of fats during
alternately heating the storage medium and then
margarine manufacture.
withdrawing heat from it. Refractory walls act as
& Reboiler: One stream, a bottoms stream from a
storage media for the heat.
distillation column and the other a hot utility (steam . What are the recommended selection criteria, based on
or hot oil) or a process stream.
area, among shell and tube, double pipe, and coil types
& Kettle Reboiler: Heat exchanger in which boiling of
of heat exchangers?
a liquid takes place on the shell side, outside tubes. 2
& A < 2m , select a coiled heat exchanger.
Steam or other heating medium flows through tubes.
2
2
& 2m < A < 50 m , select a double pipe heat
Used for energy supply to distillation process by
exchanger.
partial boiling of the bottom stream. This is a shell 2
and tube heat exchanger in which tube bundle is & A > 50 m , select a shell and tube heat exchanger.
submerged in the shell side boiling liquid, as in ➢ The coiled heat exchanger is very compact and is
electric kettles with the enclosed heater element frequently used when space is limited. The deci-
submerged in the liquid. sion between the heat exchanger types is not as
& Thermosiphon Reboiler: In thermosiphon reboiler, distinct as indicated.
boiling/vaporizing liquid is inside tubes and heating . Name different types of shell and tube heat exchangers.
medium is on the shell side. Two-phase flow is & Double pipe heat exchanger: Annulus formed by
involved in the tubes.
the outside tube may be considered to be shell side
& Barometric Condenser: Vapors produced in an
space and the exchanger may be treated as a single
evaporator are condensed in a direct contact con- tube exchanger for small duties.
denser in which vacuum is created to assist vapor- & Single and multipass straight tube bundle exchan-
ization in the evaporator at relatively lower tem-
gers: Fixed tube sheet and floating head types.
peratures. Principle of a barometer is involved with
& U-bundle heat exchangers.
the liquid leg acting as the liquid column in a
& Reboilers: Kettle and thermosiphon types.
barometer. Above the liquid column, there will be
vacuum in a closed system involving vapor space in & Finned tube heat exchangers.
the evaporator. Vapor from the evaporator is drawn . Can we use polymers for heat exchanger construction?
by the vacuum into the condenser into which water If yes, what are their benefits?
is introduced through nozzles for condensing & Polymeric materials have a number of features that
the vapors with direct contact between the two make them suitable for use in heat exchangers.
fluids. & Low cost and corrosion free. When streams contain
& Jacketed Vessel: An outer jacket is provided on a
chlorides and other corrosives,polymeric exchangers
cylindrical vessel through which heating or cooling have an advantage.
medium is circulated while the liquid in the vessel is & Thin structures are possible with reduced weight.
heated or cooled. Such vessels are normally
& Their smooth and noncrystalline surface finish
equipped with mixers to mix the fluids inside the
results in low DP and low fouling and scaling.
vessel, for example, to carry out batch chemical
& Surface properties can be changed for improved
reactions.
wettability.
& Coiled Vessels: Vessels in which heating or cooling
& Their thermal conductivity can be improved by
is required are equipped with tubular coils, through
incorporating heat conducting fillers and fibers made
which heating or cooling fluid circulated. This is an
from graphite, boron nitride, and carbon black. Using
alternative to a jacketed vessel.