Page 313 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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& Direct impingement of fluid jet onto the tubes from and alloys of nickel. Titanium or aluminum may also
inlet nozzle into the shell can cause cavitation, be used for special purposes.
vibration, and erosion. & Table 10.5 gives briefly, material selection for cor-
& Impingement plate/baffle is sometimes used to divert rosive and noncorrosive service for heat transfer
incoming fluid jet into the shell from impacting surfaces.
directly at high velocity onto the top row of tubes. . What are the different types of tube inserts in heat
This ensures distribution of the fluid evenly and exchanger tubes? How do they work? What are their
prevents fluid-induced erosion, cavitation, and benefits and applications?
vibration. & Twisted tapes, rotating helical coils, and wire matrix
& Installing the impingement baffle inside the shell
inserts are used in shell and tube heat exchanger tubes
prevents installing a full tube bundle, resulting in to enhance heat transfer through increased turbulence
less available surface. and control fouling inside the tubes.
& It can alternately be installed in a domed area above & Twisted tapes tend to promote intensityof mixing and
the shell. The domed area can either be reducing turbulence. The partitioning and blockage of the tube
coupling or a fabricated dome. This style allows a full flow cross section by the tape results in higher flow
tube count and therefore maximizes the utilization of
shell space.
& Figure 10.31 illustrates alternative arrangements of
TABLE 10.5 Selection of Materials of Construction for Heat
impingement baffle. Exchangers
& Sometimes slotted distributor plate is used in place of
Material Exchanger Type/Typical Service
a baffle plate for the above purpose.
Noncorrosive Service
& In order to accommodate such plates, it may some-

times be necessary to omit some tubes near the plate Al and austenitic Any type, T < -100 C
chromium–nickel steel
to avoid excessive pressure drop.
Nickel steel (3.5% Ni) Any type, –100 < T < -45 C

. What are the considerations involved for the selection of

Carbon steel (impact tested) Any type, 0 < T < 500 C
materials of construction for heat exchangers? Refractory-lined steel Shell and tube, T > 500 C

& Corrosion and/or operation at elevated temperatures
are the main functional considerations in material Corrosive Service
selection.
Carbon steel Mildly corrosive fluids;
& Requirement for low-cost, lightweight, high-conduc- tempered cooling water
tivity, and good joining characteristics often leads to Ferritic carbon–molybdenum Sulfur-bearing oils above 300 C

the selection of aluminum for the heat transfer and chromium– H 2 at elevated temperatures
surface. molybdenum alloys
Ferritic chromium steel Tubes for moderately corrosive
& Stainless steel is used for food processing or fluids
service; cladding for shell
that require corrosion resistance.
or channels in contact with
& Commonly used tubes are seamless or welded with
sulfur-bearing oils
materials of construction usually alloy steels; copper Austenitic chromium–nickel Corrosion-resistant service
steel
Aluminum Mildly corrosive fluids
Copper alloys: admiralty, Freshwater cooling in surface
aluminum brass, condensers; brackish and
cupro–nickel seawater cooling
High Ni–Cr–Mo alloys Resistance to mineral acids and
chlorine-containing acids
Titanium Seawater coolers and conden-
sers including plate heat
exchangers
Glass Air preheaters for large furnaces
Carbon Severely corrosive service
Coatings: Al, epoxy resins Exposure to sea and brackish
water
Linings: lead, rubber Channels for seawater cooling
Linings: austenitic Cr–Ni steel General corrosion resistance
FIGURE 10.31 Use of impingement baffle.