Page 297 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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& U-bundle exchanger is recommended for applica- & Cleaning of inside and outside of the tubes in a
tions involving large temperature differences as it floating head exchanger is easier and, therefore,
provides for tube expansion in a simpler way than a floating head exchangers can be used for services
floating head exchanger that involves internal parts where both the shell side and the tube side fluids are
that are difficult to maintain. dirty.
. What are the advantages of U-bundle heat exchangers? . How two shell passes are arranged in a heat exchanger?
& Possibility of tube expansion is taken care of. As one Illustrate by means of a diagram?
end is free, each tube is free to expand or contract & A solid longitudinal baffle is provided to form a two-
independent of other tubes, in response to stress pass shell (see TEMA F-type shell in Figure 10.2 and
differencesdevelopedduetotemperaturedifferences. also Figure 10.5 of 2–4 floating head exchanger for
& Replaceable or removable tube bundle. Facilitates figures).
cleaning of shell and outside surfaces of tubes. . Compare arranging two shells in series and two-pass
& Use of hydraulic cleaners (water jets forced through shell.
spray nozzles at high pressurefor cleaning deposits in & Two-pass shell is cheaper than two-shell arrange-
tube interiors reduces cleaning problems). ment for the same duty and also has improved heat
& Minimum clearance between outer tube limit and transfer efficiency.
inside of shell. . What are the different classes of heat exchangers ac-
& Number of joints required is reduced, making it well cording to TEMA? Give applications of each class.
suited for high-pressure applications, with low initial & Class R: For generally severe requirements of refin-
and maintenance costs. eries and related process applications. Equipment
& Nointernalgaskets.(Requiredforfloatingheadtype.) fabricated in this class is designed for safety and
durability under rigorous service and maintenance
& Replaces floating head exchanger for applications
conditions.
involving large temperature differences.
& Class C: For generally moderate requirements of
. What are the negative points in the use of U-bundle heat
commercial and general process applications.
exchangers?
& Class B: For chemical process service.
& Difficult to clean inside of tubes, especially in the
curved ends, as the U-bends require flexible-end drill & TEMA R is the most restrictive and TEMA C is the
shafts for cleaning. least stringent. TEMA B and R are very similar in
scope. TEMA R includes the requirement for con-
& Therefore, U-tube heat exchangers should not be
fined joints where recesses must be machined in the
used for services with a dirty fluid inside tubes, as
flanges and tube sheets. Spiral-wound gaskets with a
mentioned earlier.
ring construction also meet this TEMA R require-
& U-tube bundles do not have as much tube surface as
ment. TEMA R also requires a greater minimum
straight tube bundles due to the bending radius.
thickness for some components.
& Because of U-tube nesting, it is difficult to replace a
. ‘‘TEMAClass C heat exchanger is used for heavier duty
tube, particularly the interior tubes, and often
applications compared to TEMA Class R exchanger.’’
requires the removal of outer layers. Such tubes are
True/False?
often plugged, reducing heat transfer area.
& False.
& No single tube pass or true countercurrent flow is
. List the important heat exchanger parts and
possible.
connections.
& Tube wall thickness at the U-bend is thinner than at
& Shell, tubes, shell cover, channel, channel cover, tube
straight portion of tubes.
sheets, baffles, nozzles, tie rods and spacers, pass
& Draining of tube circuit is difficult when mounted
partition plates, impingement baffle plate, longitudi-
with vertical position with the head side up.
nal baffle plate, expansion joints, supports, drain
. Compare U-bundle and floating head heat exchangers.
connections, vents and relief valve connections,
& Floating head design involves more complexity than instrumentation connections, and so on.
a U-bundle design with the consequence that a
. Give a summary for the selection of heat exchangers,
floating head exchanger is costly. Also its mainte-
with respect to their types.
nance costs are higher.
& Table 10.1 gives selection of heat exchangers with
& Cleaning of inside tubes is difficult in a U-bundle
respect to their types.
exchanger.