Page 308 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 289

Rod baffles.
FIGURE 10.23

➢ The normal rod diameter is 6.35 mm and each tube
is supported on all four sides at several points
along the exchanger.
➢ Not good for large diameter shells.
& Twisted Tubes: Tubes are twisted into a helical
shape with an oval cross section, so that each tube
is supported over its entire length by multiple
contact points with adjacent tubes. The end sections
are kept circular to permit mounting in standard
tube sheets.
➢ Due to higher turbulence both inside and outside of
the tubes, higher heat transfer rates are achieved,
reducing heat transfer area requirements for the FIGURE 10.24 Baffle cut.
same duty, thus offsetting increased costs.
➢ Fouling rates are reduced and tube vibrations are
. What is meant by baffle cut? Illustrate.
eliminated.
& Baffle cut is the height of the segment that is cut in
➢ Flow distribution will not be evenin both rod baffle
each baffle to permit the shell side fluid to flow across
and twisted tube designs for large diameter shells.
the baffle. Baffle cut is expressed as the percentage of
With a single inlet nozzle, tubes near the inlet have
shell inside diameter not covered by the baffle
more than the average flow and those opposite to
(Figure 10.24).
inlet will have low flows for an appreciable dis-
tance down the exchanger. . What is the advantage and disadvantage of using (i)
higher baffle cut and (ii) lower baffle cut than the
. Under what circumstances, disk and doughnut baffles
normally recommended value of 20–25% of shell
are used in a heat exchanger?
diameter? Give brief answers.
& Where DP available is very small, that is, where there
(i) Larger Cut: Less DP.
is pressure drop problem.