Page 358 - Fluid mechanics, heat transfer, and mass transfer

HEAT TRANSFER EQUIPMENT INVOLVING PHASE TRANSFER
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because of ratio of surface areas is about 2.5, finned
to bare.
. When a 1–2 heat exchanger is used as a vaporizer, is it
advisable to vaporize all the feed? If not, explain why.
& No. It results in formation of scales.
. What is the normally recommended ratio of liquid
circulated to vapor generated in a reboiler?
& 4:1.
. What is the advantage in using recirculation of liquid in
a vaporizer/reboiler/evaporator?
& Less fouling of heat transfer surfaces.
. How is the submergence of tube bundle ensured in a
Column internal reboiler. kettle reboiler?
FIGURE 11.8
& By using a vertical partition baffle plate acting as a
weir between the tube bundle and the liquid outlet, its
kettle reboiler, except it does not have shell and the height being more than the bundle diameter as illus-
connecting piping. trated in Figure 11.9.
& Hydraulic problems are few, unlike with a kettle
. What are the factors that are to be considered in the
reboiler for which sizing and location of feed and design of the vapor space in a kettle reboiler?
vapor lines can give rise to operating problems.
& Entrainment from kettle reboilers occurs when the
& Disadvantages are limited heat transfer area can be
disengagement space above the bundle is insufficient
accommodated, requires large flange and internal to deentrain liquid droplets. One important factor is
supports, tubes are short and hence bundle costs are to provide enough space to decrease the vapor ve-
high and column has to be shut down if the reboiler is locity sufficiently for most of the liquid droplets to
to be serviced as no alternative operation is possible. fall back by gravity to the boiling surface.
. What are the advantages of using low-fin tubes in a & Also, vapor outlet nozzle should be sized adequately
kettle reboiler? to avoid excessive velocities and consequent entrain-
& In a kettle reboiler with low DT and in fouling media, ment into the column.
low-fin tube gives rise to more nucleation sites per & The extent of entrainment separation depends on the
unit of surface than a bare tube. nature of the vapor destination. A distillation column
& This is because finning operation involving alteration with a large disengaging space, low column efficien-
of tube surface with tools with tiny imperfections cy, and high reflux rate does not require as much
being introduced on finished fin surface. kettle vapor space as a normal reboiler.
& Therefore for the same DTs, there will be more ➢ Several criteria and rules of thumb for preventing
bubbles generated from a finned surface than for a entrainment are available, the most common of
comparable length of bare tube surface, not only which is keeping the top tube row not higher than

Kettle reboiler.
FIGURE 11.9