Page 338 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 319

combustion and formation of CO, soot, and other & Larger exchanger sizes result in decrease in flow
products, which can be hazardous leading to the velocities.
need to additional soot blowing to prevent deposits ➢ Decreased flow velocities result in decreased heat
on heat transfer surfaces, furnace explosions, and transfer coefficients.
creating air pollution problems. ➢ Also at low velocities, fouling deposits increase,
➢ Plugging or replacing such a tube, preventing flow,
further reducing heat transfer coefficients.
becomes a priority to avoid any intolerable ➢ Plugging of tubes can result, especially when
consequence.
liquids are viscous.
. What are the consequences of tube vibration in a heat
➢ Second aspect, (ii), is very critical.
exchanger? What type of heat transfer application is
➢ Net result can be increased A might result in
prone to tube vibrations?
reduced Q, instead of increased Q.
& Bundle vibration can cause leaks due to tubes being
➢ Another Aspect for High-Viscosity Liquids: De-
cut at the baffle holes or being loosened at the tube
creased velocity reduces h and hence Q, which
sheet joint.
means lower tube wall temperatures, thereby
& There are services that are more likely to cause
increasing m W . This has a further decreasing effect
bundle vibration than others. The most likely service on h as h / (m/m W ) 0.14 . This effect is very consid-
to cause vibration is a single-phase gas operating at a erable for very high-viscosity liquids.
pressure of 700–2000 kPa. This is especially true if . What alternatives can be suggested for overcoming the
the baffle spacing is greater than 46 cm (18 in.) and
problem mentioned in the above question in cases
single segmental.
where A is to be increased.
& Most flow-induced vibration occurs with the tubes
& Velocities should be increased by increasing number
that pass through the baffle window of the inlet zone. of tube passes. It is always better to use a higher DP
The unsupported lengths in the end zones are nor-
exchanger, although it involves higher pumping costs
mally longer than those in the rest of the bundle.
that are much less than loss of energy recovery.
& For 19 mm (3/4 in.) tubes, the unsupported length can
& Use of larger tube lengths increases A for the same
be 1.2–1.5 m (4–5 ft). The cure for removable bun-
number of tubes, with marginal increase in shell costs
dles, where the vibration is not severe, is to stiffen the
compared to use of larger diameter shells with more
bundle. This can be done by inserting metal slats or
number of tubes. While there are limitations in
rods between the tubes. Normally this only needs to
increasing tube lengths beyond certain limits, use of
be done with the first few tube rows.
multiple shells in series can be an alternative.
& Another solution is to add a shell nozzle opposite the
& Another way to increase Q by increasing A will be
inlet so as to cut the inlet fluid velocity by half.
through the use offinned tubes in place of plain tubes.
For nonremovable bundles, this is the best solution.
Surface areas offinned tubes will be higher compared
Adding a distributor belt on the shell would be a very
to plain tubes, involving no flow area increase,
good solution if it is not very expensive.
thereby not affecting flow velocities.
. Write the equation for rate of heat transfer in terms of
. ‘‘When cooling a highly viscous liquid with water in a
overall heat transfer coefficients.
tubular heat exchanger, tube wall temperature is ap-
Q ¼ UAFDT lm : ð10:32aÞ proximately equal to cooling water temperature.’’ True/
False?
. Give an equation for heat transfer rate for varying & True. Combined heat transfer resistance for water
overall heat transfer coefficient. Give the assumption film þ metal wall will be negligible compared to that
involved in the applicability of the equation. for viscous liquid film.
Q ¼ A½U 2 DT 1 U 1 DT 2 Š=ln½U 2 DT 1 =U 1 DT 2 Š: . What are the detrimental effects of using multiple tube
passes?
ð10:32bÞ
& Multiple tube passes can lead to a reduction of
The subscripts refer to the ends of the heat exchanger. number of tubes that can be accommodated in a
. ‘‘According to the equation, Q ¼ UAFDT lm , increase in given shell. In other words, increasing the shell
A by increasing number of tubes in a shell and tube heat diameter will be more for a given number of tubes
exchanger should result in increase in Q.’’ Comment resulting in increased equipment cost.
and explain. & Use of multiple tube passes results in the thermal
& Exchanger size and hence fixed costs will increase. contacting of the streams not being pure counterflow,