Page 339 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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which reduces the effective mean temperature driv- . What are the means by which one can increase DT in a
ing force and possibly can lead to a temperature heat exchanger involving a heating medium?
cross, where the outlet temperature of the cold stream
& By using heat transfer fluids.
is higher than the inlet temperature of the hot stream.
. ‘‘Increasing U increases Q according the equation,
This reduces the effective mean temperature differ-
Q ¼ UAFDT lm .’’ Comment.
ence, necessitating use of two or more exchangers
0.8
& Increase U by increasing V (U / V ).
operating in series.
& V is increased on tube side by increasing number of
& Tube bundles having more than four tube passes,
tube passes.
introducing pass partition lanes into the bundles,
result in increased bundle bypassing and a reduction & V is increased on shell side by decreasing baffle
in shell side heat transfer coefficient. spacing.
& Part of tube side pressure drop is wasted in the return & Negative aspect in the above approaches is increased
headers, as no heat transfer will take place in the pressure drop.
headers. & But pumping energy is much lessthan energytransfer
. ‘‘Increasing DT in a heat exchanger increases heat in an exchanger.
transfer rate according to the equation, Q ¼ UAFDT.’’ . What are theways by which U can be increased in a heat
What are the limitations involved in this approach? exchanger?
& Instead of using ambient cooling water supply to & U can be increased by increasing turbulence in the
condensers, refrigeration could be used. But it very flowing fluids on tube and/or shell side. It must be
much increases process costs. understood that pressure drop also increases and
& Use of high steam temperatures in heaters involves whether such an increase is considered to be under
high steam pressures. allowable DP limits.
➢ High Steam Pressures:Low l V and therefore high & By reducing fouling rates through pretreatment of the
steam consumption compared to using low-pres- fluids or increasing cleaning schedules.
sure steam. & By eliminating stagnant areas inside the exchanger
➢ High Pressure Steam: by judicious design.
- More valuable for power generation. . What are the causes for overpressure development and
- Turbine exhaust steam is at low pressure with failure of heat exchangers?
high l V . & Corrosion/erosion of exchanger internals resulting
- Exchanger is to be designed for high pressure, in a heat transfer surface leak or rupture leading
which is expensive with poorer safety. to possible overpressure of the low-pressure
side.
➢ Increased DT will also result from using super-
heated steam as heating medium but it is not & Differential thermal expansion/contraction between
normally preferred for the following reasons: tubes and shell resulting in tube leak/rupture (fixed
tube sheet exchanger).
- Superheated steam is more valuable for power
generation involving steam turbines. & Excessive tube vibration resulting in tube leak/
rupture and possible overpressure of the low-pressure
- Heat transfer coefficients are very low in the
side.
desuperheating section of the heat exchanger,
steam acting as a gas during this cooling process. & Excessive heat input resulting in vaporization of the
cold side fluid.
- Heat given out by the steam during desuper-
heating process is sensible heat that is very low & Loss of heat transfer due to fouling, accumulation of
compared to latent heat liberated during con- noncondensables, or loss of cooling medium.
densation. For example, steam at 1 bar absolute & Ambient temperature increase resulting in higher

with a 50 C superheat transfers about 101 kJ/kg vaporization rate in air heated exchanger.
while cooling from the superheat to the satu- & Cold side fluid blocked in while heating medium
ration temperature. But it transfers about continues to flow.
2485 kJ/kg of latent heat on complete
. A process liquid at high pressure is being heated by
condensation.
steam with pressure lower than the process liquid. A
➢ Use of Dowtherm or other high-temperature heat pinhole develops on a tube due to corrosion. What
transfer fluids requires special boilers and piping, happens to the operation of the exchanger? Explain
which is expensive for normal applications. and discuss.