Page 337 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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- Decreased heat transfer area for condensation. & Cooling tower water is typically available at a max-

- Decreased Q. imum temperature of 30 C.
& Summer Conditions. . What is the principal effect of tube bundle vibration in a
➢ Case I: Control system is designed to maintain shell and tube exchanger? How is it minimized?
constant process fluid outlet temperature, high & Causes damage to the tubes and results in leakages,
cooling water velocities, and high h on cooling especially near tube sheets.
waterside. & Fluid entering from inlet nozzle impinges on tube
➢ Case II: Cooling water rate is kept constant. Pro- bundle and enhances vibration.
cess fluid outlet temperature increases. Vaporiza- & Minimized by avoiding/minimizing cross flow by the
tionlossesonstoragewillincrease.Anypumponthe use of tube support baffles that promote longitudinal
exit side of the exchanger creates NPSH problems. flow.
. ‘‘Temperature-sensitive fluids passing through a heat & Special attention is given to minimize impingement
exchanger can be a source of waste products. Reducing of inlet fluid through shell inlet nozzle (by the use of
tube wall temperatures is one way to prevent formation impingement baffle at inlet nozzle).
of such wastes.’’ What are the ways to reduce tube wall . If a tube breaks within a shell (might occur while
temperatures? removing a tube from bundle), what is the suggested
& Use of lower pressure steam for heating. course of action?
& Desuperheating plant steam reduces tube wall tem- & When tube breaks, it is very difficult to remove or
perature and increases effective area for heat transfer replace it.
by increasing heat transfer coefficient (h for con- & In such cases, the holes on both tube sheets are
densing steam is about 10 times that for superheated plugged to avoid mix-up of the two fluids. This
steam). involves less loss of downtime compared to the
& Use of staged heating minimizes degradation of heat- replacement of the tube.
sensitive fluids. & On turn around for maintenance of the plant or unit,
& Using air-fincooling instead of water cooling reduces the plugged tube(s) may be replaced.
cooling water requirements. . When a pinhole or holes develop in a tube, what are the
& Using online cleaning techniques keeps tube surfaces likely consequences?
cleanthatmakesitpossibleuseoflowerpressuresteam. & The consequences depend on the nature of fluids,
& Controlling cooling water temperature reduces scale their temperature and pressure levels, and the type of
formation. service.
. ‘‘Maximum recommended temperatures for cooling ➢ In a heat exchanger, shell side fluid gets contam-
water in heat exchanger outlets are about 45 C.’’ If inated with tube side fluid or vice versa, depending

temperatures exceed this value, what are the upon which side fluid is at higher pressure. Ingress
disadvantages? of one fluid into the other causes contamination,
flow/pressure variations, fouling, corrosion, reac-
& More fouling.
tion, and other problems.
& More evaporation losses.
➢ If shell side fluid is a condensing organic vapor, for
. ‘‘Maximum recommended velocities through nozzle
example, like a petroleum product and tube side is
connections and piping associated with shell and tube
having cooling water, the condensate forms two
heat exchangers will be more for low-viscosity liquids
layers and mutual solubilities contaminate both
compared to high-viscosity liquids.’’ True/False?
the fluids that require treatment as moisture in the
& True.
oil can be detrimental for the oil product and oily
. Water cooling has been normal practice in industry, but waters create environmental problems. If the con-
of late air cooling is acquiring importance. Give reasons densing vapor is miscible with water, there will be
for this shift. concentration changes that will affect quality.
& Water shortages. ➢ If the tube side fluid is at a high pressure, the shell
& Costs involved in treatment and disposal of waste- side gets pressurized with the consequence of
water requirements originating from water cooling failure/explosion of the shell as it is not normally
systems to meet pollution control/regulations. designed to withstand high pressures.
. What are the maximum temperatures at which cooling ➢ In a tube still heater, leakage of oil into the firebox
water is normally available? alters combustion process, with incomplete