Page 302 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 283

of installing an expansion bellows in the exchanger & If pressure drop requirements are low, using four or
shell. more tube passes should be avoided as this will
. What are the relationships between velocity and (i) heat drastically increase pressure drop.
transfer coefficient and (ii) pressure drop through tubes & It is important to realize that the total pressure drop
in a heat exchanger? How are tube side velocities for a given stream must be met.
optimized in a heat exchanger? & The distribution of pressure drop in the various heat
& Velocity strongly influences heat transfer coefficient. exchangers for a given stream in a particular circuit
For turbulent flow, the tube side heat transfer coef- may be varied to obtain good heat transfer in all the
ficient varies to the 0.8 power of tube side velocity, heat exchangers. Low DP exchangers perform poorly
whereas tube side pressure drop varies to the square for effective heat transfer, as these involve low fluid
of velocity. velocities.
& Thus, with increasing velocity, pressure drop in- & High fouling fluids require higher velocities to pre-
creases more rapidly than does the heat transfer vent deposits, with the consequence of higher DP
coefficient. requirements.
& Consequently, there will be an optimum velocity & Also it must be noted that higher viscosity liquids
above which it will not be useful to increase it any require higher DP for good heat transfer.
further. & Normally, a pressure drop of 70 kPa per shell is
& Furthermore, very high velocities lead to tube vibra- permitted for liquid streams. If, for example, there
tion and erosion. are five such exchangers through which a particular
& Tube side pressure drop rises steeply with an increase stream is flowing, a total pressure drop of 340 kPa for
in the number of tube passes. Consequently, it often the circuit would be permitted.
happens that for a given number of tubes and two & If the pressure drop through two of these exchangers
passes, the pressure drop is much lower than the turns out to be only 80 kPa, the balance of 260 kPa
allowable value, but with four passes it exceeds the would be available for the other three.
allowable pressure drop. . What are the typical pressure drops in a shell and tube
& If, in such circumstances, a standard tube has to be heat exchanger?
employed, the designer may be forced to accept a & In most applications, pressure drops are higher on the
rather low velocity. However, if the tube diameter and tube side compared to the pressure drops on the shell
length varied, the allowable pressure drop can be side.
better utilized and a higher tube side velocity & Typical pressure drops are 30–60 kPa (5–8 psi) on the
realized.
tube side and 20–30 kPa (3–5 psi) on the shell side.
. Compare pressure drops in 1–1 and 1–2 heat
. Give equations for estimation of tube side pressure
exchangers.
drops in shell and tube heat exchangers.
& In a 1–1 heat exchanger, the flow cross-sectional area
& Table 10.2 gives tube side pressure drops in shell and
for the fluid is the sum of the cross-sectional areas of
tube heat exchangers.
all the tubes, whereas in a 1–2 exchanger only half the
. ‘‘Maximum recommended velocities through nozzle
total tube cross-sectional area is available for the
connections and piping associated with shell and
flow. Therefore, tube velocity doubles in a 1–2
tube heat exchangers will be more for low-viscosity
exchanger over that in a 1–1 exchanger.
liquids compared to high-viscosity liquids.’’ True/
& As pressure drop is proportional to the square of
False?
velocity, doubling the velocity results in four times
& True.
increase in pressure drop in a 1–2 exchanger, com-
. What are the consequences of low fluid velocities in a
pared to that for a 1–1 exchanger. Allowance to this
heat exchanger?
is to be given for the decrease in flow cross section
due to increased fouling in a 1–1 exchanger, on & Poor heat transfer coefficients.
account of lower velocities, which increases pres- & Increased fouling rates.
sure drop due to fouling over that in a clean & Tube plugging for high-viscosity liquids.
exchanger.
. What is the normal recommended range of liquid
. What are the considerations involved in deciding the velocities in heat exchanger tubes?
value of allowable pressure drop for a heat exchanger? & Not less than 0.9 m/s and not more than 4 m/s.
& Allowable pressure drop for a given heat exchanger
Generally 1–2 m/s.
cannot be arrived at in isolation.