Chemical Engineering May 2017 - 58

Estimating the duty. In a heating
system, the design heat load is
arrived at by considering the minimum
ambient temperature, resulting
in a conservative estimate of
the requirement of utilities, such as
steam. Many references are available
for calculating the tank heating
or cooling loads [4, 5]. The requirement
of surface area of the heating
coils should be considered assuming
the lowest operating steam temperature.
If the tank is provided with
an agitator to facilitate uniform mixing,
then the case of failure of agitation
should also be considered while
estimating the surface area.
Factors, such as heat added by
agitators and through external pumping
systems, heat losses through the
auxiliary systems of the tank (like
external cooling or heating pipe network,
vapor equalization line, vapor
recompression system), should be
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carefully evaluated to optimize the
tank heating or cooling requirements.
Apart from the heat losses or gains
from the surroundings (including the
soil), the total heating or cooling load
must consider the requirements, if
any, of the temperature rise of the
incoming fluid and the tank material
to the desired holding temperature of
the tank.
Calculation procedure for heating
coils. The following description illustrates
a step-by-step calculation procedure
for heating coils using steam.
Step 1. Determine the heating (or
cooling) duty to be served by the coil.
Step 2. Determine the overall heattransfer
coefficient between the
medium inside the coil (in this case
steam) and the bulk fluid. Typical
overall heat-transfer coefficient values
can be taken from Table 1 [7].
Otherwise, the values can be calculated
empirically from Nusselt's
correlations for specific systems [4].
Such heat-transfer coefficients are
experimentally derived. The coefficients
could also be cross-checked
using commercially available heatexchanger
design software.
In addition, application of agitation
(in situ heating) or an eductor system
(Figure 2) with external pumped
recirculation allows further improvement
in the rate of heat transfer.
Step 3. Determine the temperature
difference between the steam and
the process fluid.
Step 4. Determine the heat transfer
area required.
Step 5. Select the diameter and determine
the length of the pipe that forms
the coil. Because of the difficulties in
providing accurate values of the overall
heat-transfer coefficient and the nonavailability
of effective heat-transfer
surface area due to condensate flow, it
is typical to add a margin to the above
calculated heat-transfer area.
Step
6.
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58
mended steam velocities passing
through the heating coils are in
the range of 20-25 m/s. For higher
steam loads and heat-transfer areas,
the steam path could be divided into
several parallel paths to reduce the
steam-side velocity and temperature
variation. In very long coils, a significant
pressure drop occurs along the
length of the coil. In such cases, the
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Chemical Engineering May 2017

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