Chemical Engineering June 2018 - 75

Ref. 1. In fire tube boilers, the fluegas-side
heat-transfer coefficient
governs the overall heat transfer coefficient
U. An idea of the tube-side
coefficient hi in a superheater will
help determine its tube wall temperature
and pressure drop. In a fluid
heater, it helps to determine the film
temperature of the thermic fluid.
Tube side coefficient may be estimated
by the following:
Nu = 0.023 Re0.8 Pr0.4
Nu = hidi/12k
Re = 15.2wdi/µ
[Re = 3,600ρ Vdi/µ/12. V = 576 w/
[3,600π di2ρ.
Simplifying Re = 15.2 w/di/µ ]
Pr = µCp/k
All properties estimated at fluid
bulk temperature.
Simplifying further, hi = 2.44 w0.8
C/di
1.8
where factor C = (Cp/µ)0.4 k0.6
C is available in tables and charts
in order to quickly estimate hi for various
fluids (see Refs. 1 and 2).
a: For wet air (% volume H2O = 1,
N2 = 78, O2 = 21)
Importance of streams in economizer
and superheater
One of the important pieces of
data to be furnished by boiler vendors
in their description of a superheater,
economizer or a fluid heater
is streams, or the number of tubes
carrying the entire steam or water
flow of a boiler. Often one is required
to estimate the steam or water velocity
inside the tubes, heat transfer
coefficient, pressure drop or tube
wall temperatures. Many boiler vendors
do not provide this crucial data
or drawings which show baffle plates
in the header, if used. Hence when
confronted with solving a tube failure
problem in a superheater, one
gets stuck. Partitions in headers can
mask this data and hence plant engineers
should get this data from the
boiler vendor for future analysis. The
Figure 10 shows a few examples of
what streams are.
APPENDIX 2
Heat-transfer coefficients outside
plain tubes
Fishenden and Saunders equation
74
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JUNE 2018
for convective heat transfer coefficient
hc outside plain tubes takes
the form:
Nu = 0.35FhRe0.6Pr0.3
The Fh factor depends on the tube
arrangement, whether inline or staggered,
and is shown in Table B1. It
can be seen that the correction factor
is nearly the same for both inline
and staggered when Sl/d is above
2, which is the arrangement typically
used in boiler practice. It also
is used to improve ligament efficiency
in drums. However if fluegas
pressure drop is considered, it is far
higher for staggered compared to
inline arrangement. Hence, rarely will
one see a staggered arrangement
for plain tubes, because from a heattransfer
perspective, there is no significant
benefit while operating costs
in the form of fan power consumption
will be much higher [1,2]
A conservative correlation used for
both inline and staggered arrangements
is:
Nu = 0.33 Re0.6Pr0.33
Nu = hcd/12k
Re = Gd/12µ
Pr = µCp/k
All gas properties for heat transfer
are evaluated at gas film temperature
which is the average of gas and tube
wall temperature and lower than the
average gas temperature.
Simplifying the above,
hc = 0.9G0.6Fg/d0.4
where
Fg = k0.67 Cp0.33/µ0.27
Another correlation widely used is
that of Grimson's [1]
Nu = BReN
where B and N are shown in Table
B1.
Examples of using these correlations
are given in Refs. 1 and 2.
Finned tube bundles
Correlations for finned tube bundles
are more involved and shown in
Refs. 1 and 2 with worked out examples.
The
gas property factor affecting
gas side heat transfer is given by
[1,2,4]
Fg = k0.67 Cp0.33/µ0.32
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Chemical Engineering June 2018

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