Chemical Engineering June 2018 - 59

Feature Report
Performance Prediction for
Industrial Boilers
Understanding boiler performance calculations can allow engineers to improve the
operation of their facility's steam system and better engage with boiler-system vendors
Cleaver Brooks
Viswanathan
Ganapathy
Boiler consultant
IN BRIEF
DATA FOR BOILER
PURCHASES
BASIC ENERGYTRANSFER
EQUATIONS
WHICH COEFFICIENT
GOVERNS U?
TWO CALCULATION
CATEGORIES
OFF-DESIGN
CALCULATION METHODS
EVAPORATOR
PERFORMANCE
CHALLENGING THE
HRSG VENDOR
CONCLUDING REMARKS
C
hemical plants, petroleum refineries,
power plants and cogeneration
systems use oil-and-gas-fired
steam generators, fluid heaters and
waste-heat boilers of various types. Understanding
how a boiler behaves with variations
in parameters such as capacity, fluegas flow,
inlet gas temperature or steam pressure can
help plant engineers evaluate boilers better
and understand why failures occur in crucial
boiler components, such as superheaters,
evaporators or economizers. Also, better understanding
of boiler behavior can help plant
engineers identify the reasons for decreases
in efficiency. Figures 1 and 2 show heat-recovery
steam generators (HRSGs) of naturaland
forced-circulation designs behind gas
turbines. Figure 3 shows a typical fire tube
boiler used in a chemical plant and Figure 4
shows an oil-and-gas fired steam generator.
In many of today's cases, plant engineers
rush to the vendor who supplied the equipment
to ask opinions and get repair help, even
if the boiler component is only experiencing
minor performance problems. In this scenario,
the vendor company is not likely to find faults
with its own equipment design, even if there
are flaws present. Rather, the vendor is likely
to divert the attention of the plant engineers to
A
Surface area, ft2 (subscripts i and o refer
to inside and outside)
C-factor Thermal conductance factor Cmin/Cmax
Cmin, Cmax (WCp)min , (WCp)max
Cp
d
Fg
h
K
Tube diameter, in. (subscript i for inside)
Factor considering gas properties used in
heat transfer coefficient calculation
Heat transfer coefficient, Btu/ft2h°F (subscripts
i for inside, o for outside, c for convective,
n for non-luminous)
Constant used in simplified equations considering
tube geometry data (subscript m
denotes thermal conductivity of tube metal
at operating temperature)
k
58
Thermal conductivity of fluid, Btu/ft h°F
U
ε
η
Q
W
w
Specfic heat (subscript g for gas, s for
steam, w for water)
FIGURE 1. Shown here is a modular heat-recovery steam generator
(HRSG) system at a chemical manufacturing facility
the operations group or to the plant operating
parameters. In other cases, the plant may not
get a timely reply from the equipment vendor
if the problem occurs several years after the
equipment is installed.
Because boiler calculations are quite involved,
many plant engineers are unable or
unwilling to undertake them on their own.
For many engineers, the basic heat-transfer
principles covered in university degree programs
are forgotten once they leave the institution
behind for an industry job. This article
NOMENCLATURE
T
t
∆T
Temperature of hot fluid, °F (subscripts 1
and 2 indicate temperature at inlet and exit,
respectively)
Temperature of cold fluid, °F (subscript s is
for saturation temperature)
Temperature difference, °F (log-mean temperature
difference, LMTD)
Overall heat-transfer coefficient, Btu/ft2h°F
(subscripts i and o for inside and outside, c
for corrected)
Effectiveness, used in NTU method
fin effectiveness, fraction
Duty, Btu/h (subscripts c for convective, r
for external radiation, n for non-luminous)
Total flow, lb/h (subscripts i for inside, o for
outside, h and c for hot and cold fluids)
Flow per tube, lb/h
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Chemical Engineering June 2018

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