Chemical Engineering June 2018 - 71

Cmax = 500,000 × 0.995 × 0.259
= 128,825.
C = 0.6543 NTU= 8.03 × 38,698 /
84,296 = 3.6863
ε = [1-exp{-3.6863 × (1 - 0.6543)}]
/ [1-0.6543 × exp{-3.6863 × (1 -
0.6543)}] = 0.8818
Q3 = 0.8818 × 84296 × (511 -
230) = 20.89 million Btu/h
tw2 = 230 + 20.89 × 106 / 84296
= 478°F, Tg2 = 511 - 20.89 × 106 /
128,825 = 349°F
Total energy transferred = 20.99 +
58.7 + 20.89 = 100.58 million Btu/h
Corrected steam flow = 100.58 ×
106 / (1,463.78 - 199.7) = 79,560
lb/h
Let us go with 79,560 lb/h as the
expected steam from this HRSG in
guarantee case. Since the estimated
value is far lower than that guaranteed,
the plant has a strong case
to inform the HRSG supplier that
there is a need to verify the design
and correct it. The exit gas temperature
guaranteed by them is also on
the low side by a significant margin
(349°F versus 337°F). The energy
absorbed 102 million Btu/h as
guaranteed by them also cannot be
met with the existing surface areas.
The plant can show that the duty of
the HRSG in the guaranteed case
is only about 100.5 million Btu/h.
The HRSG supplier will argue about
measurement errors, calibration of
instruments and so on. But there is a
strong case that the HRSG will generate
only about 79,500 lb/h steam
at 900°F based on how it is presently
performing. Plant engineers should
familiarize themselves with such calculation
processes. An excel work
sheet or VBA coded excel can help
in more accurate results instead of
manual calculations.
Superheater Example 2
A finned-tube counterflow superheater
in a petroleum refinery is
experiencing high tube-wall temperatures
as measured at the steam
outlet end. See data in Table 9. The
operator is thinking of making it a
parallel flow unit to lower the tube
temperatures. What will be the performance
and tube wall temperature
if it is made a parallel flow unit? How
70
Arrangement
Case
TABLE 9: SUPERHEATER OPERATING DATA
Counter flow
Operating
Parallel flow
Proposed
Gas flow, lb/h
Gas inlet temp, ˚F
Gas exit temp, ˚F
Steam inlet temp, ˚F
Steam flow, lb/h
Steam press exit, psig
Steam exit temp, ˚F
Maximum tube wall
temp, ˚F
Surface area-total, ft2
Tube OD,ID, tube length
Fins/in, ht, thick
Rows wide × deep ×
streams
Turbine exhaust flow,lb/h
Gas temperature in, F
Gas temperature out, F
Duty, MM Btu/h
Gas pr drop, in wc
Paramtherm1 flow, lb/h
Inlet temperature, F
Exit temperature, F
Pressure drop, psi
Tube size, OD x ID
Fins/in. × height × thickness
Tubes/row
No deep
Length, ft
Trans and long pitch, in.
Surface area, ft2
Arrangement
Streams
176,000
1,472
839
508
132,000
700
894
1,123
10,195
2 × 1.75 × 10.2
4 × 0.75 ×.075 × 0.157
24 × 8 × 24
TABLE 10:THERMAL FLUID HEATER QUOTES
Vendor 1
150,000
950
551
15.94
3.2
80,000
250
547
9
2
5 × 0.75 × 0.075
15
10
8.5
4.5 × 4.5
7,740
Staggered
5
950
551
15.94
2.2
80,000
250
547
12
2
2 × 0.75 × 0.075
15
14
8.5
4.5 × 4.5
4,895
Staggered
5
Average U, Btu/ft2h˚F
TABLE 11: RESULTS OF CALCULATIONS FOR THERMAL FLUID HEATER
Vendor 1
5.9
Fluid heat transfer coefficient @ 400˚F
fluid heat transfer coefficient @550˚F
Fg at average gas temperature of 750˚F
Fg at maximum gas temperature of 950˚F
Maximum U, Btu/ft2h˚F
Max heat flux inside tubes, Btu/ft2h
Film temp rise, ˚F
Max film temperature, ˚F
much should the steam flow be reduced
to get the same steam temperature
as before for process?
Fluegas analysis: vol.% CO2 =
8 vol.%, H2O = 18 vol.% ,N2 = 72
vol.%, O2 = 3 vol.%.
The plant is experiencing high
tube-wall temperatures and failures
in its superheater. and wants to understand
why this is happening. It
wants to know if parallel flow design
will help as a temporary measure and
what will be its performance? If the
same steam temperature of about
233
387
0.134
0.141
6.21
32,550
84
634
Vendor 2
9.33
233
387
9.82
23,257
60
610
880˚F is required in parallel flow arrangement
for process reasons, can
they reduce the steam flow and by
how much? What will be the maximum
tube wall temperature in both
cases?
Solution: Let us first understand
why they have a problem. High finned
superheaters increase the heat flux
inside the tubes as discussed in
several articles (Refs. 1 and 4) and
hence should be designed with caution.
The
ratio of external to internal
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2018
Vendor 2
150,000
176,000
1,472
927
508
132,000
700
833
890
Parallel flow
Lower steam flow
176,000
1,472
977
508
105,000
700
890
933
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Chemical Engineering June 2018

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