Chemical Engineering June 2018 - 69

p
,
g
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 below shows a few examples of what streams are.
b:Saturated and superheated steam
sured, let us use these to obtain the
exhaust gas flow.
From steam tables, we can obtain
the following: enthalpy of final steam
at 863°F, 515 psia is 1,446.2 Btu/lb.
Enthalpy of feed water at say 550
psia, 230°F is 199.5 Btu/lb. Energy
absorbed by steam (no blowdown
used to simplify the calculations) =
55,600 × (1,446.2 - 199.5) = 69.32
million Btu/h.
Heat loss on the gas side is taken
as 0.5%. Exhaust gas flow = 69.32
× 106/ [0.995 × 0.2667 × (1,000 -
350)] = 401,880 lb/h (0.2667 is the
gas specific heat at the average gas
temperature of 675°F).
Let us do a similar energy balance
at the superheater to determine the
gas temperature leaving the superheater.
Energy to steam = 55,600
× (1,446.2 - 1,204.8) = 13.42 million
Btu/h (saturated steam enthalpy
being 1,204.8 Btu/lb). At the average
gas temperature of say 940°F, Cp =
0.275. Hence, gas temperature leaving
superheater = 10,00 - 13.42 ×
106 / (401,882 × 0.995 × 0.275) =
878°F.
Energy absorbed by the economizer
= 55,600 × (451.8 - 199.5) =
14.03 million Btu/h
Gas temperature entering economizer
= 350 + 14.03 × 106 / (401,882
× 0.995 × 0.2591) = 485°F.
Evaporator duty by difference =
69.32 - 14.03-13.42 = 41.87 million
Btu/h.
Let us compute the U values for
each section from field data, as now
we can get LMTD for each section
and we also know the surface area
and duty of each section.
LMTD SH = [(1,000-863) - (878-
472)] / ln[(1,000-863)/(878-472)] =
248°F.
LMTD evaporator = [(878-485) / ln
[(878-472) / (485-472] = 115°F
LMTD economizer = [(120-16) /
ln(120/16)] = 52°F
U1 = 13.42 × 106 / (8,941 × 248) =
6.06 Btu/ft2h°F (superheater)
U2 = 41.87 × 106 / (86,379 × 115)
= 4.21 Btu/ft2h°F (evaporator)
U3 = 14.03 × 106 / (38,698 × 52) =
6.97 Btu/ft2h°F (economizer)
Using these computed values of U
from field data, let us verify the steam
generation in the guarantee case
68
200
220
240
260
280
300
320
340
360
380
400
400
heat transfer coefficient steam
correct for di and w using (1.75/di)1.8 (w/10000)0.8
700
1000 psia
400 psia
500
600
temperature,F
700
800
900
1000
FIGURE 10. The graph shows the heat-transfer coefficient of steam as a function of pressure and temperature
Heat
transfer Coefficient for Water inside Tubes
600
700
800
900
1000
1100
1200
1300
1400
1500
Tube ID=1.75 in and flow/tube=10,000
lb/h. Correct for other flows and tube IDs
by multiplying by [(1.75/di)1.8(w/10000)0.8]
600
700
800
900
1000
1100
1200
1300
1400
1500
100
200
400
Figure A1: Heat transfer Coefficient of steam as a function of pressure and temperature
Heat transfer Coefficient for Wa
Tube ID=1.75 in and flow/tube=10,000
lb/h. Correct for other flows and tube IDs
by multiplying by [(1.75/di)1.8(w/10000)0.8]
Temperature,F
300
FIGURE 11. Heat transfer coefficient for water flowing inside tubes
Fig A2: Heat transfer coefficient for water flowing inside tubes
using the NTU method and the flow
logic for the off-design performance
calculations shown in Figure 6. We
need to confirm if 81,000 lb/h at 600
psig and 892°F as guaranteed by the
HRSG supplier is, in fact, feasible.
First, let us correct U1, U2, U3 for the
guarantee case as follows:
For
the
superheater:
(UA)p = (UA)d (Wgp/Wgd)0.65 (Fgp/
Fgd) (Wsp/wsd)0.15
(15)
Fig A3: superheater. 18 tubes/row and 9 streams
The steam flow term may be deleted
for evaporator and economizer
(UA)p
FIGURE 12. Superheater has 18 tubes/row and nine
streams
Fig A3: superheater. 18 tubes/row and 9 streams
calculations as the effect of
steam flow (evaporator) or water flow
(economizer) has minimal impact on
U. The correction factor Fg for fluegas
properties is discussed in Appendix
2 of this online section.
Superheater
From Refs. 1, 2 and 3, the following
can be observed:
U1c =
6.06 × (500,000
/
401,882)0.65 × (0.1438 / 0.1407) ×
(81,000 / 55,600)0.15 = 7.55 Btu/
ft2h°F. (0.1438 and 0.1407 are Fg
factors based on gas properties
shown in Table 7. Fg of 0.1438 is at
the average gas temperature in superheater
of 1,025°F in the guaranCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM
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500
100
600
200 700
Temperatu
300
Fig A2: Heat transfer coefficient for water flowing ins
Heat transfer coefficient,Btu/ft2hF
heat transfer coefficient,Btu/ft2hF
Heat transfer coefficient,Btu/ft2hF
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