ESL-IE-95-04-46
APPLICATIONS OF HRSG SIMULATION
V.GanapathY,ABCO Industrles,Abllene,Texas
not require the computation of
U,which is why anyone
familiar with heat balances
can perform the analysis.
Rather than use U,the term UxS
is computed for each surface
ABSTRACT
Heat Recovery steam
Generators are widely used in
cogeneration and combined
cycle plants generating steam
utilizing energy from gas
turbine exhaust. Before
planning cogan projects
,consultants should study
various options available in
terms of steam parameters and
select the optimum. simulation
helps plan such studies. In
addition,useful "what if ..
studies can be performed
without even designing the
HRSG,thus saving valuable
time.
UxS=
and then this term is
corrected for off-design
conditions by applying
correction factors for the
effect of gas flow and its
analysis [1,2,3].
Thus,the performance of a
HRSG at any condition can be
simulated even before it is
physically designed,providing
consultants and plant
engineers with valuable
engineering information about
the HRSG behaviour.
This article outlines the
applications of HRSG
simulation and how plant
engineers,consultants can
benefit from such studies.
The limitations are that
the HRSG should be of the
convective type,which is the
case with 90-95 % of HRSGs
built to-day and that the gas
stream should be clean.
nr.rRODUCTION
It is not necessary to
physically design a HRSG in
terms of surface area,tube
size,fin configuration etc in
order to evaluate its
performance under different
modes of operation and
different gas/steam
conditions.
APPLICATIONS
The theory behind simulation
is well discussed in the
references and hence will not
be repeated here. The
applications will be discussed
so that those interested can
benefit.
HRSG designers arrive at the
surface area S of each
component of the HRSG using
the equation
S
= Q/U
Q/~T
1 BR sa PKRTORHANCK
Specifications for HRSG are
often written without an idea
of its capability. For
example,the exit gas
temperature from the HRSG is
impacted by steam pressure and
pinch and approach points. The
~T
where Q is the duty ,U the
overall heat transfer
coefficient and ~T,the log
mean temperature difference.
The simulation process does
304
Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995
ESL-IE-95-04-46
consumption is excessive.
exit gas temperature and hence
steam production cannot be
arbitrarily selected.
3. STUDY TEMPERATURE
PROFILES,
By looking at the
possibility of multiple
pressure operation and
placement of modules in the
gas path,the exit gas
I
temperature from the HRSG ca~
be lowered. Simulation helps
to locate different modules
such as evaporator,
superheater,economizer at
different places in multiple
pressure HRSGs and see which
combination generates the moat
steam.
Table 1 shows how the steam
pressure and temperature
affects the duty. It may be
seen that as the pressure
increases,the exit gas
temperature also increases.
Also as the steam temperature
increases,the exit gas
temperature increases,
decreasing the steam
production. The economizer
acts as a heat sink in
lowering the exit gas
temperature and as the steam
generation falls in the
evaporator section,the duty of
the economizer also
reduces,thus increasing the
exit gas temperature.
Fig 1 and 2 show the
gas/steam temperature profil~
analysis fram two HRSGs for
the same gas parameters. The
objective is to generate a
given quantity of HP
steam,while maximizing the LP
steam generation. In Fig 1,tqe
HRSG consists of the HP stag~
followed by LP stage each wi~
its own economizer and in fig
2,the economizer feeds both ijP
and LP stages,thus offering ~
larger heat sink and hence
lowering the stack gas
temperature. The LP steam
generation is also much
higher.
Thus using simulation
studies,one can understand
what a HRSG is capable of
doing at a given inlet gas
temperature and steam
pressure. A lot of engineers
are under the impression that
300 F exit gas temperature can
be attained in single pressure
HRSGs but as seen fram the
table,this is possible only at
low pressures and at high
inlet gas temperatures.
f.BRSG FIELD PERFORMANCE
often when HRSGs are test~d
in the field,the gas
I
parameters to which they were
designed cannot be duplicate4
due to various factors such 4s
different ambient
conditions,load of the
plant,gas turbine
operation,variations in fuel
analysis etc.
2.EVALUATX GAS 'l'URBINXS
Another application of HRSG
simulation is that it can be
used to evaluate different gas
turbines and the optimum
machine may be selected based
on desired steam conditions.
Tables 1 and 2 show the
parameters of a few gas
turbines and their impact on
HRSG performance in unfired
and fired modes. It is
possible to rule out certain
machines if they cannot
deliver the minimum steam
demand or if the fuel
However it is possible to
evaluate the actual
performance of the HRSG and
relate it to the guarantee
point using HRSG simulation.
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Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995
ESL-IE-95-04-46
By suggesting different
pinch and approach points,the
gas/steam temperature profile
and steam generation of the
HRSG at the plant operating
condition may be simulated.
Once this "design" is arrived
at, its off-design performance
at any other condition may be
evaluated using the simulation
process; hence by seeing what
happens at the simulatd
guarantee conditions, one can
study if the HRSG was designed
to deliver the guaranteed
steam; the margins for
measurement may also be
considered in such studies.
This method gives a good idea
if the HRSG performance is
close to what was predicted.
~
. S'l'XAM WeIHE
RJ:J'J:RJ:NCJ:S
1.V.Ganapathy, "Waste heat
boiler Deskbook," 1992
Fairmont Press,Atlanta
2.V.Ganapathy,"Simplified
approach to HRSG performance
evaluation",presented at the
ASME International Power
generation Conference, Oct 6
10,San Diego.
3.V.Ganapathy,"Temperature
profiles determine HRSG steam
production",Power Engineering
May 1993.
SKUC~ION
HRSG simulation may be used
to arrive at optimum steam
parameters such as
pressure, temperature. By
considering different
pressures/temperatures and the
resulting steam generation and
plant efficiency, one can fine
tune the steam parameters and
hence select the right steam
turbine for a given
application. As seen from
Table 1 higher steam pressure
results in lesser steam
production but the steam
turbine performance could be
better than a low pressure
unit.
CONCLUSION
HRSG simulation has various
applications and may be used
by consulting engineers,plant
engineers and those planning
COGEN projects. In order to
help perform such studies,the
author has developed a
software "HRSGS",which is
available from him.
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Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995
ESL-IE-95-04-46
Table 1: HRSG exit gas temperatures vs steam pressure
steam press,pslg
temp,F
exit gas,F
100
sat (338)
300
150
sat (3660
313
250
sat (406)
332
400
sat (448)
353
400
600
367
600
sat(490)
373
600
750
398
( based on 15 F plnch,20 F approach,9OO F Inlet gas temperature
and 230 feed water with no blow down)
Table 2: Data on gas turbines and steam parameters
Gas IIow,Lb/h
165,000
137,000
165,000
Exhaust gas temp,F
900
1040
950
Steam press,psIg
200
200
200
Feed water lemp,F
230
230
230
Required steam,Lb/h
40,000
40,000
40,000
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Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995
ESL-IE-95-04-46
Table 3: Design and off-design performance of HRSG
Item
GT1
unflred
GT1
flred
GT2
unfired
GT2
flred
GT3
unflred
GT3
flr
Exhaust temp,F
900
900
1040
1040
950
950
to HRSG,F
900
1200
1040
1347
950
1194
Ivg evap,F
403
409
403
408
403
408
Ivg econ,F
309
293
281
265
299
285
steam,Lb/h
25450
40000
27300
40000
28000
40000
fuel,MM btu/h
14.41
12.63
11.76
plnch,F
15
22
15
20
15
20
approach,F
15
42
15
41
15
37
(based on 1 % heat loss, 3 % blow down; fuel Input Is on lower heating value
basis)
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Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995
nn0\.:1 rc.nrUMIVIJ-\I\lvC. -
Ue::ilgrl c;ase
roject - SAMPLE Units - British Remarks - COMMON ECO FEEDS MODULES
ESL-IE-95-04-46
amb temp - F== 60 heat loss-%= 1 gas temp to HRSG F== 900 gas flow - Lb/h== 500000
f~ vol C02 =3. H20 =7. N2 ==75.02 =15. S02=. ASME eff-% :::71.04
pstm -pinch apprch
gas temp
watJstm
duty
pres
flow
MMB/h Psia
%
F
F
in/out -F
Lb/h
in/out -F
sh
491 600
2.72
615.
31759 100
900 879
evap 879 671
28.
31759 100
180
630.
140
351 491
evap 671
381
351 366
37.93 165.
43453 100
15
15
eco 381
9.51
650.
230 351
76716 0
306
-------_._._-----.-_._---_.._------_ .._ . _ - ..._----_._-.----.--.__ ._.--_._-_..
Surf
~-----------_._._----_
..
Gas-Steam Temperature profiles
79
71
491
~~
1-0;3=66;;------\
351
---..........
306
230
5uphtr
evap
eean
evap
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Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995
HRSG PERFORMANCE - Design case
Project - SAMPLE Units - British Remarks - 2 SEPARATE MODULES
ESL-IE-95-04-46
amb temp - F= 60 heat loss-%= 1 gas temp to HRSG F= 900 gas flow - Lb/h= 500000
% vol C02 =3. H20 =7. N2 =75.02 =15. S02=. ASME eff-% =66.79
Surf
gas temp
in/out -F
sh
900 879
evap 879 706
eco 706 642
381
evap 642
eco 381
342
watJstm
in/out -F
491 600
476 491
230 476
351 366
230 351
duty
MMB/h
2.7
23.36
8.38
34.09
4.95
pres
Psia
615.
630.
640.
165.
175.
flow
Lb/h
31468
31468
32097
39050
39831
pstm pinch apprch
%
F
F
100
15
100 215
0
15
100
15
0
Gas-Steam Temperature profiles
79
42
491
76
366
342
230
230
suphtr
evap
evap
ecan
ecan
310
Proceedings from the Seventeenth Industrial Energy Technology Conference, Houston, TX, April 5-6, 1995
