HRSG SIMULATION
V.Ganapathy
What is HRSG Simulation
Knowing gas flow,temperature & analysis and steam
parameters,establish HRSG temperature profiles,duty and steam
flow. This is the design case,where for each surface we solve for:
UA=Q/  T
In the off-design case,we know the new gas flow & temperature.It is
desired to obtain the temperature profiles and steam flows. Calculation
is tedious as surface area is indirectly known. Correct (UA) for effects of
gas flow,temperature and analysis and solve for: Q=(UA)c T
In typical design,we compute U and then A for each surface.In
simulation we compute (UA) and hence there is no need to physically
the HRSG in terms of tube size,fin density etc.Hence anyone familiar
with heat balances can perform these studies.
Consultants,plant engineers,project planners
can use this method to evaluate HRSG
performance without even knowing its
size!They need not also contact a HRSG
supplier!
Applications of HRSG Simulation
•Obtain design temperature profiles
•Obtain off-design HRSG
performance(unfired/fired) at
different gas/steam conditions
•Evaluate different gas turbines
during initial project planning stages
•Maximize energy recovery by
modifying HRSG configuration
•maximize energy recovery by
adding secondary heat recovery
such as deaerator,condensate
heater
Ideal tool for cogen/combined
cycle plant evaluation
•Evaluate field performance and
relate it with HRSG performance
guarantees
Saves time for consultants
•Write better HRSG specifications
by knowing HRSG’s capabilities
No need to physically design the
HRSG!
Limitation:Convective type surfaces
with no external radiation
Pinch and Approach Points
Note: This is the Design
mode ..We cannot preselect pinch and
approach points in offdesign mode!
pinch point,F
bare finned
a. evap type
approach point,F
b.inlet gas temp,F
1200-1800
130-150
700-1200
80-130
30-60
10-30
40-70
10-40
Facts •Pinch
about
Pinch
and
Approach
Points
and Approach points are selected in unfired mode at
“Design” gas flow,exhaust gas temperature.These are called
“design” pinch and approach points
•Once selected,they fall in place in other cases of gas
flow/inlet gas temperature/steam conditions,whether unfired
or fired.
•Pinch/approach points increase with inlet gas temperature
•They cannot be arbitrarily selected
-temperature cross can occur
-low pinch point may not be physically feasible unless extended surfaces are used
-affected by inlet gas temperature
-economizer steaming is a concern ;suggest minimum approach at coldest ambient
HRSG conditions
-steam temperature can be achieved in fired conditions if it is achieved in unfired
conditions
• HRSG surfaces are determined once design pinch/approach points are selected
Why HRSG exit gas temperatures cannot be
assumed
Exit gas temperature cannot be assumed as in conventional fired steam generators as
temperature cross can occur.Looking at the superheater and evaporator,we have:
WgxCpgx(tg1-tg3)=Ws(hso-hw2)
(1) Looking at the entire HRSG,
WgxCpgx(tg1-tg4)=Ws(hso-hw1)
(2) [blow down and heat loss neglected]
Dividing (1) by (3) and neglecting effect of variations in
Cpg with temperature,we have:
(tg1-tg3)/ (tg1-tg4)= (hso-hw2)/ (hso-hw1)=K
For steam generation to occur and
for a thermodynamically feasible
temperature profile,two conditions
must be met: If pinch and
approach points are arbitrarily
selected,one of these may not be
met.
tg3>ts and tg4>tw1.
Pinch=20F,approach=15 F,gas
inlet=900 F,feed water=230 F
(3)
Psig
stm temp,F
sat temp,F
K
exit gas,F
100
sat
338
.904
300
150
sat
366
.8704
313
250
sat
406
.8337
332
400
sat
448
.7895
353
400
600
450
.8063
367
600
sat
490
.7400
373
600
750
492
.7728
398
Temperature calculations
Example 1: Determine HRSG exit gas temperature when inlet gas temperature=900 F,steam
press=100 psig.use 20 F pinch and 15 F approach points.
Solution:K=0.904 sat temp=338 F.hence tg3=358F.tw2=323 F.(900-358)/(900-tg4)=0.904 or
tg4=300 F
Example 2: what is tg4 when steam press=600 psig and temp=750 F?
Solution:K=0.7728.sat temp=492 F.tw2=477F.tg3=512 F.(900-512)/(900-tg4)=0.7728 or tg4=398
F.So 300 F stack temperature is not feasible!
Example 3:Why can’t we obtain 300 F at 600 psig,750 F steam?
Solution:K=0.7728 Let us compute tg3 from:(900-tg3)/(900-300)=0.7728 or tg3=436 F.This is
called temperature cross!
Example 4:What should be done to get 300 F stack temperature?
Solution:Increase tg1 by firing.say tg1=1600 F.(1600-tg3)/(1600-300)=0.7728 or tg3=595 F and
pinch=103 F.
Example 5:If tg1=800F,what is tg4 at 100 psig sat?
Solution:(800-358)/(800-tg4)=0.904 or tg4=312 vs 300 F when tg1 was 900F.
Example 6:With 1600 F gas inlet,can we use 20 F pinch?
Solution:(1600-512)/(1600-tg4)=0.7728 or tg4=192 F,which is below 230 F.Not feasible!
That’s why pinch & approach points should not be selected in the fired mode!We have no idea in
what range they can fall.
Temperature Profile Calculations
The following procedure describes “Design”
temperature profile calculations for HRSGs.
Assume pinch and approach points.
Saturation temperature ts is known by
assuming a pressure drop through the
superheater. tg3=ts+pp and tw2=ts-ap
Energy absorbed by sh +evap =
Q12=WgxCpgx(tg1-tg3)xhl =Ws[(hso-hw2)+bd(hf-hw2)] .Ws is then computed.
Q1=Superheater duty=Ws(hso-hv)=WgxCpgx(tg1-tg2) .Q1 and tg2 are thus obtained
From above,Q2=(Q12-Q1)=evaporator duty is obtained.
Economizer duty=Q3=Ws(1+bd)(hw2-hw1)=WgxCpg(tg3-tg4)xhl ; from this both Q3 and
tg4 are obtained.
hl=heat loss is on the order of 0.5 to 1 %
bd=blow down varies from 1 to 7 % depending on feed water quality and boiler water
conditions.
Simplified HRSG Performance
Using the concept that firing in a HRSG is
100 % efficient,we can evaluate the
performance in fired case for estimation
purposes.
Example:160,000 lb/h of exhaust at 950 F
enters a HRSG to generate 600 psig
steam at 750 F from 230 F
water.Determine unfired steam production
and also burner duty,firing temperature
and exit gas temperature when generating
35,000 lb/h of steam at 600 psig,750 F.
Solution:Using 25 F pinch and 20 F approach,compute energy absorbed by
SH+evap=160,000x0.27x(950-517)x0.98=18.33 MM Btu/h=Ws(1378.9-455.4) or
Ws=19,850 lb/h. Energy absorbed by HRSG=19,850x(1378.9-199.7)=23.4 MM
Btu/h=160,000x0.98x0.268x(950-tg4) or tg4=393 F.
Fired case: Energy absorbed by steam=35000x(1378.9-199.7)=41.27 MM Btu/h.
Additional fuel energy required=(41.27-23.4)=17.87 MM Btu/h.
Oxygen consumed=17.87x106/(160000x58.4)=1.91 % So there is plenty of oxygen left.
Firing temperature=17.87x106=160000x0.3x(T-950) or T=1322 F
Exit gas temperature=1322-41.27x106 /(160000x.275x.98)=364 F
Design & Off-design calculations
DESIGN
•unfired
•establishes configuration
•establishes surface areas indirectly
•only one case
•zero desuperheater spray
•pinch and approach points selected
OFF-DESIGN
•zero economizer steaming
•unfired/fired/fan mode/combination
WHATIF STUDIES
•several cases possible
•steam pressure variations
•computes desuperheater spray
•firing temperature restrictions
•pinch and approach points computed
•effect of fuels
•economizer steaming possible
•performance testing
•effect of gas turbine load
•variations in ambient temperature
A simple example of simulation
The energy transferred to the evaporator is given by:
Q=WgCp(T1-T2)=UST=US (T1-T2)/ln[(T1-ts)/(T2-ts)] ; simplifying,
ln[(T1-ts)/(T2-ts)]=US/WgCp . In a fire tube boiler,U  Wg0.8. For a water tube
boiler,U  Wg0.6 ,neglecting the effects of temperature.
Then, Wg0.2ln[(T1-ts)/(T2-ts)]=K1 for a fire tube boiler
and Wg0.4ln[(T1-ts)/(T2-ts)]=K2 for a water tube boiler
Example: A water tube boiler is designed to generate
steam at 250 psig with 100,000 lb/h of flue gas at 1000
F.Exit gas temperature is 500 F.What is the exit gas
temperature when 90,000 lb/h of flue gas enters the boiler
at 970 F and steam pressure is 200 psig?
Solution: First compute K2 using design conditions...
1000000.4ln[(1000-406)/(500-406)]=184.4=K2
In the off-design case,900000.4ln[(970-388)/(T2388)]=184.4 or T2=473 F.Duty and steam generation may
be computed from this.
[406 and 388 F are saturation temperatures
corresponding to 250 and 200 psig respectively.]
Example of a HRSG simulation
Example:140,000 lb/h of turbine exhaust gases at 980 F enter a HRSG generating sat
steam at 200 psig.Determine the steam generation and temperature profiles if feed
water temperature is 230 Fand blow down=5%.
Solution: Let us choose a pinch point of 20F and approach of 15 F.Sat
temperature=388F. Gas temperature leaving evaporator=408 F and water temperature
entering it is 373 F.Evaporator duty=140000x.99x.27x(980-408)=21.4 Mm Btu/h. [ 1%
heat loss and average specific heat of 0.27 Btu/lbF is assumed]
Enthalpy absorbed in evaporator=1199.3-345+.05x(362.2-345)=855.2 Btu/lb
[1199.3,345 and 362.2 are enthalpies of sat steam,water entering evaporator and
saturated water respectively]. Hence steam generation=21.4x106/855.2=25,000 lb/h
Economizer duty=25000x1.05x(345-198.5)=3.84 Mm Btu/h .gas temperature
drop=3840000/(140000x.253x.99)=109 F.Hence exit gas temperature=408-109=299 F
Off-design Performance
Simulate the HRSG performance with a 165,000 lb/h of gas flow at 880 F.Steam
pressure =150 psig.
Using the model for evaporators discussed elsewhere,ln[(980-388)/(408388)]=Kx140000-0.4 or K=387.6 Under new conditions: ln[(880-366)/(Tg366)]=387x165000-0.4 =3.1724 or Tg=388 F.Evaporator duty=165000x.99x.27x(880388)=21.7 MM Btu/h
In order to determine the steam flow,the feed water temperature to evaporator must
be known.Try 360 F.Then steam flow=21.7x106/[1195.7-332)+.05x(338.5332)]=25,110 lb/h. Economizer duty(assumed) Qa=25110x1.05x(332-198.5)=3.52MM
Btu/h.Compute (US)d=Q/T for economizer based on design conditions. Q=3.84x106
T =[(408-373)-(299-230)]/ln[(69/35)]=50 F.(US)d=3840000/50=76800. Correct this
for off-design case. (US)p=(US)dx(165000/140000).65=85200.The effect of variations
in gas temperature is minor and not considered. The energy transferred =(US)p xT.
Based on 360F water exit temperature,the economizer duty=3.52MM Btu/h and gas
temperature drop=3520000/(165000x.99x.253)=85 F or exit gas =388-85=303
F.T=[(303-230)-(388-350)]/ln[(73/28)]=47 F or transferred duty=85200x47=4.00 Mm
Btu/h.As this does not match the assumed value of 360F and duty ,another iteration is
required. It can be shown at 366 F,the balance is obtained.
Superheater performance
Performance of a superheater is obtained from: Q=(US)pT T=[(Tg1-ts2)-(Tg2ts2)]/ln[(Tg1-ts2)/(Tg2-ts2)] (US)p is obtained from design (US) values as follows:
(US)p=Wg0.65FgK1(Ws/Wsd)0.15 where K1=a constant obtained from design case =
Q1/(T Wg0.65Fg) where Fg = (Cp0.33k0.67/m0.32).Basically we are correcting for the
effects of: 1.Gas flow 2. gas analysis, gas temperature and hence gas properties, which
is significant if the superheater operates say in unfired and fired modes. Similar
constants K2,K3 may be evaluated for evaporator and economizer.
Example:In design mode, gas flow=150,000 lb/h.Gas in=900F and leaving SH=842F.
steam flow=18510 lb/h.steam pressure=450 psig. steam in=460F,out=650F.duty=2.34
MM Btu/h Cp=.273,m=.0826 ,k=.029. Fg=.2730.33x.0290.67/.08260.32=0.135.T=[(842460)-(900-650)]/ln[(842-460)/(900-650)] = 311F.Hence
K1=2.34x106/(311x.135x1500000.65) =24.1
Off-design:steam flow=18050 lb/h,gas flow=165000,gas in=840F.steam pressure=450
psig.Let exit steam temp=640F.Duty=18050x(1325-1204.4)=2.177MM Btu/h. Exit
gas=840-2177000/165000/.99/.271=791F. Since gas temperatures are close, Use same
Fg=0.135. (US)p=1650000.65x0.135x24.1x(18050/18510)0.15=7974. T=[(840-640)(791-460)]/ln[(840-640)-(791-460)]=260F. Hence Qt=7974x260=2.074MM Btu/h. This is
close to the assumed value, else another iteration would be required. The NTU method
may also be used here by using the new US term.
HRSG Performance Calculations
Performance may
be obtained even if
HRSG geometry is
unknown using
simulation concept.
Why are HRSGS inefficient?
•Low steam/gas ratios
•Low inlet gas temperatures(900 F vs 3300 F)
•Temperature profiles depend on steam
pressure and temperature
•Higher the pressure,lower the steam generation
•Higher the steam temperature,lower the steam
generation (and higher the exit gas temperature)
Improving HRSG Efficiency
•Design with lower pinch and approach points
•Use of secondary surfaces such as condensate
heater,heat exchanger,deaerator
•Consider multiple pressure HRSG
•Use supplementary firing
•Optimize temperature profiles by rearranging
surfaces
Improving HRSG performance
Bottom line is to
lower the exit gas
temperature!
RESULTS OF A SIMPLE STUDY
Data
base
975
Cond htr Heat
exch
975
975
LP
evap
975
Gas inlet temp,F
Stack gas temp,F
374
310
323
297
Steam to turbine,Klb/h
80
80
80
80
Steam to deaerator
10250
1730
3400
0
Feed water temp,F
240
240
151
240
Electric power,kw
6528
6830
6770
6890
Gas flow=550,000 lb/h pinch=20 F approach=20F,make up=60 F,cond
pr=2.5 in hg,steam at 620 psig,650F
HRSG
simulation
Knowing gas flow,temperature,analysis and steam parameters,establish HRSG
temperature profiles,duty and steam flows.In the design case,solve for:UA=Q/ T.In
the off-design case knowing the new gas parameters,use the NTU method to establish
performance using Q=(UA)T.Correct for UA using new gas parameters. We do not
have to compute U. Hence there is no need to know the tube size,fin details,HRSG
mechanical data;anyone can perform such calculations and evaluate HRSG
performance in unfired,fired modes,evaluate burner duty,optimize temperature
profiles,predict part load performance,review performance different gas turbines...
HRSG Temperature profile
HP stage is followed by LP section. Not a very efficient design
HRSG Temperature profile
Using common Economizer concept,we improve energy recovery
Why Steaming occurs in HRSG
100 % load
Economizers
HRSG performance at Low Load
HRSG performance at 40 % load. Note steaming in
economizer and also the high exit gas temperature.
HRSG Simulation-unfired case
HRSG simulation-fired case
Effect of ambient temperature on
HRSG performance
Multiplication factor on steam flow is 0.1
Evaluating HRSG performance
HRSG performance is evaluated at different gas flow,exhaust
temperature conditions to see if the performance is reasonable.
Evaluating HRSG performance
Design
basis
Evaluating Operating Data
Note:Actual steam flow is 68,700 lb/h and exit gas temperature is 380 F,while it should have
been about 364 F.Hence further evaluations are necessary to check if HRSG design is
adequate. The gas flow was estimated based on steam duty and inlet/exit gas temperatures.
TWO OR SINGLE PRESSURE HRSG-case 1
We are trying to see if a 2 pressure HRSG is
required. Customer wants about 40,000 kg/h,30
kg/cm2 steam and 3000 kg/h steam at 6 kg/cm2
in fired mode and about 3500 kg/h LP steam in
unfired mode,which is taken off the drum and
pressure reduced..
Two or Single pressure HRSG-case 2
HRSG makes 40,000 kg/h HP steam at
400 C and 30 kg/cm2 and 3000 kg/h
steam is taken off the drum for process
and pressure reduced to 6 kg/cm2
Two or Single pressure HRSG-case 3
Here we have a dual pressure HRSG.
Two or Single Pressure HRSG-case 4
Two or Single Pressure HRSG?case 5
Here we see what happens if LP steam pressure
were 3 instead of 6 kg/cm2
Two or Single Pressure HRSG?-case 6
Note the stack gas temperature with lower
LP steam pressure.
Summary -two or single pressure?
OPTION
DESIGN A
DESIGN A
DESIGN B
DESIGN B
DESIGN C
DESIGN C
Case
HP steam flow,kg/h
HP steam temp,C
Process steam,kg/h
Stack temp,C
Firing temp,C
HP/LP steam press
Fuel,MW
Unfired
23434
370
3500
177
500
30/6
0
Fired
40000
400
3000
161
681
30/6
12.6
Unfired
24800
370
3500
164
500
30/6
0
Fired
40000
400
3000
163
681
30/6
12.6
Unfred
26340
370
3900
139
500
30/3
0
Fired
40000
400
3000
137
655
30/3
10.64
It may be seen that as long as the HRSG operates in the fired mode,the
single pressure system has the same performance as the dual pressure unit
when process LP steam is at 6 kg/cm2a,thus saving lot of expenditure, field
costs,operational costs etc. A pressure reducing station replaces a complete
LP evaporator,which could cost several hundred thousand dollars.If the LP
steam pressure were different,then the outcome will be different.When
process steam is at 3 kg/cm2a,then dual pressure looks attractive as seen in
columns 5 and 6.HP steam is at 30 kg/cm2a.So there is an optimum LP
pressure below which multiple pressure is justified. We cannot simply go for
dual pressure without doing this analysis. If the HRSG runs more often in
unfired mode,then a dual pressure may be warranted even at 6 kg/cm2
Effect of Exhaust gas analysis
ABOVE: TYPICAL EXHAUST GAS. BELOW: WITH STEAM INJECTION.
Note the difference in steam generation
Multiple Pressure Level HRSG
GT exhaust vs Fan Operation
HRSG temperature profiles
Q 1: Exhaust gas flow is 100,000 kg/h at 550 C. % volume
co2=3,h2o=7,h2o=75,o2=15. Steam at 60 kg/cm2a and 450 C is required. Feed water
is at 110 C and blow down=1 %. Using say 10 C pinch and approach, arrive at the
HRSG temperature profiles and steam generation.
Q 2:Repeat the calculation at 10
kg/cm2a for saturated as well as
superheated steam and discuss
the findings. If pinch and
approach increase by 5 C, 10 C,
how much duty we lose and also
the steam generation for Q 1
above?
Table shows enthalpy in btu/lb
vs temperature.
Temp,F
200
400
600
800
1000
enthalpy
34.98
86.19
138.7
192.48
247.56
If gas flow changes to 80,000 kg/h and steam pressure to 45
kg/cm2a in operation, what is the HRSG performance, duty, steam
temperature and ASME efficiency?
ASME efficiency=energy absorbed by steam/water/fluids or duty/(gas flow x
enthalpy+fuel input on LHV basis)
EFFECT OF PART LOAD AND HIGH LOAD
What are the effects of part load operation of gas turbine on HRSG and effect of
supplementary firing? Discuss.
MULTIPLE PRESSURE HRSG
HRSG performance at part loads
HRSG WITH REHEATER
Module 1
2
3
4
5
6
7
8
9
10
SMALLER HRSG – UNFIRED CASE
UNFIRED AND FIRED HRSG PERFORMANCE-ALSTOM
SMALLER HRSG – FIRED CASE
ACTUAL DESIGN
RESULTS FOR MODULE 1
MULTIPLE PRESSURE HRSG
Mod 7 feeds 4. mod 8 feeds 7. mod 10 feeds 8 and 9 mod 5 fed by mod 9. mod 13 feeds 11 and 12
HRSG PERFORMANCE SUMMARY
HRSGS program
•Up to 6 pressure levels or thirteen modules
•Complex configurations can be built up using common eco/sh
concept in a few minutes
•Unfired/GT exhaust or fresh air fan fired performance
•Gas turbine or fan operation or both combined
•Automatically computes firing temperature,fuel consumption
given desired steam flow in off-design case
•Steaming in economizer can be evaluated at part loads!
•ASME PTC efficiency computed
•All versions of Windows
•User friendly. Get design as well as off-design performance of
HRSGs in a few minutes! Try the free DEMO!
Programs for Boiler/HRSG Engineers
• Coil program- $ 750 :
http://vganapathy.tripod.com/coilpgm.html
• Programs for steam plant engineers- $ 400
http://vganapathy.tripod.com/boil1.html
• HRSG simulation program-$ 1500
http://vganapathy.tripod.com/hrsgs.html
(Prices are for single user. Download free demo for
coil program/hrsg simulation from above sites)