HRSG%20SIMULATION

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HRSG SIMULATION
V.Ganapathy
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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
UAQ/ ? 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!
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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
• Evaluate field performance and relate it with
  HRSG performance guarantees
• Write better HRSG specifications by knowing
  HRSGs capabilities

Ideal tool for cogen/combined cycle plant
evaluation No need to physically design the
HRSG! Saves time for consultants
LimitationConvective type surfaces with no
external radiation
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Pinch and Approach Points
Note This is the Design mode ..We cannot
pre-select pinch and approach points in
off-design mode!
pinch point,F approach point,F
a. evap type bare finned
b.inlet gas temp,F 1200-1800
130-150 30-60 40-70 700-1200
80-130 10-30 10-40
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Facts about Pinch and Approach Points

• Pinch 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

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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-tg
3)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 (3)
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.
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
tg3gtts and tg4gttw1.
Pinch20F,approach15 F,gas inlet900 F,feed
water230 F
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Temperature calculations
Example 1 Determine HRSG exit gas temperature
when inlet gas temperature900 F,steam press100
psig.use 20 F pinch and 15 F approach
points. SolutionK0.904 sat temp338 F.hence
tg3358F.tw2323 F.(900-358)/(900-tg4)0.904 or
tg4300 F Example 2 what is tg4 when steam
press600 psig and temp750 F? SolutionK0.7728.s
at temp492 F.tw2477F.tg3512 F.(900-512)/(900-tg
4)0.7728 or tg4398 F.So 300 F stack temperature
is not feasible! Example 3Why cant we obtain
300 F at 600 psig,750 F steam? SolutionK0.7728
Let us compute tg3 from(900-tg3)/(900-300)0.7728
or tg3436 F.This is called temperature
cross! Example 4What should be done to get 300 F
stack temperature? SolutionIncrease tg1 by
firing.say tg11600 F.(1600-tg3)/(1600-300)0.7728
or tg3595 F and pinch103 F. Example 5If
tg1800F,what is tg4 at 100 psig
sat? Solution(800-358)/(800-tg4)0.904 or
tg4312 vs 300 F when tg1 was 900F. Example
6With 1600 F gas inlet,can we use 20 F
pinch? Solution(1600-512)/(1600-tg4)0.7728 or
tg4192 F,which is below 230 F.Not
feasible! Thats why pinch approach points
should not be selected in the fired mode!We have
no idea in what range they can fall.
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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. tg3tspp
and tw2ts-ap Energy absorbed by sh evap
Q12WgxCpgx(tg1-tg3)xhl Ws(hso-hw2)bd(hf-hw2)
.Ws is then computed. Q1Superheater
dutyWs(hso-hv)WgxCpgx(tg1-tg2) .Q1 and tg2 are
thus obtained From above,Q2(Q12-Q1)evaporator
duty is obtained. Economizer dutyQ3Ws(1bd)(hw2-
hw1)WgxCpg(tg3-tg4)xhl from this both Q3 and
tg4 are obtained. hlheat loss is on the order
of 0.5 to 1 bdblow down varies from 1 to 7
depending on feed water quality and boiler water
conditions.
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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. Example160,00
0 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.
SolutionUsing 25 F pinch and 20 F
approach,compute energy absorbed by
SHevap160,000x0.27x(950-517)x0.9818.33 MM
Btu/hWs(1378.9-455.4) or Ws19,850 lb/h. Energy
absorbed by HRSG19,850x(1378.9-199.7)23.4 MM
Btu/h160,000x0.98x0.268x(950-tg4) or tg4393
F. Fired case Energy absorbed by
steam35000x(1378.9-199.7)41.27 MM Btu/h.
Additional fuel energy required(41.27-23.4)17.87
MM Btu/h.Oxygen consumed17.87x106/(160000x58.4)
1.91 So there is plenty of oxygen left. Firing
temperature17.87x106160000x0.3x(T-950) or
T1322 F Exit gas temperature1322-41.27x106
/(160000x.275x.98)364 F
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Design Off-design calculations

• DESIGN
• unfired
• establishes configuration
• establishes surface areas indirectly
• only one case
• zero desuperheater spray
• pinch and approach points selected
• zero economizer steaming

• OFF-DESIGN
• unfired/fired/fan mode/combination
• several cases possible
• computes desuperheater spray
• pinch and approach points computed
• economizer steaming possible

• WHATIF STUDIES
• steam pressure variations
• firing temperature restrictions
• effect of fuels
• performance testing
• effect of gas turbine load
• variations in ambient temperature

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A simple example of simulation
The energy transferred to the evaporator is given
by QWgCp(T1-T2)US?TUS (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)184.4K2 In the
off-design case,900000.4ln(970-388)/(T2-388)184
.4 or T2473 F.Duty and steam generation may be
computed from this. 406 and 388 F are saturation
temperatures corresponding to 250 and 200 psig
respectively.
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Example of a HRSG simulation
Example140,000 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 down5. Solution Let us choose a
pinch point of 20F and approach of 15 F.Sat
temperature388F. Gas temperature leaving
evaporator408 F and water temperature entering
it is 373 F.Evaporator duty140000x.99x.27x(980-40
8)21.4 Mm Btu/h. 1 heat loss and average
specific heat of 0.27 Btu/lbF is
assumed Enthalpy absorbed in evaporator1199.3-34
5.05x(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 generation21.4x106/855.225,000
lb/h Economizer duty25000x1.05x(345-198.5)3.84
Mm Btu/h .gas temperature drop3840000/(140000x.25
3x.99)109 F.Hence exit gas temperature408-10929
9 F
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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)/(408-388)Kx140000-0.4
or K387.6 Under new conditions
ln(880-366)/(Tg-366)387x165000-0.4 3.1724 or
Tg388 F.Evaporator duty165000x.99x.27x(880-388)
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
flow21.7x106/1195.7-332).05x(338.5-332)25,110
lb/h. Economizer duty(assumed)
Qa25110x1.05x(332-198.5)3.52MM Btu/h.Compute
(US)dQ/?T for economizer based on design
conditions. Q3.84x106 ?T (408-373)-(299-230)/l
n(69/35)50 F.(US)d3840000/5076800. Correct
this for off-design case. (US)p(US)dx(165000/1400
00).6585200.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 duty3.52MM
Btu/h and gas temperature drop3520000/(165000x.99
x.253)85 F or exit gas 388-85303
F.?T(303-230)-(388-350)/ln(73/28)47 F or
transferred duty85200x474.00 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.
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Superheater performance
Performance of a superheater is obtained from
Q(US)p?T ?T(Tg1-ts2)-(Tg2-ts2)/ln(Tg1-ts2)/(
Tg2-ts2) (US)p is obtained from design (US)
values as follows (US)pWg0.65FgK1(Ws/Wsd)0.15
where K1a 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. ExampleIn design
mode, gas flow150,000 lb/h.Gas in900F and
leaving SH842F. steam flow18510 lb/h.steam
pressure450 psig. steam in460F,out650F.duty2.3
4 MM Btu/h Cp.273,m.0826 ,k.029.
Fg.2730.33x.0290.67/.08260.320.135.?T(842-460)
-(900-650)/ln(842-460)/(900-650) 311F.Hence
K12.34x106/(311x.135x1500000.65)
24.1 Off-designsteam flow18050 lb/h,gas
flow165000,gas in840F.steam pressure450
psig.Let exit steam temp640F.Duty18050x(1325-120
4.4)2.177MM Btu/h. Exit gas840-2177000/165000/.9
9/.271791F. Since gas temperatures are close,
Use same Fg0.135. (US)p1650000.65x0.135x24.1x(18
050/18510)0.157974. ?T(840-640)-(791-460)/ln
(840-640)-(791-460)260F. Hence
Qt7974x2602.074MM 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.
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HRSG Performance Calculations
Performance may be obtained even if HRSG geometry
is unknown using simulation concept.
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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)

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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

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Improving HRSG performance
Bottom line is to lower the exit gas temperature!
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RESULTS OF A SIMPLE STUDY
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HRSG simulation
Knowing gas flow,temperature,analysis and steam
parameters,establish HRSG temperature
profiles,duty and steam flows.In the design
case,solve forUAQ/? 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 dataanyone
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...
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HRSG Temperature profile
HP stage is followed by LP section. Not a very
efficient design
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HRSG Temperature profile
Using common Economizer concept,we improve energy
recovery
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Why Steaming occurs in HRSG Economizers
100 load
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HRSG performance at Low Load
HRSG performance at 40 load. Note steaming in
economizer and also the high exit gas temperature.
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HRSG Simulation-unfired case
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HRSG simulation-fired case
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Effect of ambient temperature on HRSG performance
Multiplication factor on steam flow is 0.1
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Evaluating HRSG performance
HRSG performance is evaluated at different gas
flow,exhaust temperature conditions to see if the
performance is reasonable.
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Evaluating HRSG performance
Design basis
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Evaluating Operating Data
NoteActual 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.
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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..
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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
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Two or Single pressure HRSG-case 3
Here we have a dual pressure HRSG.
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Two or Single Pressure HRSG-case 4
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Two or Single Pressure HRSG?case 5
Here we see what happens if LP steam pressure
were 3 instead of 6 kg/cm2
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Two or Single Pressure HRSG?-case 6
Note the stack gas temperature with lower LP
steam pressure.
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Summary -two or single pressure?
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
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Effect of Exhaust gas analysis
ABOVE TYPICAL EXHAUST GAS. BELOW WITH STEAM
INJECTION. Note the difference in steam
generation
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Multiple Pressure Level HRSG
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GT exhaust vs Fan Operation
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HRSG temperature profiles
Q 1 Exhaust gas flow is 100,000 kg/h at 550 C.
volume co23,h2o7,h2o75,o215. Steam at 60
kg/cm2a and 450 C is required. Feed water is at
110 C and blow down1 . Using say 10 C pinch and
approach, arrive at the HRSG temperature profiles
and steam generation.
Q 2Repeat 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 efficiencyenergy absorbed by
steam/water/fluids or duty/(gas flow x
enthalpyfuel input on LHV basis)
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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.
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MULTIPLE PRESSURE HRSG
HRSG performance at part loads
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HRSG WITH REHEATER
Module 1 2 3 4
5 6 7
8 9 10
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SMALLER HRSG UNFIRED CASE
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UNFIRED AND FIRED HRSG PERFORMANCE-ALSTOM
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SMALLER HRSG FIRED CASE
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ACTUAL DESIGN
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RESULTS FOR MODULE 1
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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
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HRSG PERFORMANCE SUMMARY
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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!

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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)