The example shows how one can use the HRSG simulation methods to improve the efficiency of a system.
problem: A gas turbine HRSG is required to generate a desired quantity of High Pressure steam and maximize the Low pressure steam. The question is what should be the HRSG configuration.
data:
exhaust  flow=500,000 lb/h
exhaust temperature=900 F
gas analysis:% vol CO2=3,h2O=7,N2=75,O2=15
HP steam: 31,500 lb/h at 600 psig,600 F
LP steam: maximum in the unfired mode at 150 psig sat.
feed water=230 F
assume heat loss=1 %
Solution:
The HRSGS program was used to simulate different configurations. Two are presented below. In Option a,there are three modules. HP superheater with evaporator,LP evaporator,follwed by the common economizer.The HP pinch point was selected to give the desired steam flow,while low pinch and approach points were used for the LP section to maximize the LP steam flow. In option b,the HP section has its own superheater,evaporator and economizer,followed by the LP section with its own evaporator and economizer. Tables a and b show the results from the program.
 

Table showing results of Option a

surface	gas in	gas out	wat in	stm out	duty	press	flow	% stm	pinch	approch
suph	900	879	491	600	2.72	615	31759	100	-	-
evap 1	879	671	351	491	28.00	630	31759	100	180	140
evap 2	671	381	351	366	37.93	165	43453	100	15	15
econ	381	306	230	351	9.51	650	76716	0	-	-

[units: temperatures-F,duty-MM Btu/h,pressure-psia,flow-lb/h]

Table showing results of Option b

surface	gas in	gas out	wat in	stm out	duty	press	flow	% stm	pinch	approch
suph	900	879	491	600	2.70	615	31468	100	-	-
evap 1	879	706	476	491	23.36	630	31468	100	215	15
eco 1	706	642	230	476	8.38	640	32097	0	-	-
evap 2	642	381	351	366	34.09	165	39050	100	15	15
eco 2	381	342	230	351	4.95	175	39831	0	-	-

ANALYSIS
It can be seen that in option a,we have a larger heat sink in the economizer feeding the two modules. This economizer handles the water flow equivalent of the two evaporators and hence is able to pull down the gas temperature from LP evaporator exit to a much lower value.In option b,the heat sink is not as large,as the economizer handles only the LP flow.Hence the exit gas temperature in option a is lower and thus more energy is recovered. Thus using the HRSGS program,one can simulate different configurations,modules and rearrange their locations to maximize energy recovery.

Click here for more information on HRSGS program
V.Ganapathy's Home page
Generating steam efficiently in cogeneration plants
Understanding HRSG temperature Profiles