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   Check Boiler,HRSG Operating costs !

Before you buy a steam generator or HRSG,make sure that you are making a good comparison of various options/offers,particularly the operating costs. Often purchase managers evaluate the initial costs of the equipment and neglect the long term effects of operating costs. Low price option may look attractive initially but during the life time of the equipment,which could be 15 to 25 years,the decision to go with that option could be a bad one.The following two examples give an idea of the importance of operating costs in boiler operation.
HRSG
Table below shows the performance of two gas turbine HRSG designs,generating low pressure saturated steam,for the same gas inlet conditions.Designs A and B  are designed with different pinch points and the effect on fuel input and steam generation is shown.HRSG A is designed with a pinch point of 41 F,while B with 22 F.The steam production and gas pessure drop are higher with design B. In the fired mode,while generating 30,000 lb/h, design A requires 8.1 MM Btu/h fuel,while B requires only 6.9 MM Btu/h(The example is also discussed in my books).

                             HRSG options with different Pinch Points
 
   design A-unfired   design A -fired design B-unfired design B-fired
 gas flow,lb/h 150,000 150,000 150,000 150,000
 inlet gas temp,F  900   1086 900 1062
 temp to eco,F  407   419  388  393
 exit gas temp,F  332  329  309 302
gas pr drop,in wc  4.2  4.6 5.4 5.8
steam flow,lb/h  22,107  30,000 22,985 30,000
feed water in,F  230  230 230 230
temp to evco,F  351 337 352 340
burner duty,MM  0 8.1 0 6.9
pinch point,F  41 53 22 27
approach point,F  15 29  14 26
surf area evap  13,227  13,227  16,534  16,534
surf are eco  5948 5948 8922 8922
       [steam press=150 psig,fuel fired=natural gas, gas turbine capacity=4500 kw]

A simple evaluation
Let the cost of fuel=2.7 $/MM Btu(LHV),cost of steam=3$/1000 lb and electricity=5c/kwh.Assume that the unit operates 50 % of the time in unfired and fired modes,or 4000 hours in each mode per year.The cost advantage of design B over A is:

due to extra steam generated in unfired mode=878x3x4000/1000= $ 10,536
due to additional fuel in fired mode=1.2x4000x2.7=$ 12,960
due to loss in electrical power output of the gas turbine(using the fact that about 4 in wc additional gas pressure drop is worth 1 % of power)=1.2x4500x.05x8000/400=$ 5,400
Hence design B offers($ 10,536+12,960-5400)=$ 18,096 in value over design A each year..! Based on the differential costs,one can then determine if it is worth going for the more expensive design B or stay with design A. It is the author's experience that designs with lower cost of operation are more attractive financially over the long term,though they could be more expensive to begin with.

STEAM GENERATOR
Two designs are available for a  packaged steam generator,a custom designed boiler design A and a standard design B.In the custom designed boiler option,tube spacing of evaporator tubes is increased to reduce the gas pressure drop. More energy is absorbed in the economizer to bring the efficiency to the same value as in design B.

                Performance of Standard vs Custom designed boiler
 
    design A   design B
 gas temp to evaporator,F  2176  2176
 gas temp lvg convection,F  565  520
 gas temp lvg economizer,F  320  320
 gas pr drop in evap,in wc   7.5  13
 gas pr drop in econ,in wc  1.2  1.1
 evaporator surface,ft2   5900  5900
 economizer surface,ft2  8960  8400
 transverse pitch,evap,in   4.5   4.0
  fan power , kw  113  144
 annual operating cost,$   45,200  57,600
steam =125,000 lb/h at 150 psig sat. Fuel=natural gas,15% excess air,12 % FGR. Burner,duct loss=11 in wc. Electriity cost= 5cents/kwh
Evaporator: 2 in tubes,13 tubes/row,96 deep,9 ft long
Economizer: 2 in OD,18 tubes/row,10 ft long,fins 0.75 in high,.05 thick serrated. 3.5  and 4.0 fins/in  design A & B.
Furnace: 32 ft long,10 ft high,7 ft wide

Hence there is a significant savings in operating costs with the custom designed option. Even if the difference in initial boiler costs is say  $ 25,000,the payback is less than 2 years. Failing to evaluate the operating cost could be a costly error.Operating cost differentials  could also arise in the form of lower efficiency (higher exit gas temperature or excess air).Hence consulting engineers or plant managers making decisions solely based on initial costs are doing a disservice to the owner of the steam plant.Plant owners should be wary of operating costs,potential increases in cost of fuel,electricity or steam down the road and ensure those responsible for purchasing HRSGs or steam generators have given proper consideration to operating cost differentials.

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