Understand boiler performance characteristics


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Packaged boilers.

Completely shop-assembled packaged
boilers (Fig. 1) are used to generate steam up to
200,000 lb/h, 1,000 psig and 850°F. Slight deviations
in these parameters are feasible on a case-by-case
basis and depend on permitted boiler shipping
dimensions. Typically, these steam generators fire
natural gas and distillate oils with burners located in
the furnace's front wall. A superheater and economizer
may be used if superheated steam is required and a
higher efficiency level is sought.

Heat-recovery steam generators.

Large HRSGs are generally the convective type (Fig. 2).
But they can resemble large utility boilers with radiant
furnaces if the firing temperature is above 1,600°F to
1,700°F. The duct burner is located ahead of the HRSG,
which consists of a superheater, evaporator and
economizer. Additional modules may be required in
multi-pressure units. If a constant steam temperature
is required at all loads, the design philosophy is to
make sure that the steam temperature is achieved at
unfired conditions. In the fired condition, although
steam generation is larger, steam temperature is also
higher due to a higher inlet gas temperature.
Desuperheating may be restored to control the steam
temperature. Simulation methods can be used to
predict HRSG performance at different conditions.