Page 177 - Handbook of Energy Engineering Calculations

MM Btu/h (1.18 MW).
Since the assumed and transferred duty do not match, i.e., 3.52 MM Btu/h
versus 4.01 MM Btu/h, another iteration is required. Continued iteration will
show that when Q = Q = 3.55 MM Btu/h (1.04 MW), and the temperature of
t
a
the water leaving the economizer, = 366°F (185.6°C) (saturation) and exit-
gas temperature = 301°F (149.4°C), the amount of steam generated = 25,310
lb/h (11,491 kg/h).

Related Calculations. Studying the effect of gas inlet-temperature and gas
flows on HRSG performance will show that at lower steam generation rates
or at lower pressures that the economizer water temperature approaches
saturation temperature, a situation called “steaming” in the economizer. This
steaming condition should be avoided by generating more steam by

increasing the inlet gas temperature or through supplementary firing, or by
reducing exhaust-gas flow.
Supplementary firing in an HRSG also improves the efficiency of the

HRSG in two ways: (1) The economizer acts as a bigger heat sink as more
steam and hence more feedwater flows through the economizer. This reduces
the exit-gas temperature. So with a higher gas inlet-temperature to the HRSG,
we have a lower exit-gas temperature, thanks to the economizer. (2)
Additional fuel burned in the HRSG reduces the excess air as more air is not

added; instead, the excess oxygen is used. In conventional boilers we know
that the higher the excess air, the lower the boiler efficiency. Similarly, in the
HRSG, the efficiency increases with more supplementary firing. HRSGs used

in combined-cycle steam cycles, Fig. 24, may use multiple pressure levels,
gas-turbine steam injection, reheat, selective-catalytic-reduction (SCR)
elements for NO control, and feedwater heating. Such HRSGs require
x
extensive analysis to determine the best arrangement of the various heat-
absorbing surfaces.