Page 180 - Handbook of Energy Engineering Calculations

generators where the rate of steam generation is predetermined and can be
achieved, steam-flow determination in an HRSG requires an analysis of the
gas/steam temperature profiles. This requirement is mainly because we are
starting at a much lower gas temperature—900 to 1100°F—(482.2 to
593.3°C) at the HRSG inlet, compared to 3000 to 3400°F (1648.9 to

1871.1°C) in a conventionally fired boiler. As a result, the exit-gas
temperature from an HRSG cannot be assumed. It is a function of the
operating steam pressure, steam temperature, and pinch and approach points

used, Fig. 25.

FIGURE 25 Gas/steam temperature profiles.

Typically, the pinch and approach points range from 10 to 30°F (5.56 to
16.6°C). Higher values may be used if less steam generation is required. In

this case, we will use 20°F (11.1°C) pinch point (= T − t ) and 10°F
s
g3
(5.56°C) approach (= t − t ). Hence, the gas temperature leaving the
s
w2