Page 148 - Handbook of Energy Engineering Calculations

GAS-TURBINE HEAT-RECOVERY STEAM-GENERATOR
(HRSG) CHOICE

Choose a suitable heat-recovery boiler equipped with an evaporator and
economizer to serve a gas turbine in a manufacturing plant where the gas
flow rate is 150,000 lb/h (68,040 kg/h) at 950°F (510°C) and which will
2
generate steam at 205 lb/in (gage) (1413.5 kPa). Feedwater enters the boiler
at 227°F (108.3°C). Determine if supplementary firing of the exhaust is
required to generate the needed steam. Use an approach temperature of 20°F
(36°C) between the feedwater and the water leaving the economizer.

Calculation Procedure:

1. Determine the critical gas inlet-temperature

Turbine exhaust gas (TEG) typically leaves a gas turbine at 900−l000°F
(482−538°C) and has about 13 to 16 percent free oxygen. If steam is injected
into the gas turbine for NO control, the oxygen content will decrease by 2 to
x
5 percent by volume. To evaluate whether supplementary firing of the
exhaust is required to generate needed steam, a knowledge of the temperature
profiles in the boiler is needed.

Prepare a gas/steam profile for this heat-recovery boiler as shown in Fig.
11 TEG enters on the left at 950°F (510°C). Steam generated in the boiler at
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205 lb/in (gage) (1413.5 kPa) has a temperature of 390°F (198.9°C), from
steam tables. For steam to be generated in the boiler, two conditions must be
met: (1) the “pinch point” temperature must be greater than the saturated
steam temperature of 390°F (198.9°C), and (2) the temperature of the
saturated steam leaving the boiler economizer must be greater than that of the

feedwater. The pinch point occurs somewhere along the TEG temperature
line, Fig. 11, which starts at the inlet temperature of 950°F (510°C) and ends
at the boiler gas outlet temperature, which is to be determined by calculation.
A pinch-point temperature will be assumed during the calculation and its

suitability determined.