Page 173 - Handbook of Energy Engineering Calculations

generated, compared to other options for adding capacity. Further, compared
to building a new power plant, the permitting process for repowering it is
typically much shorter and less complex. The HRSG will often have a
separate firing capability such as that discussed in this calculation procedure.
These comments from Power magazine were prepared by Steven Collins,

Assistant Editor of the publication.

SIMULATION OF HEAT-RECOVERY STEAM GENERATORS

(HRSG)

A gas turbine exhausts 140,000 lb/h (63,560 kg/h) of gas at 980°F (526.7°C)
2
to an HRSG generating saturated steam at 200 lb/in (gage) (1378 kPa).
Determine the steam-generation and design-temperature profiles if the
feedwater temperature is 230°F (110°C) and blowdown = 5 percent. The
average gasturbine exhaust gas specific heat is 0.27 Btu/lb°F (1.13 kJ/kg°C)

at the evaporator and 0.253 Btu/lb°F (1.06 kJ/kg°C) at the economizer. Use a
20°F (11.1°C) pinch point, 15°F (8.3°C) approach point, and 1 percent heat
loss. Evaluate the evaporator duty, steam flow, economizer duty, and exit-gas
temperature for normal load conditions. Then determine how the HRSG off-

design temperature profile changes when the gas-turbine exhaust-gas flow
becomes 165,000 lb/h (74,910 kg/h) at 880°F (471°C) with the HRSG
2
generating 150-lb/in (gage) (1033.5 kPa) steam with the feedwater
temperature remaining the same.

Calculation Procedure:

1. Compute the evaporator duty and steam flow
Engineers should be able to predict both the design and off-design
performance of an HRSG, such as that in Fig. 23, under different conditions
of exhaust flow, temperature, and auxiliary firing without delving into the

mechanical design aspects of tube size, length, or fin configuration. This
procedure shows how to make such predictions for HRSGs of various sizes
by using simulation techniques.