Page 182 - Handbook of Energy Engineering Calculations

5. Determine the economizer duty and exit-gas temperature
The economizer duty, Q = (rate of steam generation, lb/h)(l + blowdown
3
expressed as a decimal) (enthalpy of water leaving the economizer − enthalpy

of feedwater at 240°F) = (18,502)(1.02) (431.2 − 209.6) = 4.182 MM Btu/h
(1.225 MW).
The HRSG exit-gas temperature = (480, the exit-gas temperature at the
evaporator computed in step 1, above) − (economizer duty)/(gas-turbine

exhaust-gas flow, lb/h)(1.0 − heat loss)(exhaust gas specific heat) = 371.73°F
(188.9°C); round to 372°F (188.9°C). Note that you must compute the gas
specific heat at the average gas temperature of each of the heat-transmission
surfaces.

6. Compute the ASME HRSG efficiency

The ASME Power Test Code PTC 4.4 defines the efficiency of an HRSG
as: E = efficiency = (energy absorbed by the steam and fluids)/(gas flow ×
inlet enthalpy + fuel input to HRSG on LHV basis). In the above case, E =
6
(16.65 + 4.182)(10 )/(150,000 × 220) = 0.63, or 63 percent. In this
computation, 220 Btu/lb (512.6 kJ/kg) is the enthalpy of the exhaust gas at
900°F (482.2°C) and (16.65 + 4.182) is the total energy absorbed by the
steam in MM Btu/h (MW).

Related Calculations. Note that the exit-gas temperature is high. Further,

without having done this analytical mathematical analysis, the results could
not have been guessed correctly. Minor variations in the efficiency will result
if one assumes different pinch and approach points. Hence, it is obvious that

one cannot assume a value for the exit-gas temperature—say 300°F
(148.9°C)—and compute the steam generation.
The gas/steam temperature profile is also dependent on the steam pressure
and steam temperature. The higher the steam temperature, the lower the
steam generation rate and the higher the exit-gas temperature. Arbitrary

assumption of the exit-gas temperature or pinch point can lead to temperature
cross situations. Table 3 shows the exit-gas temperatures for several different
steam parameters. From the table, it can be seen that the higher the steam

pressure, the higher the saturation temperature, and hence, the higher the exit-
gas temperature. Also, the higher the steam temperature, the higher the exit-
gas temperature. This results from the reduced steam generation, resulting in