Size combined-cycle Heat Recovery Steam Generators (HRSG) with thermodynamic rigor. Compute evaporator pinch point, economizer approach temperature, live gas cooling curves, steam generation rates, and overall energy recovery efficiency with interactive Temperature-Enthalpy (T-Q) profile rendering.
Gas Turbine Exhaust & Boiler Parameters
HRSG Thermodynamic Performance
Live Temperature vs. Heat Transferred (T-Q) Diagram
Real-time thermodynamic temperature profiles showing exhaust gas cooling curve alongside economizer heating, isothermal drum evaporation, and superheater warming lines with pinch & approach points.
Fatal Traps & Engineering Pitfalls in HRSG Design
1. Steaming in Economizer Tubes (Under-sizing Approach Margin)
Selecting an approach temperature difference below 5°C risks premature vapor generation inside economizer tubes during gas turbine load transients or cold ambient air operation. Two-phase flow causes devastating steam hammer, tube chattering, flow distribution maldistribution, and severe tube-to-header weld cracking.
2. Cold-End Acid Dew-Point Condensation & Low Stack Temperature
Excessively maximizing heat recovery to drive stack gas temperature below 105°C–120°C triggers sulfuric acid and moisture condensation if sulfur is present in fuel or trace mercaptans in pipeline gas. Cold-end finned tubes will perforate from corrosion in less than 18 months unless feedwater preheating loops maintain metal temperatures above the dew point.
3. The Pinch Point Diminishing Returns Trap (Surface Area Explosion)
Attempting to lower the pinch point below 8°C produces diminishing steam gains while evaporator tube surface area scales asymptotically as 1/ΔT_pinch. Below 6°C, the added capital cost of titanium or high-alloy finned tubing, structural framing, and gas-side pressure drop penalty far outweighs the marginal turbine kilowatt-hours generated.
4. Ignoring Flue Gas Specific Heat (Cp) Temperature Dependence
Using ambient air Cp (1.005 kJ/kg·K) instead of combustion exhaust gas Cp at elevated temperatures (1.10–1.18 kJ/kg·K depending on humidity and gas turbine fuel/air ratio) introduces a 10% to 15% error in steam generation sizing. Exhaust gas contains substantial H2O (7–11 vol%) and CO2 (3–5 vol%), raising heat capacity significantly above dry air.
5. Gas-Side Backpressure Penalties on Gas Turbine Output
Adding excessively dense finned tube banks (e.g. 6–7 fins/inch) to squeeze out extra heat increases gas-side backpressure on the gas turbine. Every 1.0 kPa (4 inches water gauge) of HRSG gas backpressure drops gas turbine electrical power output by approximately 0.3% to 0.5% and worsens heat rate, negating the minor heat recovery bonus.
Thermodynamic Derivations & Governing Equations
The sizing of a Heat Recovery Steam Generator is bounded by the second law of thermodynamics. The heat transferred across any section must maintain a positive temperature driving force everywhere along the flow path.
T_gas,pinch = T_sat(P_drum) + ΔT_pinch
2. Economizer Water Exit Temperature Condition:
T_water,eco_out = T_sat(P_drum) - ΔT_approach
3. High-Pressure Evaporator + Superheater Heat Balance:
Q_sh_evap = m_gas · Cp_gas · (T_gas,in - T_gas,pinch) · (1 - f_loss)
m_steam = Q_sh_evap / [ (h_steam(P_drum, T_sh) - h_water(T_water,eco_out)) ]
4. Economizer Energy Balance:
Q_eco = m_steam · [ h_water(T_water,eco_out) - h_water(T_feed) ]
T_gas,stack = T_gas,pinch - [ Q_eco / (m_gas · Cp_gas · (1 - f_loss)) ]
5. Overall HRSG Heat Recovery Efficiency:
η_HRSG = (T_gas,in - T_gas,stack) / (T_gas,in - T_ambient)