For the studied combined-cycle case, increasing HRSG air-side pressure loss is worth evaluating only if the added heat recovery or cost reduction outweighs the gas-turbine penalty and remains within exhaust-pressure and stack-temperature limits. A higher heat-transfer coefficient alone does not prove either benefit.
Reject the quick fixes that confuse pressure loss with useful recovery
- Raise gas velocity and assume steam output rises. Faster gas flow can raise the air-side heat-transfer coefficient, but it does not create additional exhaust heat. If the HRSG already meets a stack-temperature or temperature-pinch limit, more heat-transfer area or a higher coefficient may not lower the stack temperature or increase useful steam generation.
- Double pressure loss and cut tube quantity by the same percentage. The proposed estimate of roughly 40% more velocity and roughly 10% more air-side heat-transfer coefficient is a preliminary scaling argument, not a design result. Tube layout, gas distribution, heat-transfer surfaces, pressure boundary design, and the limiting temperature all affect the installed cost.
- Treat lost exhaust pressure as recoverable turbine work. A pressure drop across the HRSG is not automatically converted into useful output. Added exhaust-system resistance raises gas-turbine backpressure and can reduce gas-turbine performance; compare that penalty against any bottoming-cycle gain.
Do not change the HRSG design from these estimates alone. Use them to define a comparative design study.
Set the case around the actual pressure and output limits
The design discussion gives a current HRSG total-pressure-loss range of 3–4%, defined as ΔPt / Pt_inlet. The stated gas-turbine exhaust temperature is on the order of 700°C (about 1300°F). These are case inputs, not universal HRSG targets.
Keep total-pressure loss, turbine discharge pressure, and static pressure distinct in the study. Record the pressure stations and reference conditions used for every value. Confirm with the gas-turbine supplier how added HRSG resistance changes turbine exhaust pressure and performance for the specific operating condition; a percentage pressure loss by itself does not establish the resulting turbine penalty.
The case includes two different benefit thresholds that should not be conflated. The initial question asks whether about 0.1% more steam-turbine output could follow a one-percentage-point increase in pressure loss. A later case estimate says the bottoming system would need about 0.2% more enthalpy recovery for each 1% increase in ΔPt / Pt_inlet to offset the gas-turbine efficiency loss in that particular comparison. Define the output metric and baseline before comparing these figures.
Check whether a thermal constraint blocks more recovery
More air-side heat transfer helps only when heat-transfer resistance is limiting and the HRSG has a usable temperature range in which to transfer the additional heat. Identify the controlling section and temperature constraint before attributing a benefit to higher velocity.
| Observed or proposed condition | What it may indicate | What to check |
|---|---|---|
| Air-side heat-transfer coefficient appears limiting | Higher gas velocity could improve local heat transfer, but may also raise pressure loss. | Compare modeled heat-transfer coefficients, gas distribution, and pressure loss by HRSG section. |
| Stack temperature is at its design minimum | Further heat extraction may be constrained by the cold end, not by air-side coefficient. | Check minimum stack-temperature requirements and the cold-end design basis. |
| Temperature pinch or approach controls steam generation | Added surface or higher gas-side coefficient may not produce the expected steam-cycle gain. | Review temperature profiles and pinch/approach points through the proposed HRSG design. |
| Tube-fin fouling raises exhaust pressure over time | Operating margin may be needed for degradation, separate from clean-design pressure loss. | Use the plant’s fouling and cleaning basis, and confirm turbine discharge-pressure limits. |
The discussion cites a clean-design turbine-exhaust pressure of about 12 in. H₂O, a trip at +22 in. H₂O, and the difference as margin for gradual fouling associated with ammonium bisulfate accumulation after NOx removal. Treat those as figures recalled for one context, not general limits. Verify actual alarm, trip, clean-condition, and fouling values from the plant design documents and turbine controls.
Test the cold-end limit before claiming an efficiency gain
The cold end can set a hard boundary on heat recovery. The case discussion identifies stack-temperature restriction, acid-dew-point concerns tied to trace sulfur from natural-gas odorant, and tube material corrosion resistance as relevant constraints. If the existing design is already at its allowable stack temperature, raising the air-side coefficient may not permit more extraction without changing the cold-end material or design basis.
Also distinguish heat quantity from power value: heat recovered at lower temperature generally has less ability to produce power than heat supplied at higher temperature. A study that counts only additional recovered enthalpy can therefore overstate the steam-turbine benefit. Calculate steam-cycle output from the proposed heat input and operating conditions rather than using enthalpy recovery as a direct proxy for electrical output.
Compare pressure-loss cases with a coupled design study
Evaluate at least the existing configuration and the proposed higher-loss configuration using the same gas-turbine exhaust condition, ambient condition, fuel basis, and output accounting. The source itself points to preliminary designs of HRSGs with different pressure losses as the practical way to resolve the tradeoff.
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Lock down the baseline. Record the existing
ΔPt / Pt_inlet, pressure station definitions, exhaust temperature and flow, steam-cycle output, stack temperature, and turbine backpressure response. - Define the constraint set. Identify the controlling pinch/approach, minimum allowable stack temperature, materials at the cold end, fouling allowance, and gas-turbine operating or trip limits.
- Model alternative HRSG designs. Change the gas-side velocity and pressure-loss target in a preliminary thermal and mechanical design. Do not treat a velocity or heat-transfer coefficient scaling as a substitute for section-by-section thermal and pressure-drop calculations.
- Include the full plant balance. Compare gas-turbine output penalty against steam-turbine gain at the same operating point. Report net combined-cycle output and the bottoming-system recovery metric separately.
- Estimate installed cost from the design. Quantify tube and fin surface, casing or pressure-boundary implications, supports, and other affected equipment. The proposed 10% reduction in piping is a hypothesis to test, not a cost estimate.
Keep velocity and cost estimates inside their assumptions
The case proposes that a roughly 40% increase in air velocity might follow from doubling HRSG pressure loss, with an approximately 10% air-side heat-transfer-coefficient increase. These are rough estimates made to frame a cost question. Pressure loss depends on geometry and operating conditions, and a change in one HRSG design variable does not guarantee the same velocity ratio or heat-transfer response in another design.
Likewise, fewer tubes do not automatically mean a cheaper unit. A higher pressure drop can affect the upstream exhaust system and turbine operating margin; higher internal gas pressure may alter casing or other mechanical requirements. Have the HRSG designer check the applicable mechanical design and pressure boundary rather than assuming the shell-cost effect is either negligible or decisive.
Verify the selected design at the limits that govern operation
For each candidate, verify the thermal solution at the specified exhaust condition and confirm the lowest stack temperature, controlling pinch/approach, section pressure drops, and turbine discharge pressure. Recheck the calculation for the fouled condition used in the plant design basis, not just the clean HRSG case.
Then verify the performance comparison against the agreed accounting boundary: gas-turbine loss, steam-turbine gain, and net combined-cycle result. If the expected benefit disappears when the stack-temperature floor, fouling margin, or turbine limit is applied, higher pressure loss does not deliver the proposed recovery benefit for that design.
FAQ: Decide whether higher HRSG pressure loss is useful
What happens if HRSG pressure loss increases by one percentage point?
The gas turbine can incur a backpressure-related performance penalty, while increased gas velocity may improve air-side heat transfer. The net result depends on the turbine response and whether the HRSG has usable thermal margin.
What happens if the stack temperature is already at its minimum?
Higher heat-transfer coefficient may not permit more recovery if the cold-end temperature limit controls. Check the stack-temperature requirement, pinch/approach, and cold-end materials before claiming additional steam output.
What happens if HRSG pressure loss is doubled?
The cited rough estimate is about 40% higher velocity and about 10% higher air-side heat-transfer coefficient, but neither value is a guaranteed design outcome. Confirm both with a preliminary HRSG thermal and pressure-drop model.
When should we stop the study and call official support?
Stop before changing a design or operating limit if the turbine discharge-pressure margin, trip setting, stack-temperature floor, fouling basis, or cold-end material limit is unclear. Ask the gas-turbine OEM and HRSG designer to confirm those limits and review the coupled performance and mechanical design.