We Simulated an HRSG and a Fired Heater Convection Section. Here Is What the Numbers Say.
A short follow-up to our comparison of the two machines, this time with model output instead of principles.
In an earlier article we argued that an HRSG is, in engineering terms, a fired heater convection section scaled up, with a set of well-defined differences. Recently our team had the chance to test that claim directly: we built simulation models of a gas turbine HRSG and of a crude heater, using the same software, the same methods, and the same engineers we use to rate fired heaters every week.
Here is how the two convection sections compare, straight from the models:
| Parameter | Heater convection section | HRSG convection section |
|---|---|---|
| Flue gas pressure at convection entry | (-) 0.10 in. w.c. | (+) 5.39 in. w.c. |
| Total flue gas pressure drop | 0.272 in. w.c. | 2.349 in. w.c. |
| Flue gas temperature at convection entry | 1,700 F | 1,707 F |
| Flue gas exit temperature | 541 F | 298 F |
| Average heat flux (BOS) | 10,800 Btu/hr·ft² | 9,024 Btu/hr·ft² |
| Maximum flue gas mass velocity | 0.40 lb/sec·ft² | 1.06 lb/sec·ft² |
| Maximum tube metal temperature | 730 F | 920 F |
Five things the numbers say:
The hot ends are nearly identical. The heater's convection section receives flue gas at 1,700 F; the supplementary-fired HRSG's tube bank receives it at 1,707 F. This is the clearest demonstration of the point we made in the comparison article: at the convection section, the two machines meet. The heat flux confirms it, 10,800 against 9,024 Btu/hr·ft², the same order, the same discipline of fin selection, tube metal temperature, and fouling margin.
One is pulled, the other is pushed. The heater's convection entry sits at a draft of -0.10 in. w.c., pulled by the stack. The HRSG's sits at +5.39 in. w.c., pushed by the gas turbine. Same tubes, opposite sign. That single row explains why heater work is draft management and HRSG work is backpressure management.
The HRSG's pressure drop is real money. The HRSG bank takes 2.349 in. w.c. against the heater's 0.272, about 8.6 times more, because the HRSG packs far more surface into the gas path and runs 2.6 times the mass velocity through it. On a gas turbine, that pressure drop is backpressure. Using the common rule of thumb of roughly 1% of turbine output per 4 in. w.c., the HRSG bank in this model costs about 0.6% of turbine output, continuously. Every fin and tube-spacing decision in that bank is also a megawatt decision.
The HRSG chases the heat further down. The heater releases its flue gas at 541 F; the HRSG works the gas all the way down to 298 F, another 243 F of recovery, because steam generation gives it economizer duty a process heater does not have. That is where the HRSG earns its efficiency, and also where it meets the water and acid dewpoint questions we manage at the cold end of heaters and air preheaters.
The HRSG's tubes actually run hotter here. Maximum tube metal temperature is 920 F in the HRSG against 730 F in the heater. Steam superheat service runs hotter than this heater's process duty, and it is a useful correction to the instinct that the HRSG is always the gentler machine. Metallurgy, creep life, and tube metal temperature monitoring matter on both sides of the fence.
The tools transferred without modification. The same simulation software, rating methods, and design judgment we apply to fired heaters produced the HRSG model, which is the practical version of the argument we made in the comparison article: same physics, different machine.
Every unit is different, and the numbers that matter are yours. If this comparison raises questions about your HRSG or fired heater, or you need support with either, let us know. We are glad to help.
FIS provides independent engineering, audit, and revamp services for HRSGs and fired heat-transfer equipment, including thermal simulation of both. To scope a study, contact us.
Related reading: HRSGs and Fired Heaters: What Is the Same, What Is Different and Duct Burners in HRSGs: A Practical Introduction.