Pinch Point in a Heat Recovery Steam Generator: The Temperature Gap That Sets Efficiency

What the Pinch Point Is and Where It Lives

A heat recovery steam generator is a gas-to-water/steam heat exchanger that captures the 500–650°C exhaust of a gas turbine and boils feedwater to drive a steam turbine — the 'bottoming' half of a combined-cycle plant that lifts overall efficiency from ~38% (simple cycle) to ~60% (modern combined cycle).

Inside, the gas flows past three sections in series: an economizer (preheats water), an evaporator (boils it at constant temperature), and a superheater (raises steam above saturation). Plot gas temperature and water/steam temperature against cumulative heat transferred and the two curves come closest at the cold end of the evaporator — where the boiling plateau is flattest and the gas has already given up most of its heat.

• That minimum vertical gap is the pinch point, ΔT_pp.
• It is where the second-law driving force for heat transfer is smallest.
• It sits at the evaporator outlet on the gas side / saturation line on the water side.

How It Works: The T–Q Diagram and Heat Balance

Heat only flows from hot to cold, so at every point the gas must stay hotter than the water. The pinch point is the tightest spot on that constraint. Fix ΔT_pp and you fix the gas temperature leaving the evaporator, which fixes the heat available to boil water — and therefore the steam flow.

The evaporator heat balance is:

• m_gas · c_p,gas · (T_gas,in − T_gas,pinch) = m_steam · (h_g − h_f)
• where T_gas,pinch = T_sat + ΔT_pp
• m_gas = exhaust mass flow, c_p,gas ≈ 1.05–1.15 kJ/kg·K
• h_g − h_f = latent heat of vaporization at drum pressure

Solve for steam production:

• m_steam = m_gas · c_p,gas · (T_gas,in − T_sat − ΔT_pp) / (h_g − h_f)

Because ΔT_pp subtracts directly from the numerator, shrinking it lifts steam output. The catch is on the area side: heat-transfer area A ≈ Q / (U · ΔT_LM), and as ΔT_pp → 0 the log-mean temperature difference collapses, so A → ∞. That hyperbolic cost is why zero pinch is impossible.

Key Quantities and a Worked Example

Take a single-pressure HRSG behind a mid-size gas turbine, drum at 70 bar (T_sat ≈ 285.8°C, latent heat h_g − h_f ≈ 1505 kJ/kg):

• Exhaust: m_gas = 500 kg/s, T_gas,in to evaporator = 400°C, c_p,gas = 1.10 kJ/kg·K
• Choose ΔT_pp = 10°C → gas leaves evaporator at 285.8 + 10 = 295.8°C
• Q_evap = 500 · 1.10 · (400 − 295.8) = 57,310 kW
• m_steam = 57,310 / 1505 = 38.1 kg/s

Now tighten to ΔT_pp = 6°C: gas leaves at 291.8°C, Q_evap = 500 · 1.10 · (400 − 291.8) = 59,510 kW, m_steam = 39.5 kg/s — about 3.7% more steam. But since ΔT_LM roughly halves, the evaporator surface area nearly doubles. That is the pinch-point trade in one calculation.

Choosing the Pinch in Practice

Pinch selection is an economic optimization, not a fixed number. Designers run the plant's expected duty hours against fuel price and steam value, then pick the ΔT_pp that minimizes lifetime cost.

• Unfired combined-cycle HRSGs: 8–15°C is standard. Merchant plants chasing efficiency push to 5–8°C.
• Cogeneration / process heat: often 15–25°C, since steam quantity matters less than reliability and low cost.
• Multi-pressure HRSGs (LP/IP/HP drums) apply a pinch at each pressure level to squeeze the tail of the gas curve and cut stack temperature to 70–110°C.

The approach point (economizer subcooling, T_sat − T_water,out) is set independently — typically 5–15°C — to keep the economizer from flashing to steam during off-design or low-load operation, which would block flow and cause hammer. Fin density, tube rows, and gas-side pressure drop (target < 25–35 mbar) are then tuned to hit the chosen pinch without starving the gas turbine's back-pressure budget.

How It Compares to Approach Point and Effectiveness

The pinch point is easy to confuse with its cousins, but each guards a different limit:

• Pinch point — gas vs. saturation at the evaporator cold end; sets how much steam you can make.
• Approach point — saturation vs. economizer-outlet water; a safety margin against economizer boiling, not an efficiency lever.
• Superheater approach — gas inlet vs. final steam temperature; governs steam superheat and turbine inlet conditions.
• Effectiveness (ε = Q_actual / Q_max) and NTU characterize the whole exchanger, whereas the pinch is a single local constraint.

Unlike a shell-and-tube exchanger where a single ΔT_LM suffices, a boiling section makes the temperature profile pinch internally — the limiting ΔT is not at either terminal but midway, which is exactly why the pinch analysis (Linnhoff's pinch technology, 1980s) is needed. The same idea generalizes to any process with a phase-change plateau, from ORC waste-heat units to LNG cold-boxes.

Failure Modes, Trade-offs, and Why It Matters

Get the pinch wrong and the consequences are real hardware problems, not just spreadsheet errors:

• Too small a pinch: huge, expensive evaporator; higher gas-side pressure drop robs the gas turbine of power; fouling or fin degradation erodes the razor-thin margin and can violate the second-law constraint, stalling heat transfer.
• Too small an approach: economizer steaming during startup or low load — two-phase flow causes water hammer, tube vibration, and thermal fatigue cracking at headers.
• Too large a pinch: gas leaves the evaporator hot, stack temperature rises, and 1–3 efficiency points are thrown away up the chimney.

Because a single point of combined-cycle efficiency on a 400 MW plant is worth millions of dollars per year in fuel, the pinch point is one of the most scrutinized numbers in HRSG design. It is the crisp, quantitative expression of the fundamental tension in all heat recovery: you can always recover more heat by adding area, but never all of it, and never for free.