A pressure-containing heat‑exchange assembly placed in the exhaust path of a gas turbine (or other high‑temperature exhaust source) that transfers heat from flue gas to feedwater to generate saturated or superheated steam for a steam turbine or industrial process in combined‑cycle or cogeneration plants.

Principle

An HRSG converts otherwise-wasted exhaust enthalpy into usable steam by routing hot flue gas through staged heat‑transfer surfaces (economizer, evaporator, superheater) so that mass and temperature of exhaust determine steam production and pressure levels; multiple pressure levels and gas‑side/steam‑side arrangements set achievable steam parameters and thermal efficiency.

Demonstration

Illustrative scenario — Situation: A combined‑cycle power plant operates a gas turbine at rated load. Recognition: Plant control identifies continuous high‑temperature exhaust flow. Action: Exhaust is routed through the HRSG’s economizer, evaporator and superheater circuits; feedwater is pumped and boiled into steam at designed pressure levels. Consequence: Steam enters the steam turbine to generate additional power and raises overall plant thermal efficiency compared with a simple‑cycle gas turbine.

Misapplication

Treating any heat exchanger on an exhaust duct as an HRSG and assuming identical steam output. The error ignores required steam‑cycle components (steam drum, appropriate pressure stages, water treatment, design pinch points, and flue‑gas chemistry) and may lead to overestimating steam production or operating life.

Consequence

When correctly integrated, an HRSG increases combined‑cycle thermal efficiency and enables steam export to processes or a steam turbine; it also imposes requirements for water treatment, steam‑cycle controls, pressure vessel design, and maintenance to manage fouling, corrosion and fatigue.

Reversal

If exhaust temperature or mass flow is below the design threshold, an HRSG cannot produce useful steam; in such cases steam production may be infeasible without supplemental firing (duct burners) or the HRSG may be bypassed during start‑up or low‑load operation. Also, retrofitting an HRSG into an existing plant can be constrained by steam‑cycle integration limits.

Boundary

Clearly within: a staged heat‑exchange assembly integrated with gas‑turbine exhaust, steam drum(s) and blowdown/drainage, designed to deliver steam at specified pressures. Boundary case: a simple economizer that only preheats boiler feedwater but lacks evaporator/superheater sections — it recovers heat but is not a full HRSG. Clearly outside: isolated air‑to‑air recuperators or indirect heat recovery equipment not producing steam for a steam cycle.

Semantic Tension

Maximizing recovered heat (efficiency) versus minimizing complexity and capital cost: higher steam pressures and multi‑pressure HRSGs recover more energy but increase capital, controls and water‑chemistry constraints.

Synthesis

An HRSG is the steam‑cycle interface that converts gas‑turbine exhaust energy into controllable steam; its value is measured not by heat recovered alone but by how its pressure levels, materials and controls integrate with the steam turbine or process while managing water‑steam risks.