Definition
Gas-air heater and gas-gas heater
GAHs preheat combustion air with flue-gas heat (like an air heater). GGHs transfer heat between two flue-gas streams, typically reheating scrubbed gas before the stack.
- Subject
- HRSG and gas path
- Also known as
- GAH, GGH, gas-gas heater
A gas-air heater (GAH) transfers heat from hot flue gas to combustion air. A gas-gas heater (GGH) transfers heat between two gas streams, usually hot untreated flue gas and cooler treated flue gas. The names are plant-specific, but both describe heat exchangers in dirty gas service.
Where they appear
GAH language is common in cement, metallurgical and some industrial boiler systems where the unit preheats air using exhaust gas. GGHs are common around wet flue-gas desulphurisation and some SCR layouts, where cleaned gas must be reheated before the stack to improve plume rise, avoid visible condensation or reach catalyst temperature.
Main designs
Rotary regenerative heaters use a slowly rotating matrix that alternately passes through hot and cold gas streams. Tubular or plate heat exchangers keep streams separated by metal surfaces. Heat-pipe or coil systems may be used where leakage must be minimised. Each design trades leakage, fouling tolerance, footprint, pressure drop and maintainability.
Failure modes
Common problems include fouled baskets or tubes, acid dew point corrosion, ammonium salt deposits, seal leakage, cold-end plugging, erosion, uneven gas distribution, drive failure on rotary units and high pressure drop. GGH leakage can contaminate cleaned gas with untreated gas, while GAH leakage can disturb combustion air balance.
Cleaning relevance
Deposits often form where gas cools through sticky temperature ranges. Sonic horns can help with dry ash or friable salt deposits in accessible casings, but wet, acidic or condensed deposits may need washing, material upgrades or temperature-control changes. The cleaning plan must consider whether the surface is rotary, fixed, fragile or corrosion-limited.
Performance checks
Useful checks include gas inlet and outlet temperatures, pressure drop on both sides, leakage tests, drive current for rotary heaters, seal condition, washing effectiveness and corrosion inspection at cold-end surfaces. If a heater is downstream of ammonia injection, ammonium salts should be considered when pressure drop rises. If it is downstream of wet scrubbing, droplet carryover and acid condensation may dominate. Acoustic cleaning is most credible when deposit samples are dry enough to fracture or shed.
Operating variables and fouling risk
Gas-air heaters and gas-gas heaters recover heat across dirty gas streams, so performance depends on temperature approach, leakage, pressure drop, ash loading, acid dew point, seal condition, basket or tube geometry and the cleaning system fitted. In rotary units, seal leakage can mix gas streams and change oxygen or moisture balance. In tubular units, ash can bridge between tubes or collect in low-velocity lanes.
Cold-end fouling is a common limitation. Sulphur trioxide, ammonia slip, chloride salts or moisture can create sticky deposits that increase pressure drop and corrode metal. Operators track inlet and outlet temperatures, differential pressure, leakage, fan load, sootblower history and wash-water results. Acoustic cleaners can help with dry ash on baskets, ducts or nearby hoppers, but they are less effective once ammonium salts or acid-bound deposits have formed a sticky layer.
Maintenance checks
Inspection should cover seals, baskets or tubes, wash nozzles, drains, access doors, sootblower alignment and casing leakage. A heater that is clean on average can still have a plugged sector or a leaking seal that drives local cold-end corrosion.
Related terms
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Related terms
4 terms
- Air heaterAn air heater (also air preheater, APH) recovers low-grade heat from flue gas to preheat combustion air. Cold-end fouling and corrosion are the dominant operational challenges.
- Ljungstrom air preheaterA Ljungstrom air preheater rotates heat-transfer baskets between flue gas and combustion air. Cold-end fouling, ABS, corrosion and leakage drive maintenance.
- Electrostatic precipitatorAn ESP removes particulate from flue gas by charging dust and collecting it on plate electrodes. Sonic horns are widely used to dislodge ash from plates and to keep hoppers from bridging.
- Selective Catalytic ReductionSCR reduces NOx by injecting ammonia upstream of a catalyst. Temperature, mixing, catalyst condition and ash control determine performance.
References