Waste Heat Recovery Boiler (WHRB)
Recovering flue gas heat from DG, kilns, incinerators and process gas.
WHRBs recover heat from high-temperature exhaust streams (DG sets, gas turbines, sponge iron kilns, incinerators) to raise steam without additional fuel — a major energy-efficiency lever.
How it works
- 1
Hot flue/exhaust gas enters the WHRB inlet plenum.
Exhaust from a DG set, gas turbine, sponge-iron kiln, incinerator or furnace enters the inlet plenum at 400–900 °C. The duct includes an expansion joint, refractory lining and often a dust knock-out or spark arrestor. Gas velocity and distribution across the coil face are engineered to avoid erosion on one side and dead zones on the other.
- 2
Gas flows over evaporator, superheater & economizer coils.
Gas then passes in sequence over superheater, evaporator and economizer coils — hottest gas meets the hottest water/steam surface to keep the temperature difference, and therefore heat transfer, high all the way through. Finned tubes are used only where the gas is clean; dusty kiln or incinerator gas needs bare tubes and generous pitch so ash cannot bridge.
- 3
Feed water is heated, evaporated and superheated by cross-exchange.
Feed water from the deaerator is preheated in the economizer, evaporated in the evaporator bank (natural or forced circulation through the drum), and dried/superheated in the superheater. No fuel is burned at all — every kilogram of steam is recovered energy, which is why a WHRB typically pays back in 1–3 years.
- 4
Cooled gas exits via stack; steam feeds the process/turbine.
Cooled gas at 160–220 °C leaves via the ID fan and stack. The outlet temperature is deliberately held above the acid dew point (roughly 130–150 °C for sulphur-bearing gas) so that sulphuric acid does not condense on the cold-end tubes; pushing for a few extra degrees of recovery can destroy the economizer.
- 5
Bypass damper allows operation independent of source load.
A three-way bypass damper lets exhaust go straight up the stack when the WHRB is off, under maintenance, or when the source must keep running independently. Interlocks force the bypass open on low drum level, high steam pressure or ID fan trip so the coils are never dry-fired.
Key components
Refractory-lined duct with expansion joints and a three-way diverter/bypass damper with position feedback and fail-safe actuator. Damper seal leakage is a hidden energy loss and a safety issue — stroke-test monthly.
The main heat transfer surface where feed water boils. Natural circulation via drum, downcomers and risers, or forced circulation with a pump on compact units. Tube erosion from ash-laden gas and waterside scaling are the two life-limiting mechanisms.
First bank in the gas path, exposed to the hottest gas. Raises steam above saturation for turbine use or long-distance distribution; needs a spray attemperator and careful metallurgy (T11/T22) since it sees the highest metal temperature.
Last coil in the gas path, preheating feed water and dropping stack temperature. Most vulnerable to cold-end corrosion and fouling; feedwater inlet temperature is deliberately kept high (via deaerator or recirculation) to protect it.
Steam or compressed-air retractable/rotary blowers that periodically clean ash from the coils. Skipping soot blowing shows as rising ΔP across the banks, rising stack temperature and falling steam output — the classic WHRB performance decay.
Separates steam from water, houses the internals, feed distributor, chemical dosing and blowdown connections, and provides the water inventory that buffers the swings in exhaust flow from the upstream source.
Typical operating parameters
Problem solving videos
Safety notes
- •Bypass damper interlocks — protect against dry-firing.
- •Expansion joint inspection — hot gas leak risk.
- •Low-water trip must be tested regularly.
Maintenance schedule
- •Daily: soot blower operation, drum level, ΔP across banks.
- •Monthly: damper stroke test, expansion joint check.
- •Annual: internal inspection, tube thickness, refractory.
Problem solving matrix
| Issue | Likely cause | Fix |
|---|---|---|
| Steam output drop | Tube fouling / low gas flow | Soot blow, verify source |
| High stack temp | Fouled economizer | Offline clean, inspect coils |
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