Controlling air preheater leakage
Air preheater leakage control is the work of minimising the air that bypasses combustion by leaking directly into the flue-gas side of a regenerative or recuperative air heater. Excess leakage dilutes the flue gas, loads the induced and forced draught fans, and masks the heater's true heat-recovery performance.
What it is
An air preheater recovers heat from flue gas to warm combustion air, but in regenerative types the rotating or static seals between the air and gas sides always leak some air across. Leakage control keeps that crossover to its design minimum through seal setting, monitoring and timely repair. The objective is to recover heat without parasitically pumping combustion air straight into the stack.
Why it is done
Air that leaks across the preheater never reaches the burner — it is heated, then carried out of the stack, wasting fan power and confusing combustion control because the measured flue-gas oxygen rises from dilution rather than true excess air. High leakage forces the draught fans to handle far more mass than the process needs, consuming electrical energy, and it lowers the gas-side temperature drop so the heater looks less effective than it is. Controlling it recovers fan power and restores honest combustion measurement.
How it is done
Leakage is quantified from the oxygen rise between the heater inlet and outlet on the gas side, giving a percentage that can be trended. Seals — radial, axial and circumferential on rotary units — are inspected and reset to their design clearances, with worn seal strips renewed. Differential pressure across the heater is monitored as a fouling and leakage indicator, and soot-blowing or washing is scheduled to keep the matrix clean. After seal work the oxygen-rise measurement is repeated to confirm the improvement, and the figure is tracked to flag drift.
- Quantify O2 rise
- Inspect seals
- Reset clearances
- Renew worn strips
- Monitor differential pressure
- Re-measure leakage
What to watch for
The classic error is reading flue-gas oxygen downstream of the preheater and trimming combustion to it, so leakage air is mistaken for excess air and the burner is run too lean. Letting seals wear without periodic resetting allows leakage to climb unnoticed until fan power and stack temperature give it away. Over-tightening seals against a warped rotor causes rubbing, heat and further damage.
Related practices
Running a compressed-air leak survey programme
Retrofitting waste-heat recovery
Retrofitting variable-speed drives
Related topics
Waste Heat Recovery in Industry: Methods and Where It Pays · Flue Gas Loss · Economiser · Boiler Efficiency
Common in: Power Generation · Cement · Steel & Metals · Chemicals