What combustion control helps prevent oxygen-deficient conditions, especially during load changes?

• A. Automatic fuel limiting
• B. Cross-limiting
• C. Oxygen trim
• D. Load-following control

Answer: (B)

The idea being tested is how to keep the furnace in balance so there isn’t not enough oxygen when the load changes. Cross-limiting works by tying the fuel and air controls together so a change in one path is limited by the other. If the load step would push more fuel toward the burners, the air side is automatically constrained to ensure there’s enough oxygen available for that extra fuel. Conversely, if air flow changes, fuel is adjusted so the overall air–fuel ratio stays within safe bounds. This coordinated restraint helps maintain an adequate oxygen level in the furnace during transients, preventing oxygen-deficient conditions.

Automatic fuel limiting focuses on capping fuel but doesn’t ensure the air side follows suit to keep the proper ratio. Oxygen trim aims to hold a target oxygen level at the exit but may not address the dynamic coupling during rapid load changes. Load-following control drives the boiler output to meet demand but doesn’t specifically safeguard the combustion air–fuel balance under transients.