Flame Detection Physics
How burner management systems (BMS) reliably distinguish a live flame from background heat and radiation.
Reliable flame detection is the cornerstone of combustion safety. A flame detector must respond instantly to flame loss (typically within 1-3 seconds) to close fuel valves and prevent explosive accumulation of unburnt fuel. Different technologies exploit different physical properties of the combustion reaction.
1. Ionization (Flame Rod)
Physics Principle
Flame Rectification: A flame is not just hot gas; it is a plasma containing free ions and electrons. When an AC voltage is applied between a metal rod (flame rod) and the grounded burner nozzle, the flame acts as a rectifier.
- Ion Flow: The large ground area of the burner head compared to the small rod area allows current to flow more easily in one direction.
- Signal: The BMS detects this tiny rectified DC current (typically 2-10 microamps). If the flame goes out, the circuit opens instantly.
Application
- Best for: Gas burners (clean blue flame).
- Limitations: Cannot be used with oil (soot deposits short-circuit the rod) or high temperatures (rod melts).
2. Ultraviolet (UV) Detection
Physics Principle
Combustion reactions emit radiation in the UV spectrum (185-260 nm), specifically from OH- radicals in the flame front.
- UV Tube: A gas-filled tube with two electrodes. When a UV photon hits the cathode, it releases an electron, causing a temporary avalanche current (Geiger-Müller principle).
- Discrimination: The sensor is "solar-blind" and does not react to visible light or hot refractory brick (which emits IR, not UV).
Application
- Best for: Both Gas and Oil burners. Very versatile.
- Limitations: Spark ignition (also emits UV) can trick it. Smoke/oil mist can block UV. X-rays can trigger false positives.
3. Infrared (IR) Detection
Physics Principle
All flames emit heat (Infrared). However, hot walls also emit IR. To distinguish flame from background heat, IR detectors look for the flicker frequency.
- Flicker Analysis: Flames flicker at characteristic frequencies (typically 10-100 Hz). Glowing bricks are static (0 Hz).
- Lead Sulfide (PbS) Cell: A photo-resistor that changes resistance when exposed to IR radiation.
Application
- Best for: Heavy Oil, Coal, Waste incineration (where smoke blocks UV).
- Limitations: Must be carefully tuned to avoid picking up background heat or other burners.
Comparison Table
| Technology | Spectrum | Gas | Oil | Key Advantage |
|---|---|---|---|---|
| Ionization Rod | N/A (Electrical) | Excellent | No | Fail-safe, cheap |
| UV Scanner | 185-260 nm | Good | Good | Ignores hot walls |
| IR Scanner | Infrared | Fair | Excellent | Sees through smoke |