Fuel Atomization Principles
The critical first step in liquid fuel combustion: transforming bulk liquid into a combustible mist.
Liquid fuels (Light Oil, Heavy Fuel Oil) cannot burn in their liquid state. They must first be vaporized.Atomization is the process of breaking the fuel jet into millions of tiny droplets (typically 20-100 microns), drastically increasing the surface area exposed to heat and air, enabling rapid vaporization and stable combustion.
1. Pressure Jet Atomization (Mechanical)
Working Principle
The most common method for light oil and smaller heavy oil burners. Fuel is pumped at high pressure (typically 7-20 bar for light oil, up to 30 bar for heavy oil) through a nozzle with a tangential slot or swirl disk.
- Swirl Chamber: The fuel enters a spin chamber tangentially, creating high rotational velocity.
- Orifice: The centrifugal force pushes the fuel against the walls, creating a hollow core. As it exits the tiny orifice, the liquid film expands into a conical sheet.
- Breakup: Surface tension instability causes the sheet to tear into ligaments and finally into droplets.
Key Characteristics
- Simple, robust design (no moving parts in nozzle).
- Highly dependent on fuel viscosity (requires preheating for HFO).
- Turn-down ratio is limited (approx 1:3) because flow varies with the square root of pressure.
2. Rotary Cup Atomization
Working Principle
Ideal for high-viscosity heavy oils and dirty fuels. A high-speed rotating cup (3000-5000 RPM) spins the oil into a thin film.
- Centrifugal Force: Oil flows onto the inner surface of the spinning cup and spreads to the rim.
- Primary Air Shear: As the oil film flies off the rim, a high-velocity air stream (Primary Air) from an annular nozzle hits it, shearing the film into fine droplets.
Key Characteristics
- Can handle very high viscosity fuels (up to 45-50 cSt at nozzle).
- Excellent turn-down ratio (up to 1:10) as atomization is independent of oil pressure.
- More complex mechanical maintenance (high-speed motor/belt).
3. Steam/Air Assisted Atomization (Y-Jet)
Working Principle
Uses a secondary fluid (compressed air or steam) to impact the fuel stream and shatter it into droplets. Common in large industrial boilers and marine applications.
- Internal Mixing (Y-Jet): Oil and steam mix inside the nozzle before exiting.
- External Mixing: Steam hits the oil jet after it exits the nozzle.
- Energy Transfer: The expansion energy of the compressible fluid (steam/air) does the work of atomization.
Key Characteristics
- Best atomization quality for largest capacities.
- Low oil pressure requirements.
- Consumes steam (operating cost) or compressed air.
- Excellent turn-down ratio.
Comparison Summary
| Method | Best For | Turndown | Complexity |
|---|---|---|---|
| Pressure Jet | Light Oil, Small/Medium HFO | Low (1:3) | Low |
| Rotary Cup | Heavy/Dirty Oils, Large Boilers | High (1:10) | High |
| Steam Assisted | Very Large Boilers, Marine | Very High | Medium |