Oil is the awkward member of the atomization family. It is viscous, it heats and thins as you work it, and it loves to leave coke behind on any surface that runs hot. A liquid atomizer nozzle that sprays water cleanly all day will coke shut on fuel oil in a shift if you treat the two fluids the same. Oil atomization is a discipline, not just a nozzle choice — and the payoff is a burner that lights clean, a lube film that is even, and a coating free of spits and streaks.
Why oil is a special atomization problem
Three things make oil harder than water. It is more viscous, so it resists shearing into fine droplets and needs more air or more pressure to get there. It carries heat poorly at the film, so local hot spots bake residue onto the cap. And it is usually burned, which means any droplet that is too big or too slow becomes soot or unburned carry-over instead of heat. Air atomizing handles the viscosity problem better than a pressure jet does, because the air does the shearing rather than relying on oil pressure alone — but it hands you a coking problem you have to manage on purpose.
Air atomization versus pressure jet for oil
A pressure-jet (monarch) oil burner nozzle forces heated oil through a swirl chamber at high pressure to form a hollow cone. It is simple and needs no air supply, but it demands tight viscosity control and a clean oil, and it struggles with heavier grades. An air atomizing nozzle for oil uses a small air stream to shear the oil into a fine, even cone at modest oil pressure. That finer, more uniform spray lights faster, burns more completely, and tolerates a wider viscosity range — at the cost of an air supply and a coke-prone cap. For heavy fuel oil, waste oil, and any grade that fouls a pressure jet, air atomization is the route that keeps the burner alive.
The coking trap
Coke forms where hot gas meets wetted metal and the oil film bakes instead of burning. The usual culprits: oil hitting the cap face downstream of the orifice, a cap that runs too hot, and a droplet that is too large to fully vaporise before it lands. The defenses are mundane and effective — keep the oil hot enough to stay fluid but not so hot it cracks, keep the air swirl tight so no oil wets the cap, and size the air-to-oil ratio so droplets are small enough to burn in flight. A liquid atomizing nozzle with external mix is often the safer geometry for dirty or heavy oil, because the oil never sits in a pressurised chamber where it can bake.
Droplet size and combustion
Combustion quality tracks droplet size. Too coarse and droplets fall out as soot or puddle on the burner tile; too fine and you can over-atomise, cooling the flame and wasting air. For oil the target is a tight distribution of small droplets — that is why air atomizing wins, it gives you a controllable SMD rather than whatever the pressure jet happens to produce at your viscosity. An internal-mix air atomizer in the small range lands 10 to 25 micron droplets, which for light oil and lube duties is often the sweet spot; heavier oil moves toward 25 to 60 microns on external mix, where fluid tolerance matters more than ultimate fineness.
Viscosity and preheat
Viscosity is the lever you actually pull on the plant floor. Heat the oil and it atomises finer for the same air; let it cool and the same nozzle coarsens and cokes. Most heavy-fuel installations preheat to a target viscosity band at the nozzle, not to a fixed temperature, because the right temperature depends on the grade. Treat the viscosity at the cap as the real spec, not the tank temperature. Heating the oil almost always buys more atomisation quality than adding air pressure does, and it costs less in air. A practical band: keep heavy fuel in the low-single-digit cSt range at the cap, and you will find the atomizer behaves instead of fighting you.
Air-to-oil ratio
The air-to-oil (or atomizing-air-to-fuel) ratio sets droplet size and pattern. Too little air and the spray is coarse and the flame is lazy; too much and you dilute the flame, cool it, and burn air you pay for. The ratio also interacts with the burner’s combustion air, so the atomizing air is only part of the total — but it is the part that decides droplet size. Size it for the droplet you need, then let the combustion-air system do the burning. This is where atomizing pressure matters: hold it steady at the cap and the ratio holds; let it sag under load and the flame goes coarse with it.
A preheat and ratio worked example
Say a burner fires 40 kg/h of heavy fuel at roughly 4 cSt at the cap. At that viscosity an external-mix air atomizer needs about a 0.4 to 0.6 kg air per kg oil ratio to land a clean 30 to 50 micron cone. If the preheater only reaches 10 cSt, the same air gives 60-plus micron droplets and the flame soots — so the fix is preheat, not more air. Push the air ratio to 1.0 and you cool the flame and waste compressor capacity for no combustion gain. The lesson holds across grades: hit viscosity first, then set the air ratio for the droplet, and leave it.
Reading a sooty flame
Soot is a symptom, not a cause. A sooty, lazy flame on an air atomized burner usually means one of three things: the oil cooled and coarsened since commissioning, the atomizing air pressure sagged under load, or the cap has begun to coke and is throwing the pattern off. In that order, check viscosity at the cap, check pressure at the cap, then pull the cap. Most “the burner is dirty” calls are actually “the oil is cold” or “the air header sagged.” Fix the fluid and the air before you condemn the nozzle.
Burner, lube and coating are different duties
- Burner duty wants a fine, even cone that lights clean and leaves no soot. Internal mix on light oil, external mix on heavy or dirty oil.
- Lube duty wants a thin, uniform film on a chain or roll — low liquid flow, modest fineness, no streaks. Small internal-mix or adjustable units shine here.
- Coating duty with oil-based media wants repeatable film thickness and an air cap that will not coke between runs. Adjustable external mix, with heat if the oil is heavy.
The fluid is the same; the success metric is not, so the nozzle geometry and the air ratio follow the duty, not the grade alone.
Material and heating
Hot, coke-prone oil wants 316L stainless and a cap you can pull and clean, not a sealed body. Where the oil is heated in line, keep the heater upstream of the nozzle and insulate the last run so viscosity at the cap matches viscosity at the set point. A pneumatic atomizer with air actuation also lets you cycle the oil line on and off without a liquid valve, which matters on batch burners that fire and shut down repeatedly.
The oil-rated models, with flow, air and droplet figures, are listed on the BoreJet air atomizing nozzles page. If you are fighting coke or soot on a burner, send the oil grade and preheat temperature to our application team and we will size the air-to-oil ratio that stops it.
Frequently asked questions
Air atomizing or pressure jet for heavy fuel oil? Air atomizing, almost always. Heavy oil fouls a pressure jet; air shear handles the viscosity and gives a controllable droplet size.
Why does my burner coke the cap? Usually oil wetting the cap face, a cap running too hot, or droplets too large to burn in flight. Tighten the air swirl, hold viscosity at the cap, and raise the air-to-oil ratio.
Does heating the oil help atomization? Yes, markedly. Heating drops viscosity, which fines the spray more cheaply than adding air pressure. Control viscosity at the cap, not tank temperature.
Can I cycle an oil line on and off without a liquid valve? With a pneumatically actuated atomizer, yes — the air shuts the liquid line without a valve in the oil path.
My flame is sooty but the nozzle is new — why? Check viscosity at the cap and air pressure at the cap before blaming the nozzle. Cold oil or a sagging air header causes most soot, not a worn part.