Oil Atomization Done Right: Avoiding Coke, Soot and Uneven Burn

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.

The discipline has four levers: viscosity at the nozzle, air-to-oil ratio, atomizing air quality and pressure stability, and the start-stop routine that bakes the cap or keeps it clean. This guide walks all four, then shows what changes when the same technology moves from a burner to an oil-mist lube system.

Why oil is a special atomization problem

An air oil lubrication system atomizes oil into a fine mist that travels through the air line to bearings, chains and slides. The same atomization physics as a burner, at a fraction of the flow: typical oil mist rates of 0.5–5 ml/h per lubrication point versus litres per hour for firing.

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.

The same three properties matter when the oil is a lubricant rather than a fuel, with the stakes inverted: instead of burning every droplet, you want each one to land where it lubricates and nothing to bake into varnish on a hot chain or bearing.

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.

Selection point	Pressure-jet nozzle	Air atomizing nozzle
Droplet size control	Fixed by orifice, swirl chamber and oil pressure	Adjustable through the air-to-oil ratio
Viscosity tolerance	Tight: a few cSt of drift coarsens the cone	Wide: air shear absorbs a wider band
Atomizing air supply	None needed	Compressor, dryer, header and regulator
Coke risk at the cap	Moderate, in the swirl chamber	Real, at the cap face: must be managed
Load turndown	Narrow, pattern degrades at the edges	Wide, re-set the ratio and it holds
Best duty	Light, clean, steady grades	Heavy, waste, variable and dirty grades

The flow-versus-pressure rule still applies: liquid flow through the oil orifice rises with the square root of oil pressure (Q ∝ √P), so doubling oil pressure does not double the rate, and on a burner that is the wrong lever anyway. Fineness comes from air and viscosity, not from forcing more oil.

The oil train around the nozzle

The nozzle is the last few millimetres of a system, and most oil atomization failures are upstream of it. A typical burner or lube train runs: day tank → strainer → heater (for heavy grades) → fine filter → pump → nozzle, with atomizing air as a separate parallel supply. Every element sets what the nozzle sees.

• Strainer and filter. Oil systems foul with tank scale, polymerized residue and, on waste-oil duty, solids that would lodge in a small orifice. A filter change is cheaper than a cap pull.
• Heater. Heavy grades are heated so viscosity at the cap lands in the atomizer’s working band. The heater must be sized for the coldest ambient start, not the running load, or the burner lights coarse on winter mornings.
• Pump and relief. Steady oil pressure matters because the ratio is set against it. A relief valve that hunts turns the flame into a heartbeat.
• Atomizing air supply. Separate from combustion air. It only shears the oil; it does not feed the flame. Its quality and stability decide droplet size.

Meter pressure at the nozzle, not at the pump or the compressor. The last metre of pipe is where pressure is lost, and it is the pressure at the cap that decides the spray.

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.

Coking is also a temperature story: thermal cracking of hydrocarbon films accelerates sharply above roughly 300–350 °C at the wetted metal, which is why a cap in the flame path cokes faster than one in a cool airstream. The fix is alignment and swirl, keeping the flame off the cap, rather than a different material. 316L resists the chemistry; only geometry and temperature control stop the 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.

The reason fineness is a combustion spec is geometry. Burning happens at the droplet surface, and specific surface scales as 6/d: a litre of oil as 1 mm droplets carries about 6 m² of surface, while the same litre atomised to 50 microns carries about 120 m²: twenty times the surface, and with it twenty times the rate at which heat can be released. Evaporation time scales with the square of droplet diameter, so a 50 micron droplet burns roughly four times faster than a 100 micron one. Every micron of SMD you buy is combustion margin you do not have to buy with excess air.

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.

Viscosity follows temperature along a predictable curve (the Walther/ASTM D341 relation plots log-log viscosity against temperature as a straight line). Two lab points on the grade certificate are enough to draw the curve, and from it you can read the preheat temperature that puts the grade at your target cSt at the cap. Typical published figures for common grades, confirmed against the actual certificate:

Fuel grade	Viscosity @ 50 °C (typical)	Preheat to reach ~15–25 cSt at the cap (typical)
Light fuel oil / diesel	2–5 cSt	None: ambient is fine
Light heating oil	5–12 cSt	40–70 °C
Heavy fuel oil, IFO 180	~180 cSt	~90–110 °C
Heavy fuel oil, IFO 380	~380 cSt	~110–130 °C
Waste / used oil	Varies with source	Heat to target cSt and filter

Two traps live in that table. First, the grade on the tank may not match the certificate: blend residues drift, and waste oil moves with every batch, so measure at the cap on a schedule. Second, over-heating is as bad as under-heating: push heavy fuel past the cracking temperature and it forms insolubles and gum before it reaches the nozzle. Stay inside the band.

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 lazy; too much and you dilute and cool the flame, burning air you pay for. The atomizing air is only part of the total, combustion air does the burning, but it is the part that decides droplet size. This is where atomizing pressure matters: hold it steady at the cap and the ratio holds; let it sag and the flame goes coarse with it.

As a working rule, most oil atomizing installations run an air-to-oil ratio by mass of 0.2 to 0.6 for burner duty. Record the ratio at the cap under full load. That is the value that reproduces the spray at the next start.

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, set the ratio for the droplet, and leave it.

Atomizing air quality and supply

Atomizing air gets less respect than it deserves because it is invisible. Three quality points matter:

• Pressure stability. A sagging header coarsens the spray before any gauge at the compressor notices. Size the header, add a receiver, and meter pressure at the cap.
• Dry air. Water in the atomizing air is a nuisance in combustion, it cools the flame slightly, and worse on lube mist duty: it condenses in the mist line and washes the oil film off the part. A dryer on the lube-air line is not optional.
• Oil-free air. Compressor carry-over oil is the silent killer: indistinguishable from the process oil on the cap, it carbonises the same way and is never in your nozzle spec. If the compressor is lubricated, filter the atomizing air and test the filter.

Fit a pressure gauge and regulator at the last nozzle, set the ratio against that gauge, and log it at commissioning: when the flame soots six months later, the first check is whether the cap pressure still reads what it read on day one.

Oil mist lubrication: the fine end of the lube job

The same air atomizing technology that fires a burner lubricates one. Oil mist lubrication, the “pure mist” or “micro-fog” family of systems, replaces recirculating oil with a continuous fine mist carried by low-pressure air to the lubrication point. The principle is simple: meter a small oil flow, atomise it into droplets small enough to stay suspended in the air stream, carry it to the bearing, chain or gear case, and let it condense onto the metal it meets.

• Micro-fog phase. Very fine droplets, typically low-single-digit microns, travel suspended in the header air and can run long pipe runs without wetting out.
• Reclassification. At the point of use, a reclassifier, or the nozzle itself, coalesces the fog into droplets of tens of microns that wet the surface. A nozzle that does both jobs is why a small internal-mix or adjustable unit is the natural mist generator.
• Positive pressure. The header runs slightly above ambient, so dust, moisture and tramp oil cannot migrate into bearing housings: one of the main reasons mist systems outlast flooded lubrication in dirty plants.
• Low consumption. Oil feed is metered in grams per hour per point rather than litres per shift, typical header pressures are 0.3–1.5 bar, so the lube cost and the waste stream both shrink.

What the buyer must check on a lube mist duty differs from a burner: droplet size at the point (too fine and the mist escapes instead of depositing, too coarse and it rains out early), air dryness, and the oil’s wetting behaviour: the same coking discipline as a burner, at lower temperatures and with the stakes measured in downtime rather than flame quality. An adjustable air atomizing nozzle earns its keep here, because chain speeds and oil grades change and the mist rate must follow.

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.

A fourth cause appears on waste-oil and blended duties: the grade changed at the tank. If the flame changes after a fuel delivery, check the certificate before the nozzle.

Burner, lube and coating are different duties

Duty	Droplet target (typical)	Geometry that fits	Success metric
Burner, light oil	10–25 µm	Internal mix	Clean light-off, no soot, stable flame
Burner, heavy or waste oil	25–60 µm	External mix	No coking, full burn-out of every droplet
Oil mist lube	5–30 µm at the point	Small internal-mix or adjustable	Even film, no varnish, low oil consumption
Coating with oil-based media	20–50 µm, repeatable	Adjustable external mix	Film thickness repeatability, no spits

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.

Start-up and shut-down sequencing

Most cap coking happens in the first and last thirty seconds of a run, when the oil is present but the conditions are not yet settled. A disciplined sequence removes most of it:

• Start: open the atomizing air first, confirm it is flowing, then bring on the oil. Oil meeting a live air stream atomises on arrival; oil dribbling onto a cold cap does not.
• Run: let the preheat settle before loading the burner; a cold start at full fire is how light-off soot gets baked onto the tile.
• Stop: cut the oil first, then let the atomizing air run a purge cycle long enough to blow the residual film off the cap face. Ten seconds of purge air is cheap; a coked cap is not.
• Batch duty: on burners that cycle, a pneumatic actuator makes the purge automatic instead of operator-dependent.

Troubleshooting an oil atomizing system

Symptom	Likely cause	Fix
Sooty, lazy flame	Oil cold at the cap, or atomizing air pressure sagged	Check cSt at the cap; meter pressure at the cap; then pull the cap
Coke on the cap face	Oil wetting the cap, cap in the flame path, ratio too low	Tighten air swirl, raise ratio, check alignment and insulation
Spitting or lopsided pattern	Partially coked cap, or eroded orifice	Pull and clean the cap; replace if the orifice is worn
Flame pulsates	Oil pump cavitation or relief valve hunting	Steady the oil supply; check the strainer for starvation
Dribble after shutdown	Liquid valve leakage, no purge cycle	Repair the valve; add purge air after oil-off
Uneven lube film on a chain	Mist too coarse or too fine for deposition	Resize the nozzle for the point; check feed rate and air dryness
Water in the mist line	Condensation in the air supply	Fit a dryer; drain the header low points

A selection checklist

• □ Viscosity at the cap measured and inside the atomizer’s working band, not assumed from tank temperature
• □ Atomizing air pressure steady at the cap under full load, logged at commissioning
• □ Air supply dry and oil-free, with the filter on a change schedule
• □ Air-to-oil ratio set for the target droplet size, not “as it came”
• □ Cap inspected on a schedule; coke cleaned before the pattern drifts
• □ Preheat setpoint re-checked when the grade or supplier changes
• □ Start/stop sequence includes purge air after oil-off
• □ Spare caps, gaskets and filters on hand before the failure, not after

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, raise the ratio, and check the start/stop purge, because most coking happens at the edges of the run.

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; the ASTM D341 curve on the grade certificate tells you the preheat temperature for your target cSt.

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.

What droplet size should I target? Light oil and lube duty: 10–25 microns, internal mix. Heavy or waste oil: 25–60 microns, external mix, where fluid tolerance matters more than ultimate fineness.

How do I know the viscosity at the cap without a lab? Use the grade certificate’s two viscosity points and the Walther relation to draw the temperature curve, then read the value at your measured cap temperature. Re-verify when the grade or supplier changes.

Can the same nozzle do burner and lube duty? The technology is the same, the geometry is not: burner duty needs a cone that burns clean, lube duty needs a mist that deposits evenly. Size each duty separately rather than stretching one unit across both.

What air pressure does oil atomization need? A couple of bar at the cap is the common working point; the exact number follows from the droplet target. Meter it at the cap, not the compressor.

The oil-rated models, with flow, air and droplet figures, are listed on the BoreJet air atomizing nozzles page; for the pressure-jet side of the duty, see the oil burner nozzle guide. Fighting coke or soot, or setting up a mist lube point? Send the oil grade and preheat temperature to our application team and we will size the air-to-oil ratio that stops it. For the wider picture, start with the air atomizing nozzles overview.