The first fork in any air atomizing project is not brand or price — it is mix geometry. Before you compare flow rates or droplet charts, you have to decide whether air and liquid meet inside the body or outside it. That single decision sets the floor on your droplet size and the ceiling on what fluid you can run. Get it wrong and you either waste air chasing fineness you cannot reach, or you clog a body that was never meant for your liquid.
The question that decides everything
Ask one thing first: can my liquid tolerate being pressurised and mixed inside a small chamber with the air stream? If yes, internal mix. If no — if it is viscous, loaded with solids, or supplied at near-zero pressure — external mix. Everything else is detail on top of that answer. An air atomizing nozzle is just a way to shear liquid with air; the geometry of where that shear happens is the whole game.
How internal mix works
In an internal-mix air atomizing spray nozzle, air and liquid combine inside the nozzle body and exit through a single orifice already atomised. Because the shear happens in a confined, pressurised space, internal mix produces the finest droplets for a given air volume and the highest atomisation efficiency. A small internal-mix unit in our range puts 10 to 25 micron droplets on the table at 0.5 to 6 L/h of liquid — the fine end of what air atomizing can do.
The constraint is interaction. The liquid line sees back-pressure from the air, so the two flows are coupled: change one and you disturb the other. That is fine for clean, low-viscosity liquids pumped at a stable pressure. It is a problem for anything that must not be pressurised, anything that sets up or gels, and anything with grit that can lodge in the mixing chamber. Internal mix also demands stable air and liquid pressure, because a sag in either shifts the ratio and the pattern together — there is no independent trim.
How external mix works
External mix keeps the air and liquid streams separate until they meet just outside the cap. Droplets are slightly coarser for the same air volume, but the two flows are independent — you can set liquid flow and air flow on separate valves and they stay put. More importantly, the liquid never has to survive a pressurised mixing chamber, so external mix shrugs off higher viscosity and mildly abrasive slurries, and it runs happily on gravity feed or a very low-pressure pump.
A mid external-mix unit in our range handles 2 to 40 L/h of liquid at about 7 Nm³/h of air, landing 25 to 60 micron droplets. Coarser than internal mix at the same air, but it will keep running on fluid that would choke an internal-mix body in an afternoon.
Viscosity is the deal-breaker
This is the variable that settles most arguments. As liquid viscosity rises, droplets coarsen and you need more air to recover fineness. Internal mix reaches a viscosity wall where the coupled flow simply will not atomise cleanly. External mix climbs that wall more slowly because the liquid is not fighting back-pressure inside the body. Past a certain viscosity — heavy oil, syrup, slurry, polymer solution — external mix is not the better choice, it is the only choice. Heating the fluid to drop viscosity usually buys more fineness than adding air pressure does, whichever geometry you run.
Back-pressure and what your fluid can tolerate
Some duties cannot pressurise the liquid at all. A fragile suspension, a shear-sensitive emulsion, a line that must drain by gravity — none of these belong in an internal-mix chamber. External mix lets the liquid arrive at near-atmospheric pressure and meet the air at the cap. If your process already pumps the liquid at a few bar, internal mix is on the table; if it does not, do not force it. This is also why internal mix needs a steadier liquid supply than people expect: the back-pressure from the air can stall a weak pump, and then the ratio wanders.
Abrasion and slurry
Solids in the stream are the other wall. Internal-mix chambers and small mixing orifices are unforgiving of abrasive particles — they erode, then they drift off-spec, then they clog. External mix puts the wear at the cap face, which is easier to inspect and replace, and the larger liquid passage tolerates more solids before it chokes. For anything described as a slurry, a suspension, or “a bit dirty,” external mix is the safe default.
The fineness trade-off
If your process genuinely needs sub-25 micron droplets — fine humidification, tight spray-drying yield, delicate coating — internal mix is where that lives. If 30 to 60 microns is acceptable, external mix gets you there with far fewer fluid restrictions. Do not pay for internal-mix fineness you do not need by swallowing fluid limitations you do. A useful check: take your worst-case fluid on the hottest day, and see which geometry still atomises it. Design for the worst case, not the datasheet.
A worked example on the same fluid
Take a 20 cP coating liquor, pumped at 2 bar, needed at about 15 L/h per nozzle across a line of 30 nozzles. Internal mix would give the finest film, around 15 to 25 microns, but the coupled flow means any pressure sag on the weak pump drifts the ratio and the film thickness with it. External mix gives 30 to 50 microns, independent liquid and air trim, and shrugs off the pump sag. If the film spec tolerates 30-plus microns, external mix is the reliable pick and the maintenance is simpler. Only if the process truly needs sub-25 microns do you justify internal mix and the steadier, pressurised feed it demands.
The pressure-sag failure mode
A classic internal-mix complaint is “it was fine at commissioning, now it sprays coarse.” Nine times out of ten the air pressure at the cap has sagged under load as more nozzles were added to the same header. Because internal mix couples the flows, that sag moves both air and liquid behaviour at once, so the spray degrades faster than you would expect. Meter atomizing pressure at the cap, not the compressor, and size the header so it holds. External mix tolerates the same sag more gracefully because the liquid valve is independent — but it still coarsens, just less dramatically.
Adjustable variants for changing recipes
When the liquid or the target fineness changes shift to shift, an adjustable air atomizing nozzle earns its keep. A needle or air-cap adjustment lets you tune pattern and flow in service without swapping hardware — the same body covers a family of recipes. A mid adjustable unit in our range spans 2 to 60 L/h of liquid at 5 to 12 Nm³/h of air, 25 to 70 micron droplets. The cost is a small loss of repeatability versus a fixed, flow-matched nozzle, so reserve adjustables for lines that actually change, and use fixed geometry where the recipe is stable.
A decision table
- Clean, low-viscosity liquid, pumped, need finest droplets → internal mix.
- Viscous, slurry, abrasive, or gravity-fed → external mix.
- Sub-25 micron mandatory → internal mix if the fluid allows it.
- Recipe changes routinely → adjustable external mix.
- Liquid must not be pressurised → external mix.
- Weak or unstable liquid pump → external mix.
The model-by-model comparison, with mix type, flow, air and droplet figures, is on the BoreJet air atomizing nozzles page. If your fluid is awkward — high viscosity, solids, or a shear-sensitive emulsion — send the properties to our application team and we will point at the geometry that will actually keep running.
Frequently asked questions
Which gives finer droplets, internal or external mix? Internal, for the same air volume — typically 10 to 25 microns at the fine end versus 25 to 60 for external. The price is fluid tolerance.
Can I run external mix on gravity feed? Yes. That is the main reason to choose it — the liquid meets the air at the cap and never has to be pressurised.
My viscous fluid clogs internal mix. What now? Move to external mix and, if you can, heat the fluid to drop viscosity. Heating usually helps atomisation more than raising air pressure.
Why does internal mix drift when I add nozzles? Because the flows are coupled and air pressure sags at the cap under load. Meter pressure at the nozzle and size the header so it holds.
When is an adjustable nozzle worth it? Only when the recipe or target fineness changes often. On a stable line, a fixed flow-matched nozzle repeats better.