Air-atomizing (two-fluid) nozzle: internal-mix and external-mix air caps, siphon feed and the air-to-liquid ratio

01

Internal-mix or external-mix cap: liquid viscosity and whether air and liquid can be set apart

EXAIR's catalogue draws the line at viscosity. Its internal-mix nozzles mix liquid and compressed air inside the nozzle for fine atomization, need both feeds pressurized, and are rated for liquids up to 300 cP. Its external-mix nozzles bring the two streams together outside the cap with independent air and liquid control for a precise volume flow, and are listed for liquids above 300 cP and for high-volume spraying of heavy coatings. [1]

Inside an internal-mix body the air and the liquid share the same passages, so one supply changes the other. In a tangential-air internal-mix atomizer tested by Yadav and Kushari, water flow at a fixed 68.9 kPa liquid pressure fell from 9.9 g/s to 3.02 g/s as air pressure was raised from 137.8 to 344.73 kPa; the authors attribute this to air taking up more of the flow area inside the atomizer. [2]

Editorial calculation: (9.9 − 3.02) / 9.9 ≈ 0.69, so in that test about 69% of the liquid flow was lost by raising the air pressure alone. With an internal-mix cap, a flow-rate figure is only meaningful when quoted together with both the air and the liquid pressure. [2]

EXAIR also lists no-drip versions of both cap types that positively stop liquid flow when the compressed air is shut off. [1]

State the liquid's viscosity at spraying temperature, whether air and liquid flow must be adjustable separately, and whether the spray must stop without dripping when the air is switched off.

02

Air-to-liquid mass ratio against Sauter mean diameter

The air-to-liquid mass ratio (ALR) is the main control on droplet size in a two-fluid nozzle. Yadav and Kushari ran their internally mixed atomizer from an ALR of 0.0277 to 0.623 and measured with a phase Doppler analyser that droplet size fell as ALR rose at a given liquid pressure; at the centreline, 50 mm below the tip, the Sauter mean diameter (SMD) dropped from 158.5 to 49.9 µm as air pressure went from 137.8 to 344.73 kPa. [2]

The same study reports sprays with centreline SMD below 60 µm already at an ALR of about 0.1. At an ALR of 0.269 the SMD was 60–120 µm across the central ±10 mm of the spray, with much larger droplets at the edges where velocities were lower, so a single SMD figure for a nozzle depends on where in the spray it was measured. [2]

Kim and co-authors, studying the mixing chamber of an internal-mix atomizer by laser diffraction, found that chamber length changed the internal flow pattern and the SMD, and that at air/water mass ratios below 0.2 the SMD became smaller as the liquid mixed more fully with the air inside the chamber. [3]

Ikeuchi notes for its BIMV flat-spray pneumatic nozzle that the spray distribution also depends on the air-water ratio: mountain-shaped at a low ratio, becoming even as the ratio rises. [4] Ikeuchi's catalogue grades its pneumatic nozzles by droplet class: dry fog below 10 µm, fine fog 10–100 µm and fine drizzle 100–300 µm. [4]

Editorial calculation: a nozzle spraying 1 L/min of water (about 16.7 g/s) at an ALR of 0.1 needs about 1.67 g/s of air; at an assumed air density of 1.2 kg/m³ that is about 1.4 L/s, or roughly 83 L/min of free air, before any allowance for pattern air or leakage. Doubling the ALR doubles the compressor load. [2]

03

Siphon and gravity feed when no liquid pump is available

Where there is no liquid pressure, the air stream itself can draw the liquid. EXAIR's siphon-fed air-atomizing nozzles work with gravity-fed liquids or lift liquid from as much as 36 in (91 cm) below the nozzle, are rated for viscosities up to 200 cP, and are aimed at thin coatings. [1]

A pressure-fed nozzle gives a wider operating window. Ikeuchi's BIMV pressure-fed flat-spray nozzle is specified for a mean droplet size of 100 µm or less, with a large turn-down ratio at liquid pressures of 0.1–0.3 MPa, spray capacities of 0.017–3.3 L/min across the range, air consumption of 7–245 L/min, spray angles of 45–110°, free passage of 0.2–2.2 mm, and Rc 1/8 or Rc 1/4 connections. [4]

The 0.2 mm lower end of that free-passage range means strainers on both lines are part of the specification for the smallest caps. Air consumption also belongs in the comparison: across the BIMV range it spans a factor of 35 (editorial calculation: 245 / 7). [4]

04

Data to send for a two-fluid nozzle

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Liquid: [name; viscosity at spray temperature; solids or fibres]
Feed: [pressure-fed (liquid pressure) or siphon/gravity (lift height)]
Air: [available pressure; free-air volume per nozzle]
Spray: [flow per nozzle; target droplet size and how it is measured; pattern and angle]
Mixing: [internal or external; no-drip shut-off needed]
Mounting: [thread size; distance to target; materials]

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