An air curtain only works if its discharge air jet reaches across the doorway opening, and that depends on the velocity profile leaving the nozzle, not just the rated volume flow.
The Jet Reach Principle
An air curtain projects a thin, high-velocity sheet of air down across a door. As the jet travels it entrains room air, grows thicker, and slows, so the velocity at the floor must still be enough to oppose the infiltrating outdoor plume. If the nozzle velocity is too low or the nozzle is poorly shaped, the jet breaks up before reaching the floor and the door leaks. A well-formed profile is fairly uniform across the width, with a sharp edge that resists mixing, because a ragged edge entrains room air too early and the jet dies short of the floor.
Discharge velocity is typically 5โ12 m/s at the nozzle for a doorway curtain, higher for cold-store or industrial doors where the density difference between inside and outside is large and the jet must fight both infiltration and buoyancy. The velocity is a means to an end: what matters is the residual velocity at the floor, which is why profiling beats quoting a single number.
The jet is only half of the equation; the pressure difference across the door is the other half. Wind, stack effect in a tall building, and unbalanced mechanical ventilation all push air through the opening, and a curtain sized for still conditions will be blown open by a few pascals of differential. A tall building in winter can develop 10 to 25 Pa across a ground-floor entrance from stack effect alone, which is why entrance curtains in high-rise lobbies need more discharge momentum than the same door in a single-storey shop. Balancing the building so the lobby is not held at negative pressure often does more for infiltration than upgrading the curtain.
Measuring and Adjusting
A pitot or thermal anemometer traverses the nozzle face to map the profile. Uniformity across the width should be within about ยฑ 15%; a dip in the middle lets infiltration through unchecked. Adjustments include nozzle angle (a slight outward tilt improves capture), fan speed, and straightening vanes that remove swirl, because swirl curls the jet back on itself and shortens its throw noticeably.
| Item | Specification | Note |
|---|---|---|
| Nozzle velocity | 5โ12 m/s | Doorway air curtain |
| Profile uniformity | ยฑ 15% | Across the width |
| Throw height | 2.5โ4.0 m | Typical door opening |
| Nozzle angle | 0โ15ยฐ outward | Improves capture |
| Swirl | Minimal | Straightening vanes used |
| Door pressure difference | ≈ 10–25 Pa | Winter stack effect, tall building |
| Sealing effectiveness | Commonly 60–80% | Against an unprotected opening |
The optimal angle is a trade: too vertical and the jet stays thin but weak at the floor; too tilted and it loses height. Field tuning finds the angle that just reaches the floor across the full width, and that angle differs for a 2.5 m shop door versus a 4 m industrial door even with the same unit.
Mounting geometry decides whether the measured profile ever reaches the doorway. The unit should sit as close to the opening as practical and cover the full clear width; a curtain 200 mm short at each side leaves two vertical gaps that behave like small open windows. Where several units are joined across a wide entrance the housings must butt together so no dead band forms at the junction, and all of them must run together, since one switched off makes a hole in the sheet. Noise limits how far discharge velocity can be pushed: entrance units are usually held near 55 to 65 dB(A) at a few metres, and a curtain that meets its throw but disturbs a reception desk will simply be switched off.
Optimization Checklist
- Traverse the nozzle: Map velocity before assuming the unit is wrong or the door leaks.
- Straighten the flow: Add or clean vanes to kill swirl that shortens throw.
- Tilt slightly out: A small outward angle improves floor capture without losing height.
- Match height: Size throw to the actual opening, not a nominal door dimension.
- Recheck after fit: A curtain mounted off-level reads differently across the width; verify on site.
- Check the door differential: Allow for stack effect and building pressure, not still-air conditions alone.
- Cover the full width: Butt multiple units together and run them as one so no vertical gap is left open.
Why the Profile Matters
A curtain with the right profile cuts infiltration, saving heating and cooling and keeping insects and dust out, while a poor one is an expensive decoration that still lets the door leak. Keruite designs air curtain nozzles for a uniform, low-swirl discharge so the jet reaches the floor at the rated opening height, and the unit performs as drawn rather than as hoped.
An air curtain is judged on whether a doorway stops feeling cold, which is a comfort outcome rather than a catalogue figure, so nozzle development needs measurement behind it. Units built in an 80,000 m² plant by a manufacturer with more than 30 years of air-moving experience are profiled in an in-house performance laboratory, so a stated throw height corresponds to a measured residual velocity at floor level. ISO 9001:2015 controls the nozzle and vane fabrication that determines uniformity, CCC certification covers the electrical build, and with deliveries into more than 50 countries units are configured for both retail entrance and cold-store duty.
Our engineers respond to selection enquiries within 24 hours, including nozzle velocity and throw sizing for air curtain applications.