Air Knife

Overview

An air knife is a precision nozzle that converts regulated compressed air into a thin, high-velocity curtain for drying, cleaning, and deflecting liquid and solid particles. The design is elegantly simple: a Plenum Chamber distributes air uniformly, then accelerates it through a Precision Slot ground to micron tolerance. At pressures of 5–8 bar, the exiting air reaches 80–120 m/s, faster than sound at sea level, creating an intense shock wave that peels liquid from surfaces or blows away contaminants. Air knives are ubiquitous in food packaging, automotive painting, print finishing, and wafer cleaning.

How it works

Regulated supply air enters the Inlet Port Assembly and flows into the Plenum Chamber, a high-volume cavity that evens out pressure across the width. The Plenum Divider Baffle baffle encourages uniform distribution so that no one part of the slot receives preferentially higher pressure than another. Air then enters the Precision Slot, a carefully machined converging passage (0.3–0.8 mm wide). As the flow area decreases, air velocity increases; by the time it exits the slot, the air has accelerated to near-sonic speeds.

The Slot Body is machined from aluminum or ductile iron to a tolerance of ±0.025 mm, ensuring that the slot width is uniform across its entire length (100 mm to meters, depending on product). The slot's Exit Lip, a sharp edge with radius < 0.05 mm, minimizes flow separation and shock losses. Just beyond the exit, a Diffuser Section with a gentle 10–15° half-angle expands the flow area. This divergence converts dynamic pressure back into static pressure, reducing back-pressure on the plenum and allowing efficient use of the supply air.

Applications

Drying: A water-coated surface is hit by the air knife jet. The shock-compressed air at the leading edge of the curtain peels the water layer, and the expanding low-pressure wake behind sucks the droplets away. Exit wetness on bottles, cans, or films is reduced from 10–50 mg/cm² to < 1 mg/cm².

Cleaning: Dust, lint, and light particles stuck to a surface are blasted loose by the high shear stress. The same knife often incorporates a recovery duct so expelled particles are immediately suctioned, containing them.

Deflecting: Light materials (labels, tissue) are blown sideways or upward by the knife, redirecting them without contact damage.

Sizing and control

Air knife capacity scales with slot length. A 500 mm knife at 6 bar might consume 800 L/min; a 3000 mm knife could draw 3000 L/min. Drying force varies roughly with pressure squared (doubling pressure quadruples force), so pressure regulators are essential for tuning performance. Some designs include Port Orifices to meter flow and reduce noise.

Noise is inherent: the air accelerating through the slot generates frequencies around 3–5 kHz. The Diffuser Section and downstream ducts provide some attenuation, but silencers on the exhaust are standard in enclosed applications.

Edge effects at the slot's ends are minimized by End Cap Assembly, which prevent air from spilling out the sides and ensure a uniform curtain across the full width.