Air Knife

Air Knife Technology

Compressed air, typically 90 to 100 psi, is converted to a supersonic jet while flowing through a nozzle especially designed for the purpose. The maximum jet velocity that can be achieved is determined by the pressure available from the compressor. Exit velocities in the range of mach 1.6 to mach 1.7 are typical for most portable compressors. Since the determining limit on mach number for the exiting jet stream is the available pressure, higher mach numbers can only be achieved by using higher compressor pressures.

Since the emerging jet stream diameter is the same as the nozzle exit diameter, the air stream is initially the same diameter as the nozzle exit. For this reason, some refer to this characteristic as being laser-like. But as soon as the stream leaves the nozzle, it expands concentrically, since it is surrounded by atmospheric air. High speed video shows this rapid expansion, but it also shows that this high velocity air penetrates the ground to a depth of about a foot, creating a momentary cavity of about a foot in diameter, in which the dirt is crumbled. As the jet leaves that location or the air blast is ended, the dirt falls back on itself if the tool barrel is held close to the vertical. If the Air Knife barrel is inclined away from the user, the dirt can be blasted out the ground to a depth of one to two feet, depending upon technique. Since buried pipes, cables and tree roots are not porous, the dirt is removed from them and they are not damaged.

Air Knife Operation

There is an integral air gage on the Air Knife. By observing the gage pressure, the operator can determine if they have adequate pressure. The tool is designed for a compressor of 100 psi rating, which is typical, and ideal conditions occur if the pressure at the gage is 90 psi or more. Lower pressures will affect performance, and at 75 psi, operation will be markedly poorer. The nozzle is designed for 90 psi to allow for variations in compressor performance and pressure drop in the hose.

The operator should also hold the bottom of the barrel directly on the ground and move it quickly over the area being excavated. The tool will crumble the dirt as rapidly as the operator moves the tool. Holding the end of the barrel above the ground reduces digging effectiveness.

Soils vary considerably in strength, density, moisture, etc. This tool will work effectively with all soils, but the effectiveness can vary significantly, particularly with soil moisture. Hard clays and other very hard soils will be slow during the first several inches of depth, and will produce increased scatter. But somewhat deeper, where the soil has usually retained greater moisture, the scatter will reduce, and the excavation rate will improve.

In tree root applications, it is often feasable to irrigate the surface one day prior to the work. When moisture content is not close to zero because of irrigation or natural conditions, the excavation proceeds more rapidly.

Air Knife Safety

The operator can wear normal work clothes, however many operators prefer to wear coveralls. Ear, eye and hard hat protection is required. A face shield is desirable. Other normal safety precautions associated with compressed air must also be employed.

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