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.