Supersonic Air-jet Excavation

The CEG patented supersonic nozzle turns compressed air into a high speed, laser-like jet moving at twice the speed of sound, Mach 2. All of the energy and momentum of this air moving at approximately 1200 mph is focused onto the soil dislodging it in a fraction of a second. Soil is an unconsolidated assemblage of solid particles that may or may not contain organic matter, the voids between the particles being occupied by air and/or water. The aggregate nature of soil aids the ability of the air to fracture it; while stronger materials and ones that are not porous like metal or plastic pipes or cables or even wooden tree roots are unaffected.

Although exhaust nozzles for rocket engines have been designed and built for many years, supersonic air jet excavation nozzles are different. CEG has carried out extensive research into the aerodynamic design of its earth excavation nozzles. CEG has developed its own proprietary design method and CAD-CAM interface for its air jet excavation nozzles. Unlike propulsion nozzles, the energy to accelerate the air comes primarily from the release of its compression rather than from the combustion of a fuel. Because of their small size, particular attention must be paid to the effect of the boundary layer on the nozzle profile. Special tooling and computer-aided-machining is used to manufacture the nozzles. CEG continues to refine and improve its design through detailed mathematical modeling and laboratory experimentation. As trade secrets, CEG has developed the knowledge of what the parameters of the supersonic air jet need to be to dig effectively in the many different types of soil. CEG has carried out our own tests digging up simulated hazardous waste and military ordnance.

CEG Supersonic Air-jet vs. Other Air Jets

Soil fractures from stress, i.e. force per unit area, on its surface. As shown below for the same pressure and flow, compressed air exiting from a pipe nipple, orifice, or improperly designed nozzle expands outward rapidly to 3 to 4 times the area versus the jet from the patented CEG supersonic nozzle. The flow from these competitors can even go sub-sonic as indicated by the presence of a “Mach Disk,” which can be seen in the flow if the light is right.

In head to head tests, the CEG supersonic nozzle in the AIR-SPADE® dislodged harder clay soil and dug faster than other air digging tools. Shown below is a comparison test between the AIR-SPADE ® and two competitive air excavation tools. In the test, the first 3 inches of soil with a moisture content of 23% were excavated by each tool. The AIR-SPADE ® worked up to 50% faster in the harder soil. In fact, in one test the where the competitive tool could not finish digging its clay test plot, the AIR-SPADE ® did.

Relative time required to excavate 1 cubic foot.

Air Pressure Required

The standard CEG supersonic nozzles all are designed to work at 90 psig corresponding to a Mach 2 jet. Increasing the air pressure above 90 psig on a properly designed supersonic nozzle does not lead to a proportional gain in excavation capability. For example, doubling the nozzle pressure to 180 psig increases the air jet force by only 10% and the exit momentum flux (stress seen by the soil) by 45%. Supplying higher pressure to a nozzle designed to work at 90 psig actually un-focuses the air jet degrading performance and consuming more air.

Most portable air compressors are designed to deliver air at about 90 to 100 psig to run standard air tools such as paving breakers. Industrial workers are familiar with working with compressed air at this pressure. Working with compressed air at elevated pressures is costly and difficult. To adiabatically compress 100 cubic feet of air per minute in a single stage takes theoretically 18 hp at sea level. Doubling the pressure to 200 psig increases the power needed by 40% and correspondingly increases fuel costs. Working at pressures above 150 psig requires fittings other than the Air King Universal normally supplied with rental air compressors. Air hoses must also be rated for the elevated pressure and are subject to greater working stress.

Performance vs. Soil Characteristics

Because of its unique, focused air-jet, the CEG supersonic nozzle works in most soils, even hard clays. Cohesive soils can be classified and described by unconfined compressive strength as shown below. Tests have shown the CEG supersonic nozzle to be effective in compacted soils with unconfined compressive strengths well above the values listed for hard clay.

OSHA Cohesive Soil Classifications

Consistency and Unconfined Compression Strength of Clays

Soil texture can also be classified on a triangular diagram that shows composition as a percentage of clay, silt, and sand. Shown below is the U.S. Department of Agriculture standard chart (left) with the OSHA soil classifications superimposed (right.)

Watering the work area ahead of time can be helpful sometimes. Watering reduces airborne dust if the soil is extremely dry. It also reduces the soil’s strength making the digging easier. Combined use of the CEG supersonic nozzle with a low pressure water jet is effective even with extreme cases of highly compacted or sun-baked soils.

The CEG supersonic nozzle in general will not cut through rock, since its unconfined compressive strength as shown below is much greater than for soil. In fact, soil results from the physical and chemical breakup of weathered rock. Shales, however, may be broken apart by the supersonic nozzle if the jet is directed between the laminations of the rock. Similarly, the supersonic nozzle will not dislodge hard frozen soil which may behave like pavement or concrete.

Unconfined Compressive Strength of Rock

Excavation Rate

As shown below nozzles are available that use from 15 to 225 scfm of compressed air. The amount of soil that can be dislodged in a given amount of time is roughly proportional to the amount of air used.

Excavation Rates (cu ft / min)