Air Compressor Basics


Air compressors are sized by air pressure and air flow. In the US, pressure is measured in pounds per square inch (psi). Flow is measured in cubic feet of air per minute (cfm). Virtually all air compressors nominally operate at approximately 90 - 100 psi while air flow capacities vary from model to model -- from a few cfm for small electric piston units, to hundreds of cfm for gas or diesel driven portable screw compressors. However, by far, the most popular towbehind air compressors are rated 185 cfm. At larger job sites, or where more air flow is required to run multiple air tools, 375 cfm air compressors are often preferred.

Utility AirSpade is available with three, off the shelf Supersonic Nozzles to match commercially available, tow-behind air compressors.

Note that a lower flow (cfm) nozzle may always be used on a higher capacity compressor, but not the reverse. Running a nozzle with a higher flow rating than the capacity of the compressor will result in noticeably diminished performance.

Mode Nozzle Flow (cfm) Recommended Min. Compressor Size (cfm)


105 125
150 175
225 250

Air Pressure

Utility AirSpade Supersonic Nozzles are designed to operate optimally at 90 psi in order to produce a Mach 2 air jet. Since the vast majority of portable air compressors deliver compressed air in the 90 to 100 psi range, these nozzles are well-matched. Keep in mind that increasing the air pressure above 90 psi delivered to the Supersonic Nozzle does not lead to a proportional gain in excavation capability.

For example, increasing the compressor output pressure (by say 30 percent to 117 psi) increases the air-jet force and exit momentum flux (stress seen by the soil) but only nominally. Supplying higher pressure to a supersonic nozzle designed to work at 90 psi actually de-focuses the air-jet thereby degrading performance. In addition, operating the air compressor at higher pressures will dramatically increase energy (fuel) consumption.

Tier 4 Maintenance

Most typical 185 cfm/90-100 psi tow-behind air compressors feature diesel engines small enough that they do not require diesel exhaust fluid, or a diesel particulate filter to clean exhaust emissions. But be sure to understand specific diesel engine maintenance requirements before heading down the highway.

For systems that require diesel exhaust fluid, the reservoir will probably need to be filled once for every few tanks of diesel fuel burned. Diesel particulate filters may need periodic regeneration to burn off the accumulated soot inside the filter. On some models, this may require ceasing work until the regeneration is complete.

Air Compressor Safety

Horse Whip Prevention

Potentially hazardous links in any compressed air system are the connections between the air compressor and the air hose, connections between lengths of air hose, or the connection between the air hose and the air tool. Should a coupling blow apart under pressure, the loose end of the hose can whip around in a dangerous manner. People have been killed this way.

Air hose restraint systems, such as WhipChek®, hold the two couplers together, preventing them from flailing around should a break occur. OSHA requires “positive means” such as these to secure the air tool to the air hose. See OSHA Standard 29 CFR 1926.302 (b) (1).

Safety Excess Flow Check Values

Another way to prevent hose whip is to install a safety excess flow check valve, sometimes called an air fuse. OSHA requires an air fuse on any hose with an inside diameter (ID) greater than 1/2-inch. See OSHA Standard OSHA 29 CFR 1926.302 (b) (7).

Installed at the output of the air compressor, should the air fuse detect a sudden drop in pressure, it closes shut and prevents additional compressed air from entering the hose. Once tripped, air fuses automatically reset once the fault in the air hose has been corrected.

Air fuses are sized for the pressure and flow rate required by the air tools in use. One-size does not fit all. Consequently, air fuses are not a standard feature supplied on tow-behind compressors.


Due to their gas or diesel engines, tow-behind air compressors contribute to high noise levels when running on the job site. Most compressors nominally operate in the 90 to 100 dBa range, with some units generating up to 110 dBa. To keep noise levels safe and within OSHA standards, under these conditions hearing protection is mandatory. See OSHA Standard 29 CFR 1910.95.

In addition to mechanical noise, crack the output valve of any towbehind air compressor not connected to an air hose and the noise generated by escaping air can be in excess of 120 dBa – a noise level equivalent to a commercial jet at takeoff. Products such as the Guardair Quiet-Test, attach to the output of the compressor and allow for testing the air compressor at full flow, while reducing noise levels by 20 dBa.