Compressed Air for Pneumatic Tools

Pneumatic tools are powered by compressed air. Compressor selection needs to match two parameters: flow and pressure. Flow measured in CFM, pressure measured in psi.

Workshop

Pneumatic Tool Applications

Problem Usually Is Flow

Pressure picked wrong is rare. 90 to 100 psi output capability most compressors have. And when pressure is insufficient, tool won't start. Problem is obvious.

Insufficient flow shows completely different. Tool can start, can run, just no power. Performance degrades further after few seconds of continuous work. Pause a moment, recovers to normal.

Air receiver pressure being consumed by tool faster than compressor can replenish causes this phenomenon.

Running into this situation, first reaction is usually suspecting tool quality. Replace tool, symptoms persist.

Thirty Times the Gap

Flow demand gap between pneumatic tools exceeds most people's expectations.

A brad nailer consumes a small gulp of air each trigger pull then stops. Heavy duty breaker in continuous operation needs 200+ CFM. A 50 hp screw machine's full output barely enough. Difference between them is over thirty times.

This means same air supply system, handling nailer, angle grinder, wrench all with plenty to spare. Switch to heavy duty breaker or large bore sandblasting gun, completely can't drive.

Easiest trap when upgrading from medium breaker to heavy duty breaker. Medium breakers (teens to twenties ft-lb class, Atlas Copco TEX series mid-range models are this type) need 100 to 160 CFM. Small screw machines of teens of horsepower on job sites can still manage. Heavy duty breaker straight doubles. Same compressor becomes inadequate.

Rock drills similar situation. Handheld ones 70 to 100 CFM. Leg-mounted ones approaching 140. Like breakers, both are piston repeated impact working method. Consumption is continuous not intermittent.

Sandblasting Works Backwards

Nozzle bore almost completely determines sandblasting gun consumption.

1/8" nozzle about 50 CFM. 1/4" nozzle approaching 210 CFM. 5/16" nozzle over 350 CFM. Bore doubles, cross section quadruples, flow demand rises proportionally. Not much to do with blast pot capacity or media type.

Why Pneumatic Angle Grinders Have Bad Reputation

Pneumatic angle grinders often considered less powerful than electric angle grinders.

Electric angle grinder plugs into 120V and gets stable power output, unaffected by external conditions. Pneumatic angle grinder completely different. Performance output depends on whether inlet flow and pressure meet spec. Manufacturer's rated power and speed are measured under specific supply conditions. On-site conditions insufficient, performance naturally discounted.

Pipe pressure drop is most common problem. Compressed air from compressor outlet through dozens of feet of hose and multiple fittings to reach tool inlet, pressure loss can reach 30 psi. Compressor outlet 100 psi, at tool handle only 70 psi left. Torque and speed both noticeably drop. Multiple tools sharing one main line is worse. Simultaneous starts compete for air.

4" wheel angle grinder flow demand under 21 CFM. 7" wheel needs about 35 CFM. Dynabrade product manuals specify recommended pipe ID and max length. Following this avoids most performance problems.

Die grinders, bench grinders, polishers have lower flow demand than angle grinders. Supply requirements not as strict. Air drills depend on chuck size. 3/8" under 18 CFM. 1/2" approaching 28. Drilling steel consumes more than drilling wood.

Rotary

Angle Grinders

Fastening

Impact Wrenches

Spray

Paint Guns

Can't Tighten the Bolt

Pneumatic wrench flow demand isn't high. 1/2" size (auto repair mainstay, Ingersoll Rand 231C model sold for decades) under 10 CFM. 3/4" about 14. 1" only around 28.

What's sensitive is pressure.

Torque output and inlet pressure have roughly linear relationship. Each 15 psi drop in inlet pressure, torque loss can reach 15%. A wrench rated 600 ft-lb, inlet pressure drops from 90 psi to 75 psi, actual output might only be 500 or so.

When bolt won't tighten, may not be tool problem. Fittings loose? Hose deteriorated? Filter element due for change?

Screwdrivers and rivet guns have even lower flow demand. About 5 CFM. Almost no pressure on the system.

Nail Guns

Brad nailers are the exception among impact tools.

Each trigger pull only consumes a small gulp of air. After firing, tool stops consuming until next trigger pull. This pulse type load is friendly to air supply system. Nailer rated 7 CFM, actual operation because of aiming, repositioning, loading intervals, average consumption well below rated value.

A small piston machine under 35 CFM with 13 gallon receiver, supporting two Senco or MAX nailers continuous operation no problem at all.

Rivet guns slightly higher. Each rivet needs multiple consecutive hammer strikes to form. Flow demand reaches 21 CFM.

Traditional Spray Guns and HVLP

Paint spray guns have two tech routes.

Traditional spray guns use higher pressure (45-60 psi) to atomize paint. Flow demand under 14 CFM. Fine atomization, good paint finish quality. Paint mist scatter is severe. Paint transfer efficiency only 30-40%.

HVLP guns (High Volume Low Pressure) drop pressure to 30-45 psi, raise flow to 14 or even 18 CFM. Low speed high volume airflow pushes paint onto workpiece surface. Bounce back and scatter both reduced. Transfer efficiency up to 60%+. SATA and DeVilbiss mid to high end products basically take this route. Some regions' environmental regulations already mandate this.

Above 115 psi

Most pneumatic tools work at 75 to 100 psi.

Pneumatic jacks are exception. Use air-over-hydraulic structure. Compressed air drives built-in hydraulic pump, hydraulic pump pushes cylinder to lift. 20-ton class jack flow demand under 7 CFM, but pressure needs 115 to 145 psi. If compressor max output is only 115 psi, after pipe pressure drop, pressure reaching jack may not be enough. Lift speed slows down or can't complete action.

Tire inflation guns also need around 115 psi. Single use no problem. Tire service shops need to consider peak periods with multiple inflation guns working simultaneously.

Blow guns rated parameters are low. Around 7 CFM. Holding trigger continuously, actual load will be higher than expected. Small compressors might get dragged into sustained full load operation.

Pneumatic shears and pneumatic saws for sheet metal and pipe work. Flow demand 10 to 21 CFM, working pressure 90 psi.

Calculation

Multiple tools working simultaneously, add up each tool's flow demand, multiply by 0.7 as simultaneity factor (actual operation tools aren't full load every second), then multiply by 1.2 as margin factor (covers fitting leaks and pipe pressure loss), get minimum compressor discharge requirement.

Pressure determined by tool with highest requirement. If there's jack or inflation gun, need to select 145 psi+ compressor. Each station uses pressure regulator to adjust down to what that station's tools need.

Air receiver capacity depends on load fluctuation characteristics. Breakers, sandblasting guns, these continuous high flow loads need large capacity receiver to smooth pressure pulsation. Nailers, wrenches, these pulse type loads don't need much receiver capacity.

Appendix: Quick Reference Parameters

Data compiled from Atlas Copco, Ingersoll Rand, Chicago Pneumatic product manuals