Direct answer: SCFM, ACFM and FAD can all describe compressor airflow, but they do not describe the same reference state. In a rock-drilling RFQ, never compare the bare number alone. Compare the unit, reference temperature, absolute pressure, humidity basis, test standard, delivery pressure and measurement point. If any of those are missing, the two airflow ratings may not be comparable.
For DTH hammers and pneumatic rock drills, the practical question is not “Which compressor has the larger CFM number?” It is “Can the complete package deliver the tool’s required airflow while maintaining the required pressure at the tool under the actual altitude, temperature, hose and simultaneous-use conditions?”
SCFM vs ACFM vs FAD: Quick Reference
| Term | What it describes | Qualifier you must obtain | Typical procurement risk |
|---|---|---|---|
| ACFM | Actual volumetric flow at stated local conditions and a stated point in the system. | Actual pressure, temperature, humidity/composition and measurement location. | Treating an inlet ACFM or discharge ACFM as though it were a normalized tool-demand value. |
| SCFM | Flow converted to a stated set of “standard” conditions. | The exact standard temperature, absolute pressure and humidity basis. | Assuming every supplier uses the same definition of “standard.” |
| FAD | Air actually delivered by a compressor, expressed as an equivalent volume at defined compressor-inlet or reference conditions. | Test standard, reference inlet conditions, rated discharge pressure, package boundary and tolerance. | Comparing FAD with theoretical displacement, or comparing FAD values tested at different pressures. |
| Unqualified CFM or m³/min | A volume-per-time unit with no stated reference basis. | Whether it means actual, standard, normal or free-air delivery—and at what pressure. | Building a compressor shortlist from numbers that cannot be normalized. |
Why the Same Airflow Number Can Represent Different Air Mass
Air is compressible. A cubic metre or cubic foot of air contains a different mass when its absolute pressure, temperature or moisture content changes. That makes gas-volume ratings different from a simple liquid-volume comparison.
SCFM and other normalized units solve part of this problem by reporting the measured flow as an equivalent volume at defined reference conditions. ACFM reports the volume at the actual stated conditions. FAD reports useful compressor delivery by referring delivered flow back to defined inlet or reference conditions. Each can be technically valid, but only if the basis is clear.
This is why a conversion based only on “1 cubic foot equals 0.0283 cubic metres” can be misleading. That factor converts volume units; it does not reconcile different pressure, temperature, humidity or test bases. For a defensible comparison, first put both ratings on the same reference basis, then convert the units.
What ACFM Means in a Compressed-Air System
ACFM means actual cubic feet per minute. It describes the physical volume flowing at the actual conditions defined for the measurement point. Those conditions may be at the compressor inlet, at a discharge flange, in a distribution pipe or near a tool. The point matters because the same air mass occupies a much smaller volume after compression than it does near atmospheric pressure.
An ACFM statement is incomplete unless it identifies:
- measurement location;
- absolute pressure, not only gauge pressure;
- air temperature;
- humidity or gas composition where relevant; and
- whether the value is measured, calculated or corrected.
For rock drilling, do not take an ACFM value measured at compressor intake conditions and compare it directly with an actual-volume value inside a pressurized hose. Ask the compressor supplier and drilling-tool supplier to state what their airflow figures represent.
What SCFM Means—and Why “Standard” Must Be Defined
SCFM means standard cubic feet per minute. It expresses a gas flow as the volume that the same air mass would occupy at a chosen standard temperature, absolute pressure and humidity. This makes SCFM useful for comparison when every party uses the same standard.
The catch is that “standard” is not universal. Different industries, regions and documents can use different reference temperatures, pressures or humidity assumptions. A datasheet that prints only “SCFM” has therefore left out part of the specification.
Ask for a statement such as: “SCFM referenced to [temperature], [absolute pressure] and [humidity basis].” Do not silently assume that a North American tool datasheet, an international compressor catalogue and a local flowmeter all use identical conditions.
What Free Air Delivery (FAD) Means
Free air delivery is a compressor-capacity rating. It represents air actually delivered by the compressor, expressed as the equivalent volume at defined inlet or reference conditions. In practical terms, FAD is intended to describe usable compressor output rather than the swept volume of the compression element.
Atlas Copco explains that FAD is commonly determined from outlet delivery and converted back to free-air conditions. Its current technical explanation cites reference conditions of 20°C, 1 bar absolute and 0% relative humidity unless otherwise stated, and notes that comparisons should normally use the same ISO 1217 basis. Kaeser likewise explains that ISO 1217 evaluates positive-displacement compressor flow and power, with the delivered flow referred back to inlet conditions.
However, the label “FAD” alone is still not a complete purchase specification. Confirm:
- the edition or annex of the test standard;
- reference inlet conditions;
- rated discharge pressure;
- whether the rating covers the complete compressor package or only a compression element;
- the standard discharge point; and
- the applicable tolerance.
FAD Is Not Theoretical Displacement
Theoretical displacement describes the geometric volume swept by a compressor element. It does not automatically account for internal leakage, valve losses, heating, controls, filters, separators or other package effects. FAD is intended to represent delivered performance and is therefore the more relevant starting point for matching a compressor to drilling demand.
If one quotation gives “piston displacement” and another gives tested FAD, the larger number does not identify the stronger package. Ask both suppliers for delivered flow on a common standard and at the same operating pressure.
Why Unqualified “CFM” or “m³/min” Is Not Enough
CFM and m³/min are units, not complete definitions. They can be attached to actual, standard, normal or free-air-delivery conditions. The same applies to L/s and m³/h. A prefix or note—such as S, A, N or FAD—changes the meaning.
| Datasheet wording | Can a buyer compare it immediately? | Required follow-up |
|---|---|---|
| “900 CFM” | No | Ask whether it is SCFM, ACFM, inlet flow, FAD or displacement; obtain pressure and reference conditions. |
| “25 m³/min FAD at stated operating pressure, ISO 1217” | Closer, but verify details | Confirm reference conditions, complete-package boundary, tolerance and site derating. |
| “1,500 ACFM” | No | Obtain measurement point, absolute pressure, temperature and humidity/composition. |
| “Tool demand: 800 CFM” | No | Confirm the tool model, pressure at tool inlet and the airflow basis used by its manufacturer. |
How to Compare Compressor Airflow Ratings for Rock Drilling
- Identify the exact drilling equipment. Record the DTH hammer or pneumatic rock-drill model, bit/hole range and supplier-approved operating data.
- Obtain the tool’s required pressure and flow together. A flow figure without its operating pressure does not define the duty point.
- Record the tool-demand basis. Determine whether the manual uses SCFM, FAD, normal flow, actual flow or an unqualified unit.
- Obtain the compressor’s delivered-flow curve or rated points. Use FAD or equivalent delivered performance at the required pressure—not motor power or displacement alone.
- Normalize both sides. Convert compressor capacity and tool demand to the same reference temperature, absolute pressure, humidity basis and units.
- Apply site corrections. Use compressor-manufacturer correction data for altitude, inlet temperature, humidity and any other limiting condition.
- Add distribution losses and simultaneous demand. Include hoses, pipes, couplings, filters, lubricators, water injection and every tool that can operate at once.
- Check pressure at the tool under load. Receiver pressure or no-flow pressure is not the same as operating pressure at the drill inlet.
- Confirm reserve and control behavior with the OEM or supplier. Required margin depends on the compressor, duty cycle, controls, leakage and operating pattern; do not apply a universal percentage by guesswork.
Pressure and Flow Are Separate Requirements
Pressure is the force potential available to operate the pneumatic cycle; flow describes how much air is supplied over time. A compressor can reach the required pressure with the outlet closed yet fail to maintain that pressure when the drill consumes air. Conversely, a large low-pressure airflow rating does not prove that a high-pressure DTH hammer can operate correctly.
For quotation comparison, require a flow value at the required operating pressure. For field verification, measure pressure as close to the tool inlet as safely practical while the tool is working, following the equipment manufacturer’s test procedure. If pressure collapses only under load, investigate compressor capacity, controls, restrictions, leakage and line sizing rather than treating the static gauge value as proof of supply.
How Altitude, Temperature and the Air Line Change the Result
At higher altitude, compressor inlet air is less dense. A fixed-displacement compressor ingests less air mass per cycle unless its design and controls compensate. High inlet temperature also reduces density. The result is that a package selected from sea-level catalogue data may not supply the same normalized airflow at a high, hot mine or quarry.
Distribution adds another layer. Long or undersized hoses, restrictive couplings, dirty filters, poorly selected lubricators, leaks and multiple branches can reduce the pressure available at the drill. These losses do not change the definition of SCFM or FAD, but they change whether the advertised compressor capacity reaches the tool as useful working air.
Use the compressor manufacturer’s site-rating or derating method, and use the drilling-tool supplier’s permitted inlet-pressure and air-quality limits. Avoid generic altitude factors or hose-loss estimates when the actual model and layout can be checked.
A Practical Comparison Example Without False Precision
Suppose Supplier A quotes a compressor as “25 m³/min FAD at the required discharge pressure under ISO 1217 reference conditions.” Supplier B quotes “920 CFM” with no pressure or reference basis. A direct metric conversion might make the figures look similar, but that is not an engineering comparison.
The correct next step is to ask Supplier B whether 920 CFM is delivered flow, standard flow, actual inlet flow or displacement; obtain the rated pressure, reference temperature, absolute pressure, humidity, standard and package boundary; then convert both values to one agreed basis. Only after that should the buyer apply site corrections, line losses and simultaneous demand.
The point of this workflow is not to create more paperwork. It prevents a high-looking catalogue number from becoming a low-performing drill at the jobsite.
Compressor-Airflow RFQ Checklist for Drilling Projects
| RFQ item | Information to state or request |
|---|---|
| Drilling system | DTH hammer or pneumatic rock-drill model, hole diameter/depth and operating method. |
| Tool duty point | Required airflow and pressure at the tool inlet, with the manufacturer’s reference basis. |
| Flow terminology | SCFM, ACFM, FAD, Nm³/min or another defined basis; reject unqualified CFM. |
| Reference conditions | Temperature, absolute pressure and humidity/composition. |
| Compressor test basis | Standard and edition/annex, rated pressure, measurement point, package boundary and tolerance. |
| Site conditions | Elevation, expected inlet-temperature range, humidity, dust and ventilation constraints. |
| Distribution system | Hose/pipe ID and length, couplings, filters, receiver, lubricator, water injection and branch layout. |
| Demand profile | Number of simultaneous tools, continuous/intermittent duty and other compressed-air consumers. |
| Verification | Performance curve or rated points, site correction method and proposed loaded-pressure test. |
Common Airflow-Rating Mistakes
- Comparing SCFM and FAD by the printed number alone. First confirm both reference bases and operating pressures.
- Assuming “standard” has one worldwide definition. Require the actual temperature, absolute pressure and humidity basis.
- Converting units before normalizing conditions. A cubic-foot-to-cubic-metre conversion does not correct gas density.
- Buying by compressor horsepower or kilowatts. Power input does not prove delivered flow at the required pressure.
- Using displacement as delivered capacity. Ask for tested FAD or an equivalent delivered-flow rating.
- Ignoring the package boundary. Element-only data can omit losses included in a complete-package rating.
- Using receiver pressure as proof of tool supply. Check pressure and flow under load at the drilling end.
- Ignoring altitude and temperature. Apply the compressor manufacturer’s site correction.
- Adding a universal reserve percentage. Margin should reflect duty, controls, leakage, future demand and OEM guidance.
Frequently Asked Questions
Is SCFM the same as CFM?
No. CFM is only a volume-per-time unit unless its conditions are qualified. SCFM is CFM normalized to a stated set of standard conditions. Because more than one standard basis is used, the SCFM reference conditions must still be stated.
Is FAD the same as compressor displacement?
No. Displacement is a theoretical geometric capacity. FAD represents air actually delivered and referred to defined inlet or reference conditions. For matching a compressor to drilling-tool demand, delivered performance at the required pressure is the more useful value.
Can compressor FAD be compared directly with a tool’s SCFM requirement?
Only after confirming that both values use compatible reference conditions and that the compressor FAD is stated at the tool’s required operating pressure. Then apply site derating and distribution losses before deciding capacity.
Does altitude change a compressor’s SCFM or FAD available to the drill?
Altitude reduces inlet-air density and can reduce the air mass a compressor delivers. The size of the effect depends on compressor design, controls, temperature and required discharge pressure. Use the manufacturer’s model-specific site-rating data.
Should a buyer size pressure or airflow first?
Neither should be sized alone. Define the required pressure and airflow as one duty point, then verify that the compressor and distribution system can maintain both at the tool under load.
Technical Sources
- Atlas Copco — Understanding air compressor measurements, flow and free air delivery
- Kaeser — ISO 1217 and positive-displacement compressor performance
- Compressed Air Best Practices — Measuring installed compressor performance and FAD
Next Step: Normalize the Air Data Before Requesting a Drilling-System Quote
If you are matching compressed-air supply to a DTH hammer or pneumatic rock drill, send PerfoMax the tool model, required pressure and flow basis, site elevation and temperature, hose layout, simultaneous tool count and current compressor datasheet. The goal is to identify missing specifications and confirm the drilling-side requirements before an equipment order is locked.