Why the same air can have different flow numbers
Air is compressible, so a fixed mass of air occupies different volumes as pressure and temperature change. A flow meter at a compressor discharge may report the volume inside a pressurised pipe, while a compressor catalogue may convert that same mass flow back to an inlet or reference state. The numerical value changes even though the amount of air delivered has not. Humidity can also matter when a calculation is intended to describe dry air rather than the moist inlet stream.
CAGI defines volumetric cfm as a volume rate and distinguishes actual cfm from standard cfm. Its glossary describes actual capacity at rated operating conditions, generally at the compressor inlet, and identifies it as free air delivered (FAD); it defines standard cfm as free air converted to 14.5 psia, 68°F and 0% relative humidity. Those definitions are useful anchors, but a quotation still needs to state its adopted standard because other regions and instruments may use different conventions.
- Mass flow answers how much air is moving; volumetric flow answers how much space that air occupies at stated conditions.
- A bare number such as 500 cfm is incomplete unless the measurement point, pressure basis, temperature, humidity basis and rating pressure are supplied.
- FAD and SCFM are reference conversions, not extra air created by the compressor.
FAD and actual CFM: follow the inlet basis
Free air delivery (FAD) expresses delivered air as an equivalent volume at a defined free-air or inlet reference condition. CAGI uses actual cfm (acfm) for the actual volume of air or gas delivered at rated speed, pressures and temperatures, generally at inlet conditions, and notes that it is also known as FAD. In practice, the phrase FAD is often used on European-oriented data sheets, while acfm or inlet cfm may appear in North American documentation. The label alone does not settle the reference basis.
Atlas Copco's technical explanation gives a common FAD reference of 1 bar(a), 20°C and 0% relative humidity, while describing normal litres per second at 1.013 bar(a) and 0°C. Use those values only as the cited example, not as a universal rule. Ask the supplier to name the exact standard, reference inlet pressure, reference temperature and humidity assumption used for its FAD figure. Also ask whether the number is measured at the package outlet or recalculated from an inlet measurement.
Do not confuse FAD with the pipe volume at working pressure. If a machine supplies 10 m³/min FAD, the compressed volume inside a line at 8 bar(g) will be materially smaller than 10 m³/min. A receiver fill test therefore needs absolute pressure, temperature and the starting condition; a discharge flow rating needs the supplier's stated measurement method and rated pressure.
CFM, ACFM and SCFM: identify the word before the unit
CFM simply means cubic feet per minute. It can describe actual cubic feet per minute (ACFM), inlet cfm, or standard cubic feet per minute (SCFM), depending on the document. ACFM is tied to conditions prevailing at a defined point, commonly the inlet or discharge. SCFM converts the flow to a standard reference set so that data from different sites can be compared more readily. The conversion does not remove the need to check the standard: CAGI's glossary uses 14.5 psia, 68°F and 0% relative humidity, while another supplier or project specification may define a different standard.
Pressure must be absolute in a gas-law conversion. Gauge pressure reads relative to local atmosphere, so 7 bar(g) is not 7 bar(a). For a rough ideal-gas comparison at the same humidity basis, a reference volume can be related to an actual measured volume by Vref = Vactual × (Pref / Pactual) × (Tactual / Tref), where every pressure is absolute and temperature is in kelvin or degrees Rankine. This is an illustrative relationship; real metering may require compressibility, moisture and instrument corrections.
A supplier's rated capacity may also be stated at a discharge pressure. That is not the same as a free-air flow at zero pressure. Capacity can change with pressure, speed, inlet conditions, controls and cooling conditions, so request the capacity curve or test point rather than selecting from motor horsepower alone.
| Term | What it usually communicates | RFQ confirmation needed |
|---|---|---|
| FAD | Delivered air restated at a defined free-air or inlet reference condition | Reference pressure, temperature, humidity, standard or test method, and measurement location |
| ACFM | Actual volume per minute at stated operating conditions | Actual pressure and temperature, point of measurement, gas composition and whether pressure is absolute |
| SCFM | Free air converted to a named standard reference state | The exact standard basis; do not assume all SCFM definitions match |
| Nm³/min or Nl/s | Normalised SI volume at a named normal state | Normal pressure and temperature, humidity basis, and whether the supplier means mass-equivalent normal flow |
| m³/min at line conditions | Physical pipe volume at the stated pressure and temperature | Line pressure, temperature, elevation or atmospheric pressure, and meter location |
Normalised flow: Nm³/min is not a universal number
Normalised flow is intended to put air on a chosen normal state, commonly written Nm³/min or Nl/s. The capital N is a warning to look for the definition, not a guarantee that every document uses the same one. The Atlas Copco example uses 1.013 bar(a), 0°C and 0% relative humidity for normal conditions. Other contracts may use a different temperature, pressure or moisture convention. A normalised flow can be useful for mass-balance work, but the basis must travel with the number.
When comparing Nm³/min with FAD, first convert both values to a common reference state or obtain the mass-flow basis from the supplier. Do not apply a simple m³-to-ft³ factor and call the result SCFM: the unit conversion changes length units, while the reference-state conversion changes volume through pressure and temperature. If the application is sensitive to moisture, request whether the value is wet, dry or corrected to a stated relative humidity.
An illustrative conversion method with explicit assumptions
Suppose a meter reports 100 ACFM at 20°C and 1 bar(a), and you want an illustrative equivalent at 14.5 psia and 68°F. Convert units, use absolute pressure, and apply the gas-law relationship. Because the example reference temperature and pressures are close, the result will be near 100, subject to exact unit conversion, humidity and instrument uncertainty.
Now consider 100 ACFM at 8 bar(g) and 40°C. Convert gauge to absolute pressure using local atmospheric pressure, convert 40°C to kelvin, and apply the stated reference conditions. The equivalent free-air flow will be several times larger than the pressurised pipe volume. Therefore, '100 cfm at 8 bar' cannot be compared safely with '100 SCFM' without more information.
For procurement, put the required flow, pressure, reference state and tolerance in the RFQ, then require each bidder to return a data sheet on that basis. Use a conversion as a sense check, not as a substitute for a required acceptance test.
Convert plant demand into a comparable compressor requirement
Start with the end uses, not the compressor nameplate. Record each tool or process's required flow, minimum pressure at the point of use, operating hours, duty pattern and whether the demand is continuous, cyclic or a short peak. Separate simultaneous demand from connected load. Then account for treatment equipment, distribution losses and an explicitly stated design margin. Those additions should be expressed on the same flow reference as the demand; otherwise the apparent margin may be meaningless.
Map the system from compressor outlet through cooling, receiver, dryer, filters and distribution to the most demanding point. A filter or dryer can have a pressure drop that changes the pressure available at the process, while a receiver can support a transient without changing the compressor's steady-state FAD. If a demand is intermittent, describe its event duration and repetition rate. A ten-second blow-off peak and an all-shift pneumatic load may need different storage and control responses even if their average flow is similar.
For multiple compressors, ask for both individual and combined operating points. Confirm whether the quoted flow is per unit, total installed flow, or expected available flow with one unit unavailable. A control sequence can change which machines load, unload or vary speed. The RFQ should therefore request the usable flow at the specified pressure, not only a sum of catalogue capacities.
RFQ criteria for an unambiguous flow figure
A good RFQ makes every bidder solve the same definition problem. State the required flow in one nominated basis and ask bidders to show any alternate units only as labelled conversions. Ask for the rated operating point and the method or standard behind it. If the plant elevation or climate is significant, include site atmospheric pressure, inlet temperature range and humidity range. Do not accept a bare CFM number as evidence that the machine meets the process requirement.
Request the following items in the technical return:
- Required flow as FAD, SCFM or Nm³/min with exact reference pressure, temperature and humidity basis; state whether the basis is wet or dry.
- Minimum and normal working pressure at the point of use, plus the pressure at which the compressor capacity is guaranteed or tested.
- Measurement point: compressor inlet, package outlet, aftercooler, dryer outlet or point of use; include any stated pressure drop.
- Flow tolerance, turndown or control range, continuous demand, peak demand, event duration and expected simultaneous use.
- Site elevation or atmospheric pressure, inlet temperature range, cooling-water or ambient conditions and electrical supply where they affect rating.
- Whether the figure is per compressor, total installed capacity, or available capacity after a standby or redundancy requirement.
- Applicable test standard, calibration or acceptance method, and the supplier's treatment of humidity, leakage and measurement uncertainty.
- Required accessories and their guaranteed outlet conditions, including receiver, dryer, filters, drains and isolation valves.
Common flow-selection pitfalls
Common errors include comparing a standard number with a line-condition number, adding connected tool ratings as if every tool operates simultaneously, and using motor size as a proxy for air delivery. Power can compare energy performance after the operating point is fixed, but it does not define required flow or pressure.
Write conversion assumptions beside every result. A receiver stores air and can reduce short peaks, but the compressor must replenish it. Treat supplier-specific capacity, efficiency, availability, certification and warranty as data to obtain and verify in the quotation; the comparison criteria above help set a consistent basis.
Data to confirm before purchase
Before selecting equipment, obtain a supplier data sheet that states the flow reference, absolute pressure basis, reference temperature, humidity treatment, rated discharge pressure and measurement location. Confirm that the dryer, filters, receiver and piping are included or excluded consistently. Where the application is safety-critical or production-critical, have the final flow basis and acceptance test reviewed by the responsible engineer.
SELECTION & OPERATION
Frequently asked questions
Is FAD the same as CFM?
Not necessarily. FAD is a way of expressing delivered air as an equivalent free-air or inlet volume, while CFM only states cubic feet per minute. CFM may mean actual cfm, inlet cfm or SCFM. Ask for the reference pressure, temperature, humidity, measurement point and rated pressure before comparing the numbers.
Can I convert Nm³/min directly to SCFM?
You can convert them only after identifying both reference states. A simple length-unit conversion is insufficient because normalised and standard volumes also depend on absolute pressure, temperature and humidity. Put both values on one declared basis, or ask the supplier for the mass-flow or common-reference result.
Why is compressor FAD higher than the volume in my 8 bar(g) pipe?
FAD refers the delivered air back to a lower-pressure free-air or inlet condition. The same mass of air occupies less space in a pressurised pipe, so the line-condition volumetric flow is smaller. The comparison must use absolute pressures and stated temperatures; gauge pressure alone is not enough.
What flow unit should I put in an air compressor RFQ?
Use the unit already used by your process or plant standard, but define its reference state completely. State the required flow, minimum point-of-use pressure, rated pressure, wet or dry basis, temperature, humidity, measurement location, peak profile and tolerance. Ask every bidder to return the guaranteed or tested capacity on that same basis.
Technical references
- CAGI Resource Library glossary ↗
Definitions of volumetric cfm, actual cfm/acfm, free air delivered (FAD), standard cfm, rated capacity and the stated SCFM reference conditions of 14.5 psia, 68°F and 0% relative humidity.
- Understanding air compressor measurements: work, power, and flow ↗
Distinction between normal flow and FAD, illustrative reference states of 1.013 bar(a) and 0°C for normal flow versus 1 bar(a) and 20°C for FAD, and the need to size by flow and pressure rather than motor size alone.