Compressed Air Flow Unit Conversions – Air Compressor Manufacturers

CFM and m³/min differ by a factor of 35.

American equipment manual says 50 CFM. Convert to m³/min, that's about 1.4. Someone takes the document and plugs 50 straight in as m³/min. Sizes the compressor based on that. Power oversized by 30-plus times. Reverse it, treat 1 m³/min as 1 CFM. Air supply configured is only 3% of the requirement. Equipment can't even start.

1 m³/min = 35.31 CFM. Round to 35. Reverse: 1 CFM = 0.0283 m³/min. Round to 0.028, or 28 L/min.

Domestic compressor nameplates use m³/min. Foreign brands selling in the domestic market also label m³/min. CFM is basically from North American equipment. Pneumatic tools, spray equipment, packaging machines. American manufacturers' manuals don't convert. Multiply by 0.0283 yourself.

Industrial compressed air system — Proper unit conversion is critical for compressor sizing

L/min is used for small components like cylinders, solenoid valves, pneumatic fittings. Air consumption written in m³/min would be 0.002, 0.005. Write it as 2 L/min, 5 L/min, easier. 1000 L = 1 m³. Medical devices, lab instruments, air source requirements are usually tens to hundreds of L/min.

m³/h is the convention in chemical and pharmaceutical. Material balance calculated hourly. Annual air consumption statistics also use m³/h. m³/h divided by 60 gives m³/min.

L/s is used in some European countries. 1 L/s = 0.06 m³/min.

From	To	Multiply by
m³/min	m³/h	60
m³/min	L/min	1000
m³/min	L/s	16.67
m³/min	CFM	35.31
CFM	m³/min	0.0283
L/min	m³/min	0.001
L/s	m³/min	0.06
m³/h	m³/min	0.0167

Unit conversion, look up the table and done. Standard conditions are a different matter.

Air can be compressed. The same batch of air at 1 atmosphere takes up 10 cubic meters. Compress to 8 atmospheres, only 1.25 cubic meters. "Flow rate 10 m³/min" without stating pressure and temperature conditions is useless.

The industry agreed on "standard conditions" as a reference. ISO standard is 20°C, 101.325 kPa. FAD values on compressor nameplates mostly follow this.

More than one definition exists. German DIN uses 0°C. American sources sometimes use 15°C, sometimes 20°C. The difference between 0°C and 20°C is about 7%. Two compressors both rated FAD 10 m³/min. One rated at 0°C, one at 20°C. The one rated at 0°C actually delivers 7% more. Small projects, doesn't matter. Large compressor stations with a dozen machines, 7% accumulated is a difference of hundreds of thousands of dollars in investment.

FAD is Free Air Delivery. "Displacement" on compressor nameplates refers to FAD.

FAD is not the flow rate in the discharge pipe.

A compressor with FAD 10 m³/min, discharge pressure 7 bar gauge. 7 bar gauge plus atmospheric is 8 bar absolute. Pressure goes up 8 times, volume shrinks to 1/8. Flow inside the discharge pipe is 10 ÷ 8 = 1.25 m³/min.

FAD 10 m³/min means this compressor sucks in 10 cubic meters of atmosphere per minute. Or the compressed air it discharges, released back to atmospheric pressure, expands to 10 cubic meters. FAD describes throughput capacity. Not the actual volume in the pipe.

Sizing compressors, look at FAD. Air-consuming equipment's consumption is usually also stated at standard conditions. Equipment manual says "air consumption 2 m³/min," meaning each minute it consumes compressed air equivalent to 2 cubic meters of atmosphere. A compressor with FAD 2.5 m³/min can handle it.

Compressor equipment — FAD ratings help match compressor capacity to equipment requirements

Sizing pipe diameter, calculate at working-state flow. Pipe diameter calculations look at actual flow velocity inside the pipe. Velocity kept to 15 to 20 m/s. Too high, pressure drop is big. System with FAD 10 m³/min at 8 bar absolute, pipe calculated at 1.25 m³/min. Not 10.

Flow meter on the pipe reads working-state flow. Pipe pressure 6 bar gauge (7 bar absolute). Flow meter shows 2 m³/min. Convert to standard conditions: 2 × 7 = 14 m³/min. Someone sees the flow meter showing 2, thinks a compressor with FAD 3 m³/min is enough. Off by a factor of four or five.

Adding up air consumption from multiple devices, unify units first. Spray gun 50 CFM. Cylinder 800 L/min. Blow-off nozzle 30 m³/h. Convert to m³/min: 1.42, 0.8, 0.5. Adds up to 2.72 m³/min. Size the compressor with a safety factor, 1.1 to 1.3, depending on pipe network condition and whether equipment runs simultaneously.

Mass flow in kg/h or kg/min. Used for thermodynamic calculations. Standard-condition air density 1.2 kg/m³. 10 m³/min = 12 kg/min = 720 kg/h. Won't come up in everyday sizing.

Gauge pressure vs. absolute pressure. Compressor nameplate says 7 bar, that's gauge. Absolute pressure adds atmospheric. Sea level add 1 bar. High altitude, atmospheric pressure is below 1 bar. Go by local conditions. Using gauge pressure directly in calculations, error is over 12%.

m³/min	CFM	L/min	m³/h
0.5	17.7	500	30
1	35.3	1000	60
2	70.6	2000	120
5	176.6	5000	300
10	353.1	10000	600
20	706.3	20000	1200
50	1765.7	50000	3000

Technical documents, when writing flow rates, include the conditions. "10 m³/min (20°C, 101.325 kPa)." Whoever reads it next doesn't have to guess what standard was used. International project bid documents, state upfront which version of standard conditions is being used. Otherwise suppliers each report their own way and you can't compare.

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SCFM is Standard CFM. CFM at standard conditions. Some American manufacturers distinguish SCFM and ACFM (Actual CFM, working state). Some mix them up. Get the documents and confirm.

NL/min is Normal Liter per minute. Used by Japanese and some European manufacturers. N indicates standard conditions. See NL/min, treat as L/min. Verify whether the reference temperature is 0°C or 20°C.

Air density changes with temperature and pressure. Standard conditions (20°C, 101.325 kPa), dry air density 1.205 kg/m³. Rounding to 1.2 is fine. Other conditions, use the ideal gas equation: ρ = ρ₀ × (P/P₀) × (T₀/T). ρ₀ is standard density. P₀ and T₀ are standard pressure and absolute temperature (293.15 K). P and T are actual conditions.

Temperature in absolute scale. Kelvin. 20°C = 293.15 K. 0°C = 273.15 K. Celsius plus 273.15.

Compressor catalogs sometimes write "displacement 10 m³/min @ 7 bar." The pressure after the @ is discharge pressure. Not the pressure at which flow was measured. FAD is always converted to inlet conditions (approximately atmospheric). Regardless of discharge pressure. An 8 bar discharge machine with FAD 10 m³/min. A 12 bar discharge machine can also be FAD 10 m³/min. Difference is the latter uses more power. Flow in the discharge pipe is smaller (10 ÷ 13 = 0.77 m³/min).

Industrial air system piping — Proper sizing ensures optimal system performance

VFD compressor FAD is a range. A 4 to 10 m³/min VFD screw compressor. Full load 10 m³/min. Minimum stable operation 4 m³/min. Continuously adjustable in between. Size by matching maximum demand to full-load FAD. Confirm minimum demand is above the lower limit. Avoid frequent load-unload cycling.

Centrifugal compressor flow adjustment range is narrower than screw. Surge line limits minimum flow. Usually 70% to 80% of rated. Sizing margin can't be too big. Otherwise at low load you're venting or recirculating. Wasting energy.

High altitude. Atmospheric pressure is low. Air is thin. At 3,000 meters elevation atmospheric pressure is about 0.7 bar. Compressor intake density drops. Same FAD rating, actual output capacity is reduced. Manufacturers provide altitude correction factors, or directly offer high-altitude models. Size using corrected FAD, or divide air demand by the correction factor before selecting the machine.

Pipe pressure drop means end-point pressure is below compressor discharge pressure. Main pipe pressure drop kept within 0.1 bar. Branch adds another 0.1 bar. Total drop no more than 5% of discharge pressure. Pressure drop too big, end-point equipment doesn't have enough pressure. Only option is to raise compressor discharge pressure. Raising 1 bar costs about 7% more electricity. Larger pipe diameter costs more upfront. Saves on electricity long-term. Do the total math.

Receiver tank volume is related to flow. Rule of thumb: tank volume (m³) = compressor FAD (m³/min) × coefficient. Coefficient between 0.1 and 0.3, depending on demand fluctuation and control method. FAD 10 m³/min compressor paired with 1 to 3 m³ receiver tank. Tank too small, pressure swings wildly. Too big, takes up space, high upfront cost.

Dryer and desiccant dryer processing capacities are also rated as FAD. Size dryer at 100% to 120% of total system air demand. Leave margin for peaks and equipment aging. Inlet temperature above design value, ambient temperature above design value, dryer processing capacity drops. Manufacturer catalog has correction curves.

Filter flow ratings sometimes show maximum flow, not rated flow. At maximum flow, pressure drop is already very high. In practice, size at 70% to 80% of rated flow. Pressure drop stays in reasonable range. Reference conditions for filter flow ratings also need attention. Some rate at 1 bar, some at 7 bar. Factor of 7 difference.