CFM, SCMH & Beyond: Understanding Compressor Specifications for Reciprocating and Screw Compressors
IntroductionTwo compressors can look identical on paper and perform very differently once installed — because a spec sheet is only useful if you understand exactly what each number means and under what conditions it was measured. CFM, SCMH, FAD, working pressure, specific power: these terms show up on every compressor datasheet, but they're easy to misread or compare incorrectly across manufacturers and compressor types.
In this guide, we break down the key parameters used to specify reciprocating and screw compressors, what they actually measure, and how to use them to compare options correctly.
Flow Rate Parameters: CFM, SCMH, and FAD
CFM
Cubic Feet per Minute — a volumetric flow rate measurement of how much gas a compressor moves in one minute. The most common flow unit on compressors specified to imperial or US-influenced standards.
SCMH
Standard Cubic Meters per Hour — the metric equivalent of CFM, referenced to standard conditions (commonly 20°C and 1 atmosphere, though the exact reference varies by standard). Widely used on Indian compressor, biogas, and process gas datasheets. As a rough conversion, 1 CFM ≈ 1.7 SCMH.
FAD
Free Air Delivery — arguably the most important, and most misunderstood, flow specification. FAD is the actual volume of air delivered, converted back to inlet conditions, giving a true measure of output unlike raw displacement, which only reflects swept cylinder or rotor volume.
ACFM, ICFM, and SCFM are related but distinct terms worth knowing:
- ACFM (Actual CFM) — measures flow at actual operating conditions at a specific point in the system
- ICFM (Inlet CFM) — measures flow at the compressor's inlet conditions — closely related to FAD
- SCFM (Standard CFM) — measures flow converted to standard reference conditions, similar in concept to SCMH but in imperial units
When comparing two compressors, always confirm whether the quoted flow figure is FAD, displacement, or a standard-conditions equivalent — comparing displacement on one machine to FAD on another will make an inferior compressor look competitive on paper.
Pressure Parameters
Working Pressure (Discharge Pressure)
Typically expressed in bar, kg/cm²(g), or psi, representing the pressure the compressor delivers at its outlet. The “(g)” denotes gauge pressure — relative to atmospheric pressure — as opposed to absolute pressure, which includes atmospheric pressure in the total.
Compression Ratio
The ratio of absolute discharge pressure to absolute suction pressure for a given stage. Higher ratios per stage increase discharge temperature and mechanical stress, which is why high-pressure applications (CBG filling at 200–250 bar, or PET blowing at 30–60 bar) typically require multiple stages rather than one.
Power and Efficiency Parameters
Motor Power (kW / HP)
The electrical input power the compressor's motor is rated for — but on its own says little about efficiency, since two compressors with the same motor rating can deliver very different amounts of compressed air per unit of energy consumed.
Specific Power
Often expressed as kW per 100 CFM or kW per m³/min of FAD, this is the metric that actually tells you how efficient a compressor is. Lower specific power means more compressed air delivered per unit of energy — the number to compare, not motor rating alone.
Volumetric Efficiency
Describes how much of a compressor's theoretical displacement is actually delivered as usable FAD, accounting for internal leakage, valve losses, and clearance volume. It naturally declines as compression ratio increases — another reason multi-stage designs suit higher pressures.
Parameters Specific to Reciprocating Compressors
- Number of stages — Reciprocating compressors compress air or gas across one or more stages, with intercooling between stages in multi-stage designs. More stages allow higher overall pressure with lower mechanical stress and heat generation per stage.
- RPM (Revolutions Per Minute) — The operating speed of the crankshaft. Lower RPM designs generally see less wear and longer service life, at the cost of a larger physical footprint for a given output.
- Displacement vs. FAD — Displacement is the theoretical swept volume of the cylinders per unit time; FAD is the actual delivered output after accounting for volumetric efficiency losses. Always compare FAD, not displacement, between machines.
- Duty cycle — Whether the compressor is rated for continuous (24×7) operation or intermittent duty. Applications like CBG plants, PET bottling, and cold storage refrigeration typically require continuous-duty-rated machines.
Parameters Specific to Screw Compressors
- Single-stage vs. two-stage — Single-stage screw compressors compress air in one rotor pass; two-stage designs compress in two steps with intercooling, achieving lower specific power at the same output pressure — Colt's two-stage screw range, for example, delivers up to 35% energy savings over less efficient configurations.
- Oil-injected vs. oil-free — Oil-injected screw compressors use oil for sealing, cooling, and lubrication between rotors; oil-free designs avoid oil contact with the air stream entirely, which is essential for PET, food, and pharmaceutical applications.
- Fixed-speed vs. VFD (Variable Frequency Drive) — Fixed-speed screw compressors run the motor at a constant speed; VFD models adjust motor speed to match real-time demand, reducing energy waste during partial-load operation.
- Motor efficiency class (IE2/IE3/IE4) — International Efficiency (IE) classes rate motor efficiency, with higher numbers indicating greater efficiency. Colt's two-stage screw range, for instance, uses IE4 permanent magnet motors for super premium efficiency.
Air Quality Parameters
Pressure Dew Point
The temperature at which moisture in compressed air begins to condense at system pressure. A lower pressure dew point means drier air; refrigerated dryers typically achieve 2–4°C, while desiccant dryers achieve much lower dew points for critical applications.
ISO 8573-1 Air Quality Classes
This standard classifies compressed air quality by particle count, water content, and oil content, using a class rating (such as Class 0 for zero oil content) that lets buyers specify air quality precisely rather than relying on vague terms like “clean” or “dry.”
Ambient and Environmental Parameters
Maximum Ambient Temperature Rating
The highest surrounding air temperature at which the compressor is designed to maintain rated performance. This matters significantly in Indian conditions, where equipment rated only for 40°C ambient can underperform or trip on hot summer days — Colt's screw compressor range, for example, is engineered for ambient temperatures up to 48°C.
Altitude Derating
Compressor and motor performance typically decreases at higher altitudes due to lower air density, so equipment specified for sea-level conditions may need derating or resizing for high-altitude installations.
How to Compare Compressor Quotes Using These Parameters
When comparing quotes from different manufacturers, checking these points prevents comparing incompatible numbers:
- Confirm flow figures are FAD (or an equivalent standard-conditions figure), not raw displacement
- Compare specific power (kW per 100 CFM or per m³/min), not just motor kW rating
- Check working pressure is quoted at the same point in the system (compressor outlet vs. point of use)
- Verify duty cycle rating matches your actual operating pattern (continuous vs. intermittent)
- Confirm ambient temperature rating matches your site conditions, especially in high-temperature environments
- For oil-free requirements, confirm the ISO 8573-1 class the equipment is certified to, not just a generic “oil-free” claim
Why These Parameters Matter When Choosing Between Reciprocating and Screw Compressors
Reciprocating compressors typically deliver higher pressure per stage and handle wider gas composition variation, making them well suited to biogas, ammonia refrigeration, and very high-pressure applications like CBG cylinder filling. Screw compressors typically offer lower specific power and quieter, continuous operation at moderate industrial pressures (7–16 kg/cm²(g)), making them the default choice for general plant compressed air. Understanding FAD, specific power, and duty cycle ratings lets you match the right compressor type — not just the right brand — to your actual application.
Conclusion
A compressor spec sheet is only as useful as your ability to interpret it correctly. Understanding the difference between CFM and SCMH, FAD versus displacement, specific power versus motor rating, and how duty cycle and ambient temperature affect real-world performance puts you in a position to compare quotes accurately and avoid under-specifying equipment for your actual operating conditions. Colt Group's reciprocating and screw compressor ranges are documented with these parameters clearly stated, reflecting over four decades of engineering experience across India's industrial compressed air and gas compression applications.
If you'd like help interpreting a compressor specification or comparing options for your application, get in touch with Colt Group to discuss your requirements with their engineering team.
Frequently Asked Questions
CFM (Cubic Feet per Minute) and SCMH (Standard Cubic Meters per Hour) both measure volumetric flow rate, just in different unit systems — CFM in imperial units and SCMH in metric units referenced to standard conditions. As a rough conversion, 1 CFM is approximately 1.7 SCMH.
FAD (Free Air Delivery) is the actual volume of air a compressor delivers, converted back to inlet conditions, accounting for internal losses. Displacement is only the theoretical swept volume of the cylinders or rotors. Comparing FAD, not displacement, gives an accurate picture of real compressor output.
The “(g)” denotes gauge pressure — pressure measured relative to atmospheric pressure, as opposed to absolute pressure, which includes atmospheric pressure in the total reading.
Specific power (typically kW per 100 CFM or kW per m³/min) measures how much energy a compressor uses to deliver a given volume of compressed air. It's a better indicator of energy efficiency than motor power alone, since two compressors with the same motor rating can have very different specific power figures.
A single-stage screw compressor compresses air in one rotor pass, while a two-stage design compresses in two steps with intercooling between them, achieving lower specific power (better efficiency) at the same discharge pressure.
Higher compression ratios per stage increase discharge temperature and mechanical stress. Multi-stage compression with intercooling allows a compressor to reach high final pressures — such as 200–250 bar for CBG cylinder filling — more efficiently and reliably than a single stage attempting the same pressure rise.
ISO 8573-1 is an international standard that classifies compressed air quality by particle count, water content, and oil content into defined classes. Specifying a class (such as Class 0 for oil content) gives a precise, verifiable air quality requirement rather than a vague “oil-free” claim.
Compressors rated only for moderate ambient temperatures can underperform, trip, or wear out faster in India's high summer temperatures. Equipment rated for higher ambient temperatures (such as up to 48–50°C) maintains rated performance and reliability under local conditions.