Air compressors: CFM, free air delivery and tank size explained

An air compressor spec sheet usually has three or four headline numbers: an air flow in CFM or litres per minute, a maximum pressure in bar or psi, a tank size in litres and a motor power. They are not all equally useful. This guide uses the makers’ own figures to show which ones to compare.

Displacement or intake versus free air delivery

The biggest air figure on many compressors is the piston displacement (sometimes called intake or suction capacity): how much air the pump draws in. What reaches your tool is less. SIP publishes both figures for its compressors, which makes the gap easy to see:

• SIP QT 24 litre: 8 CFM (226 l/min) displacement, 6.5 CFM (184 l/min) free air delivery.
• SIP AIRMATE 50 litre: 9.6 CFM (272 l/min) displacement, 4.7 CFM (131 l/min) free air delivery.
• SIP QT 5 litre battery & mains: 2.8 CFM displacement, 2.3 CFM (65 l/min) free air delivery.

Notice that the 50 litre AIRMATE has the bigger displacement figure but the smaller free air delivery (FAD). Compare compressors on FAD.

Einhell goes a step further for its TE-AC 240/24 Silent. The model name comes from its 240 l/min intake, but Einhell also lists the output at different pressures: 166 l/min at 0 bar, 120 l/min at 4 bar and 92 l/min at 7 bar. Delivered air falls as working pressure rises, so if your tool works at around 6 bar, the figure near that pressure is the one that matters.

Converting CFM and litres

SIP gives both units, so you can see the conversion: 8 CFM is 226 litres a minute — roughly 28 litres per CFM. If a tool maker gives its air use in one unit and the compressor maker in the other, that is close enough to compare.

Match the compressor to the tool

SIP’s buying guide says knowing what CFM your compressor needs to produce is vital to ensure power for all your air tools. The tool’s maker states how much air it uses; the compressor’s FAD needs to cover it. SIP suggests its QT 24 litre for inflation, nailing, stapling and airbrushing, and lists the QT 5 litre for nailing, stapling, airbrushing and tyre inflation.

Tank (receiver) size

SIP explains that the tank stores air and affects run time: larger receivers run for longer at more consistent CFM. A bigger tank doesn’t make the pump deliver more air; it stores more, so you get longer before the pump has to catch up. SIP pitches the 50 litre AIRMATE at light trade and automotive use as well as DIY, even though its FAD is lower than the 24 litre QT’s.

Pressure

Pressure is given in bar and psi (SIP lists 8 bar as 116 psi). The Einhell lets you regulate up to 10 bar; the SIP QT 5 litre runs from 1.4 to 9 bar. Most compressors here have two gauges — tank pressure and regulated working pressure — so you can set what the tool needs.

Direct drive, belt drive and rotary screw

SIP’s guide describes three designs:

• Direct drive: the pump crankshaft is connected straight to the motor shaft — simple, reliable and cost-effective. The SIP compressors in this guide are all direct drive.
• Belt drive: a belt turns the pump, which suits jobs needing more air for longer, and the pump runs slower and quieter.
• Rotary screw: twin screws give continuous air, often for several users at once.

Oil-free, noise and power supply

Every compressor in this guide is oil-free. SIP says oil-free pumps mean low maintenance and cleaner air to tools. Noise varies a lot: SIP rates its QT 24 litre at 59 dB and the AIRMATE at 62 dB(A), and Einhell gives 70 dB(A) for the TE-AC 240/24 Silent.

SIP notes that smaller compressors usually run from a normal 230 V 13 A supply, while larger units need more, up to three-phase power. The three SIP compressors here are listed for a 230 V 13 A supply, and the SIP QT 5 litre can also run from a battery.

Looking after it

Water condenses in a compressor’s tank. Einhell fits a drain tap so it can be let out easily; follow the maker’s manual for draining and other maintenance. If you use a compressor at work, HSE notes that pressure equipment must meet the Pressure Systems Safety Regulations as well as PUWER — see machinery safety at home and at work.

Quick checklist

1. Find the air use of the tool you’ll run most.
2. Compare compressors on free air delivery, ideally at your working pressure.
3. Choose the tank size for how long your bursts are.
4. Check the pressure range, noise level and plug.

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