Flow Units

Flow (volume rate) determines the speed of work. This is where it gets complicated because air is compressible.

The Fundamental Problem

Air is compressible. 100 liters of air at 100 psig occupy only ~14 liters at atmospheric pressure.

When we talk about flow, we must specify at what conditions we're referring.

Unit	Name	Reference conditions
scfm	Standard Cubic Feet per Minute	14.7 psia, 68°F (20°C), 0% RH
Nm³/h	Normal cubic meter per hour	1.013 bar, 32°F (0°C), 0% RH
Sm³/h	Standard cubic meter per hour	1.013 bar, 59°F (15°C), 0% RH
sl/min	Standard liters per minute	1.013 bar, 68°F (20°C), 65% RH

Caution!

scfm and Nm³/h are NOT directly convertible because they use different reference temperatures.

Term	Meaning
SCFM	Flow converted to standard conditions
ACFM	ACTUAL flow at operating conditions
ICFM	Flow at compressor INLET
FAD	Free Air Delivery - "free" air flow

ACFM = SCFM \times \frac{P_{std}}{P_{actual}} \times \frac{T_{actual}}{T_{std}}

Example:

ACFM = 100 \times \frac{14.7}{114.7} \times \frac{555}{528} = 13.5 \text{ acfm}

100 scfm becomes only 13.5 cfm of actual flow in the pipe!

FAD is the most common measure for compressor capacity. It represents the free air flow the compressor can deliver.

To compare compressors

Always compare FAD under the same reference conditions.