ACFM (actual cfm) is the volume of air moved per minute by a fan regardless of air density. According to the fan laws, a fan will move the same volume of air at a given RPM, regardless of air density. As the air gets denser, the fan does more work, and brake hp increases, but ACFM remains constant.
SCFM (“standard” cfm) is the volume of air moved by a fan at a given RPM when that fan is at sea level and the blower temperature is 70 deg F (for HVAC applications – the definition may be different for compressors, etc ). For HVAC calculations, Dry Air Mass Flow (DAMF, measured in lb dry air per hour) is used, because cooling BTUH is easily calculated by:
Cooling BTUH = DAMF x (h1 – h2), where h1 and h2 are the entering and leaving enthalpies, respectively.