Sizing a compressor: altitude, temperature and humidity

A compressor draws an actual volume of air (ACFM, actual cubic feet per minute), but a need is stated in SCFM (standard cubic feet, referred to 1 bar and 20 °C). The gap between the two comes from the density of the intake air: the hotter, more humid, or thinner (altitude) the air, the more volume must be drawn to deliver the same useful amount.

ACFM = SCFM × (P_std / P_dry_air) × (T_site / T_std)

Symbol	Meaning	Value / unit
SCFM	Useful flow required (reference conditions)	SCFM
ACFM	Actual volume to draw at site conditions	ACFM
P_std	Reference pressure	100 kPa (1 bar)
T_std	Reference temperature	293.15 K (20 °C)
P_dry_air	Dry-air pressure at site = atmospheric pressure (ISA per altitude) − water-vapour pressure	kPa
T_site	Intake air temperature	K

This is the ISO 1217 (Annex C) intake-conditions correction: you order the machine on its ACFM capacity at actual conditions, and reason the need in SCFM. To tell the two units apart, see Flow units.

Why correct?

Three effects lower the intake air density and thus inflate the required ACFM:

Altitude — atmospheric pressure falls with altitude: less air mass per volume drawn.
Temperature — hot air is expanded: the same volume holds less mass.
Humidity — water vapour takes up part of the drawn volume at the expense of useful dry air (and there is more of it the hotter the air).

Because these three effects stack, you size on the worst case: the hottest, most humid air of the year (and the site altitude).

Worked example

Need: 500 SCFM. Québec site at 100 m altitude.

Intake conditions	Required ACFM	vs 500 SCFM
Hot, humid summer — 30 °C, 70 % RH	≈ 532 ACFM	+6.4 %
Cool, dry winter — 10 °C, 40 % RH	≈ 485 ACFM	−3.1 %

So size the machine on ≈ 532 ACFM (the summer worst case), even though the “nominal” need is 500 SCFM. Picking 500 with no correction means running short of air on heatwave days.

Altitude changes everything in the mountains. The same need (500 SCFM, 30 °C, 70 % RH) at 1,000 m altitude requires ≈ 595 ACFM (+19 %). In Québec, at low altitude, it’s mainly summer heat and humidity that drive the correction.

Beyond flow

Capacity (ACFM/SCFM) is only one criterion: the real working pressure, the demand profile (steady vs peaks — see the buffer receiver), the target air quality and energy efficiency all matter just as much. To recognise the machine in place, see Identifying an air compressor.

With the Onyx M3 tools

Calculator — Compressor sizing — enter the SCFM need, the altitude and the temperature and humidity ranges: it computes the required ACFM at the min and max conditions and recommends the worst case.
Calculator — Standardised flow — converts SCFM ↔ ACFM ↔ Nm³/h for a given condition.

References

ISO 1217 — Displacement compressors – Acceptance tests (Annex C) — intake-conditions correction (pressure, temperature, humidity)
CAGI — Compressed Air & Gas Handbook — capacity, reference conditions and selection
ISA model (standard atmosphere) — atmospheric pressure by altitude

Frequently asked questions

What's the difference between SCFM and ACFM when choosing a compressor?

SCFM (standard cubic feet) refers the flow to reference conditions (1 bar, 20 °C): the right unit to state a NEED and compare machines. ACFM (actual cubic feet) is the volume actually drawn at site conditions. In hot, humid or high-altitude air the air is less dense, so more volume must be drawn (ACFM > SCFM) to deliver the same useful SCFM.

How do I correct a compressor's capacity for altitude and temperature?

ACFM = SCFM × (P_std / P_dry_air) × (T_site / T_std), with P_std = 100 kPa, T_std = 293.15 K (20 °C). P_dry_air = site atmospheric pressure (which falls with altitude) minus the water-vapour pressure (which rises with temperature and humidity). This is the ISO 1217 (Annex C) correction.

Which conditions should I size on?

The WORST case: the hottest, most humid air and, where relevant, the highest altitude — that's where density is lowest and the ACFM demand is highest. A compressor that holds the worst case holds the rest of the year.

Is altitude a major factor in Québec?

Rarely: most Québec sites are at low altitude, where the effect is small. The dominant factor here is hot, humid summer intake air. At high altitude (mountain sites), the correction instead becomes major.