Air Density Correction for Altitude and Temperature (ACFM/SCFM)
How much thinner the air is at altitude and temperature, and the ACFM, SCFM, and fan-static corrections that follow.
Example
You enter
- Site elevation (ft) 5000
- Air temperature (°F) 70
- Actual airflow ACFM (cfm, optional) 1000
- Sea-level rated fan static (in-wc, optional) 0.5
You get
- Density factor DF 0.8321
- Standard airflow SCFM 832
- Delivered fan static 0.416 in-wc
Checked against published worked examples from: Greenheck.
Details, formula, and sources
ScopeDensity factor DF = (1 - 6.8754e-6 elev)^5.2559 x 530/(460+T), converting ACFM to SCFM = ACFM x DF and correcting the 1.08 sensible constant and the rated fan static. 5,000 ft is ~17% thinner; 120 F rooftop air ~9% thinner at sea level. A correction factor; the fan curve governs.
Formula or tablealt = (1 - 6.8754e-6 elev)^5.2559; temp = 530/(460 + T_F); DF = alt x temp; SCFM = ACFM x DF; const = 1.08 x DF; sp = rated_sp x DF.
Source and editionThe air density correction for altitude and temperature from the ASHRAE Handbook - Fundamentals, by name.
Free accessThe barometric altitude factor and the absolute-temperature density ratio are standard ASHRAE air-property relations. The fan curve and equipment ratings at the actual condition govern.
What governsEstimate. AHJ and licensed professional govern.
API field nameselev_ft, T_F, acfm, rated_sp, DF, SCFM, sp_corr
- Density factor DF = (1 - 6.8754e-6 elev)^5.2559 x 530/(460 + T_F)ASHRAE Fundamentals
- Corrections SCFM = ACFM x DF, sensible constant 1.08 x DF, fan static rated x DFASHRAE
- Dry standard air dry air, standard atmosphere; verify against the equipment's fan curvescope of this tile