Standard Air Constants

Standard air is defined as dry air at 70°F (21°C) and 29.92 in. Hg column (101.325 kPa). The air constants below apply specifically to standard air and can be used in most AC calculations. If very precise data are required, the constants below can be adjusted to correspond to actual dry air conditions. Note that in English system units, the weight of air may be expressed in lbm or lb, which both imply lb-mass (lb is more commonly used in HVAC engineering than lbm). Note also that ft3 is also commonly expressed as cf and cf per minute is commonly expressed as cfm.

Specific volume of standard air = 13.33 ft3/lbm (0.833

Density of standard air = 0.075 lbm/ft3 (1.2 kg/m3)

Specific heat of standard air = 0.24 Btu/lbm • °F (1.00kJ/kg • °C or 0.278 Watt-h/kg • °C)

Average latent heat of water vapor = 1,054 Btu/lbm (2,451 kJ/kg or 681 W-h/kg)

Use of these standard air constants is often sufficient in performing HVAC engineering calculations. Such calculations are, however, limited to the accuracy of the assumption of standard air conditions. To refine the accuracy of such calculations, one must adjust factors such as air density (and specific volume), specific heat, and latent heat. These can be found in psychrometric charts designed for non-standard conditions, including charts for high and low temperatures and barometric pressures.

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