TABLE OF CONTENTS The discussion, calculations, and examples presented in this Guide are generally in terms of volume flow rates based on standard air properties. The density of moist air varies with local barometric pressure (which varies with altitude), temperature, and moisture content. Accurate calculation of energy transfer and psychrometric properties requires that flow rates be specified in terms of mass rather than volume. However, common design practice uses engineering formulas based on volume flow rates, and volume values are often required for selection of coils, fans, ducts, and other components.
This issue is typically resolved by expressing airflow volumes in terms of standard airflow. Standard air is defined as dry air at 70 F and 14.696 psia, with a density of 0.075 lb/ft 3 (20 C and 101.325 kPa absolute, with a density of 1.204 kg/m). When the air density is significantly different from standard, the actual flow rate is adjusted to the mass-equivalent standard flow rate, according to the ratio of the actual density to the standard density.
| (4-1) |  |
or
| (4-2) |  |
where
| Q std = | standard airflow rate, scfm (L/s) |
| Q act = | actual airflow rate, acfm (L/s) |
| ? std = | standard air density, 0.075 lb/ft 3 (1.2 kg/m 3) |
| ? act = | actual air density, lb/ft 3 (kg/m 3) |
| v std = | standard specific volume, 13.33 ft 3/lb (0.833 m 3/kg) |
| v act = | actual specific volume, ft 3/lb (m 3/kg) |
The air...
