minute (NCFM) refers to air at normal conditions, such as 14.7 psia, 68°F, 36% relative humidity. However, there is not an agreed upon definition for what constitutes normal conditions. Free air delivery Free air delivery (FAD) indicates delivered air, referred to as "free air", at inlet conditions. Centrifugal fan rating Ratings found in centrifugal fan performance tables and curves are based on standard cubic feet per minute (SCFM). Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds mass per cubic foot, with the atmospheric pressure at sea level of 29.92 inches of mercury and a temperature of 70°F. Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and brake horsepower. The volume of air will not be affected in a given system because a fan will move the same amount of air regardless of the air density. If a centrifugal fan is to operate at a non-standard density, then corrections must be made to static pressure and brake horsepower. At higher than standard elevation, usually sea level, and higher than standard temperature (70°F), air density is lower than standard density (0.075 pounds per cubic foot). Centrifugal fans that are specified for continuous operation at higher temperatures need to be selected taking into account air density corrections. Again, a centrifugal fan is a constant volume device that will move the same amount of air at two different temperatures. If, for example, a centrifugal fan moves 1,000 CFM at 70°F it will also move 1,000 CFM at 200°F. The air volume delivered by the centrifugal fan is not affected by density. However, since the 200°F air weighs much less than the 70°F air, the centrifugal fan will create less static pressure and will require less brake horsepower. When a centrifugal fan is specified for a given CFM and static pressure at conditions other than standard, an air density correction factor must be applied to select the proper size fan to meet the new condition. Since 200°F air weighs only 80% of 70°F air, the centrifugal fan will create less pressure. To get the actual pressure required at 200°F, the designer would have to multiply the pressure at standard conditions by an air density correction factor of 0.8 (i.e., 0.8/ 1.0) to get the system to operate correctly. To get the actual horsepower at 200°F, the designer would