TABLE OF CONTENTS A fan is a gas flow producing machine which operates on the same basic principles as a centrifugal pump or compressor. Each of these devices, including fan, converts rotational mechanical energy, applied to the shaft, to total pressure increase of the moving gas. This conversion is accomplished by changing the momentum of the fluid.
The American Society of Mechanical Engineers power test codes limit fan definition to machines that increase the density of the gas by no more than 7% as it travels from inlet to outlet. This is a rise of about 30 inches of water pressure based on standard air. For pressure higher than 30 in water gauge, the air-moving device is a compressor, or pressure blower. Fans for heating, ventilating and air conditioning, even on high velocity, high-pressure systems, rarely encounter more than 10-12 inches of water pressure.
Terminology, Abbreviations, and Definitions. Definitions of terms common in fan technology follow.
Standard Air: Air at a temperature of 70 F dry bulb and a barometric pressure of 29.92 inches of mercury with a density of 0.075 lb/ft 3.
Water Gauge (wg) (Fig. 14-1): The measure of pressure above atmospheric expressed as the height of a column of water in inches (as in Fig. 14-1, atmospheric at sea level equals 407.1 inches of water or 33.97 feet of water).
cfm and scfm: The cubic feet per minute ( cfm) of air...
