25.12 Fans and Blowers Fans and blowers are essential parts of most refrigeration and air conditioning systems. The purpose of any fan or blower is to move a specific volume of air at a specified velocity (speed). The amount of air moved (volume) is measured in cubic feet per minute (cfm). The volume of air that a fan can move is determined by several factors. If any one of these factors changes, the volume of air moved also changes: • •• Speed of rotation. •• Number of fan blades. •• Blade pitch (angle, in degrees). •• Blade length. • Blade width and shape. In HVACR applications, there are two general types of fan blades used to move air: • •• Axial (propeller) blades are used for fans. • Centrifugal (squirrel cage) blades are used for blowers. 25.12.1 Axial Blade Fans There are two types of propeller (axial) blades: round and square. See Figure 25‑62. Each type serves a definite purpose. The round blade is best suited for free-air applications, such as pedestal fans. It does not perform well operating against pressure. To obtain proper airflow, the movement of inlet air and discharge air must be unrestricted. The round blade has excellent efficiency in free- air conditions and is almost noiseless because the air enters and exits at different angles along the blade length. The square-type propeller blade is designed to operate against medium and high pressures in such applications as evaporators and condensers. The discharge blade tip is at full diameter, providing an air seal at the corners. The square blade fan is very efficient when placed in a cowling or orifice that surrounds the blade tips. A gap of about ¼″ (6 mm) is permitted between the blade tips and the orifice. Fan Blade Rotation Proper rotation of a fan blade is determined at the factory and cannot be changed in the field. Most propeller-type blades are mounted directly B A Goodheart-Willcox Publisher Figure 25‑62. Square blades and round blades are used for different applications. Square blade fans are used in most air conditioning applications. A—Square blades. B—Round blades. onto the motor shaft, so the rotation of the fan blades must match the rotation of the motor. Rotation of fan blades is said to be clockwise (CW) or counterclockwise (CCW) as viewed from the discharge air side. Rotation for draw-type fan blades (those that pull air back over the motor) must be viewed from the motor side. Rotation for blow-type fan blades must be determined from the front of the blades, with the motor behind. Most fan blades are not designed to be reversed (mounted backward). Reversing a fan blade causes severe reduction in the volume of air moved, increases motor and fan blade fatigue, and causes system problems. It is always best to replace a fan blade with an exact duplicate, but some allowance 518 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.