Propeller Fans have sheet metal blades, usually four or more in number, which are curved to provide an aerodynamic lift and are generally arranged without blade overlap (PREMAIRPROP™ blades overlap). These fans operate best at free delivery or at very low static pressures. Propeller Fans are suitable for conditions which require movement of a large volume of air against low resistance to flow. These fans are recommended for flushing cooling air through small enclosures such as radio cabinets, instrument panels, and communications equipment shelters. They come in a variety of voltages and frequencies and are available with lead wires only. Most units are available with an optional internal Fan Performance Sensor (FPS) or external Low Speed Warning Device (LSWD).