Inside it looks like a doodle. But if the "scroll" air compressor's swirly internals catch the eye, performance definitely grabs the attention. Absent are the clicking valves, vibration, and unreliable seals that characterize reciprocating compressors. The scroll's rotary motion delivers quiet, clean, and reliable performance. The operating element of a scroll compressor is made up of two identical involutes, which form right- and left-hand components. One scroll is indexed or phased 180 degrees with respect to the other so the scrolls mesh. This indexing creates crescent shaped gas pockets, bounded by the involutes and base plates of both scrolls. In operation, one scroll remains fixed; the other is attached to an eccentric driven by the motor shaft. As the moving scroll orbits the fixed scroll, the gas pockets formed by the meshed scrolls follow the spiral inward to compress air, which is discharged through an outlet in the center of the scrolls. Running in reverse, the scroll compressor becomes an expander or air motor.