Inside a Scroll Compressor
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Sometimes, people will say that a malfunctioning or underperforming scroll compressor has “bad valves.” However, scroll compressors don’t have “valves” in the traditional sense; reciprocating compressors have suction and discharge valves that work with the pistons.
Instead of having a piston (or set of pistons) inside the compressor shell, a scroll compressor has two scrolls. The top scroll is fixed (stationary scroll), and the bottom scroll moves and is called an orbiting scroll. The orbiting scroll doesn’t actually spin as many people think; it oscillates and uses more of a side-to-side motion to compress the gas between the scrolls.
The scrolls are metallic, so oil lubrication is important to keep them quiet and prevent them from wearing out. There is also a discharge check valve on the stationary scroll. That valve opens and closes, which prevents the refrigerant from going back into the compressor during the off cycle. If that discharge check valve fails, you may hear the compressor run backward in the off cycle.