Scroll Compressors & Things to Check for Overheating with Jeff Kukert

Scroll compressors compress refrigerant in the head, and high-side pressure is contained by the compressor’s top cap, muffler plate, the fixed and orbiting scrolls, the IPR valve, and the floating seal. Low-pressure gas enters from the outer edge of the scrolls, and the high-pressure gas is discharged from the center.

A fixed scroll doesn’t move and is bolted to the main bearing housing, and the orbiting scroll oscillates within the fixed scroll, which compresses the gas during that motion. The scrolls have axial and radial compliance, which means they can lift up and down slightly (1 mm) and side to side, respectively. An Oldham coupling transforms the rotating motion of the motor into an orbiting motion on the orbiting scroll. A floating seal has a ridge that pushes against the muffler plate and separates the pressures during operation (it floats down and equalizes pressure when the compressor isn’t running); it also acts as a protection device against excessive compression ratio.

Oil lubricates all of the bearings, including the lower, main, and drive bearings, and it must be between the straddle bearing and the orbiting scroll. The scroll compressor also has various safeties, including the TOD (a bimetal disk that flexes in response to high temperature) and an IPR (a pressure safety).

Prevention is the best cure when it comes to compressor failure. Best practices to avoid compressor failure later include purging and flowing with nitrogen, checking to make sure a system is leak-free, and pulling a deep vacuum on a system. Apart from using nitrogen while brazing, make sure you avoid overheating the metal based on the material (especially copper-clad steel) and use alloys with silver. When crankcase heaters are necessary, be sure to install them correctly, referring to the Copeland AE bulletins.

Compression ratio is a major performance indicator, and high compression ratios cause high discharge pressures, which lead to overheating. Overheating causes oil to lose lubricity and break down, which leads to premature failure. Low suction pressure and high head pressure cause high compression ratios, and common causes include refrigerant losses, restrictions, pressure drops in the suction line, and inadequate airflow on the high side. Compressor performance sheets, available on the Copeland mobile app, can also give you some insight into the performance data we can yield from our compression ratios.

Strategies that control compression ratio include liquid and vapor injection. Liquid injection is common in refrigeration and injects liquid refrigerant into the scroll set after a metering assembly; it has a DTC valve (mechanical) or a stepper valve (electronic). Vapor injection takes cool vapor refrigerant (from a heat exchanger that also increases subcooling) and inserts it into the compressor head via ports.

We can also be diligent and check condenser airflow, verify subcooling (especially via a sight glass), ensure coil cleanliness, and make sure our electrical connections and components are in good working order.

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