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What Is a Scroll Compressor?

By Paul Reed
Updated May 17, 2024
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A scroll compressor is used in air conditioning and refrigeration systems to compress refrigerant gas for cooling. The design of a scroll system has been in existence since the early 20th century, but precision machining techniques did not support commercial development until the 1970s. A scroll consists of two spiral sections, one stationary and one that orbits in a circle, creating a compression effect needed for refrigeration.

Refrigeration systems require a compressor to operate correctly, because refrigerant gas needs to be compressed, then expanded to reduce the gas pressure. This pressure drop, combined with the refrigerant changing from a liquid to a vapor, lowers the temperature of the gas circulating inside metal coils and provides cooling. Low-pressure gas then returns to the compressor, and the cycle is repeated until the desired temperature is reached.

The scroll compressor design consists of two scroll or spiral-shaped parts, one welded into the compressor body, and the other connected to an electric motor. When the motor starts, the moving scroll turns in an orbital motion, which might be described like the motion of a marble inside a pan. The motion is circular, rather than a back-and-forth motion found in reciprocating compressors.

As the scroll orbits around the stationary part, pockets of refrigerant gas are trapped between the two scroll parts. The scroll compressor gas inlet is on the outside of the widest part of the scroll, and the high-pressure exit is at the center. Trapped gas moves around the scroll spiral, moving into an increasing small area, which results in a higher pressure. When the gas reaches the center of the scroll unit, it is at the desired discharge pressure and exits the compressor.

A scroll compressor can be very durable, because there are few moving parts and the unit is not subject to some kinds of compressor damage. Reciprocating compressors using pistons can be damaged if liquid refrigerant enters, because liquid does not compress and can damage or even destroy a compressor. A scroll will accept some liquid, because the moving scroll section is not locked against the stationary part, and can shift slightly if liquid enters. The effect of liquid on a compressor is called "liquid slugging," and a scroll compressor is a good choice if slugging is likely.

Reciprocating compressors use pistons and cylinders, similar to an automotive engine, to compress refrigerant gas. The pistons have rings that seal the cylinder walls and allow the gas pressure to rise. As the rings and cylinders wear, gas can bypass the pistons, resulting in lower compressor and possible compressor failure. A scroll compressor is sealed by the two scroll members sliding against each other, and they maintain compression with less gas bypass. The compression efficiency can increase over time, because the sliding scroll will polish the surfaces of the compressor and improve the sealing ability.

Most compressors have to be installed in a specific orientation, either upright for most reciprocating compressors, or sideways for screw compressors. Since the late 20th century, manufacturers have the choice of providing scroll compressors that can be mounted upright or on their side, allowing flexibility in a customer's facility. The orbiting motion of scrolls also results in lower vibration, which can make them an advantage in applications where noise or vibration need to be controlled.

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