Understanding Refrigeration Compressors — Part 2 of 3 - Rotary Screw Compressors | Blog No. 107

In modern industrial refrigeration, reliability and capacity are critical. Among the various compressor designs available, rotary screw compressors have become the preferred solution for large-scale applications. Their ability to deliver high capacity with fewer moving parts has made them a cornerstone of today’s industrial refrigeration systems.

Understanding Rotary Screw Compressors

 Rotary screw compressors were originally developed in the 1930s; however, early versions were not efficient enough for refrigeration applications. Significant advancements came later when engineers began injecting oil into the compression chamber. This oil serves multiple purposes: sealing internal clearances, absorbing heat, and lubricating moving components. Unlike reciprocating compressors, which rely on pistons and valves, rotary screw compressors utilize two intermeshing rotors to continuously compress vapor.

As the rotors turn, refrigerant vapor enters the open end of the compressor. The space between the rotor threads gradually decreases as the vapor moves toward the discharge end, increasing pressure until the vapor exits the compressor. This process results in smooth, continuous compression, rather than the pulsating compression seen in piston-based machines. 

Because rotary screw compressors can operate at high speeds—often 3,600 RPM or higher—they can achieve large capacities in a relatively compact footprint. This makes them ideal for industrial facilities with high refrigeration loads, where space and reliability are key considerations.

A helpful analogy for understanding rotary screw compression is a meat grinder. When meat is fed into the grinder, the screw threads pull it forward. As the meat moves along, the available space becomes smaller, forcing the meat through a tight opening at the end. Similarly, in a rotary screw compressor, refrigerant vapor is drawn in, trapped between the rotor threads, and compressed as the space gradually reduces toward the discharge end. 

Efficiency Considerations 

One challenge with rotary screw compressors is that they operate at a fixed volume ratio. If system conditions cause the discharge pressure to differ significantly from the compressor’s design pressure, inefficiencies can occur. This may result in over-compression or under-compression, increasing energy use and operating costs. 

To address this, manufacturers often use Variable Vi (volume index) control. Variable Vi allows the compressor to adjust its internal compression ratio to better match actual system conditions. This adjustment can be automated through system controls or manually set on the compressor, improving efficiency and reducing unnecessary energy losses. 

Rotary screw compressors have earned their reputation as the compressor of choice for large industrial refrigeration systems. Their continuous compression process, high operating speeds, and compact design make them well-suited for demanding applications. While efficiency depends on properly matching system conditions, modern design features like Variable Vi help ensure reliable and efficient performance in real-world operations.

Thank you for reading! Stay tuned for more!

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