Scroll Compressor - Partial Loading

Partial Loading

Until recently, scroll compressors could only operate at full capacity when powered. Modulation of the capacity was accomplished outside the scroll set. In order to achieve part-loads, engineers would bypass refrigerant from intermediate compression pocket back to suction, vary motor speed, or provide multiple compressors and stage them on and off in sequence. Each of these methods has drawbacks:

• Bypass short-cycles the normal refrigeration cycle and allows some of the partially compressed gas to return back to the compressor suction without doing any useful work. This practice reduces overall system efficiency.
• A two-speed motor requires more electrical connections and switching, adding cost, and may have to stop to switch.
• A variable speed motor requires an additional device to supply electrical power throughout the desired frequency range. Also variable frequency drive associated with variable speed compressor has its own electrical losses, and is a source of additional significant cost and often is an additional reliability concern.
• Compressor cycling requires more compressors and can be costly. In addition, some compressors in the system may have to be very small in order to control process temperature accurately.

Recently, scroll compressors have been manufactured that provide part-load capacity within a single compressor. These compressors change capacity while running.

Reciprocating compressors often have better unloading capabilities than scroll compressors. Reciprocating compressors operate efficiently in unloaded mode when flow to some of the cylinders is completely cut off by internal solenoid valves. Two stage reciprocating compressors are also well suited for vapor injection (or so-called economized operation) when partially expanded flow is injected between the first and second compression stages for increased capacity and improved efficiency. While scroll compressors can also rely on vapor injection to vary the capacity, their vapor injection operation is not as efficient as for the case of reciprocating compressors. This inefficiency is caused by continuously changing volume of the scroll compressor compression pocket during the vapor injection process. As the volume is continuously being changed the pressure within the compression pocket is also continuously changing which adds inefficiency to the vapor injection process. In case of a two stage reciprocating compressor the vapor injection takes place between the two stages, where there is no changing volume. Both scroll and reciprocating compressors can be unloaded from mid-stage compression, however reciprocating compressors are also more efficient for this mode of unloading than scroll compressors, because the unloaded port dimensions in case of scroll is limited by the internal port size, which would not be the case for a reciprocating compressor where unloading again occurs from between the two stages.

Emerson manufactures a scroll compressor that is capable of varying the refrigerant flow as per requirement. Instead of fixing the scrolls together permanently, the scrolls are allowed to move apart periodically. As the scrolls move apart, the motor continues to turn but the scrolls lose the ability to compress refrigerant, thus motor power is reduced when the scroll compressor is not pumping. By alternating the two different working states: the loaded state and the unloaded state. A solenoid contracts and expands the rotating scroll and/or the fixed scroll, using axial compliance. The controller modifies the load time, and the unload time, matching the capacity of the compressor to the load requested. This type of scroll compressors while offering variable capacity control, normally down to 20% of the full flow, can suffer from a significant loss of efficiency especially toward the lower range of the capacity control.