A scroll compressor (also called spiral compressor, scroll pump and scroll vacuum pump) is a device for compressing air or refrigerant. It is used in air conditioning equipment, as an automobile supercharger (where it is known as a scroll-type supercharger) and as a vacuum pump.
A scroll compressor operating in reverse is known as a scroll expander, and can be used to generate mechanical work from the expansion of a fluid.
Many residential central heat pump and air conditioning systems and a few automotive air conditioning systems employ a scroll compressor instead of the more traditional rotary, reciprocating, and wobble-plate compressors.
Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid between the scrolls. Another method for producing the compression motion is co-rotating the scrolls, in synchronous motion, but with offset centers of rotation. The relative motion is the same as if one were orbiting.
Another variation, is with flexible (layflat) tubing where the archimedean spiral acts as a peristaltic pump, that operates on much the same principle as a toothpaste tube. and have casings filled with lubricant to prevent abrasion of the exterior of the pump tube and to aid in the dissipation of heat, and use reinforced tubes, often called 'hoses'. This class of pump is often called a 'hose pump'. Furthermore, since there are no moving parts in contact with the fluid, peristaltic pumps are inexpensive to manufacture. Their lack of valves, seals and glands makes them comparatively inexpensive to maintain, and the use of a hose or tube makes for a relatively low-cost maintenance item compared to other pump types.
The scroll compression process is nearly one hundred percent volumetrically efficient in pumping the trapped fluid. The suction process creates its own volume, separate from the compression and discharge processes further inside. By comparison, reciprocating compressors leave a small amount of compressed gas in the cylinder, because it is not practical for the piston to touch the head or valve plate. That remnant gas from the last cycle then occupies space intended for suction gas. The reduction in capacity (i.e. volumetric efficiency) depends on the suction and discharge pressures with greater reductions occurring at higher ratios of discharge to suction pressures.
Scroll compressors have fewer moving parts than reciprocating compressors. This simplifies risk assessment and theoretically increases reliability.
In 2006 a major manufacturer of food service equipment, Stoetling, chose to change the design of one of their soft serve ice cream machines from reciprocating to scroll compressor. They found through testing that the scroll compressor design delivered better reliability and energy efficiency in operation.
Scroll compressors utilize different methods of protection inside the compressor to handle difficult situations. Some scroll designs utilize valves at different points in the compression process to relieve pressure inside the compression elements.
A reciprocating compressor can run in either direction and still function properly, whereas a scroll compressor must rotate in one direction only in order to function. This can be important during extremely short periods of power loss when a scroll compressor may be forced to run backward from the pressure in the discharge line. Only single phase scroll compressors would run in reverse during short power interruption. If this happens, the scroll compressor will stop pumping. Running scroll compressor in reverse for several minutes would normally not damage the compressor. The three phase compressor, as compared to single phase compressors, would continue to operate in a forward direction after short power interruption. However, it is important to properly wire the three phase compressor during the initial installation. If during the installation the polarity is inadvertently reversed then the three phase compressor would run backward and the damage to the compressor may result if it goes unnoticed for long period of time. One of the ways to mitigate the flooded operation of the compressor on start up, is to actually run the compressor for several minutes in the reverse direction before turning the compressor in the forward direction. The short reverse run on the start up would expel any liquid accumulated inside the compressor pumping element back into the crankcase, as well as as preheat the liquid stored in the crankcase by dissipated motor heat. Expelling the liquid from the pumping element and preheating any liquid refrigerant in the crankcase prior to initiating the normal run in the forward direction significantly alleviates problems with the flooded start.
Recently, scroll compressors have been manufactured that provide part-load capacity within a single compressor. These compressors change capacity while running. One method is to delay the start of compression. The beginning stages of compression are vented back to suction. This reduces the amount of fluid that will be compressed. The rest of the compression process is normal.
Emerson manufactures a scroll compressor under the "Digital Scroll" trade name that is capable of stopping discharge, intermittently. 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. Even though the capacity of the scroll compressor can be varied down to 10%-20% of its normal capacity, the power consumption is still substantial. The digital scroll compressors are still not nearly as efficient as compressors operated by variable speed drive.