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. 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.
A scroll compressor operating in reverse is a scroll expander, and can generate mechanical work.
History
Léon Creux first patented a scroll compressor in 1905 in France and the US (Patent number 801182). Creux invented the compressor as a rotary steam engine concept, but the metal casting technology of the period was not sufficiently advanced to construct a working prototype, since a scroll compressor demands very tight tolerances to function effectively. In the 1905 patent, Creux defines a co-orbiting or spinning reversible steam expander driven by a fixed radius crank on a single shaft. However, the scroll expander engine could not overcome the machining hurdles of radial compliance inherent to achieving efficiency in scroll operation that would not be adequately addressed until the works of Niels Young in 1975. The first practical scroll compressors did not appear on the market until after World War II, when higher-precision machine tools enabled their construction. In 1981, Sanden began manufacturing the first commercially available scroll compressors for automobile air conditioners. They were not commercially produced for room air conditioning until 1983 when Hitachi launched the world's first air conditioner with a hermetic scroll compressor.
Design
A scroll compressor uses two interleaving scrolls to pump, compress or pressurize fluids such as liquids and gases. The vane geometry may be involute, Archimedean spiral, or hybrid curves.
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. An eccentric shaft can provide the orbital motion but the scroll must be prevented from rotating, typically with an Oldham-type coupling, additional eccentric idler shafts, or a bellows joint (particularly for high-purity applications). 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.
Leaks from axial gaps are prevented by the use of spiral-shaped tip seals, placed into grooves on the tips of both spirals. These tip seals also help lower the friction and can be replaced when worn down. Some compressors use the pressurized discharge gas to push both scrolls together, eliminating the need for tip seals and improving sealing with use; these compressors are said to wear-in instead of wear-out, but other parts such as the Oldham ring can still wear out.
Another variation is with flexible (layflat) tubing where the archimedean spiral acts as a peristaltic pump, which operates on much the same principle as a toothpaste tube. They 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 pumper'.
Engineering comparison to other pumps
These devices are known for operating more smoothly, quietly, and reliably than conventional compressors in some applications.
Rotations and pulse flow
The compression process occurs over approximately 2 to 2½ rotations of the crankshaft, compared to one rotation for rotary compressors, and one-half rotation for reciprocating compressors. The scroll discharge and suction processes occur for a full rotation, compared to less than a half-rotation for the reciprocating suction process, and less than a quarter-rotation for the reciprocating discharge process. Reciprocating compressors have multiple cylinders (typically, anywhere from two to six), while scroll compressors only have one compression element. The presence of multiple cylinders in reciprocating compressors reduces suction and discharge pulsations. Therefore, it is difficult to state whether scroll compressors have lower pulsation levels than reciprocating compressors as has often been claimed by some suppliers of scroll compressors. The more steady flow yields lower gas pulsations, lower sound and lower vibration of attached piping, while having no influence on the compressor operating efficiency.