Definitions

wobble plate

Scroll compressor

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.

History

Léon Creux first patented a scroll compressor in 1905 in France and the US (Patent number 801182). Creux originally 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. The first practicable scroll compressors therefore did not appear on the market until after World War II when the development of higher precision machine tools permitted their construction and were not commercially produced for air conditioning until the early 1980s.

Design

A scroll compressor uses two interleaved 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. 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.

Engineering comparison to other pumps

These devices are known for operating more smoothly, quietly, and reliably than conventional compressors in some applications. Unlike pistons, the orbiting scroll’s mass can be perfectly counterbalanced, with simple masses, to minimize vibration. However, Oldham coupling mass that ensures proper position of the orbiting scroll cannot be balanced. The scroll’s gas processes are more continuous. Additionally, a lack of dead space gives an increased volumetric efficiency.

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. The more steady flow yields lower gas pulsations, lower sound, lower vibration, and more efficient flow.

Valves

Scroll compressors never have a suction valve, but depending on the application may or may not have a discharge valve. The use of a dynamic discharge valve is more prominent in high pressure ratio applications, typical of refrigeration. Typically, an air-conditioning scroll does not have dynamic valves. The use of a dynamic discharge valve improves scroll compressor efficiency over a wide range of operating conditions, when the operating pressure ratio is well above the built-in pressure ratio of the compressors. However, if the compressor is designed to operate near a single operating point, then the scroll compressor can actually gain efficiency around this point if there is no dynamic discharge valve present (since there are small additional discharge flow losses associated with the presence of the discharge valve).

Efficiency

The isentropic efficiency of scroll compressors is slightly higher than that of a typical reciprocating compressor when the compressor is designed to operate near one selected rating point. The scroll compressors are more efficient in this case because they do not have a dynamic discharge valve that introduces additional throtlling losses. However, the efficiency of a scroll compressor that does not have a discharge valve begins to decrease as compared to the reciprocating compressor at higher pressure ratio operation. This is a result of so called under-compression losses that occur at high pressure ratio operation of the positive dispalacement copressors that not have a dynamic discharge valve.

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.

Vulnerabilities

Scroll compressors are more vulnerable to introduced debris, as any debris need to pass through at least two closed compression pockets. The scrolls that operate without radial and/or axial compliance are even more prone to the damage caused by foreign objects. However, scrolls do not have suction valves, which is one of the most vulnerable parts of the reciprocating compressor to liquid flooding.

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.

Size

Scroll compressors tend to be very compact and smooth running and so do not require spring suspension. This allows them to have very small shell enclosures which reduces overall cost but also results in smaller free volume. This is a weakness in terms of liquid handling. Their corresponding strength is in the lack of suction valves which moves the most probable point of failure to the drive system which may be made somewhat stronger. Thus the scroll mechanism is itself more tolerant of liquid ingestion but at the same time is more prone to experience it in operation. Small size of a scroll compressor and quiet operation allows for the unit to be built into high power density computers, like IBM mainframes. Scroll compressors also simplify the piping design, since they require no external connection for the primary coolant.

Partial loading

Until recently, scroll compressors operated 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 (called hot-gas bypass), vary motor speed, or provide multiple compressors and stage them on and off in sequence. Each of these methods has drawbacks. Hot gas bypass short-cycles the normal refrigeration cycle and allows some of the compressed gas to return directly to the compressor without doing any useful work. This practice reduces overall system efficiency. A variable speed motor requires more electrical connections and switching, adding cost. Compressor cycling requires more compressors, increasing cost.

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.

References

See also

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