Scroll compressor

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Mechanism of a scroll pump

Operation of a scroll compressor

A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involute, archimedean spiral, or hybrid curves.^[1]^[2]^[3]^[4]^[5] 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.

The device was first patented in 1905^[6] but did not become commercially produced for air conditioning until the early 1980’s.^[7] It waited for manufacturing to develop cost effect methods for high-volume precision machining of these uniquely shaped parts.

These devices are known for operating more smoothly, quietly, and reliably than conventional compressors in some applications.^[8] Unlike pistons, the orbiting scroll’s mass can be perfectly counterbalanced, with simple masses, to minimize vibration. The scroll’s gas processes are more continuous. The compression process occurs over approximately 1½ 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. And the air-conditioning scroll does not have dynamic valves, gaining flow efficiency and reduced sound versus other compressors.^[9]^[10]

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 occuring at higher ratios of discharge to suction pressures.

As for reliability, scroll compressors have fewer moving parts than reciprocating compressors which, theoretically, should improve reliability. Conventional scroll compressor designs have smaller available free volume (to accommodate refrigerant flooding) compared to reciprocating compressors and have a greater tendency to ingest liquid refrigerant in flooding conditions. Liquid flooding is known to occur in residential split systems that are often installed by service technicians not familiar with (or not concerned with) the proper method for charging a split system. Thus, residential split systems are often overcharged, which can lead to refrigerant flooding. Reciprocating compressors are more robust to such errors. However, scrolls do not have suction valves, which is one of the most vulnerable parts of the reciprocating compressor to liquid flooding. As a result, the reliability of scroll compressors in residential cooling and heating applications has proven to be very competitive. Still, scroll compressors are primarily used in residential heating and cooling systems to meet government-mandated efficiency standards particularly for higher tonnages and customer noise requirements, not reliability requirements.

In general, an accurate reliability comparison is not really available. Manufacturers typically do not publish their reliability data, holding it as proprietary and confidential. What information does get published is usually limited in scope and typically supports one product or the other as part of a marketing campaign. The strength of reciprocating machines in terms of liquid handling is in the large free volume of the shell enclosure, which is mainly a result of the spring suspension required by reciprocating compressors to limit vibration and noise and the weakness of reciprocating machines is in the valve system. Design measures which normally increase the robustness of the suction valves (thicker reeds, lower valve lift) unfortunately also reduce the efficiency of the compressor so the ultimate design is a balancing act between efficiency and reliability.

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. Comprehensive, objective reliability comparisons are, as a rule, not available from any manufacturer.

The twenty or so years since scroll compressors were introduced to the market in the 1980's have seen a very heated competition between the newer scroll and the older reciprocating compressor types which accounts in large part for the many different views on, for example, reliability. The driving force behind this may be traced to the high investment (in both R&D and capital tooling) needed to enter scroll compressor production and the relative difficulty of adapting existing production equipment from reciprocating to scroll compressors. Manufacturers which were heavily invested in reciprocating compressor production have extracted maximum value from their investment by reinvesting in reciprocating technology improvements. Prior to the introduction of scroll compressors, progress in reciprocating technolgy was relatively stagnant compared to more recent activity. However, it is worth noting that since the introduction of the scroll compressor the majority of investment in new production capacity has been for the scroll type and most investment in reciprocating types has been of a sustaining nature, that is in support of existing production capacity. Very few manufacturers have built new reciprocating compresssor factories while many have built scroll factories. A company's decision to enter scroll production is often timed at least in part on the obsolescence of existing production equipment.

Many residential central heat pump and air conditioning systems and a few automotive air conditioning systems employ a scroll compressor instead of the rotary, reciprocating, and wobble-plate compressors traditionally used.

In 2006 a major manufacturer of food service equipment, Stoetling, chose to change the design of one of their softserve 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. ^[11]

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.

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.

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 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.

-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.

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.

Another method is to stop discharging, 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 them. However, they are no longer able to compress refrigerant, thus no work is performed and far less energy is used. The compressor "engages" on a percentage of a given period equal to the load percentage required. This type of scroll compressor capacity control through on-off cycling is proprietary to a single major manufacturer which advertises it under the "Digital Scroll" tradename.

[edit] References

^ Tojo, Kenji: “Scroll Compressor With Means For End Plate Bias And Cooled Gas Return To Sealed Compressor Spaces,” U.S. Patent 4216661, 1980.
^ Tischer, J., Utter, R: “Scroll Machine Using Discharge Pressure For Axial Sealing,” U.S. Patent 4522575, 1985.
^ Caillat, J., Weatherston, R., Bush, J: “Scroll-Type Machine With Axially Compliant Mounting,” U.S. Patent 4767293, 1988.
^ Richardson, Jr., Hubert: “Scroll Compressor With Orbiting Scroll Member Biased By Oil Pressure,” U.S. Patent 4875838, 1989.
^ Etemad, S., Yannascoli, D., Hatzikazakis, M: “Scroll Machine With Wraps Of Different Thicknesses,” U.S. Patent 4834633, 1989.
^ Creux, L: “Rotary Engine,” U.S. Patent 801,182, 1905.
^ David T. Gerken; John L. Calhoun (03 2000). Design Review of Cast Aluminum Scroll Compressor Components ('fee required'). SAE 2000 World Congress. SAE International. Retrieved on February 21, 2007.
^ Scroll Compressors Cool the Cost of Cooling Milk. Wisconsin Public Service Corporation - A WPS Resources Company (07 2003). Retrieved on February 21, 2007.
^ Jim Wheeler (11 1988). "Scroll Compressors: The Inside Story". Contracting Business: 36.
^ James W. Bush; John P. Elson (07 1988). "Scroll Compressor Design Criteria for Residential Air Conditioning and Heat Pump Applications". Proceedings of the 1988 International Compressor Engineering Conference 1: 83-92.
^ Jill Russell (02 2006). Commercial Foodservice Equipment, A Continuous Cool. Appliance Magazine. Retrieved on January 10, 2007.