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(For a more detailed review, please refer to The News’ July 3, 2000 article, “Scroll Technology Starts to Take Off.”)
Short HistoryThe scroll idea has been around for quite some time. It was in 1905 that the first patent for a scroll-type compressor was applied for and granted. How-ever, back then it was impossible for designers, engineers, and manufacturers to mass produce such compressors at a reasonable cost.
Within the last 15 to 20 years, however, computer-aided design and manufacturing have made it possible to mass produce a low-cost scroll in high volumes, despite its complicated geometry.
The demand for a quieter, more reliable compressor system was forced on compressor designers because of rising energy costs, federal energy regulations, and increased consumer awareness of sound levels.
The scroll compressor started in residential application, but now its applications have spread to low-temperature systems.
REFRESHERScroll technology is relatively simple to understand. The spiral-shaped parts (scrolls) fit inside one another. These parts are sometimes referred to as “involute spirals.”
One of the spiral-shaped parts stays stationary while the other orbits around it. This is a true orbiting motion, not a rotating motion.
The orbiting motion causes continuous crescent-shaped gas pockets to be formed. It also causes the gas pockets to become smaller and smaller in volume as they near the center of the scroll form. (See Figure 3.)
Once it reaches the center, the gas pocket is discharged out of a port of the nonorbiting (fixed) scroll member. Several crescent-shaped gas pockets are compressed at the same time, providing for a small and continuous compression cycle.
The intake, compression, and discharge phases are simultaneous and continuous.
AdvantagesAn important advantage to scroll compressor design is the fact that a scroll compressor discharges its gas to a zero volume, eliminating any carryover of trapped discharge gases in a clearance volume. Thus, there is no re-expansion of discharge gases to cause unwanted volumetric inefficiencies.
When liquid refrigerant, oil, or small particles enter between the two scrolls, the mating scroll parts can actually move apart in a sideways direction. This radial movement eliminates high-stress situations and allows for the right amount of contact force between mating parts.
The scroll requires no valves, so it does not have any valve losses that contribute to inefficiencies. Because a considerable distance separates the suction and discharge ports or locations, heat transfer is reduced between the two.
Because of the scroll’s continuous compression process and the fact that it has no valves to create valve noise, the scroll produces very low gas pulsation noises with hardly any vibration.
The scroll has few moving parts. This enhances its reliability and efficiency.
BREAKTHROUGHSRecently, Copeland introduced 20- and 25-hp scroll compressors for use in rooftop units, chillers, and vertical self-contained and custom-engineered systems from 20 to 200 tons in capacity. These compressors include features such as:
Soon 18-hp models will be available, the company said. As early as March 2002, selected hp models will be available for use with a full range of refrigerants (22, 407C, and 410A). Tandem compressor configurations will also be available.
Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI, and author of the book Troubleshooting and Servicing Modern Air Conditioning and Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at firstname.lastname@example.org (e-mail).
Publication date: 10/01/2001