Variable wall thickness boostsscroll compressor's performance

Willowbrook, IL--For many years, engineers realized that they could improve compressor efficiency by employing a spiral-shaped inlet member known as a scroll device. Unfortunately, the inherent complexity and limited performance of scrolls restricted them to low-horsepower and low-compression-ratio applications.

Conventional scroll air compressors can generate compression ratios of roughly 3:1. But industrial refrigeration systems and pneumatic gas sources need compression ratios ranging from 7:1 to 10:1 or even higher. Previous attempts to boost scroll-type compression ratios failed, mainly because of internal heat buildup and manufacturing difficulties.

Now, however, that's changing. An innovative new configuration allows engineers to design scroll compressors capable of serving in medium- and high-horsepower and high-compression-ratio applications.

The new design eliminates many problems by employing a radical new geometry. It includes five main parts: a fixed scroll; a movable scroll that orbits around the fixed scroll; a shaft with bearings; an Oldham ring; and a housing. This system departs from the conventional, however, in the shape of the scroll members.

Unlike conventional scrolls, which use a simple spiral shape, the new system employs a geometry in which the wall thickness of the spirals varies. The new geometry minimizes the number of spiral turns (2.5 to 3.5 turns) of the scroll vane, and at the same time achieves a 7:1 or higher compression ratio.

When two of these scroll members are inter-fit with one another at an angular and radial offset, a number of line contacts develop between their spiral curved surfaces. As one scroll orbits (it does not rotate) around the other, the line contacts shift along the curved surfaces. This shift causes the volume of contained gas pockets to change more rapidly than in a conventional scroll device--increasing or decreasing in response to the direction of the scroll. Result: gas compresses or expands.

By designing the Scroll Air Compressor in this way, engineers at the Mindtech Corp. improved its efficiency and manufacturability. The reason: The Scroll Air Compressor achieves a 7:1 compression ratio with only three turns of the spiral. In contrast, conventional scroll compressors need at least six turns of the spiral to reach 7:1. And those six turns would add substantial complexity to the manufacturing process.

Fewer turns also shorten the dwell time of the gas inside the scroll. And because the compressed gas resides inside the chamber for a shorter time, less heat is dissipated throughout the rest of the device. Result: It runs more efficiently. "When you compress the gas faster, the hot air escapes and the fresh comes in," explains Shimao Ni, inventor of the Scroll Air Compressor and president of Mindtech. "The fewer turns you use, the less it heats up."

Efficiency and manufacturability aren't the only advantages, however. During high loading conditions--caused by incompressible fluid or jamming caused by contaminants--the orbiting scroll yields radially to protect the device. And during normal operation, axial gaps between the tips and bases of the scroll members are hydromechanically sealed off. Thus, the Scroll Air Compressor eliminates the detrimental effects of frictional power loss; namely vibration, noise, and wear caused by contact between the tips and bases of the scroll members.

Ni says that the design may find applications in commercial refrigeration and air conditioning, which require compression ratios of 7:1 or higher. Existing systems intended for such applications do not use scroll technology and are therefore larger, noisier, and less efficient than they could be. Conventional systems also employ discharge valves, which further reduce efficiency and reliability.

Ni says the key lies in the new system's ability to reach the high compression ratios and other requirements needed for such applications. "The difficulty has always been to reach a high compression ratio with a small number of turns," Ni says. "Using this technique, we've achieved it."

Additional details...Contact Shimao Ni, Mindtech Corporation, 7884 B Quincy Ave., Willowbrook, IL 60521, (630) 887-8817.