Oil-Free Scroll Compressor Opens New Applications

More efficient scroll compressors may be about to break into food processing, medical systems, and other ultra-clean applications, thanks to the development of new oil-free, "floating scroll" compressor design.

The new configuration, developed by engineer Sam Ni of Scroll Laboratories (Romeoville, IL; http://rbi.ims.ca/3846-563), adds to the breadth of applications for the technology because it eliminates the need for lubricants and simultaneously offers a 30-percent efficiency boost over more traditional piston-type compressors. As a result, Ni foresees its use not only in food processing and medical applications, but also in cryogenics, textile manufacturing, and fuel-cell stacks. "It can run at high speed, maintain good sealing qualities, and not consume a lot of power," Ni says. "That opens it up to a lot of applications where it hasn't been used before."

Ni achieved those characteristics by effectively balancing forces and moments on the compressor and by introducing a device called a synchronizer. The synchronizer is critical to the design because it enables the two key parts-the fixed scroll and orbiting scroll-to achieve a condition called radial compliance, in which physical contact between the parts results in good, gas-tight, sealing.

Ni complemented that radial compliance by employing a back-to-back scroll configuration that also results in axial compliance. Axial compliance is a key, Ni adds, because it allows the two scrolls to barely touch one another, even as they maintain compliance. The concept, which consists of one scroll's inner surface lightly touching the other's outer surface, has been designated the "floating scroll" by the company.

Together, the compressor's axial and radial compliance are critical, Ni says, because they enable the new compressor to be highly efficient. Achieving high efficiency in this way is significant because Ni accomplished it with a so-called "three-crank-handle" design, which is inherently oil-free. Other scroll compressors have been compliant in the past, he says, but those compressors required lubrication because they were subjected to greater frictional forces. Because Ni's new device combines the three-crank-handle design and floating scroll concept, friction is virtually non-existent and no lubrication is required. "There are other compressors, but they have to run at very high speed and low efficiency," Ni says. "Or they require lubrication."

Ni says his company's compressor can be employed in medical respirators and nebulizers, where oil-free operation is critical. At the same time, he says, Scroll Labs' oil-free compressor can achieve efficiencies about 20-30% higher than the compressors previously used in those applications. What's more, it's good for about 10,000 hours in those applications, compared to about 1,000 hours for the piston-type compressors.

Ni also says the new design can be used in the textile industry, where oil stains can't be tolerated, as well as in fuel cell stacks, where oil is forbidden because it can poison a catalyst. "When you eliminate the oil from a compressor, it creates new possibilities," Ni concludes. "Most air applications need oil-free designs."

Sam Ni, founder and president of Scroll Labs, has spent most of his professional career in compressor design. He received his Ph.D. in mechanical engineering from the University of Illinois, where his studies in thermal physics earned him the ASHRAE Homer Adams Award for graduate research. During his career, he has published 14 technical papers on scroll compressor design and earned 11 scroll-compressor-related patents. Ni is a past winner of a grand prize Design News' Excellence In Design contest.