Originally Published MPMN May 2005
Breaking Sound, Weight, and Battery-Life Barriers
A rotary scroll compressor helps oxygen concentrator offer new level of independence and quality of life for breathing-impaired patients
|The InogenOne concentrator uses a rotary scroll compresssor from Air Squared as its prime mover.|
Start-up company Inogen Corp. has developed a new oxygen concentrator that is not only truly portable, but also nearly triples the time respiratory patients requiring supplemental oxygen can move about freely. According to the company, the Inogen One concentrator creates a new class of independent oxygen devices by providing three key quality of life benefits for patients. They are: light weight, long battery life, and low noise.
Inogen achieved these goals largely by breaking away from the industry-standard piston pump and selecting a rotary scroll compressor as the concentrator’s prime mover, says Geoff Deane, Inogen’s vice president of engineering/CTO. Developed by Air Squared (Cincinnati; www.airsquared.com) and licensed exclusively to Inogen for use in oxygen concentrators, the scroll compressor delivers more than 80% volumetric efficiency, while drawing less than 40 W. The Inogen One operates in the sub-40-dBa range, a fraction of what’s considered acceptable in the industry, Deane stated.
Existing products for patients with chronic obstructive pulmonary disease (COPD) are almost exclusively stationary, designed for in-home use. Though they are reliable and low-cost, they sacrifice on quality of life in the areas of noise and vibration. The only portable device available previously had a battery life of less than 50 minutes and a noise level of 55 dBA, and required overhaul at 3000 hours.
“To achieve the kind of miniaturization and reliability we were targeting, we had to optimize each subsystem, starting with the compressor, which consumes about 80% of the energy,” says Deane. “We spent a lot of development effort to attain higher efficiency in the pressure swing adsorption cycle. And we invested in development of a very sensitive conserver, as well as miniaturization of the electronics and the software we use to manage operation of the device. This is all with the goal of reducing the amount of work the machine has to do.”
Critical performance criteria for the compressor included low noise and vibration, long life, oil-free operation, light weight, low energy use, and ability to deliver precise flow and pressure. Deane says the team researched more than a dozen alternatives including helical, rotary vane, and piston pumps. “Until we came upon Air Squared’s rotary scroll compressor, a piston pump was the only device that came close in the pressure and size range we needed, and it would have required work to isolate the noise and vibration caused by valves and balance issues,” Deane added.
Scroll gives new spin to compressor technology
The scroll compressor design is based on a 1900-era patent and 1970s refrigeration compressor technology. Under development by Air Squared, it has evolved into a new kind of air compressor that runs quietly and oil-free. The operating elements of a scroll compressor consist of two identical involutes, which form right- and left-hand components. One scroll is indexed or phased 180° with respect to the other to allow the scrolls to mesh. This indexing creates crescent-shaped gas pockets, bounded by the involutes and base plates of both scrolls.
In operation, one scroll remains fixed; the other is attached to an eccentric, driven by an electric motor. As the moving scroll orbits around the fixed scroll, the pockets formed by the meshed scrolls follow the spiral toward the center and diminish in size. The compressor inlet is at the periphery of the scrolls. The entering gas is trapped in two diametrically opposed gas pockets and compressed as the pockets move toward the center, where the discharge port is located in the fixed scroll. No valves are needed because the discharge port is isolated from the inlet. This reduces noise and improves the durability of the unit.
Because scroll compressors use true rotary motion, they can be dynamically balanced for nearly vibration-free operation. Air delivery is continuous, which virtually eliminates inlet or discharge pulsation and associated noise. Reliability is inherent, because there are only two primary moving parts, with no inlet or discharge valves to break or make noise, and no associated valve losses.
“We researched the scroll compressor in depth, and determined it could give us a big advantage right off the starting line because we had very little noise or vibration to isolate,” says Deane. “Air Squared conducted additional development and customization for us to reduce energy consumption of the compressor by 40% through tweaking the manufacturing process and through a proprietary technology–now patented–that greatly improved the sealing of the oil-free machine.” Air Squared also developed a way of fine-tuning the discharge flow of the compressor so that each unit delivered the set rate with an accuracy of ±0.2 L/min.
The scroll compressor inside the Inogen One weighs just 2 lb, including its motor, and measures 4.5 ¥ 3 ¥ 3.3 in. The entire concentrator weighs less than 10 lb, including its battery, and measures about 12 ¥ 6 ¥ 12 in.
Its pulse-dose delivery system delivers 1–5 L/min in nine flow settings, varying the speed of the compressor from 1500 to 2700 rpm to meet demand. A lithium-ion battery provides 2–3 hours of independent use on a 3-hour recharge, with ac and dc capability for stationary and mobile operation, while concurrently charging the battery.
Inogen received FDA marketing clearance for its new oxygen concentrator in May 2004, and began shipping in October.
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