Magnetic bearings break down market barriers, expand horizons

July 12, 2000
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Like superheroes, companies maintain their competitive edge in a dynamic and rapidly changing global marketplace by enthusiastically and relentlessly pursuing innovation. Consider the following: Batman has a utility belt; Spiderman has web slingers; Wonder Woman has an invisible jet.

York International has a new compressor in its arsenal — one that literally defies gravity.

Far-fetched, futuristic flight of engineering or cartoon fancy? Electromechanical sleight of hand? Not really: Truth is stranger than fiction. Recently, York debuted its new “ML” compressor, a successful marriage of magnetic bearings and its “M” compressor, the company’s most reliable large-tonnage refrigeration compressor.

By outfitting this industry workhorse, with more than 40 years of global service, with an equally proven technology, York has taken conventional compressor science a leap forward. What’s more, the ML has allowed York access to previously untapped growth opportunities in new markets.

This latest engineering development from York marks the operational debut of a completely oil-free system for this centrifugal compressor. The ML compressor now operates without the possibility of lube oil entering the process gas, resulting in completely oil-free gas compression.

Here’s a look at how the ML works and how the project came to fruition.

'Bearing' the burden of the world

Bearings, in general, are devices that support something that moves, like a shaft or piece of rotating machinery. While relegated to a quiet life of mechanical obscurity in an engineer’s specification guide, the world of mechanics revolves on bearings.

From behemoth hydro-electric turbines, laser jet printer motors, jet engines, and roller blades, to the drive axles in every automobile rolling on the road today, the bearing is either directly, or indirectly, responsible for keeping the world’s machinery in continual motion.

But nowhere does the bearing play a more important role than cooling and refrigeration equipment. So much of this equipment spins, rolls, slides or glides on bearings that York has spent millions to take the technology to the next level.

Perhaps the most significant achievement of the ML compressor is the merging of two mature technologies, the M compressor with more than 40 years of proven global service, and a magnetic bearing design with over two decades of active service.

Anatomy of the frictionless bearing

The ML compressor delivers up to 26,000 inlet cfm and outputs of up to 15,000 hp.

The compressors are customized for each project with one of three standard casing diameters, and up to eight stages of compression. The compressor can be driven by a variety of methods and fuel sources including electricity, steam, natural gas engines, steam turbines, and gas turbines. The compressor’s open motor design eliminates refrigerant compatibility issues often associated with hermetic designs.

The ML’s electromagnetic bearing system consists of two distinct components: the stationary bearing assemblies — including the radial, axial and auxiliary bearings — and an energy-efficient control system. Both combine to provide five-axis rotor support and control with no mechanical contact between the rotor and stator.

The radial bearings are comprised of a rotor outfitted with ferromagnetic laminated sleeves inline with the stator magnets and sensors. Rotor position is monitored continuously by the sensors, resulting in instantaneous control feedback to the electromagnets delivering variable bearing forces to maintain the proper rotor orbit.

This technology senses the rotor’s inertial forces and applies the appropriate bearing forces for seamless speed transitions.

The axial bearing is based on the same principles as its radial counterpart: The rotor contains a thrust disc perpendicular to the rotation axis, stationary electromagnets both fore and aft of the thrust disk, and stationary position sensors.

In addition, auxiliary “anti-friction” bearings are used to support the compressor rotor during coastdown in the unlikely event that the magnetic bearing controller operation is interrupted. These auxiliary bearings are stationary and non-contacting during normal operation.

To mitigate power interruption concerns, the ML compressor is equipped with an uninterruptible power supply that ensures that the rotor remains levitated during coastdown. The anti-friction safety bearings and uninterruptible power supply alleviate any concerns of contact with the magnetic bearings.

The bearings have an infinite life since contact is never made with the rotor — an attribute that eliminates normal wear and tear inherently associated with mechanical friction.

Once commissioned, the bearing controller never needs mechanical adjustment. Overall, thanks in part to the lessening of parasitic loads, York’s active magnetic bearing system affords the compressor a very low energy-consumption profile.

Moreover, since the compressor rotor is levitated by electromagnetic force, the conventional bearing lube oil system is eliminated along with the attendant oil reservoir, pumps, coolers, check valves and associated auxiliary lubrication components.

Beyond gee whiz

For over two decades, magnetic bearings have been deployed in a number of industrial processes like turbo-expanders, turbines, pumps, fans, blowers, and a wide assortment of manufacturing-related end uses.

But once you get past the gee-whiz science required to cheat the forces of gravity, market forces proved just as difficult to overcome.

The project began with the qualified assumption that there was a lucrative market in oil production and natural gas compression for compressors that could deliver reliable service without contamination — a market barrier that represented a significant cost premium to end-users.

To meet the petroleum industry’s stringent requirements for purity, the compressor would have needed very costly external bearings and an elaborate network of equally expensive sealing and auxiliary lubrication devices. York’s research and development project planners realized that this exercise would have required a capital-intensive, total re-engineering effort.

There was a niche to be filled and York was well aware of the old marketing credo that “nature abhors a vacuum.” The decision to implement magnetic bearings was not aimed at servicing our existing market; it was designed to open up an entirely new market that we were not participating in. It was a design approach that was cost-justified even when we evaluated the risks associated with it.

Thanks to the forces of electromagnetism, the ML compressor now operates without the possibility of lube oil entering the process gas, resulting in completely oil-free gas compression, even at dizzying speeds of 14,000 rpm.

Since the compressor rotor is levitated, conventional bearing lube oil systems are eliminated along with attendant oil reservoirs, pumps, coolers, check valves, and associated ancillary lubrication components.

By lessening parasitic loads, York’s active magnetic bearing system affords the compressor a very low energy consumption profile, which reduces greenhouse gas and power plant emissions.

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