One of decade's most significant advances in centrifugal compressor bearing technology has been introduced within the past year for large-tonnage refrigeration and gas-compression applications. Harnessing the forces of electromagnetism to levitate the compressor's rotor, thereby eliminating the need for traditional oil-film lubrication and mechanical bearing contact, the technology also reduces vibration and associated parasitic loads significantly.

The innovation brings together two mature designs: the S2M ActidyneR magnetic bearing system; and the York International (York, PA) multistage centrifugal model "M" compressor.

The following explores the resulting MillenniumR (ML) compressor system's operation and details how magnetic bearing technology is poised to deliver operational-, energy efficiency- and reliability-related benefits to commercial-comfort cooling applications.

Bearing the Burden of the World

Bearings, in general, are mechanical devices that support something that moves, like a shaft or other piece of rotating machinery. While often relegated to a quiet life of obscurity in an hvac engineer's specification guide, the world literally revolves, slides, glides, or rolls on bearings.

From behemoth hydroelectric power turbines, laser jet printer motors, jet engines, ventilation dampers, and roller blades to the front axles and transaxles in every automobile rolling on the road today, the bearing is either directly or indirectly responsible for keeping the world's machinery in motion.

But nowhere do mechanical bearings play a more important role than in commercial cooling and refrigeration compressor operation, especially in large multistage refrigeration systems that are responsible for generating up to 9,000 refrigeration tons (TR). These refrigeration systems are typically found in larger industrial and institutional cooling plants, district-cooling facilities, and in a variety of industrial process refrigeration applications.

As with all multistage centrifugal compressors, a journal bearing is used at opposing ends of the compressor rotor to maintain proper radial positioning; a thrust bearing is also used to control the axial (along the rotor) position. The interaction between the rotor and the bearings is defined in terms of stiffness and damping, and the resulting radial rotordynamic characteristic is referred to as the rotor's orbit.



Figure 1. The control loop block diagram for the magnetic bearing system.

Rising To Fill a Need

The forces acting on the bearing system can result in material degradation and a finite service life, requiring periodic bearing inspections and, in some cases, replacement intervals. Dedicated lube-oil pumping, filtration, and distribution systems required by mechanical bearings are a major operational and maintenance consideration for commercial chillers. These systems require ongoing maintenance attention and monitoring.

For over two decades, magnetic bearings have been employed in a number of industrial-related processes like turbo-molecular pumps, high-speed spindles, centrifugal and axial-flow compressors, electric motors, pumps, fans, and blowers. While not widely known, their installations number in the tens of thousands. These compressors often endure internal operating temperatures ranging from -175°F (-115°C) to 525°F (274°C) and equally grueling external environments, the likes of which an office building's chiller will never know.

This project began with the qualified assumption that there was a clear need for oil production and natural gas compressors that could deliver reliable service without cross-contamination between compressor lubricants and process gases - a market barrier that represented a significant cost premium to endusers. Further, to meet the American Petroleum Institute's (API) turbo-machinery standards for gas compressors, two "external" bearings at opposing ends of the rotor would have been required, each supported with a costly and complex oil lubrication system. By virtue of the external nature of such bearings, two gas seals would be required to prevent fugitive emissions from escaping into the equipment room or the atmosphere.

Figure 2. The compressor's shaft position is monitored continuously to detect any deviation from the normal position. This technology is designed to determine the rotor's inertial forces then apply the appropriate bearing forces for seamless speed transitions.

System and Component Operation

The novel design approach that drove the ML magnetic bearing development is its standardization. The mission was to develop a line of standardized magnetic bearings that would be used for all ML configurations in all applications. The result is one physical bearing size for each casing diameter, and two material options predicated on the required rotor load range. This compressor family consists of 26-, 38- and 55-in. casing diameter sizes, which accommodate from one to eight stages of compression.

The bearings have an infinite life since contact is never made with the rotor, an attribute that eliminates the dependency on a lube-oil system and the potential wear associated with mechanical friction. Once commissioned, the bearing controller never needs mechanical adjustment.



General System Operation

This electromagnetic bearing system consists of two main components: the stationary bearing assemblies (including the radial, axial, and auxiliary bearings) and an energy-efficient control and positioning system. Both combine to provide five-axis rotor support and control with no mechanical contact between the rotor and stator.

The magnetic bearings are configured with four opposing radial electromagnets in two axes at each journal bearing; two opposing magnets act on the thrust disk. This arrangement produces forces of attraction that support the compressor rotor within a 0.025-in. gap between the rotor and bearings. Sensors continuously monitor the position of the rotor and modulate the current required by the stator magnets to maintain the proper rotor position.

Unlike conventional lube-oil bearing systems, the rotordynamic stiffness and damping of a magnetic bearing system may be programmed into the bearing controller. This feature allows design engineers to actually define the rotor's critical speed. What's more, the compressor shaft is levitated even before it starts to rotate.

The radial bearings are comprised of a rotor outfitted with ferromagnetic laminated sleeves that are in line with the stator magnets, actuators, and position sensors. The laminations are mounted on the shaft to help conduct the magnetic flux to produce attractive forces for levitation. The rotor's position is monitored continuously by these sensors, which results in instantaneous control feedback to the electromagnets delivering variable bearing forces to maintain the proper rotor orbit.

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.

Shaft Positioning and Balancing

The Actidyne bearing is equipped with an automatic balancing system (ABS) that allows the compressor rotor, when operating above its first critical speed, to rotate about its inertial center instead of its geometric center. This technology senses the rotor's inertial forces and applies the appropriate bearing forces for seamless speed transitions.

The ABS makes use of a notch filter in the controller that eliminates all damping and stiffness at the synchronous or rotor speed only. The result is a rotor with an inertial orbit and virtually no seismic vibration.

Overall, the ABS reduces the dynamic bearing loads transmitted to the bearing casing. Unlike its industrial predecessors, no additional external vibration monitoring probes, cables, or signal-conditioning equipment is required with the magnetic bearings.



Coastdown/Power Loss Protection

Auxiliary "anti-friction" bearings are used to support the compressor rotor during coastdown in the unlikely event that the magnetic bearing controller's operation is interrupted.

These auxiliary bearings are stationary and noncontacting during normal operation. A rotor-to-bearing clearance of 0.012 in. accounts for approximately half the clearance of the magnetic bearing. The anti-friction auxiliary bearings are angular contact by design to carry loads in the radial and thrust axes. In the unlikely event of a complete loss of control to magnetic bearings, the rotor coasts down, supported by the auxiliary bearings, with no contact with the magnetic bearings. During a partial controller failure, in which one axis of bearing control may be compromised, the remaining bearing axes continue to function with auxiliary bearings being used intermittently. Events in which the auxiliary bearing makes intermittent contact is called a "touch." These bearings, which are installed outboard of the magnetic bearings for ease of inspection and maintenance, may sustain many hundreds, and the controller records each one.

To mitigate power interruption concerns, the compressor is also outfitted with an optional uninterruptible power supply (UPS) that ensures that the rotor will remain levitated during coastdown. The anti-friction auxiliary bearings and UPS alleviate any concerns of contact with the magnetic bearings.



Diagnostics

The magnetic bearing system routinely provides real-time operating data to the user such as: bearing temperatures, amplifier and cabinet temperatures, rotor position at each of five axes, rotor speed, and UPS battery voltage and bearing currents. Bearing current, a function of the applied load, allows the chiller operator to monitor the effect of process and load changes on the compressor. This is a significant diagnostic advantage over fluid-film systems in which only the rotor position or orbit is monitored, not the forces acting on the rotor.

Extensive "drop" testing has shown that the compressor is extremely robust and tolerant of significant aerodynamic and systemic disturbances, such as surge and liquid ingestion (slugs). Additionally, the magnetic bearing system supplies extensive data and proactive information regarding disturbances, such as surge detection, with an appropriate alarm and message displayed on the controller.

Operating data may be obtained via an RS232 or RS485 link to a remote control center or building automation system; the control system is capable of monitoring all channels.

In the event that disturbances detected are in excess of the compressor's design tolerances, the magnetic bearing system safely shuts down the compressor and displays the shutdown event.



Cost and ROI Issues

Lube-oil system monitoring on a chiller typically includes oil filter pressure differential, oil temperatures and pressures, and lube-oil composition. The oil cooler is subjected to heat transfer degradation, depending on the cooling-water conditions. Furthermore, oil filters must be changed when the pressure differential, influenced by the magnitude of filtered particulates, exceeds the filter's design limit.

With traditional mechanically lubricated bearings, oil viscosity creates frictional losses during initial start-up and also during normal operation. These losses can result in a substantial power penalty. On a magnetic bearing-equipped chiller, total system energy consumption is reduced 1% to 3% by avoiding friction losses alone, yielding annual savings of close to $50,000.

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 that draw power.

For the ML compressor, the typical maximum total electrical losses are 10 hp, which translates to 7.4 kW, according to the following breakdown:

  • 5 hp (3.7 kW) is required to power the electronic control cabinet;
  • 0.5 hp (0.4 kW) is needed for resistive, current, and voltage losses;
  • 0.7 hp (0.5 kW) goes towards hysteresis and eddy current losses; and
  • Roughly 3.8 hp (2.8 kW) is consumed by windage losses. When compared to a conventional bearing-outfitted compressor, the system provides a net reduction of about 37 kW, a net savings of 49 hp. The annual cost in electric energy to perform this function is about $1,000/hp, or $49,000/yr. Without considering the load on water pumps for lube oil system cooling water, etc., the operational savings alone could justify an upgrade to this new technology. Depending on the application, an enduser could recover any incremental or premium costs within three to five years solely on the merits of energy savings.

    With the elimination of the lubrication system and the costs associated with maintaining ancillary pumps, a user's return on investment becomes greater.



    Harnessing the Power of Electromagnetism

    By tapping electromagnetic forces, this new generation of compressor now operates without the possibility of mechanical bearing contact, even at dizzying rotational speeds of 14,000 rpm.

    The introduction of magnetic bearing technology to large tonnage commercial chillers allows the commercial market sector to receive the same operational and efficiency benefits currently realized by the oil and gas industry. The complete elimination of the lube-oil system from the chiller operation brings many other inherent benefits, including:

    • Reduced seismic vibration and increased reliability;
    • 50% to 75% reduction in friction horse-power;
    • No residual oil to foul heat exchanger tubes;
    • Reduced environmental impact from residual shaft seal oil disposal;
    • Real-time rotordynamic diagnostics of vibration, orbit, and rotational forces;
    • Less ancillary equipment means reduced equipment room space;
    • Reduced costs and delivery time compared to other magnetic bearing compressors; and
    • Lower total life cycle costs.

    By lessening parasitic loads, the completely oil-free S2M Actidyne active magnetic bearing system affords the compressor a very low energy consumption profile, which reduces greenhouse gas and power plant emissions as well. ES