This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Frictionless compressors: Are they right for you?

The Turbocor compressor uses magnetic bearings to provide essentially frictionless operation. (Photo by Danfoss Turbocor, Inc.)

Compressors that run on frictionless bearings are an enticing prospect. Danfoss Turbocor, Inc. is now marketing a line of compressors that use magnetic bearings to provide essentially frictionless operation. These Turbocor compressors have been on the market for about three years, and now McQuay International is incorporating the compressors into its new line of frictionless chillers.

Using innovative technology that levitates the compressor shaft in a magnetic field, the compressors operate without metal-to-metal contact, making them more efficient, and eliminating the need for an oil management system. The design has recently received three prestigious awards: the ASHRAE/AHR Expo "Energy Innovation" Award in 2003, the Natural Resources Canada's Energy Efficiency Award in 2003, and the U.S. Environmental Protection Agency’s Climate Protection Award in 2004.

How are they different?

Here are some of the features, benefits and concerns to help you decide whether frictionless compressors or frictionless chillers are right for you.

Magnetic bearings: A digitally controlled magnetic bearing system, consisting of both permanent magnets and electromagnets, replaces conventional lubricated bearings. The only rotating part—the compressor shaft—is levitated and held in place by magnetic bearings, eliminating metal-to-metal contact, essentially eliminating friction. Four positioning signals, sampling six million times per minute, hold the levitated position to within 0.00005" of center. The advantages of this technology include:

• No friction. This can improve energy efficiency by 2 to 4 percent.
• No metal-to-metal contact, therefore less wear on moving parts. Turbocor claims that maintenance costs are about half what they are for conventional technology.
• The frictionless bearings, in conjunction with variable frequency drive control, allow the shaft to spin at high speeds and give good speed control.
• Very little vibration. Because of there is physical contact, vibrations are small and tend to be self-dampened.

The magnetic bearing system, however, is more expensive, and it takes some energy to operate the system. The levitation system on current models uses approximately 180 watts, or about 0.5 percent of the operating energy. Contrast this to 2 to 4 percent for conventional bearings, along with a lubrication system that (according to Turbocor) can use up to 10,000 watts.

Oil-free design: The biggest benefit to eliminating oil from the system is that it eliminates the need for a lubrication system, which can include oil pumps, sumps, oil separators, heaters, coolers, etc. In comparing the costs of two proposed systems, it is important to consider lubrication in estimating both the capital and operating costs.

The other disadvantage of having oil in the system is that it can reduce the effectiveness of heat transfer in the coil. However, this effect is small. While Turbocor suggests that the presence of oil can reduce heat transfer effectiveness by 15 percent or more, studies by Trane and ASHRAE suggest that this amount of loss in heat transfer will only reduce overall system performance by 1 percent or less.

Low noise level: The Turbocor compressors, by all reports, make significantly less noise than the comparable screw compressors. What noise there is tends to be high frequency, which is relatively easy to attenuate.

VFD control: The big potential savings of this system comes from the integrated variable frequency drive. This provides good speed control, allowing the system to run at up to 48,000 RPM. Because there is no friction in the motor, it can also operate efficiently at low loads. The compressor has automatically controlled inlet guide vanes that unload further in low-load conditions.

These features give the system an excellent integrated part-load value rating. Many chillers in HVAC applications are running at part load most of the time, making them an excellent application for a frictionless system. Turbocor claims that it has achieved an IPLV rating as low as 0.375 kW/ton, compared to 0.63 kW/ton for the Turbocor at full load, with 0.5 kW/ton considered a very good rating. This is the key to a successful application of Turbocor. If the chiller will be operated at part load a good proportion of the time, a frictionless chiller is worth considering.

Soft start: An added benefit of the integrated VFD is that the motor has a built-in soft start. Turbocor uses a unique soft-start method that ramps the motor up gradually, making inrush current effectively the full load current. Turbocor’s Web site lists the inrush current as 2 amps, or 5 amps in some places, meaning that the ramping up process starts with this very low current.

It is not clear how this information is useful, since the information about inrush current is generally used for sizing circuit breakers. The breaker would simply have to be sized for the full load amperage.

Smaller and lighter than conventional compressors: Using permanent magnets in the motor, rather than electrical windings as in an induction motor, reduces the size and weight significantly. Also, by running the shaft at high speeds, it is able to use a smaller, lighter, shaft. According to the manufacturer, the 265-lb.Turbocor compressor is one-fifth of the weight and half the size of an equivalent conventional compressor.

Uses a centrifugal compressor: Centrifugal compressors tend to be more efficient than screw or scroll compressors and use speed control more effectively, but they are usually only available in larger sizes. The Turbocor's smaller shaft allows it to take advantage of centrifugal compressor technology in a smaller size than is normally available.

Is a frictionless compressor the right choice?

That depends. The compressor's design is clearly innovative, elegant and efficient, and it appears to be a quality product. The idea of using magnetic bearings is provocative, but the feature in itself is rarely enough to justify the 50 to 70 percent price premium you are likely to pay for a frictionless compressor. However, with associated benefits, it may be well worth considering.

Remember to take into consideration not having to include an oil management system, and perhaps easier and quicker installation. In an application where the chiller runs at part load much of the time, such as many HVAC applications, the efficiency may be much better. In that case, the IPLV rating will be a better indication of the relative performance than the full-load rating. It is probably a good idea to have an engineer do a full analysis of the relative costs based on your particular application. The more your chiller will be running at part load, the more attractive this product will be. Reduced maintenance costs may tip the scales in favor of going frictionless.

For further information, contact the Power Line at 1-800-769-3756 or submit your questions online.