Frictionless Compressors

Are they right for you?

Compressors that run on frictionless bearings are an enticing prospect. Dan Foss Turbocor, Inc. is now marketing a line of compressors that use magnetic bearings to provide essentially frictionless operation. These compressors have been on the market for about three years, and now McQuay International is incorporating the compressors into their 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. Turbocor has recently received three prestigious awards for this design: 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 McQuay’s frictionless chillers) are right for you. Each feature is discussed in more detail below.

· Magnetic bearings.
· Oil-free design.
· Low noise Level
· VFD control.
· Soft start.
· Smaller and lighter than conventional compressors.
· Uses a centrifugal compressor.

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 2-4%.
• 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 (VFD) control, allow the shaft to spin at high speeds (up to 48,000 RPM) and give good speed control.
• Very little vibration. Because of no 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% of the operating energy. Contrast this to 2-4% 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 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. The effect of this, however, is small. While the Turbocor website suggests that heat transfer effectiveness can be reduced by 15% or more by the presence of oil, this will most likely translate into a somewhat smaller effect on overall system efficiency.

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 up to 48,000 RPM and (because there is no friction in the motor) it can also operate efficiently at low loads.

In addition, 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 (IPLV) rating. Many chillers in HVAC applications are running at part load a vast majority of the time, making them an excellent application for the frictionless system.

Turbocor claims that they have achieved an IPLV rating as low as 0.375 kW/Ton, compared to 0.63 kW/Ton for the Turbocor at full load, and 0.5 kW/Ton being considered a very good rating. Herein lies 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 having the integrated VFD is that the motor has a built-in soft start. Turbocor uses a unique method of soft start that ramps the motor up gradually, making inrush current effectively the full load current. On Turbocor’s website, they list the inrush current as 2 amps, or 5 amps in some places, meaning, presumably, 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.

Small and Lighter

Using permanent magnets in the motor, rather than electrical windings as in an induction motor, reduces the size and weight significantly. In addition, by running the shaft at high speeds, they are able to use a smaller, lighter, shaft. According to the manufacturer, the Turbocor compressors, at 265 lbs., are 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 take advantage of speed control more effectively, but they are usually only available in larger sizes. By using the smaller shaft, they are able to take advantage of the centrifugal compressor technology in a smaller size than is normally available.


Is it a Right Choice - Baby?
That depends. The design of these compressors is clearly innovative, elegant, and efficient, and all indications are that it is a quality product. The idea of using magnetic bearings is provocative, but it turns out that this feature in itself is rarely enough to justify considering the 50-70% 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. Certainly in an application where the chiller runs at part load much of the time, such as in 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, but it will probably be worthwhile 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.