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Frictionless Compressor Technology
Chillers cleaner, quieter, and more energy-efficient
A new compressor technology introduced since 2003 in the US, may have a
significant effect on the future of mid-range chillers and rooftop
applications in water-cooled, evaporatively cooled, and air-cooled
chilled-water and direct-expansion (DX) systems. Designed and optimized
to take full advantage of magnetic-bearing technology, the compressor
is worldwide already awarded several times for its innovative and energy
saving performance.
The compressor is key to a new water-cooled centrifugal-chiller design,
with Air-Conditioning and Refrigeration Institute (ARI) tests indicating
integrated part-load values (IPLVs) not normally seen with conventional
chillers in this tonnage range. This article describes this new
compressor technology and its first use in an ARI-certified chiller
design.
Bearings
Traditional centrifugal compressors use roller bearings and
hydrodynamic bearings, both of which consume power and require oil and a
lubrication system. Some years ago, ceramic roller bearings, which
avoid issues related to oil and reduce power consumption, were
introduced to the HVAC industry. The lubrication of these bearings is
provided by the refrigerant itself.
Magnetic-bearing technology is significantly different. A digitally
controlled magnetic-bearing system, consisting of both permanent magnets
and electromagnets, replaces conventional lubricated bearings. The
frictionless compressor shaft is the compressor’s only moving component.
It rotates on a levitated magnetic cushion (Figure 1). Magnetic bearings
- two radial and one axial - hold the shaft in position (Figure 2).
When the magnetic bearings are energized, the motor and impellers,
which are keyed directly to the magnetic shaft, levitate.
Permanent-magnetic bearings do the primary work, while digitally
controlled electromagnets provide the fine positioning. Four positioning
signals per bearing hold the levitated assembly to a tolerance of
0.00002 in. As the levitated assembly moves from the center point, the
electromagnets’ intensity is adjusted to correct the position. These
adjustments occur 6 million times a minute. The software has been
designed to automatically compensate for any out-of-balance condition
in the levitated assembly.
Power Failures
When the compressor is not running, the shaft assembly rests on
graphite-lined, radially located touchdown bearings. The magnetic
bearings normally position the rotor in the proper location, preventing
contact between the rotor and other metallic surfaces. If the magnetic
bearings fail, the touchdown bearings (also known as backup bearings)
are used to prevent a compressor failure.
The compressor uses capacitors to smooth ripples in the DC link in the
motor drive. Instantaneously after a power failure, the motor becomes a
“generator,” using its angular momentum to create electricity (sometimes
known as back EMF) and keeping the capacitors charged during the brief
coastdown period. The capacitors, in turn, provide enough power to
maintain levitation during coastdown, allowing the motor rotor to stop
and delevitate. This feature allows the compressor to see a power outage
as a normal shutdown.
Oil-free design
Oil management, particularly as it pertains to the lubrication of
compressor bearings, is a critical issue in refrigeration-system design.
But with magnetic bearings, this issue is avoided. Only a very small
amount of oil is required to lubricate other system components, such as
seals and valves; often, however, experience shows that even this small
amount of oil is not needed. Avoiding oil-management systems means
avoiding the capital cost of oil pumps, sumps, heaters, coolers, and oil
separators, as well as the labor and time required to perform
oil-related services. Reports indicate that for many installations,
compressor-maintenance costs have been cut by more than 50 percent.
Most air-cooled products (including chillers, rooftop units, and
condensing units) use DX evaporators. Most DX systems allow oil to
travel through the refrigeration circuit and back to the compressor oil
sump. Great care must be taken during design to provide oil return,
particularly at part load, when refrigerant flow rates are reduced.
Water-cooled chillers often use flooded evaporators. In a flooded
evaporator, even small amounts of oil can coat evaporator tubes and
significantly diminish chiller performance. This can lead to an
elaborate oil-recovery system. Magnetic bearings eliminate the need for
these systems and oil management in general. In fact, the only required
regular maintenance of the compressor is the quarterly tightening of the
terminal screws, the annual blowing off of dust and cleaning of the
boards, and the changing of the capacitors every five years. Complete
service agreements and extended maintenance contracts can be provided by
the manufacturer.
The motor
Most hermetic compressors use induction motors cooled by either
liquid or suction-gas refrigerant. Induction motors have copper
windings that, when alternating current is run through them, create the
magnetic fields that cause the motor to turn. These copper windings are
bulky, adding size and weight to the compressor.
Two-pole, 50-Hz induction motors operate at approximately 3,000 rpm. A
higher number of revolutions per minute can be obtained by increasing
the frequency. Compressors that require higher shaft speeds tend to use
gears. While gears are a proven technology, they create noise and
vibration, consume power, and require lubrication.
The magnet-bearing compressor features a synchronous permanent-magnet
brushless DC motor with a completely integrated variable-frequency drive
(VFD). The stator windings found on conventional induction motors are
replaced with a permanent-magnet rotor. Alternating current from the
inverter energizes the armature windings. The stator (excitation) and
rotor (armature) change places. No commutator brushes are required.
The motor and key electronic components are internally
refrigerant-cooled, so no special cooling is required for the VFD or
the motor.
The use of permanent magnets instead of rotor windings makes the motor
smaller and lighter than induction motors. Using magnetic-bearing
technology, a 75-ton compressor weighs 265 lb—about one-fifth the
weight of a conventional compressor.
A variable-speed drive (VSD) is required for the motor to operate. The
VSD varies the frequency between 300 and 800 Hz, which provides a
compressor-speed range from 18,000 to 48,000 rpm. This avoids a gear
set. The VSD is integrated into the compressor housing, avoiding long
leads and allowing key electronic components to be refrigerant-cooled.
The VSD also acts as a soft starter; as a result, the compressor has an
extremely low startup in-rush current: less than 2 amps, compared with
500 to 600 amps for a traditional 75-ton, 460-v screw compressor with a
cross-the-line starter.
With the integration of the motor, VSD, and magnetic-bearing system, the
capa-citors required for the motor and drive can be used as a backup
power source for the bearings in the event of a power outage or
emergency shutdown.
Capacity and efficiency
Among the key parameters affecting performance are capacity (tons)
and efficiency (kilowatts per ton). The compressor’s capacity ranges
from 60 to 90 tons, depending on the operating conditions. Plans call
for that range to be extended to 150 tons water-cooled and 115 tons
air-cooled by the end of 2004 with the use of R-134a refrigerant. An
R-22 version is planned for retrofit applications.
Efficiency improvements stem from a combination of the centrifugal
compressor, permanent-magnet motor, and magnetic bearings. Within the
compressor, efficiency is affected by the compressor isentropic
efficiency (the efficiency of the wheels), the motor, and the bearings.
Traditional induction motors of this size typically are in the
92-percent efficiency range. This compressor’s permanent-magnet motor
has an efficiency of 96 to 97 percent.
Efficiency is further enhanced with the use of magnetic bearings, which
avoid the friction of rubbing parts associated with traditional oiled
bearings. Conventional bearings can use as much as 10,000 w, while
magnetic bearings require only 180 w. That amounts to 500 times less
friction loss. Current development projects are expanding the range and
duty of the compressor wheels and promise to offer even greater
efficiency for water-cooled and air-cooled duties and different
capacities.
Controls
The new compressor effectively is a computer. It provides diagnostic
and performance information through Mod-bus to the refrigeration system,
which then communicates to the building automation system through
Modbus, LonWorks, or BACnet.
Chiller application
The compressor manufacturer and a major chiller manufacturer
Geoclima, Italy developed a range of water-cooled chillers, which were
introduced in 2002. The technology was introduced in Belgium by Geveke
Climate Technology NV in January 2005. The combination of
flooded-evaporator technology and an oil-free system has allowed very
close approaches and, subsequently, enhanced performance. The
integrated VFD allows excellent part-load performance as power
consumption drops off, depending on the head relief, near the cube root
of the shaft speed.
The compressor includes wheels tuned for water-cooled duty in the
dual-compressor format, which further enhances part-load performance.
Tested in accordance with ARI Standard 550/590-98, Water Chilling
Packages Using the Vapor Compression Cycle, a 150-ton (nominal) chiller
has a full-load performance of 0.629 KW per ton (5.6 COP) and an IPLV of
0.375 KW per ton (9.4 COP).
All IPLVs are weighted for standard operating conditions and the time
spent at those conditions. Specific operating points for a 150-ton
nominal-capacity chiller are shown in Figure 3.
Sound and vibration
Because the rotating assembly levitates, there essentially is no
structure-borne vibration. The magnetic bearings create an air buffer
that prevents the only major moving part—the motor rotor—from
transmitting vibration to the structure.
Similarly, sound levels are extremely low, primarily because of
refrigerant-gas movement through the compressor and the rest of the
refrigeration system. There are no tonal issues, such as those found
with some screw compressors, and the noise occurs in the higher octave
bands, where it is easier to attenuate. When two magnetic-bearing
compressors were integrated into a chiller, the sound pressure was 77
dBA at 3.3 ft under ARI Standard 575-94, Method of Measuring Machinery
Sound Within an Equipment Space.
After 10 years of development, magnetic-bearing compressors offer
economic, energy, and environmental benefits. Chief among them are
increased energy efficiency, the elimination of oil and oil management,
and considerably less weight, noise, and vibration. This initial
mid-range package offers centrifugal-compression efficiencies
previously reserved for large-tonnage systems only. Knowing the
advantages in chiller applications, this compressor technology will
replace most of the screw compressors in the current installations in
the following years. <<
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