ASHRAE Journal - February 2013 - 63

emerging technologies
Small High Speed

Centrifugal Compressors
By Mildred Hastbacka; John Dieckmann, Member ASHRAE; Antonio Bouza, Associate Member ASHRAE

his month, we are revisiting small high speed centrifugal compressors, which were first covered in Emerging Technologies in October

2003.1 In 2003, there had been technically successful development
of a 25 ton (88 kW) capacity, two-stage centrifugal compressor for
R-134a that could be used in either water-cooled chiller applications
or air-cooled chiller or unitary air-conditioning applications.

The two centrifugal impellers were
direct-driven by a permanent magnet rotor
dc motor on a common shaft. The compressor operated at variable speeds between
35,000 and 50,000 rpm and used refrigerant-lubricated ball bearings to support the
shaft. With refrigerant lubrication of the
bearings, no oil lubrication was required,
eliminating circulating oil through the
rest of the refrigeration loop. At this high
speed, the impeller diameter is quite small,
on the order of 3 in. (75 mm). Despite the
potential advantages, this technology did
not advance to commercial production.2
Since then, another configuration of
small high speed centrifugal compressor
has emerged and become an established
commercial product. As with the previous
development, the compressor has been
designed for R-134a, with two stages
to provide sufficient pressure ratio and
temperature lift to allow it to be used in
air-cooled applications. The motor and
impellers are on a common shaft, with
variable rotating speed on the order of
30,000 rpm, with impeller diameters
between 3 and 4 in. (75 and 100 mm).
A significant difference is that the
bearings are magnetic bearings, which
levitate the shaft on a magnetic field,
with no contact with a stationary bearing
half. This eliminates mechanical friction
loss and allows lubricant-free operation.
February 2013

While these compressors have extended
the range of competitive performance of
centrifugal compressors to lower capacities than traditional centrifugal chillers—
down to 60 tons (211 kW), the technology
has proven to be scalable, with high speed
centrifugal chiller products on the market
with capacities up to 700 tons (2460 kW).
This compressor configuration contributes to increased energy efficiency in several ways. From the perspective of scaling
laws, the combination of small impeller
diameter and high rotating speed is optimum for a centrifugal compressor in this
relatively small capacity range. The variable
operating speed provides excellent partload efficiency, with the speed being varied
to match the condensing temperature.
The two-stage design allows a refrigerant economizer cycle to be incorporated;
the condensed refrigerant is expanded in
two stages from the condensing pressure
to the evaporating pressure, and vapor
flashed after the first expansion stage is
directed to the second compressor stage,
requiring approximately half of the energy for compression compared to the
full compression from the evaporating
to the condensing pressures.
Typical energy savings from the refrigerant economizer cycle range from
5% to 7%. The magnetic bearings allow
unlubricated operation with low friction

loss. Eliminating lubricating oil from the
system results in incrementally better refrigerant-side heat transfer performance,
particularly in the evaporator.

Energy Savings
The U.S. Department of Energy, Federal
Energy Management Program has included water-cooled oil-free magnetic bearing
compressor technology among its “top 20
technologies for deployment.”3,4 Annual
cooling energy savings ranging from about
40% to more than 60% have been achieved
with this technology as documented by
the Navy Technology Validation (Techval) Program.5,6 Three project sites were
involved in the Navy evaluation: San Diego, Newport, and Jacksonville.6 Projects
included a compressor retrofit as well as
a new chiller and an added compressor.
Table 1 presents a high level summary
of each project, as well as energy savings
results and payback based on the total
project cost. In the San Diego project, three
existing chillers, with and without the new
compressors, were used as the basis of the
energy savings comparison. Baselines for
the Jacksonville project and for the Newport project were existing compressors.7
A significant contributor to the energy
savings is the technology’s excellent efficiency at partial loads, typical of chiller
operation.3,4,5 The longer the compressor
is run at part load and the higher the electric rate (e.g., > $0.07/kWh), the greater
the advantage offered by magnetic bearing chiller compressors.5
The incremental costs for these three
projects were reported as $24,000;
$8,000; and $13,000, respectively. Using
this incremental project cost for payback
calculations yields payback periods of
1.1 years for San Diego; 0.3 years for
Newport; and 0.8 years for Jacksonville.6
ASHRAE Journal

ASHRAE Journal - February 2013

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