MAKING WAVES I
Water-cooled, oil-free magnetic bearing compressors are now ranked among the US Department of Energy’s Federal Energy Management Program’s top 20 technologies for deployment, with trials at three Navy bases showing they can produce up to 60% energy savings. Mildred Hastbacka, John Dieckmann and Antonio Bouza explore their capabilities
The major drivers for selection of building products and technologies in this market have been life- cycle costs and return on investment
n 2003, there had been a technically successful development of a 25-tonne (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, about 75 mm. Despite the potential advantages, this technology did not advance to commercial production1
. Since then, another confi guration of
This is a modifi ed version of an article fi rst published in the ASHRAE Journal (February 2013) © ASHRAE
www.ashrae.org
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 suffi cient 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 a variable rotating speed of about
54 CIBSE Journal June 2013
30,000 rpm, with impeller diameters between 75 and 100 mm.
A signifi cant difference is that the bearings
are magnetic bearings, which levitate the shaft on a magnetic fi eld, with no contact with a stationary bearing half. This eliminates mechanical friction loss and allows lubricant- free operation. 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 (2,460 kW). This compressor confi guration contributes
to increased energy effi ciency 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 part-load effi ciency, with the speed being varied to match the condensing temperature. The two-stage design allows a refrigerant economiser cycle to be incorporated; the condensed refrigerant is expanded in two stages from the condensing pressure to an
www.cibsejournal.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72