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Oil-free turbo-compressors for refrigeration applications.

Number: pap. n. 242

Author(s) : KUS B., NEKSÅ P.

Summary

Feasibility of replacing oil-lubricated compressing equipment in CO2 based refrigeration systems with a new generation of oil-free turbo-machinery is assessed. Efficiencies of oil-free radial turbo-compressors working in its design points were predicted for various operating conditions and sizes. For each rotational speed and operating conditions an optimal capacity exists for which the machine reaches the highest efficiency and this is analogous to specific speed/efficiency relation presented in a number of publications for various turbo-machines. It should however be stressed that in case of oil-free hermetic turbo-compressor an optimal stage specific speed does not always guarantee optimized machine performance. It is dictated by strong motor and thrust bearing sensitivity to shaft speed and it is sometimes advantageous to reduce stage specific speed. As a result, the total machine efficiency can be improved while stage efficiency is reduced. In the simulated cases it is clearly shown that machines operating with high pressure ratios (exceeding 3) cannot reach high efficiencies with the proposed two stage configuration. It is concluded that the desirable oil-free operation of a refrigeration system can be achieved without sacrificing reasonably high compression efficiency targets. Usually however, a 4-stage compression is required in order to mitigate otherwise significant motor and gas bearings windage stemming from operation in high density gas and under high rotational speeds. The study proposes placing two two-stage machines in series instead of four impellers on one single shaft. Such configuration allows for better total motor length/diameter ratio (reduced windage losses) and easier startup of the system. Due to the lower work input per impeller, optimal rotational speed is also reduced.

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Pages: 8 p.

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Details

  • Original title: Oil-free turbo-compressors for refrigeration applications.
  • Record ID : 30004554
  • Languages: English
  • Source: 10th IIR-Gustav Lorentzen Conference on Natural Working Fluids (GL2012). Proceedings. Delft, The Netherlands, June 25-27, 2012.
  • Publication date: 2012/06/25

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