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A comparative study of traditional and non-traditional polyolester lubricants for carbon dioxide (R-744) applications.

Number: pap. n. R24, 2175

Author(s) : HESSELL E., CARR D., KELLEY R., et al.

Summary

Carbon dioxide (R-744, CO2) has received increased recognition as a non-ozone depleting and ultra low global warming potential refrigerant for automotive and commercial heating and cooling applications. The lubrication requirements for transcritical CO2-based equipment are different from equipment using hydrofluorocarbon (HFC) refrigerants due to both the higher system operating pressures of CO2 and molecular structure differences of CO2 vs. HFCs. In particular, the high solubility of CO2 in the traditional commercial polyol esters (POEs) used with HFCs results in significant viscosity reduction in the compressor which can result in insufficient lubrication of bearings and improper sealing of clearances. The lubricant must possess good load carrying and lubricity properties in the presence of CO2 over the total range of operating conditions of the compressor. This paper presents a comparison of traditional POEs to non-traditional POEs in bench tests; both alone and in combination with CO2. The miscibility of the lubricants in CO2 was measured over the temperature range of -40ºC to 25ºC at a concentration of 10 wt% lubricant in refrigerant and provides an indication of refrigerant/lubricant compatibility at low temperature; a requirement for good oil return. The viscosity of lubricant/CO2 mixtures was measured as a function of temperature and pressure over a range of typical conditions encountered in CO2 refrigeration applications. The results show the enhanced viscosity retention of non-traditional POEs vs. traditional POEs in the presence of CO2. The frictional properties of the pure lubricants were evaluated using the Mini Traction Machine (MTM) at 40, 80 and 120ºC. Coefficient of friction measurements as a function of entrainment speed illustrate how friction varies for each lubricant over the full range from boundary lubrication (metal-metal contact) to hydrodynamic lubrication (full fluid film). Measurements of the traction coefficient under elastohydrodynamic lubrication conditions provide an assessment of the relative energy efficiency of the lubricants. The combined results of the miscibility, viscosity and frictional property measurements highlight the challenge in designing a lubricant with the best overall balance of good compatibility and lubricity/load carrying performance in the presence of CO2.

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Details

  • Original title: A comparative study of traditional and non-traditional polyolester lubricants for carbon dioxide (R-744) applications.
  • Record ID : 30000990
  • Languages: English
  • Source: 2010 Purdue Conferences. 13th International Refrigeration and Air-Conditioning Conference at Purdue.
  • Publication date: 2010/07/12

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