GEA's Global HVAC Technology Blog Blog

GEA's Global HVAC Technology Blog

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Drivelines For High Efficiency Centrifugal Compressors

Posted September 04, 2013 12:47 PM by larhere

In my previous blog, I discussed the development of drivelines for centrifugal compressors, from designs with a stand-alone motor and coupling drive to semi-hermetic designs, all of which use gears to drive the impeller shaft.

I received a good question about possible disadvantages with the semi-hermetic design. There is one, of course, it is not possible to service the motor without evacuating the refrigerant and opening up the compressor, but overall, the advantages of the semi-hermetic design trumps this disadvantage.

A further improvement of both designs was made
possible by the use of rolling element bearings. One option is to use rolling element bearings on the high speed shaft only, since the bearing friction is higher on the high speed shaft. Other possibilities are to convert the bearings on both shafts, or on the low speed shaft only.

The advantages of rolling element bearings are:

  • Reduced bearing friction. This not only directly improves energy efficiency but also reduces the requirements for oil cooling or even eliminates the need for an oil cooler.
  • Smaller bearing clearances making it possible to reduce impeller to volute clearances and thereby improve efficiency through reduced internal leakage
  • Reduced oil flow requirement meaning reduced size of the oil pump and the lubrication system
  • Overall, conversion to rolling bearings can lead to an energy efficiency improvement of 2% to 4%

In the past there was a hesitation to use rolling element bearings since they were thought of the finite life bearings as opposed to hydrodynamic bearings which were thought of as infinite life bearings. This thinking was very logic since previously it was always possible to calculate a fatigue life for rolling element bearings, even with light loads. This was different than for other machine element subjected to fatigue. For shafts, for example, it is possible to avoid fatigue by making sure that the stresses are below the fatigue limit.

The explanation lies in the understanding of the effect of contamination and lubrication. When the theory and formula for bearing life were developed in the 1940s the effect of contamination and lubrication were not well understood. In rolling bearings operating in a dirty environment and with poor lubrication there are high local stresses in the contacts between the rolling elements and the bearing rings. Even under light external loads, the contact areas in the bearing can be highly stressed and subject to fatigue. Today, the effect of lubrication and contamination are well understood and can be predicted. Rolling bearings operation with good lubrication and controlled levels of contamination have predicted infinite life.

In low speed machinery where space for bearings is not a limitation, it is always possible to select a large bearing to achieve (perceived) improvement in reliability. In high speed machinery such as centrifugal compressors, bigger is not always better, it is necessary to balance the effect of load and the effect of speed on bearing reliability. A (too) large bearing operation at high speed is not reliable, the frictional heat and the centrifugal forces on the rolling elements can be too high. A (too) small bearing on the other hand may not have the required load capacity. Other considerations in bearing selection are bearing types and bearing arrangement. With the right design, high reliability, low friction and small operating clearances will be achieved.

In my next blog, I will discuss high speed direct drive motors, magnetic bearings and refrigerant lubricated rolling element bearings.

Editor's Note: CR4 would like to thank Hans Wallin of GEA Consulting for contributing this blog entry.


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