Mag Lev Chiller Part Load Efficiency 1

[RobsVette]

Lately I have seen quite a big deal made about the magnetic levitation chillers and there excellent part load capability. I am wondering if anyone can explain how they generate such efficiency at part load.

As I am sure everyone is familiar with by now, a high pressure centrifugal with a VFD and standard oil lubricated bearing (1 compressor per chiller) is usually in the range of .55 KW/ton and around .375 KW/ton at part load with some condenser water relief to 65F. Going down past there chillers tend to be inefficient and increase in KW/Ton. I have always attributed this to closing of inlet guide vanes as the load decreases without the ability to further reduce VFD speed.

Mag lev compressors seem to pretty much level off at part load and can maintain as low as .32 KW/ton. I assume in comparison, to an oil bearing electric chiller is that the bearing friction power becomes a greater percentage of the total power.

Multistack in particular has a chiller with 2 compressors on it that can run at below .3 KW / ton from 50% load down.

Does anyone have any experience with these chillers or can explain why they would operate so much more efficiently when operating at low load? Any insight is appreciated.

 

[RossABQ]

I'd assume Multistack is only running one of the two compressors below 50% rated load, which is then back up near its sweet spot efficiency-wise. To be a fair comparison, you have to compare to other multi-compressor units of similar capacity.

 

[mcoul098]

To properly compare the efficiency you have to compare centrifugal oil and mag bearing at the same operating point. We must consider that the mag bearing is capable of a much lower turndown than oil lubricated. Simply because the mag bearing is not constrained by oil starvation. Inherently the mag bearing chiller IPLV kW/ton will be less. At the same operating point mag bearing will require less power as stated because of the no friction losses, and also important to consider the improved heat transfer from being oil-less.

 

[DRWeig]

I think mcoul098 hit the answer to the question.

Best to you,

Goober Dave

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