Parallel operation of 2 out of 3 screw compressors 3

[nRGee]

I come across a revamp design of chiller system in which 2 of 3 screw compressors run parallely. Details of the system given below:

Original design: 2 compressors (1-running, 1 standby)
No of compressors in revamp design: 3 (2-running, 1 standby)
Refrigerant: Propane
No of condensers: 4 (Media: sea cooling water)
No of receiver drums: 2

These screw compressors are equipped with auto loading system. We expect to run each compressor at different loading based on process requirement.

what hydraulic and thermal issues can be anticipated with this design?
Is it OK to run compressors at different loading?

 

[georgeverghese]

From what I've read about these oil flooded screws in refrigeration service is that reliance on internal slide valve for load modulation is not recommended due to thermal degradation of the lube resulting from excessive internal recirculation, resulting in lube oil vapor carryover in the propane vapor phase in to the downstream condensor and process coolers. Instead, use VFD to the extent possible for load modulation. If this is followed, then running these compressors at different loadings should be okay for long term operation.

 

[LittleInch]

Depends how the auto loading system functions and what your variability on flow is.

I would have thought that if your flow demand is nearly always > the capacity of one machine then you operate in an equal flow / speed mode on both.

I can't see the advantage of running one at 100% and the other at say 15% of flow. Run both at 57.5%??

The issue will be running one at a high flow and the other at a very low flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[quark]

Good suggestion by LittleInch. If you are running your chillers at different loads and your chilled water flowrate is constant then you have differing leaving temperatures. This is your thermal issue. If you alter flowrates and have a common header then you have hydraulic issue. Better to split the load equally.

 

[georgeverghese]

If you plan to go with VFD, check that the VFD can give you 10:1 turndown on speed or better, else an aux. motor cooling fan may be required. Modify the load sharing algorithm so that the lower speed machine does not go below 10% of design speed. Since screw compressors are pd machines, I suspect you may need this aux. cooling fan for each motor.

 

[nRGee]

Thanks for your responses and valuable inputs.

@georgeverghese: as you mentioned, we don't have VFD in current setup and we also faced thermal degradation of lube oil problem in the past. Point taken. We need to study further to provide VFD considering cost factor and other details.

@LittleInch: Great post... we will try to maintain both the compressors at same load.

@Quark: Thanks for your insights...helped me to comprehend thermal and mechanical issues better.