Centrifugal Compressor Performance Curves 1

[Loubaby101]

I working with a York centrifugal compressor and was wondering why when I lower the condenser discharge head, does the amp draw go down? I thought, like a centrifugal pump, as I lower the head, or go further out on the curve (more flow), that the h.p./amp would go up, not down. this is an LKB-65 compressor with inlet guide vane control as well as hot gas bypass.

 

[Yorkman]

With a centrifugal pump lowering discharge pressure can be related to a reduction in pressure drop/friction; increasing pump flow, increasing amperage. Don't forget you are imparting energy to a non-compressible liquid.

With a centrifual compressor your compressing a gas so that it can be condensed at a higher temperature than it was evaporated at. When you lower the condenser water temperature your accomplishing several things at the same time. First you increase the rate the gas is condensed in the condenser. This action lowers the head pressure. This lowering of head pressure decreases the lift or pressure differential(between suction and discharge)the machine has to work against.

If you plot this out on a P.E. diagrahm you can see there is decrease in the heat of compression. Less work for the machine means lower amperage at the motor.

At light evap loads and on York machines it is prefered to run at lower condenser water temperatures reducing head pressures to keep the performance curve away from the stall/surge line. Hope that helps

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Loubaby101]

Mr. Gest,
Thanks for your time and response. However, we do not have control over condenser water temp, it comes off the plant cooling tower and it flucuates with ambient conditions. Besides lowering condenser water temp, what other factors would make the heat of compression/amps lower?
Thanks,
Lou

 

[Yorkman]

Lou,
Sorry about the late response. Your pretty limited in lowering the compression ratio after that. You might want to make sure that the hot gas bypass is tuned or calibrated to energize at the correct time/pressure usualy a function of evaporator pressure. Also making sure that you're running at design condenser water and chilled water flow rates.
A good rule of thumb for evaporator flow @ 0% glycol is about 2.5 GPM/ton. The condenser water flow would be about 3 GPM/ton.
If the flow rates are low at the condenser or the tubes are fouled, the operating head pressures can go up.

Or if the flow rates are too low at the evaporator this can reduce evaporator pressures. Using glycol in a system where it was not designed can also cause reduced heat transfer and lower evaporator pressures, raising chilled water flow here can be helpful if the machine is tripping on low evap pressure because of poor heat transfer.

Air/non-condensables can also contribute to higher than normal operating pressures. Make sure that your purge is operating properly.

Those are a few things to look for when assesing the operation of your machine. Hope that is of help.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Loubaby101]

Thanks AJ,
I revised the hot gas bypass system during a recent shutdown (icreased size and relocated) and it can run at almost no load and no ill effects at this time. thanks for your recommendations.
Lou

 

[EmeraldCoastHVACR]

Actually the nothing dictates cooling tower performance than the wet-bulb ambient temperature. Water quality should also be considered for efficient heat transfer from the condensor.

If you have more than 10 degrees temperature drop on the condensor, your system isn't running as efficiently as it should.