chiller surging? 2

[naifmbo]

hi,
what is centrifugal chiller surging?How to avoid it if it is due to low cooling load?

thanks.

 

[imok2]

Centrifigul compressor surging is a condition where the existing head (lift) exceeds the compressors pumping head (lift). Most Centrifugal Refrigeration Compressors have(develop) a lift of about 70*f. For example: SST = 35*f and SCT = 105*f so the diff = 70*f. If say the SCT goes to 110*f and the SST stays at 35*f then the lift = 75*f and you start to begin surging Soooto eliminate surging at light loads, you have to unload the compressor or drop the head pressure (cooler condenser water. In either case you need to maintain the the head (lift) within the compressore pumping head(lift)

 

[SAK9]

Surging is the phenomenon of flow reversal of gases in the compressor from discharge to suction side when the compressor operates at very low capacities.Capacity modulation is normally achieved using an inlet guide vane control unit in the compressor inlet.But at very low loads this becomes ineffective.Low refrigeration load means low flow rate of gases into the compressor.With the compressor speed remaining constant,the compressor will try to maintain the same flow rate as before.This causes the flow to reverse so that an equilibrium condition can be reached.The flow reversal will try to turn the impeller in the opposite direction causing damage to bearings and the drive shaft.The motor current also will fall and rise during surging depending on the direction of rotation.

One common solution is to use ''hotgas bypass''.Hot gas bypass means to dischrage the hot compressed gas back into the suction line.This would raise the flow through the compressor and bring to the safe operating region on its performance curve. During surging the compressor tries to do the same thing as hot gas bypass by itself by sending the gas back to its suction chamber but with disasterous results !I am not sure whether ''Hot gas bypass'' can be done on an existing installation on site.The manufacturer would be the best person to advise.Another option used to overcome surging is to use a variable speed drive unit.But this agian is not an easy solution as the control circuitry and motor needs to be designed for it.

For the above reason one needs to be careful about specifying the capacity of a centrifugal compressor.If you are not sure about your load,it is better to go for multiple centrifugal compressors.Some manufacturers have two compressors on board the machine each sized for 50% capacity to overcome this problem.

In my opinion centrifugal compressors should not operate below 30% of their capacity

 

[HVAC68]

Having selected Carrier centrifugal chillers for a long time (I no longer work with Carrier, though), I can throw some tips on how to avoid surging at an equipment selection stage itself.

If you know that the chiller will run on part loads, ask the manufacturer to select and furnish part load information of the chiller down to 20% or 30% (this will depend on how low the chiller will load down to) at CONSTANT condenser entering water temperature. If the machine is stable at these conditions at Constant condenser entering water temperatures, chances of surging are very slim. The flip side, though is a slightly expensive equipment, due to probably a bigger evaporator and/or condenser, but has the advantage of better power consumption.

One needs to know their load pattern before taking this decision.

HVAC68