Centrifugal Compressor Parallel Operation

It could be that the shut-off pressure on the 100Nm³/min unit is set a little too high. Try adjusting it downwards a little and see what happens.

Given a free hand, on rising pressure it would be good to switch the 100Nm³/min machine off first, then the 70Nm³/min one, then the 55Nm³/min one so as to use the least power.

When you have more than one compressor in parallel, the characteristics (flow-pressure) of these are not likely to match. This must be affecting the load sharing between them when you are working off target (ie at low load). In the very low load condition, as is clear by your switching off of the 100 Nm³ compressor, the centrifugal and also most of the other compressors are prone to hunt. So check up the load condition and automatically switch off one or two as required.

One paper that may interest you is

http://www.ccijapan.co.jp/products/pdfs/cci/274.pdf

The 5 kg/cme pressure seems a bit low for a compressed air system. Usually it should be set around 8.0 barg. May be your special requirement.

You may need to stagger the start-stop, load/unload pattern by a bit more margin. The loading pattern could be 3.0/4.0/5.0 barg and unloading the reverse for 55/70/100 Nm3/min. Adjust the lead/lag switch on the compressor control panel. If you don't have one, then take the signal from the PS on the air reciever. This should solve the problem.

The auoto blow off is just to get the water out of the receiver. If the air is very humid it will blow off more frequently. However if you mostly see only air and no water coming off then try adjusting the timer or replace the air trap to save some power.

The solution is all in the algorithm, here's what you do, (did it with chillers, same principle).

First start out with the smallest compressor and the other two off, have the computer monitoring the load. When the 55Nm^3 becomes loaded, turn on the 70, then turn off the 55. This will keep the 70 from hunting, the 55 would not hunt as it would be fully loaded.

Next wait for the 70 to become fully loaded and then turn on the 100 and shut off the 70 to keep it from hunting. When the 100 becomes loaded, turn on the 55, now you have 155 available.

When the demand exceeds 155, shut off the 55 and turn on the 70, now you have 170 available for use. (and no compressor hunting)

When the load exceeds the 170, turn on the remaining 55.

When shutting down the units as the demand decreases... do it in reverse order.

Good luck