Originally Posted by NH3LVR
I think that is as good an answer as I am likely to find.
Also I think it is most likely the correct one.
I am however at the limits of my knowledge on screw compressor design.
With the VI set too low don’t we have in fact under compression?
I did conclude the rattling check valve and high suction at the compressor (compared to the Suction line) was due to the lack of gas flow.
You are probably correct about the low gas flow through the machine even at low VI. I have no good way to quantify that without shutting down part of the other Engine Room, which the Customer would be reluctant to do as an experiment. Or I could startup the Old Spiral Freezer, but that would entail a long pump out to put it back in mothballs.
The plant will be in continuous operation until October, even then it is a five hour roundtrip to test anything.
Fortunately this is a Frick which measures the Suction Pressure at a port on the Compressor, rather than a FES which uses a port installed upstream of the Suction Check Valve. That might have made things more difficult to observe.
Unfortunately I may never see the inside of this machine. We will most likely get a exchange from Frick.
I have been told before by knowledgeable people (As well as the manuals) that a screw produces about 15% of its rated flow when unloaded. I wonder what the effect of liquid injection cooling is on that.
Earlier this year I started up a used 200HP FES Booster in parallel with a 250HP Frick. Everything was going well as I ran it unloaded, checking Oil Pressure etc, until someone asked why the Suction Temp was at100F+. I found I had to increase the Capacity slightly to bring down the Suction Temp. My theory was that the minimum capacity was taken up by the Liquid injection. A simple adjustment to minimum Slide Valve was the cure.
I appreciate your input.