question on refrigerant charge of water cooled screw process chiller

[keepnup]

When the compressor runs at full capacity the liquid line sight glass is full and when the slide valve reduces compressor capacity the liquid line sight glass drops to half and even 1/4 full. I have seen flashing in the sight glass with reciprocating compressors running unloaded and clear up when fully loaded, so just wondering if this is normal with screw compressors or the system is low on refrigerant? The condenser water temp fluctuates it is not heated it is regulated with a water regulator actuated by system head pressure, 134a refrigerant which was running around 116, psi back pressure was 11 psi with 36 deg chilled fluid compressor running approx 50 percent capacity. Any input would be appreciated; thanks

[r404a]

suction sounds pretty low

[keepnup]

yes a little low but very steady as it increases capacity it does not dive....capacity is being controlled by suction pressure

[KnewYork]

suction sounds pretty low

X2

That pressure corresponds to about 6*F with a product temperature of 36*F. That doesn't sound right to me.

As for the sight glass showing different conditions...the screw compressor is a positive displacement compressor just like a recip. But just because the sight glass is full doesn't mean you are correctly charged. You need to know what the manufacturer wants for sub-cooling and superheat under full load conditions.

[keepnup]

Thanks for the info you are absolutely correct system was low on refrigerant determined by only 1 deg subcooling mfg recommended 10 - 15 at full capacity. Thanks again