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[mamakda]

Hi everyone!!

We have a chiller in our industry having a screw compressor. It has a slide valve and a 4 step capacity control system, 35%, 50%, 75% and 100%.
I was initially assuming that as the capacity will shift from low to high (say 50% to 75%), the low pressure gauge will show an increase in pressure while the high pressure gauge will remain constant. 
When experienced on field, as the capacity control system switches to high value (say 50% to 75%) the low pressure goes down and the high pressure goes up.
The theoretical background tell us that as the evaporator temperature drops, the mass flow rate the refrigeration effect and the refrigeration capacity all drops down so how this system is working actually ?
You can check the curves of mass flow , refrigeration effect and capacity from INDUSTRIAL REFRIGERATION HANDBOOK from Stoecker.