how to increase the capacity of an refrigeration system

hi,
iam doing a project on refrigeration system. the problem is that the original capacity is 90 tonne. but now we are getting only up to 45 to 50 tonne. the refrigerant used is *****. it is used for cooling chlorine to -5degree. the system consist of a screw compressor (vertical,single stage) , oil seperator, condensor , reciever , expansion valve and an liqufier. an ecnomizer is there but now it is not used.
so please help me in this matter to increas the capacity of the unit to much higher value

"the refrigerant used is *****"?

are you directly cooling or are you using a heat exchanger have you tried reconecting the economizer

Get the economizer operating for a start, that will increase capacity approx 15%, Pump system down and drain oil from evaps, recommission system to suppliers original specs.