1- what is the function of the pressurization unit in chilled water system and can the system operate with out it i.e with expansion tank only
2-what the correct arrangement of pressurization unit+expansion tank+air separator.. I mean how these are linked together

PU is to maintain the initial pressure of system and to make up any leak in the system, the PU and expansion tank shall be in the same line connected to the return header of pumps, if there is an air separator on the return side, then the line of PU and expansion tank can be connected to the line of ai release valve from the air separator (there is a line come out from the top of air separator to the AAV) you can connect in between the air separator and the AAV

Yes the system can work without the PU and with expansion tank only
But if you lost the pressure! What you will do?. you need to fill again with treated water... so PU can do that automatically
But the important is the EXPANSION TANK...

I hope this useful

Please tell me why it is to be placed only in the return and how the system losses pressure?? Reason..
Thanks and reply is appreciated. ( I am very new to this field and working as HVAC site engineer