What is an expansion tank and where is is installed in Chilled Water system in HVAC?

maintain the water pressure and refill the water in chilled water system

The function of the expansion tank is to always ensure a positive head at the pump suction to avoid pump cavitation, that is why it is best to install it at the suction side of the CHW system.  Likewise for a costs considerations it is better to install it at the highest point of the CHW system where the pressure of the CHW system is lower. Re charge the bladder to the required system pressure as required. 

It also ensures that there is always sufficient make up water and pressure in the system.

expansion tank maintain pressure in chiller .when chiller system working then the water volume increase  chiller absorb water so water volume in chiller need more water.when chiller system required water in chiller so that expansion tank maintain this water volum.