Hii,please help me,I want to know the physical meaning of " the expansion tank of the chilled water system" and how it works , the theory or principle of its work, its components and any site that can help me, and thanks a lot.

The expansion tank in a chilled water system is used to provide for the expansion and contraction of the water in the system. It will have a level of air in the tank to provide for the water to expand or contract due to temperature or water pressure variations. The air will compress to absorb these changes where as the water won't.
These tanks will have a sight glass to observe the level of water and a drain line to drain the tank as well a make up water pipe through a water pressure regulating valve to fill the tank and maintain a predetermined pressure in the tank.
For more info go to: engineertoolbox.com