What type of chemicals are added to the closed loop cooling water systems?
What are their purposes? thanks

Glycol to prevent freezing and rust inhibitors are typically used.

Acid (such as sulfuric acid) to control pH. Anti-scaling chemicals of various kinds. Biocides and algaecides. Much better to consult one of the many companies that offer advice and water treating chemicals than to ask for help on an Internet forum like this.

You might find it useful to read Cooling tower operational variables