A water loop is defined to ...

A water loop is defined to be “closed”, for water treatment purposes, if the make-up rate is less than 10% of the system capacity per year. In a true closed loop, the chemical treatment is added initially, and is expected to remain in the loop for a long period of time - until the loop is drained either fully or partially.

One of the most important goals in closed loop is to minimize make-up consumption. Closed loop water treatment would generally consist of,

[1] pH adjustment additive (PH booster) 

[2] Film-forming amine, corrosion inhibitor by coating all internal surfaces of the piping and equipments with a protective thin layer (Preferable anodic corrosion inhibitor rather than cathodic inhibitor which anodic inhibitor can provide passivation to the surface of the metal), monomolecular film. Corrosion inhibitors, like molybdate, nitrite and azole are very common passivator to prevent attack of oxygen

[3] Azoles are added if there is copper in the system (Cathodic corrosion inhibitor)

[4] Ethylene glycol or propylene glycol to freeze-proof the system

[5] Oxidizing or non-oxidizing biocide, (should not react with corrosion inhibitor) – in hot water system biocides may not be necessary. 

Finally, the choice of treatment regime is based on the metallurgy of the operating system.