DX (Direct Expansion) systems are self-contained and use a refrigerant cycle to produce cooling. They are suited to small and medium facilities. CHW (Chilled Water) systems connect to a central chilled water plant and are preferred for large data centres. CDW (Condenser Water) systems use a condenser water loop and cooling tower to reject heat, and are suited to very large or hyperscale facilities. See our full comparison in the DX CRAC units explained, CHW CRAC cooling systems, and CDW CRAC cooling systems articles.