A refrigeration system is a combination of components, equipment and piping, connected in a sequential order to produce the refrigeration effect. Refrigeration systems that provide cooling for air-conditioning are classified into the following categories: vapor compression systems, absorption systems, and gas expansion systems. A refrigerant is a primary working fluid used to produce refrigeration in a refrigeration system. A cooling medium is a working fluid cooled by the refrigerant during evaporation to transport refrigeration from a central plant to remote cooling equipment and terminals. A liquid absorbent is a working fluid used to absorb the vaporized refrigerant after evaporation in an absorption refrigeration system. Hydrofluorocarbons (HFCs) contain only hydrogen, fluorine, and carbon atoms and cause no ozone depletion. HFC's azeotropics are blends of refrigerant with HFCs. Hydrochlorofluorocarbons (HCFCs) contain hydrogen, chlorine, fluorine, and carbon atoms and are not fully halogenated. HCFCs have a much shorter lifetime in the atmosphere than CFCs and cause far less ozone depletion.