Two-phase flow is encountered extensively in the HVACR industries. A combination of liquid and vapor refrigerant exists in flooded coolers@ direct-expansion coolers@ thermosiphon coolers@ brazed and gasketed plate evaporators and condensers@ and tube-in-tube evaporators and condensers@ as well as in air-cooled evaporators and condensers. In heating system pipes@ steam and liquid water may both be present. Because the hydrodynamic and heat transfer aspects of two-phase flow are not as well understood as those of single-phase flow@ no comprehensive model has yet been created to predict pressure drops or heat transfer rates. Instead@ the correlations are for specific thermal and hydrodynamic operating conditions.