What is enthalpy and why is it useful in process engineering?

Enthalpy (H) is defined as H = U + PV, combining internal energy with flow work (PV). For open systems at constant pressure, enthalpy change equals heat transferred: dH = dQ. This makes enthalpy the natural property for flow processes - heat exchangers, reactors, and separation equipment. Specific enthalpy (h) tables/charts are used for steam, refrigerants, and other process fluids. Enthalpy differences (delta_H) drive heating/cooling calculations and appear in energy balances around equipment.