In thermodynamics and steam tables, the terms hf, hg, and hfg are used to represent specific enthalpy values related to the properties of steam or water.
hf: The symbol hf represents the specific enthalpy of saturated liquid, also known as the "enthalpy of liquid" or "specific enthalpy of water." It is the amount of energy per unit mass contained in water at its saturation point, where it exists as a liquid. The value of hf is typically given in steam tables for different temperature and pressure conditions.
hg: The symbol hg represents the specific enthalpy of saturated vapor, also known as the "enthalpy of vapor" or "specific enthalpy of steam." It is the amount of energy per unit mass contained in steam at its saturation point, where it exists as a vapor or gas. The value of hg is also listed in steam tables for various temperature and pressure conditions.
hfg: The symbol hfg represents the "enthalpy of vaporization" or "specific enthalpy of phase change." It refers to the amount of energy per unit mass required to convert water from a saturated liquid state (hf) to a saturated vapor state (hg) at the same temperature and pressure. In other words, hfg represents the energy required for the phase transition from liquid to vapor or vice versa. The value of hfg is the difference between hg and hf.
These specific enthalpy values (hf, hg, and hfg) are important for calculations and analyses involving steam or water, such as determining heat transfer rates, steam turbine performance, or energy balance calculations in thermodynamic systems. Steam tables provide a convenient reference for these values at different temperature and pressure conditions, allowing engineers and scientists to access the properties of water and steam for various applications.