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The enthalpy h of water or water vapor refers to the energy contained within 1 kilogram of water or 1 kilogram of water vapor at a certain pressure and temperature; in other words, it is the sum of the internal energy u of the water or water vapor and its pressure potential energy pv (h = u + pv). The enthalpy of water or water vapor can be considered equal to the heat absorbed when 1 kilogram of water at an absolute pressure of 0.1 MPa and a temperature of 0°C is heated to the pressure and temperature of that water or water vapor. The unit of enthalpy is “joule/kilogram”. 1) Enthalpy of unsaturated water: The heat absorbed when 1 kilogram of water with an absolute pressure of 0.1 MPa and a temperature of 0°C is heated to the pressure and temperature of that unsaturated water. 2) Enthalpy of saturated water: The amount of heat absorbed when 1 kilogram of water at an absolute pressure of 0.1 MPa and a temperature of 0°C is heated to the saturation temperature corresponding to that pressure. The saturation temperature increases as pressure rises, and therefore the enthalpy of saturated water also increases with rising pressure. For example, at an absolute pressure of 3.92 MPa, the enthalpy of saturated water is 1081.9x103 joules per kilogram; at an absolute pressure of 9.81 MPa, the enthalpy of saturated water is 1399.3x103 joules per kilogram. 3) The enthalpy of saturated water vapor is divided into two types: the enthalpy of dry saturated water vapor and the enthalpy of wet saturated water vapor. The enthalpy of dry saturated water vapor is equal to the enthalpy of saturated water plus the latent heat of vaporization of water ； The enthalpy of wet saturated steam equals the sum of the proportion of saturated water in 1 kilogram of wet saturated steam multiplied by the enthalpy of saturated water, plus the proportion of dry saturated vapor multiplied by the enthalpy of dry saturated vapor. For example: When the absolute pressure is 9.81 MPa, the enthalpy of saturated water is 1399.3x10³ J/kg ； The latent heat of vaporization is 1328x103 joules/kg. Therefore, the enthalpy of dry saturated water vapor is equal to: 1399.3x10³ + 1328x10³ = 2727.3x10³ joules per kilogram. For example, in wet saturated water vapor with an absolute pressure of 9.81 MPa, the proportion of saturated water is 0.2 (i.e., humidity is 20%), while the proportion of dry saturated water vapor is 0.8 (i.e., dryness is 80%). The enthalpy of this wet saturated water vapor is then 1399.3x103x0.2 + 2727.3x103x0.8 = 2461.7x103 joules/kg. 4) The enthalpy of superheated steam is equal to the enthalpy of dry saturated steam at that pressure plus the superheating heat. For example: for superheated steam with an absolute pressure of 9.81 MPa and a temperature of 540°C, the enthalpy of the dry saturated steam is 2727.3x103 joules/kg, while the superheat energy is 750.4x103 joules/kg. Then the enthalpy of this superheated steam is: 750.4x103 + 2727.3x103 = 3477.7x103 joules/kg.