The enthalpy of water or water vapour, h, is the energy contained within 1 kg of water or 1 kg of water vapour at a given pressure and temperature, i.e., the sum of the internal energy u of the water or water vapour and the pressure potential energy pv (h = u pv). Enthalpy of water or water vapour, can be considered to be equal to the absolute pressure of 1 kg of 0.1 MPa temperature of 0 ℃ water, heated to the water or water vapour pressure and temperature of the heat absorbed. The unit of enthalpy is ‘J/kg’.

1) Enthalpy of unsaturated water

1 kg of water with an absolute pressure of 0.1 MPa and a temperature of 0 ℃, the heat absorbed by heating to the pressure and temperature of the unsaturated water.

2) Enthalpy of saturated water

1 kg of absolute pressure of 0.1 MPa temperature of 0 ℃ water, heated to the saturated water pressure corresponding to the saturation temperature of the heat absorbed.

Saturation temperature increases with increasing pressure, so the enthalpy of saturated water also increases with increasing pressure.

For example: the absolute pressure of 3.92 MPa, the enthalpy of saturated water is 1081.9x103 J/kg; in the absolute pressure of 9.81 MPa, the enthalpy of saturated water is 1399.3x103 J/kg.

3) Saturated water vapour enthalpy

There are two kinds of enthalpy of dry saturated water vapour and wet saturated water vapour. Dry saturated water vapour enthalpy is equal to the enthalpy of saturated water plus the latent heat of vaporisation of water; wet saturated water vapour enthalpy is equal to 1 kg of wet saturated steam, the proportion of saturated water multiplied by the enthalpy of saturated water multiplied by the proportion of dry saturated steam multiplied by the dry saturated vapour enthalpy and.

For example, when the absolute pressure is 9.81 MPa, the enthalpy of saturated water is 1399.3x103 J/kg; the latent heat of vaporisation is 1328x103 J/kg. Therefore, the enthalpy of dry saturated water vapour is equal to: 1399.3x103 1328x103 = 2727.3x103 J/kg.

For example: the absolute pressure of 9.81 MPa of wet saturated water vapour, the proportion of saturated water 0.2, (i.e., humidity of 20) dry saturated water vapour proportion of 0.8 (i.e., dryness of 80), then the enthalpy of this wet saturated water vapour is 1399.3x103x0.2 ten 2727.3x103x0.8 = 2461.7x103 J/kg.

4) Enthalpy of superheated water vapour

Equal to the enthalpy of dry saturated water vapour at the pressure and the sum of superheated heat.

For example: the absolute pressure of 9.81 MPa, the temperature of 540 ℃ of superheated water vapour dry saturated water vapour enthalpy of 2727.3x103 J / kg, the heat of superheat 750.4x103 J / kg.

The enthalpy of the superheated water vapour is 750.4x103 2727.3x103 = 3477.7x103 J/kg.