Water Specific Enthalpy and Entropy vs Temperature

Specific enthalpy (H) is the total thermodynamic energy content of a substance per unit mass, combining its internal energy with the work required to displace the surrounding pressure. For a flowing fluid it represents the energy available to do work. It is defined as:

H = U + p·v

where U is specific internal energy, p is pressure, and v is specific volume. For liquid water, enthalpy increases monotonically with temperature. The reference point is the triple point (0.01 °C / 32.02 °F), where specific enthalpy is defined as zero. The rate of increase near room temperature is approximately 4.18 kJ/kg·K — the specific heat capacity of liquid water.

Specific entropy (S) measures the unavailability of thermal energy for conversion to useful work. It increases whenever heat is added to a substance. For a reversible process the differential change in entropy is:

dS = δQ / T

where δQ is the incremental heat added and T is absolute temperature (K or °R). For liquid water, entropy increases with temperature and rises steeply near the critical point (373.946 °C / 705 °F). The area under a process path on a temperature-entropy (T-s) diagram equals the heat transferred, making entropy essential for steam cycle analysis.

Enthalpy and entropy appear together on Mollier diagrams (h-s diagrams), which are standard tools for analyzing steam turbines, pumps, compressors, and heat exchangers. The isentropic efficiency of a turbine or pump is calculated by comparing the actual enthalpy change to the ideal constant-entropy enthalpy change. For liquid water below 100 °C at atmospheric pressure, both properties are well approximated by saturation-curve data because liquid water is nearly incompressible.

Note: 1 kcal/kg = 1 cal/g  ·  1 kJ/kg = 1 J/g