Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983). The enthalpy for the state C can be picked directly from steam tables, whereas the enthalpy for the state D must be calculated using vapor quality: Calculate the enthalpy difference between these two states. Steam leaves this stage of turbine at a pressure of 1.15 MPa, 186☌ and x = 0.87 (point D). H l = enthalpy of saturated liquid water (J/kg)Īs can be seen, wet steam will always have lower enthalpy than dry steam.Įxample: Rankine Cycle – Thermodynamics as Energy Conversion ScienceĪ high-pressure stage of steam turbine operates at steady state with inlet conditions of 6 MPa, t = 275.6☌, x = 1 (point C). In case of wet steam, the actual enthalpy can be calculated with the vapor quality, x, and the specific enthalpies of saturated liquid water and dry steam: The specific enthalpy of saturated liquid water (x=0) and dry steam (x=1) can be picked from steam tables. The fact that the absolute value of specific enthalpy is unknown is not a problem, however, because it is the change in specific enthalpy (∆h) and not the absolute value that is important in practical problems.įormulas for Specific Enthalpy of Wet Steam For example, the specific enthalpy of water or steam is given using the reference that the specific enthalpy of water is zero at 0.01☌ and normal atmospheric pressure, where h L = 0.00 kJ/kg. Normally, the enthalpy of a substance is given with respect to some reference value. In general, enthalpy is a property of a substance, like pressure, temperature, and volume, but it cannot be measured directly.
The specific enthalpy is equal to the specific internal energy of the system plus the product of pressure and specific volume. Note that the enthalpy is the thermodynamic quantity equivalent to the total heat content of a system. It equals to the total enthalpy (H) divided by the total mass (m). The specific enthalpy (h) of a substance is its enthalpy per unit mass. Engineers use the specific enthalpy in thermodynamic analysis more than the enthalpy itself. The enthalpy can be made into an intensive, or specific, variable by dividing by the mass. For ideal gas αT = 1 and therefore:Įnthalpy Formulas in Intensive Units – Specific Enthalpy Where C p is the heat capacity at constant pressure and α is the coefficient of (cubic) thermal expansion. There are enthalpy formulas in terms of more familiar variables such as temperature and pressure: This energy storage on the atomic level includes energy associated with electron orbital states, nuclear spin, and binding forces in the nucleus.Įnthalpy is represented by the symbol H, and the change in enthalpy in a process is H 2 – H 1. On the other hand, energy can be stored in the chemical bonds between the atoms that make up the molecules. It is the thermodynamic quantity equivalent to the total heat content of a system. It is the energy contained within the system, excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields. The SI unit of enthalpy is the joule (J). Enthalpy Formulas in Extensive Units Extensive and intensive properties of medium in the pressurizer.Įnthalpy is an extensive quantity, it depends on the size of the system, or on the amount of substance it contains.