Understanding the energy content of steam is fundamental to thermodynamics, mechanical engineering, and industrial process design. Enthalpy, denoted by the symbol H (or specific enthalpy h), represents the total heat content of a system. When dealing with steam, calculating this value accurately is crucial for sizing boilers, turbines, and heat exchangers.
Saturated Steam Enthalpy Calculator
Estimate the specific enthalpy of saturated steam based on temperature and quality.
What is the Enthalpy of Steam?
Enthalpy is a measurement of energy in a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.
In the context of steam, we usually talk about Specific Enthalpy (h), which is the enthalpy per unit mass, typically expressed in kJ/kg. The total enthalpy of steam is the sum of two distinct parts:
- Sensible Heat (hf): The heat required to raise the temperature of water from 0°C to the boiling point at a given pressure.
- Latent Heat (hfg): The heat required to convert the water at its boiling point into steam without changing its temperature.
The Fundamental Formula
To calculate the enthalpy of "wet" steam (a mixture of liquid water and vapor), we use the following equation:
h = hf + (x × hfg)
Where:
- h: Total specific enthalpy (kJ/kg).
- hf: Specific enthalpy of saturated liquid (kJ/kg).
- x: Steam quality (0 = all liquid, 1 = all vapor).
- hfg: Latent heat of evaporation (kJ/kg).
Step-by-Step Calculation Guide
1. Identify the Pressure or Temperature
Steam properties are interlinked. For saturated steam, knowing the pressure automatically tells you the temperature, and vice versa. You can find these values in standard "Steam Tables."
2. Determine the Steam Quality
If your steam is "dry" (100% vapor), your quality (x) is 1. If it contains 5% water droplets, your quality is 0.95. This significantly impacts the total energy content because the latent heat of vaporization is usually much higher than the sensible heat.
3. Look Up Values in Steam Tables
Using your known temperature or pressure, locate the values for hf and hfg. For example, at 100°C (atmospheric pressure):
- hf ≈ 419 kJ/kg
- hfg ≈ 2257 kJ/kg
4. Perform the Calculation
If you have dry saturated steam at 100°C:
h = 419 + (1.0 × 2257) = 2676 kJ/kg
Superheated Steam Enthalpy
If the steam is heated beyond its saturation temperature, it becomes superheated. In this state, the quality (x) is no longer applicable. To calculate the enthalpy of superheated steam, you must use a different section of the steam tables or use the formula:
h = hg + cps(Tsuper - Tsat)
Where cps is the specific heat of steam at constant pressure and Tsuper - Tsat is the degree of superheat.