Saturated Steam Enthalpy Calculator and Formula

A saturated-steam enthalpy calculator estimates the specific enthalpy of water or steam at the saturation temperature or pressure you enter, using steam-table data. Choose the saturated-liquid or saturated-vapor value; if both values are available, the calculator can also estimate a wet mixture from its vapor quality.

Formula

For a saturated liquid–vapor mixture, specific enthalpy is calculated as h = h_f + x(h_g − h_f). Here h is mixture specific enthalpy in kJ/kg, h_f is saturated-liquid specific enthalpy in kJ/kg, h_g is saturated-vapor specific enthalpy in kJ/kg, and x is vapor quality, a dimensionless mass fraction from 0 to 1. The calculator’s worked example uses approximate steam-table endpoint values at 100 °C: h_f = 419.04 kJ/kg and h_g = 2675.5 kJ/kg. These rounded endpoint values are supplied as defaults, not recalculated for each entered temperature.

Important limitation: a usable general calculator must obtain h_f and h_g from a steam-property table or a validated water-property equation at the entered saturation condition. The compact calculator above demonstrates the mixture-quality formula using the stated 100 °C reference values; it does not adjust those endpoints when the temperature input changes. For accurate results away from 100 °C, look up both endpoints at the selected saturation temperature, then apply the formula. The temperature input’s physical range is bounded by water’s triple point and critical point; the displayed input limits are a practical range, not a claim that the fixed reference values apply throughout it.

Worked Example

Using exactly the defaults, T = 100 °C and x = 1:

• h_f = 419.04 kJ/kg.
• h_g = 2675.5 kJ/kg.
• h = 419.04 + 1 × (2675.5 − 419.04) = 2675.50 kJ/kg.

The result is 2675.50 kJ/kg, equal to saturated-vapor enthalpy for the reference condition and values used here.

Assumptions

• The substance is water in liquid–vapor equilibrium at saturation.
• Quality x means the mass fraction of the total mixture that is vapor; x = 0 is saturated liquid and x = 1 is saturated vapor.
• The reference endpoint values are approximate and correspond to water near 100 °C at its saturation pressure, about 101.4 kPa absolute.
• Enthalpy values depend on the selected reference convention, though enthalpy differences used in energy balances are generally the useful quantity.

What Saturated-Steam Enthalpy Means

Enthalpy is a thermodynamic property commonly used to track energy transferred with flowing steam or water. Specific enthalpy is energy per unit mass, typically stated in kJ/kg. It is not the same as temperature: two saturated states at different pressures have different saturation temperatures and different enthalpies, while liquid water and vapor at the same saturation condition have distinct enthalpy values.

“Saturated” identifies a state on the boundary between liquid and vapor. At a specified pressure, saturation temperature is fixed; equivalently, specifying saturation temperature fixes the saturation pressure. Below the critical point, the saturated-liquid value h_f describes liquid just about to boil, and saturated-vapor value h_g describes vapor just after the last liquid disappears.

Steam tables often also list h_fg, the enthalpy of vaporization: h_fg = h_g − h_f. It is the energy difference per unit mass between the saturated vapor and liquid at the same pressure. It is not a universal constant; it changes with saturation conditions and approaches zero at the critical point.

How to Use Steam Tables and Quality

First identify what is known: saturation pressure or saturation temperature. Find that state in a consistent table, then read the saturated-liquid and saturated-vapor enthalpies. If the state is a two-phase mixture and its quality is known, interpolate between those values using the mixture equation.

1. Confirm the pressure is absolute, not gauge, when using pressure-based tables. Gauge pressure needs local atmospheric pressure added.
2. Use the saturation row matching the specified pressure or temperature. If the exact value is between rows, interpolate carefully or use a validated property tool.
3. Record h_f and h_g in the same units and from the same property source.
4. Enter quality as a fraction: for example, 0.90 means 90% vapor by mass.
5. Use h = h_f + xh_fg and retain sensible precision through intermediate calculations.

For example, if a table gives h_f = 640 kJ/kg and h_g = 2780 kJ/kg at a particular saturation condition, a mixture with x = 0.90 has h = 640 + 0.90(2780 − 640) = 2566 kJ/kg. Those endpoint values are illustrative only; use the actual row for the system being analyzed.

Common Errors and Limits

• Confusing quality with dryness by volume: quality is a mass fraction, not the fraction of the mixture’s volume occupied by vapor. Because vapor is far less dense, even modest vapor mass fractions can occupy most of the volume.
• Using gauge pressure as absolute pressure: steam tables generally use absolute pressure. A pressure reference mismatch can select the wrong saturation state.
• Applying saturated values to superheated steam: superheated vapor is above saturation temperature at its pressure. Use a superheated-steam table or property model, not the saturated-vapor column.
• Assuming the fixed calculator defaults suit every condition: they do not. The displayed formula is useful only after inserting endpoint values appropriate to the actual saturation state.
• Mixing property references or units: table conventions and units must match; do not combine kJ/kg and Btu/lb values without conversion.

Quality is meaningful only in the two-phase region. A single-phase compressed liquid or superheated vapor does not have a liquid–vapor mass quality between zero and one. At the critical point, saturated liquid and vapor cease to be distinct phases, so the usual quality interpretation no longer applies.

From Specific Enthalpy to Energy

Specific enthalpy alone is not total energy. For a mass m in kilograms, the corresponding enthalpy amount is H = mh, in kJ, if the chosen reference convention is used consistently. For a steady-flow process, an enthalpy change can help estimate heat transfer or work, but a complete energy balance may also require kinetic energy, elevation changes, shaft work, and heat losses.

For a change of state, use the difference Δh = h₂ − h₁. Multiplying by mass gives mΔh. This difference is often more informative than either absolute table value because the zero of enthalpy is a reference choice. The same reference and compatible property data must be used for both states.

Frequently Asked Questions

What is the enthalpy of saturated steam at 100 °C?
At about 100 °C and 101.4 kPa absolute, saturated-vapor enthalpy is approximately 2675.5 kJ/kg in common steam tables. The corresponding saturated-liquid value is about 419.0 kJ/kg.

How do I calculate wet steam enthalpy?
Use h = h_f + x(h_g − h_f), with endpoint values from the same saturation condition and quality entered as a 0–1 mass fraction.

Is saturated steam enthalpy the same as latent heat?
No. Saturated-vapor enthalpy is h_g; latent heat of vaporization is h_fg = h_g − h_f.

Can I find enthalpy from temperature alone?
Temperature identifies a saturation state only when saturation is known. For a general water or steam state, pressure and phase information may also be needed.

What is the quality of dry saturated steam?
Dry saturated steam has x = 1, meaning the mixture is entirely vapor at saturation. It is not superheated steam; superheated vapor is above the saturation temperature at the same pressure.

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in t | Saturation temperature | °C | 100 | 0 | 373.946
in x | Vapor quality | - | 1 | 0 | 1
out h | Saturated mixture specific enthalpy | kJ/kg | 419.04 + x * (2675.5 - 419.04) | 2
out hf | Saturated-liquid enthalpy | kJ/kg | 419.04 | 2
out hg | Saturated-vapor enthalpy | kJ/kg | 2675.5 | 2