Steam Enthalpy Calculator
Estimate saturated-water, saturated-steam, latent, and wet-mixture enthalpy from absolute pressure in U.S. customary units. Add steam quality and mass flow to calculate vapor/liquid flow split and heat rate, with the interpolation bracket kept visible.
Define the saturation state
Pressure must be absolute. The model is intentionally limited to saturated liquid, wet steam, and dry saturated vapor.
At 100.000 psia, dry saturated steam is estimated at 327.74 °F and 1,187.49 Btu/lb. Raising 1,000.00 lb/h from the entered 180.00 Btu/lb inlet state to this saturation state requires about 1.007 MMBtu/h before equipment losses.
What this saturation calculation includes
At saturation pressure, liquid water and water vapor can coexist at one saturation temperature. The NIST pressure table lists saturated-liquid enthalpy hf, saturated-vapor enthalpy hg, and their difference hfg. The difference represents the enthalpy change associated with vaporization at that saturation condition under the table’s reference convention.
hmixture = hf + x hfg
Vapor mass flow = x × total mass flow
Heat rate above inlet = mass flow × (hmixture − hinlet)
Steam quality x is the vapor mass fraction of a saturated two-phase mixture. A quality of 0 is saturated liquid; 1 is dry saturated vapor; 0.90 means 90% of the mass is vapor and 10% is entrained or coexisting liquid under the equilibrium mixture definition. Quality is not relative humidity, volume fraction, boiler efficiency, or the percentage of water molecules that “feel hot.”
The calculator converts NIST values from MPa, degrees Celsius, and kJ/kg to psia, degrees Fahrenheit, and Btu/lb. It then performs linear interpolation in absolute pressure between the neighboring embedded pressure nodes. The bracket and fraction are displayed so the numerical method is auditable rather than hidden.
Use absolute pressure, not gauge pressure
Thermodynamic saturation depends on absolute pressure. A gauge reading reports pressure above local atmosphere. Near sea level, a 100 psig gauge reading is roughly 114.7 psia, but atmospheric pressure varies with weather and elevation. Add an appropriate measured or specified local atmospheric pressure when converting psig to psia.
Confusing 100 psig with 100 psia changes saturation temperature and enthalpy. The error is especially significant at low pressure. A vacuum gauge may use inches of mercury or another convention and must be converted to absolute pressure carefully. Never add 14.7 blindly when the project defines another atmospheric reference.
Pressure should also represent the state location being analyzed. Steam can lose pressure across valves, piping, traps, separators, and equipment. A boiler drum pressure is not automatically the same as the pressure at a distant user. Heat loss and pressure drop can produce condensation before the measurement point.
Quality belongs only inside the saturation dome
The quality equation is valid for an equilibrium liquid-vapor mixture at saturation. Superheated steam has a temperature above saturation at its pressure and is not “more than 100% quality.” Compressed or subcooled liquid is below saturation temperature at its pressure and is not negative quality. Those single-phase states require both pressure and temperature with an appropriate water-property formulation.
Dry saturated vapor is x = 1 at the vapor boundary. A tiny addition of heat at constant pressure moves it into the superheated region, where h is no longer obtained from hf + xhfg. Near the critical point, the distinction between saturated liquid and vapor disappears and hfg approaches zero; simple table interpolation deserves extra caution.
Industrial “steam quality” may be inferred through calorimetry or sampling and can be affected by nonequilibrium droplets, dissolved solids, or measurement location. Use applicable test methods and qualified analysis when efficiency, turbine protection, custody, or safety depends on the value.
Worked 100 psia saturated-steam example
The default pressure is 100 psia, equivalent to approximately 0.68948 MPa. It falls between embedded NIST pressure nodes at 0.60 MPa, about 87.02 psia, and 0.70 MPa, about 101.53 psia. The position is 89.48% of the pressure interval. Linear interpolation gives a saturation temperature of approximately 327.74 °F.
Interpolated saturated-liquid enthalpy is about 298.45 Btu/lb and saturated-vapor enthalpy is about 1,187.49 Btu/lb. Latent enthalpy is therefore approximately 889.04 Btu/lb. With quality set to 100%, mixture enthalpy equals the saturated-vapor value and all 1,000 lb/h is counted in the vapor mass flow.
Vaporizing saturated liquid at this state requires about 889,035 Btu/h for 1,000 lb/h in the ideal property balance. If the known inlet state has an enthalpy of 180 Btu/lb, the total ideal rise to dry saturated vapor is about 1,007,487 Btu/h, 1.007 MMBtu/h, or 295.27 thermal kW. Boiler fuel input would be higher because combustion, radiation, blowdown, and other losses are outside the property balance.
Reference enthalpy and energy differences
Specific enthalpy is reported relative to a reference convention; its absolute numeric zero is not a directly measurable tank of energy. Engineering balances use differences calculated with a consistent formulation and reference. The NIST/IAPWS table values are internally consistent. Mixing an inlet enthalpy from an unrelated table, reference, or fluid model can create an artificial offset.
The user-entered inlet enthalpy is accepted as a known property, not derived from temperature. Subcooled feedwater enthalpy depends primarily on temperature in many low-pressure estimates, but pressure and the selected property model still matter. Obtain hinlet from a consistent water-property source at the actual inlet pressure and temperature. Do not simply enter inlet temperature as though degrees Fahrenheit were Btu/lb.
A positive duty means energy must be added to move from the entered inlet enthalpy to the calculated mixture. A negative result means the entered baseline enthalpy is higher, signaling heat removal under that algebraic balance or an inconsistent state selection. Equipment efficiency is not applied. Heat-transfer surface, pinch temperature, fouling, condensate return, blowdown, flash steam, and distribution losses need separate models.
Interpolation accuracy and project limits
| Question | This calculator provides | Use a full property package when |
|---|---|---|
| Saturated temperature and enthalpy | Linear interpolation of selected NISTIR 5078 pressure nodes from 0.007 to 14 MPa. | Certification, high precision, near-critical work, or a contract specifies a particular implementation. |
| Wet-mixture enthalpy | Equilibrium h = hf + xhfg for 0 ≤ x ≤ 1. | The flow is superheated, subcooled, flashing dynamically, chemically contaminated, or nonequilibrium. |
| Heat rate | Steady mass flow multiplied by a specific-enthalpy difference. | Mass flow varies, heat is stored, pressure drops, equipment losses, or phase separation must be modeled. |
| U.S. customary display | psia, °F, Btu/lb, lb/h, Btu/h, MMBtu/h, and thermal kW. | Project equations require another unit standard or traceable calculation implementation. |
Linear interpolation error depends on node spacing and property curvature. The embedded nodes are denser in common low- and medium-pressure ranges and become coarser at high pressure. Results are rounded as estimates; extra displayed digits do not establish accuracy. Cross-check critical work directly against NIST, IAPWS-compliant software, or the property method required by the project.
Steam systems involve burn, pressure, rupture, water-hammer, and stored-energy hazards. Property values do not size relief devices, pressure vessels, piping, traps, controls, or boiler safety equipment. Applicable codes, manufacturer instructions, and qualified professionals govern design and operation.
Steam enthalpy FAQs
What is the enthalpy of steam at 100 psia?
For dry saturated vapor, this interpolation estimates about 1,187.49 Btu/lb at a saturation temperature near 327.74 °F. Superheated steam at 100 psia has a different enthalpy that also requires temperature.
Is 100 psig the same as 100 psia?
No. Psig is relative to local atmospheric pressure; psia is absolute. Near standard sea-level atmosphere, 100 psig is roughly 114.7 psia, but local atmospheric pressure varies. Saturation calculations require absolute pressure.
What does 90% steam quality mean?
It means 90% of the saturated two-phase mixture mass is vapor and 10% is liquid under the equilibrium quality definition. Mixture enthalpy is hf plus 0.90 times hfg. It does not mean superheated steam.
Why does latent heat decrease as pressure rises?
As saturation approaches the critical point, liquid and vapor properties converge and the enthalpy difference between them decreases. At the critical point the distinct liquid-vapor phase boundary ends. This calculator stops below that point at 14 MPa.
Can I enter steam temperature instead of pressure?
No. At saturation, either pressure or temperature fixes the saturation state, but the embedded interpolation is organized by absolute pressure. Superheated or compressed states need both independent properties and a full formulation.
Does the heat-rate result equal boiler fuel input?
No. It is an ideal fluid enthalpy-rate difference. Fuel input depends on boiler efficiency plus radiation, stack, blowdown, startup, cycling, distribution, and other losses. Use measured performance or a code-appropriate boiler model.
References
The embedded saturation nodes are transcribed from the official NISTIR 5078 pressure table, which is calculated from the IAPWS formulation. Consult these first-party sources for authoritative data and formulation scope.