STEAM · ENGINEERING REFERENCE

Steam states and heat calculations

Distinguish phase, pressure and energy terms before selecting properties and calculation methods.

Identify the state and absolute pressure

Before using a table or calculation, identify the fluid, temperature, pressure, phase and properties needed. Thermodynamic relations use absolute pressure: gauge pressure is measured relative to local atmospheric pressure, which must be added when converting to absolute pressure. A gauge reading cannot be used directly as an absolute pressure.

For subcritical liquid–vapour equilibrium, saturation temperature and pressure correspond to one another. At a specified absolute pressure, vapour above the corresponding saturation temperature is superheated. To find superheated steam properties, match both pressure and temperature.

Saturation and steam quality

In subcritical saturated water–steam equilibrium, temperature and pressure are not independent pieces of information. Knowing the saturation temperature and its corresponding pressure alone does not establish how much vapour is present: quality or another property sufficient to determine the state is also needed.

For an equilibrium wet mixture, quality x is the vapour mass fraction of the total liquid and vapour mass, with 0 ≤ x ≤ 1. At x = 0 the state is saturated liquid; at x = 1 it is dry saturated vapour; 0 < x < 1 describes wet steam. Quality is not a volume fraction and does not describe a superheated state. Real two-phase flow may have non-equilibrium, entrainment and uneven distribution that a simple equilibrium model does not capture.

Enthalpy, latent heat and other properties

At the same saturation pressure or temperature and on a consistent enthalpy reference, write saturated liquid specific enthalpy as h_f and dry saturated vapour specific enthalpy as h_g. The latent heat of vaporization is h_fg = h_g − h_f. For an ideal equilibrium two-phase mixture, specific enthalpy is h = h_f + xh_fg. These specific enthalpies are commonly expressed in kJ/kg of fluid; check units and reference states.

h_fg = h_g − h_f; h = h_f + xh_fg

The inlet-to-outlet specific enthalpy difference is used in steady-flow energy balances; it is not automatically the useful heat delivered by equipment, because heat loss, mechanical work, kinetic energy, pressure drop and other flows may also matter. Transport properties such as viscosity and thermal conductivity must be obtained separately for the relevant state and correlation; they cannot be inferred directly from enthalpy or latent heat.

Condensation example at 100 °C

At 100 °C, the saturation absolute pressure of water is approximately 101.418 kPa; the latent heat of vaporization in this site's saturation table is about 2256.4 kJ/kg. If 0.1 kg/s of dry saturated steam completely condenses to saturated liquid at this state, the magnitude of the latent heat released is approximately:

0.1 kg/s × 2256.4 kJ/kg = 225.64 kW

Taking heat absorbed by the fluid as positive, the steam-side result is −225.64 kW. This includes only isothermal phase-change latent heat and neglects pressure drop, heat loss, inlet superheat and condensate subcooling; it is not a complete equipment load. If inlet steam is wet or condensation is incomplete, multiplying the full mass flow by h_fg is not directly applicable.

View the saturated water and steam table · Use the phase-change latent heat tool

From one state to a steam system

Condensate drainage depends on the steam system's operating conditions. Steam-trap selection, installation and maintenance require consideration of pressure, load, backpressure and equipment requirements. Accumulated water or drainage faults can contribute to water hammer and other hazards.
Insulation affects heat loss and surface temperature of piping and fittings. Actual performance depends on material, thickness, surface conditions and installation; an ideal plane-wall result is not a direct prediction of piping energy savings.
Condensate recovery involves water quality, pressure, temperature, flash steam and system compatibility. Its potential must be evaluated against the system's full mass and energy balances.

These points explain concepts only. Qualified personnel must use actual conditions, equipment instructions and applicable rules for site design, selection and operation.

Tools and sources

Saturated water and steam table Saturation endpoint transport properties and heat capacity Single-phase water and superheated steam states Phase-change latent heat rate

Further reading: IAPWS IF97 industrial formulation for water and steam thermodynamic properties; U.S. Department of Energy steam-system resources. This site's saturation table uses CoolProp's IAPWS-95 model; IF97 is a related industrial formulation, not the same implementation.