⚙️ Configuration

Select Fluid / Phase Type

📊 Results

💧

Ready to Calculate

Select a fluid type on the left, enter conditions, then click Calculate

Computing properties…

📖

Thermodynamic Terms & Definitions — IAPWS-IF97

Formulas · Engineering significance · Accuracy notes · Key reference values

📖

Thermodynamic Terms & Definitions

IAPWS-IF97 properties — formulas, engineering use & accuracy

⚡Property Definitions

Specific Enthalpy

h = u + P·v

Total thermodynamic energy per unit mass — internal energy plus flow work (P·v). The key property for boiler, heat exchanger, and turbine design. For open systems: Q = ṁ·Δh.

SI: kJ/kg | Imperial: BTU/lb

Specific Entropy

ds = δQ_rev / T

Measure of molecular disorder per unit mass. Constant in ideal isentropic processes (turbines, compressors). Higher entropy = more energy degraded. Always increases in real processes.

SI: kJ/(kg·K) | Imperial: BTU/(lb·°R)

Internal Energy

u = h − P·v

Microscopic kinetic + potential energy stored in molecules, excluding flow work. Used in closed-system (piston-cylinder) energy balances. At saturation: u_f = h_f − P·v_f.

SI: kJ/kg | Imperial: BTU/lb

Specific Volume

v = V/m = 1/ρ

Volume per unit mass — reciprocal of density. Liquid water ≈ 0.001 m³/kg. Steam expands dramatically at low pressure: v_g ≈ 206 m³/kg at 0.01°C. Critical for pipe and nozzle sizing.

SI: m³/kg | Imperial: ft³/lb

Density

ρ = 1/v [kg/m³]

Mass per unit volume. Saturated liquid ≈ 958 kg/m³ at 100°C. Saturated steam at 1 bar ≈ 0.598 kg/m³ — approximately 1600× less dense. Used for flow velocity and pressure drop calculations.

SI: kg/m³ | Imperial: lb/ft³

Steam Quality

x = m_vapor / m_total

Vapor mass fraction in two-phase mixture. x = 0: saturated liquid; x = 1: dry saturated vapor. For mixture: h = h_f + x·h_fg. Turbine exhaust must have x ≥ 0.88 to prevent blade erosion.

Dimensionless · Range: 0.0 – 1.0

Latent Heat

h_fg

h_fg = h_g − h_f

Energy required to convert saturated liquid → saturated vapor at constant T and P. At 100°C: 2257 kJ/kg. Decreases with rising pressure, reaching zero at the critical point (374.14°C, 220.9 bar).

SI: kJ/kg | Imperial: BTU/lb

Degree of Superheat

ΔT_sh

ΔT_sh = T − T_sat(P)

How far above saturation temperature superheated steam is. Higher superheat → more specific work in turbines. Power plants use 50–200°C superheat. ΔT_sh = 0 marks the saturation boundary.

SI: °C | Imperial: °F

Critical Point

Tc, Pc

T_c = 374.14°C · P_c = 220.9 bar

Above this state, liquid and vapor phases become indistinguishable — h_fg = 0 and ρ_f = ρ_g = 317 kg/m³. Supercritical steam is used in advanced power plants for efficiencies above 45%.

IAPWS-IF97 reference values

Saturation Pressure

P_sat

Wagner equation — ±0.003 bar

Equilibrium pressure at which phase change occurs at a given T. At 100°C = 1.01325 bar; at 200°C = 15.54 bar; at 300°C = 85.88 bar. Used for flash calculation and steam trap sizing.

SI: bar / MPa | Imperial: psia

🔬Formulation Accuracy — IAPWS-IF97

Calculation Method & Error Bounds

Saturation Pressure:  Wagner equation (IAPWS-IF97 §8.1)  →  ±0.003 bar
T_sat from P:         Newton iteration on Wagner           →  ±0.01°C
Sat. Properties:      Interpolated reference tables (34 pts, 0.01–374°C)
                      h_f, h_fg, h_g  →  ±0.3 kJ/kg
                      s_f, s_fg, s_g  →  ±0.002 kJ/(kg·K)
                      v_f, v_g        →  ±0.1%
Compressed Liquid:    h = h_f(T) + v_f·ΔP  (IAPWS approx.)   →  ±0.5%
Superheated Steam:    Cp polynomial + sat. boundary           →  ±0.5%

For critical engineering decisions, always verify against certified IAPWS-IF97 tables.