Properties
Thermodynamic properties describe the state of a substance. Learn systems and boundaries, pressure, temperature and specific volume, the ideal-gas law, phases and saturation, quality and how to read steam tables.
Before calculating energy flows, you must describe the substance: how hot, how pressurised, how much space it takes up, whether it's liquid, vapour or both. These are properties, and two independent properties usually fix the whole state.
Air in an engine behaves like an ideal gas; water in a power plant changes phase and needs property tables. Knowing which model to use is the first skill of thermodynamics.
An analogy
A property is like a GPS coordinate for the substance: give two coordinates (say pressure and temperature) and every other property — density, energy, enthalpy — can be looked up.
Systems, boundaries and states
A closed system holds a fixed mass (gas in a sealed cylinder); an open system (control volume) has mass flowing through (a turbine, pump, nozzle). The state is described by properties. Intensive properties (p, T, v) don't depend on size; extensive ones (V, U, mass) do.
Always use absolute values
Gas laws need absolute temperature (kelvin) and absolute pressure. A tyre gauge reads 220 kPa gauge, which is about 321 kPa absolute. Forgetting this is the most common thermodynamics mistake.
Phases and saturation
At a given pressure, water boils at one temperature (100 °C at 101.3 kPa, 180 °C at 1 MPa). Between saturated liquid and saturated vapour is the wet region, described by quality x. Beyond the critical point (22.06 MPa, 374 °C) liquid and vapour are indistinguishable — supercritical power plants operate there.
Reading property tables
Steam tables list v, u, h, s for saturated states (by T or p) and for superheated vapour. Interpolate linearly between rows. Modern practice uses software (CoolProp, REFPROP, EES) but the logic is identical.
Real-gas behaviour
At high pressure or near saturation, gases deviate from pv = RT. The compressibility factor Z = pv/RT from generalized charts (reduced p and T) or equations of state (van der Waals, Peng–Robinson) corrects for it — essential for natural-gas pipelines, CO₂ and refrigerants.
Formulas
- Ideal gas law:
pV = mRT or pv = RT - Absolute temperature & pressure:
T(K) = T(°C) + 273.15, p_abs = p_gauge + p_atm - Wet mixture:
y = y_f + x · y_fg (y = v, u, h or s) - Combined gas law:
p₁V₁/T₁ = p₂V₂/T₂
Worked example: Mass of air in a tank
Given: V = 0.5 m³; p = 800 kPa (abs), T = 27 °C; R = 0.287 kJ/kg·K
Find: Mass, and pressure if heated to 60 °C.
- m = pV/RT = 800 × 0.5/(0.287 × 300.15) = 4.65 kg
- Constant volume: p₂ = 800 × 333.15/300.15 = 888 kPa
Answer: 4.65 kg; 888 kPa at 60 °C
Worked example: Wet steam enthalpy
Given: p = 100 kPa, x = 0.9; h_f = 417.5, h_fg = 2257.5 kJ/kg
Find: Specific enthalpy.
- h = 417.5 + 0.9 × 2257.5 = 2449 kJ/kg
Answer: ≈ 2449 kJ/kg
Common mistakes
- Wrong: Using °C or gauge pressure in pv = RT. — Right: Always convert to kelvin and absolute pressure.
- Wrong: Using the ideal-gas law for wet steam. — Right: Use steam tables in or near the two-phase region.
Applications
- Compressed-air systems: Receiver sizing and leak-down tests use pV = mRT.
- Steam plants: Boiler pressure sets saturation temperature; quality at the turbine exit limits blade erosion.
- Refrigerants: Property tables of R-134a, R-32, CO₂ and ammonia define system pressures and capacities.
Key takeaways
- Two independent properties fix the state.
- pv = RT with absolute T and p.
- Wet region: y = y_f + x·y_fg.
- Use tables/software near phase change.
Going further
- State postulate: For a simple compressible substance, two independent intensive properties fix the state — but in the wet region p and T are not independent, so use x.
- Mollier (h–s) chart: Plots enthalpy vs entropy; turbine expansions are near-vertical lines, making it the classic steam-turbine design tool.
- Equations of state in software: Helmholtz-energy formulations (IAPWS-95 for water) give highly accurate properties used by CoolProp and REFPROP.