Air and Steam Mixtures

Calculate the effective pressure and resulting temperature of air and steam mixtures using Dalton's Law of Partial Pressures. Understand how air contamination affects heat transfer in steam systems.

Mixture Pressure (psig)Mixture Pressure (bar)0% Air - Pure Steam (°F)0% Air - Pure Steam (°C)5% Air (°F)5% Air (°C)10% Air (°F)10% Air (°C)15% Air (°F)15% Air (°C)
20.1521910421610221310121099
50.35227108225107222106219104
100.7239115237114233112230110
201.4259126256124252122249121

Dalton's Law of Partial Pressure

The total pressure in a mixture of steam and air:

p = pa + ps

Where:

  • pa = partial pressure of air (Pa, bar, psi)
  • ps = partial pressure of steam (Pa, bar, psi)

Effective Partial Pressure

The effective partial pressure of steam:

ps_effective = (vs / V) × p

Where:

  • ps_effective = effective steam pressure, absolute (Pa, bar, psi)
  • vs = volume of steam (m³, in³)
  • V = volume of mixture (m³, in³)
  • p = absolute pressure (Pa, bar, psi)

Key Impact

Air in steam reduces the effective steam pressure, which lowers the saturation temperature. This significantly reduces heat transfer capability in heat exchangers.

Example: A steam/air mixture with 3 parts steam and 1 part air at 5 bar absolute has:

  • Effective pressure: 3.75 bar (instead of 5 bar)
  • Resulting temperature: ~139°C (instead of expected 152°C)