Air Mixed in Steam — Temperature Drop

Given the air fraction, the real steam temperature and the degrees lost against pure steam.

Unit system

Inputs

Total pressure (absolute)

What the gauge reads. A gauge cannot tell steam from air.Air fraction (by volume)

For ideal gases the volume fraction equals the mole fraction. 10–20 % is not unusual during start-up.

Actual steam temperature

130°C

Temperature lost

3.557K

Verdict	Noticeable — heat-up takes longer
If it were pure steam	133.5 °C
Partial pressure of steam	2.7 bar a

Dalton’s law of partial pressures: the steam partial pressure is the total pressure times the steam volume fraction.
This gives only the TEMPERATURE loss. Air also forms an insulating film on the surface and lowers the heat-transfer coefficient — the real effect is larger than shown here.

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Results are for engineering pre-assessment and are not a product selection commitment. Final responsibility rests with the user.