Steam Bulletin
No. 71
Which is Heavier: Air or Steam?
You might be surprised to learn that the correct answer is "it depends".

The average molecular weight of air is 29, heavier than the molecular weight of steam (18). However, the actual densities of air and steam vary greatly based on steam-to-air ratio, temperature, and pressure.
For example, at 1.0 MPaG (145 psig), air is heavier than steam at temperatures under 162 °C (324 ºF), but lighter than steam at temperatures over 162 °C (324 ºF). However at 2.0 MPaG (290 psig), the relationship between the densities of steam and air changes at 188 °C (370 ºF).
How does this affect your steam system?
When configuring your system for proper air venting, it is important to realize that air vents cannot simply be installed on the top or bottom of equipment because of the varying densities of steam and air. Proper air venting can thus require more than one air vent depending on equipment configuration.
Read on to learn about air vents and how air removal helps optimize your steam system.

- Air greatly interferes with heat transfer because it is a poor heat conductor.
- Air can lead to long start-up times because it restricts steam flow.
- Air can cause corrosion in steel piping due to rust.
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Contact your local TLV representative for details.
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Best regards,
TLV