Why is air in steam harmful?

It lowers the temperature, forms an insulating layer on the heat transfer surface, and together with condensate, leads to corrosion (carbonic acid, oxygen corrosion). A 1 mm air film creates resistance equivalent to a copper wall of about 15 m in terms of heat transfer.

Why is the temperature low if the pressure gauge is correct?

The manometer shows the total pressure of the vapor + air mixture; the temperature depends only on the partial pressure of the vapor. If the measured temperature is lower than the saturation temperature, there is air (or other non-condensable gas) in the steam.

Air in Steam – Temperature Drop and Air Ratio Calculation

The air entering the steam system (at first start-up, in vacuum zones or from the feed water) mixes with the steam. Even if the pressure of the mixture remains the same, the partial pressure of the steam decreases; the temperature also decreases to the saturation temperature at that partial pressure. In addition, the air forms an insulating film on the heating surface.

Formula (Dalton’s law)

p(steam) = p · (1 − a) T(mixture) = Saturation(p(steam)) a = 1 − psat(T) / p

Example

If 10% air by volume is mixed with 5 bar(g) of steam, the temperature will be 154.8 °C instead of 158.9 °C; that is, it drops by 4.1 °C . If 150 °C is measured at the same pressure, there is approximately 21% air in the steam.

Thermostatic air vents are placed at the ends of the lines and the upper parts of the heaters to remove the air; a thermal deaerator is used in the feed water tank. To evaluate your steam system as a whole, you can take a look at our steam boiler guide and get a free quote for your project.

Calculation of the Effect of Air in Steam on Temperature

Formula (Dalton’s law)

Example

Frequently Asked Questions