Content of Air in Steam Systems

Effect of Air on Saturation Temperature

In a pure saturated steam system, pressure and temperature have a defined relationship. When air is present in the system, part of the total pressure is created by air rather than steam. As a result, the partial pressure of steam is reduced and the actual saturation temperature is lower than the value expected from the total pressure reading.

As an indication, at an operating pressure of 7 barg, pure saturated steam has a temperature of approximately 170°C. If the air content reaches about 20% by volume, the actual steam temperature may fall to approximately 160°C.

Air Content in Steam Systems Table

The following table shows the required gauge pressure for air-contaminated steam in order to achieve the same saturation temperature as pure steam at the corresponding pressure.

Why Air Causes Problems in Steam Systems

Engineering Notes for Design and Maintenance

• Steam systems should include suitable air venting points at high points and line ends.
• During start-up, air should be removed quickly to reduce warm-up time.
• In heat exchangers and coils, air reduces heat transfer performance and may create cold zones.
• Steam trap selection should consider not only condensate discharge, but also air venting capability.
• Operating temperature should be compared with steam pressure to identify possible air contamination.

Related Equipment for Steam Systems

Philippopoulos S.A. supplies equipment for industrial and marine steam systems, including steam traps, thermostatic air vents, separators, strainers, isolation valves, control valves, pressure reducing valves, safety valves, condensate pumps and boiler house equipment.