Air and Steam Mixture

With air in the steam the surface temperatures in heat exchangers will be lower - less heat than excepted will be transferred

Air in the steam will cause the temperatures on heat exchangers surfaces to be lower than expected due to the saturation temperatures in the steam tables. The heat transfer will be lower and the system efficiency will be reduced.

This situation can de described with the Daltons Law of Partial Pressure.

Daltons Law of Partial Pressure

The total pressure of a mixture of gases is made up by the sum of the partial pressures of the components in the mixture as known from Dalton's Law of Partial Pressures:

The total pressure exerted by a mixture of gases is the sum of the partial pressures of the individual gases!

The total pressure in a mixture of steam and air can be expressed as:

P = Pa + Ps         (1)

where

Pa = partial pressure of air

Ps = partial pressure of steam

The Partial Pressure in a Mixture

The partial pressure is the pressure exerted by each component as if it was occupying the same volume of the mixture. The effective partial pressure of the steam can be expressed as:

Ps_effective = vs / V P         (2)

where

Ps_ effective = effective steam pressure, absolute

vs = volume of steam

V = volume of mixture

P = pressure, absolute

Reducing the part of steam reduces the effective steam pressure. Increasing the part of steam (until 100%) increases the effective pressure.

Example - Mix of Air and Steam

The effective pressure in a steam/air mixture made up by 3 parts steam and 1 part air, with total pressure 5 bar absolute, can be expressed as:

Ps_effective = (3 parts) / (3 parts + 1 part) (5 bar abs)

    = 3.75 bar absolute

Important! Since the steam has an effective pressure of 3.75 bar instead of the pressure of 5 bar a, the mixture would have a temperature of approximately 139 oC rather than the expected saturation temperature of 152 oC. This has a major effect on the heat transfer capability of an heat exchanger.

Resulting Temperature - Air and Steam Mixture

Resulting Temperature - Air and Steam Mixture 
Mixture Pressure 0% Air
(Pure Steam)
5% Air 10% Air 15% Air
(psig) (bar) (oF) (oC) (oF) (oC) (oF) (oC) (oF) (oC)
2 0.15 219 104 216 102 213 101 210 99
5 0.35 227 108 225 107 222 106 219 104
10 0.7 239 115 237 114 233 112 230 110
20 1.4 259 126 256 124 252 122 249 121

Related Topics

  • Steam and Condensate - Steam & condensate properties - capacities, pipe sizing, systems configuration and more
  • Thermodynamics - Thermodynamics of steam and condensate applications

Related Documents

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