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)
ps_ effective = effective steam pressure, absolute (Pa, bar, psi)
vs = volume of steam (m3, in3)
V = volume of mixture (m3, in3)
p = absolute pressure (Pa, bar, psi)
Reducing the part of steam reduces the effective steam pressure. Increasing the part of steam (until 100%) increases the effective pressure.
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.
|Mixture Pressure||0% Air |
|5% Air||10% Air||15% Air|
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